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Evaluation of a liquid ionization chamber for relative dosimetry in small and large fields of radiotherapy photon beams
Radiation Measurements 58 (2013) 79e86
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Evaluation of a liquid ionization chamber for relative dosimetry in small and large fields of radiotherapy photon beams E.M. Benítez*, F.J. Casado, S. García-Pareja, J.A. Martín-Viera, C. Moreno, V. Parra Servicio de Radiofísica Hospitalaria, Hospital Regional Universitario Carlos Haya, Av. Carlos Haya s/n, 29010 Málaga, Spain
h i g h l i g h t s When high spatial resolution is required the results confirm the suitability of the liquid chamber. Some energy dependence of the liquid detector can be appreciated in OFs and PDDs for small and large fields. For field sizes >20 20 cm2, the differences in PDDs at great depths exceed the uncertainties estimated. Some drawbacks should be considered: the time to reach stability, the high voltage supply required and the acquiring cost.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2012 Received in revised form 22 August 2013 Accepted 23 August 2013
Commissioning and quality assurance of radiotherapy linear accelerators require measurement of the absorbed dose to water, and a wide range of detectors are available for absolute and relative dosimetry in megavoltage beams. In this paper, the PTW microLion isooctane-filled ionization chamber has been tested to perform relative measurements in a 6 MV photon beam from a linear accelerator. Output factors, percent depth dose and dose profiles have been obtained for small and large fields. These quantities have been compared with those from usual detectors in the routine practice. In order to carry out a more realistic comparison, an uncertainty analysis has been developed, taking type A and B uncertainties into account. The results present microLion as a good option when high spatial resolution is needed, thanks to its reduced sensitive volume. The liquid filling also provides a high signal compared to other detectors, like that based on air filling. Furthermore, the relative response of microLion when field size is varied suggests that this detector has energy dependence, since it is appreciated an over-response for small fields and an under-response for the large ones. This effect is more obvious for field sizes wider than 20 20 cm2, where the differences in percent depth dose at great depths exceed the uncertainties estimated in this study. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Liquid ionization chamber Relative dosimetry Isooctane Uncertainty analysis
1. Introduction Commissioning and determining quality assurance for radiotherapy linear accelerators require measuring the absorbed dose to water. A wide range of detectors is available for absolute and relative dosimetry in megavoltage beams, such as air ionization chambers (in standard, mini and micro sizes), films and diamond detectors. Presently, liquid-filled ionization chambers are not the usual dosimeters used in routine practice.
* Corresponding author. Tel.: þ34 951291436; fax: þ34 951291464. E-mail addresses: [email protected], [email protected] (E.M. Benítez). 1350-4487/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2013.08.009
The first liquid ionization detectors were built in 1950s (Leo, 1994). Higher-density media than gases were studied to increase the number of ioneelectron pairs created and thus achieve a larger signal in the detection process. Additionally, the lower diffusion in liquids indicates that a lower active volume that obtained by reducing the ionization tracking and thereby better spatial resolution is obtained. In contrast to gases, transport phenomena were poorly understood because of an essential problem: The liquid contains electronegative impurities, such as oxygen, which can attach to drifting electrons and pose the technical problem of purification, which is not possible for any liquid substance. The previous situation reduces the number of liquids that are candidates for use in an ionization detector to a very select group that consists of liquefied noble gases, primarily argon and xenon,
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and a few hydrocarbons. To reduce ion recombination, the nonultra purified liquids need high electric fields. Regarding hydrocarbon filling, isooctane and tetramethylsilane (TMS) are commonly employed. The results of a comparative study of both liquids in parallel-plate ionization chambers showed that TMS has a sensitivity that is approximately 50% higher than that of isooctane. A general recombination in TMS is also slightly higher than in isooctane (Wickman and Nyström, 1992). Additionally, the ionization current in isooctane is 300 times higher than that in an air-filled ionization chamber of the same volume (Wickman, 1974). Several studies have described the development of liquid chamber prototypes for therapy beam dosimetry. In the article by Wickman and Nyström (1992), isooctane and TMS plane-parallel chambers were tested for measurements of the absorbed dose to water in radiotherapy fields. Dasu et al. (1998) obtained good results for relative dosimetry in narrow beams using a chamber filled with TMS. Eberle et al. (2003) analyzed another prototype in the online monitoring of Intensity Modulated Radiation Therapy (IMRT) fields, and Stewart et al. (2007) designed and constructed the isooctane-filled chamber named GLIC-03. More recently, González-Castaño et al. (2011) developed another isooctane chamber that presented an adequate behavior for high-precision dosimetry of photon beams. In 2009, PTW (Freiburg, Germany) launched into the market the isooctane-filled ionization chamber type 31018, with the commercial name of microLion. Several papers have been written about this chamber. One publication (Andersson and Tölli, 2011) introduced a procedure for estimating recombination losses in continuous beams, and another study described an approach for estimating the recombination losses in pulsed beams (Tölli et al., 2010). Hrbacek et al. (2011), in their work about the commissioning of a linear accelerator, used this detector to measure dose profiles for field sizes that ranged from 20 20 cm2 to 40 40 cm2. Chung et al. (2012) used the microLion chamber as a reference dose detector for non-standard fields to establish a plan-class-specific correction factor in the new formalism for reference dosimetry in such fields (Alfonso et al., 2008). In this way, the correction factors needed for small fields to convert ionization output factors into dose output factors have been calculated for the microLion chamber for Siemens and Elekta clinacs (Francescon et al., 2011), for CyberKnife (Francescon et al., 2012) and for TomoTherapy (Sterpin et al., 2012). At the time of this study, we have not found in a peerreview journal a complete test for the relative dosimetry of radiotherapy beams for the microLion chamber. Liquid chambers have a response that is considerably higher than air chambers. Thus, the manufacturer can reduce measuring active volume to achieve a high spatial resolution, which is useful in small field dosimetry, which in turn is necessary for radiosurgery and IMRT. The AAPM TG-106 report (Das et al., 2008) about accelerator-beam data commissioning equipment and procedures documented the following: “Small volume chambers tend to have different characteristics for large fields compared to small fields and should not be used for all field sizes unless it can be documented that accurate data can be acquired for all field sizes”. To test the behavior of this chamber for relative dosimetry in a water phantom, our group performed the following measurements using a 6 MV photon beam from a Varian DBX linear accelerator: percent depth dose (PDDs), output factors (OFs) and dose profiles (DPs). These quantities have been measured in the large and small fields that are used in conventional radiotherapy and special techniques, such as radiosurgery. For small fields, the correction factors should be incorporated, when available, for the Varian ClinacÒ and the set of detectors that have been considered. We compared the obtained results with those measured using the usual detectors in the routine practice. With the aim of
Fig. 1. (a) Front and (b) lateral schematic views of the liquid ionization chamber, microLion. The reference point is situated at the center of the sensitive volume.
quantifying the agreement of the comparison, we developed a procedure to estimate the measurement uncertainty, accounting for both type A and type B components. 2. Materials and methods 2.1. Experimental set-up For this study, we used 6 MV photon beams from a Varian DBX linear accelerator (Varian, Palo Alto, CA, USA). Measurements were performed from 0.6 0.6 cm2 to 40 40 cm2 by setting the secondary collimator jaws while the multileaf collimator was fully retracted. We set the dose rate at 300 MU min1, which is equivalent to 1.98 Gy min1 in reference conditions, i.e., at 100 cm SSD and 10 cm depth in water, for a field size of 10 10 cm2. We performed measurements in a water phantom included in the PTW MP3 Therapy Beam Analyzer. We positioned the detectors using the PTW TRUFIX precision attachment system, which served to accurately position the detector effective points by properly using holders and stop thimbles, thereby allowing an exchange of detectors without the need to set a new zero point. We used the electrometer, which is a dual-channel TANDEM type T10011 obtained from PTW. To acquire PDDs and DPs we employed the PTW MEPHYSTO 7.3 software. We performed all the measurements using positive polarity. We measured the temperature and pressure using a digital barometer (Tinycal Double Function T210, Gometrics, Barcelona, Spain) and a thermometer (HD 2307.0 RTD, Delta Ohm, Padova, Italy). 2.2. Detectors 2.2.1. MICROLION chamber The liquid ionization chamber is the PTW microLion, type 31018, which presents a sensitive volume of 0.0017 cm3 and has a radius and depth of 1.25 mm and 0.35 mm, respectively (Fig. 1). The electrode is made of graphite, and the front wall is composed of 0.02 mm varnish, 0.5 mm polystyrene and 0.28 mm graphite. The filling liquid is isooctane. The microLion chamber is polarized at 800 V, which is provided by the stand-alone PTW HV-SUPPLY T16036. Before irradiation, it is necessary to apply the high voltage for at least 15 min to reach the desired stability. A recommended pre-irradiation >6 Gy should be performed. The recombination factor for the microLion chamber has been estimated for several situations in the experimental set-up. A pulse
E.M. Benítez et al. / Radiation Measurements 58 (2013) 79e86
repetition frequency at 377 Hz has been established by the manufacturer, which represents a value of 0.09 mGy per pulse, calculated under reference conditions. Variations in field size and detector positioning introduce modifications with respect to this result. Estimation on the experimental set-up in our measurements gives an overall interval from 0.01 to 0.14 mGy per pulse. A Swedish working group (Tölli et al., 2010) established the results for the microLion efficiency: In the most unfavorable condition, a recombination factor of 0.993 is obtained at 800 V compared to the recombination factor of 0.997 from the direct application of the Boag theory. The influence of this effect is neglected because the measurement uncertainty is greater, as we discuss below. With regard to the energy dependence, the microLion relative response has been analyzed by varying the field size because the scattered low-energy photon contribution increases with it. For this purpose, the Semiflex air ionization chamber has been employed as a reference detector. 2.2.2. Other detectors To establish a comparison, we performed a set of relative dosimetry measurements using the Diode 60012 and the air ionization chambers Semiflex 31010, PinPoint 31016, Farmer 30006 and Roos 34001, all of which are manufactured by PTW. Several characteristics of the detectors used in this study are presented in Table 1. Similar to the microLion chamber, the other detectors are exposed to a prior irradiation >6 Gy. We evaluated the leakage current pre- and post-irradiation and the charge loss and obtained a value <0.1 pA for all of the detectors, except for the PinPoint, where the value was <1 fA. In addition to the reference chambers, we performed several measurements using a PTW Semiflex 31002 monitoring chamber. 2.3. Relative dosimetric quantities Square fields with sides of 0.6, 1.2, 2, 3, 5, 10, 20, 30 and 40 cm were considered. Every relative dosimetric quantity analyzed in this paper has required a particular measurement set-up, as we detail below.
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The OFs are obtained for the complete range of field sizes using the microLion, Semiflex and PinPoint. The Diode presents an overresponse because of the scattered photons of low energy, whose effect is more pronounced in large photon beams (Griessbach et al., 2005). For this reason, the Diode is not used for sizes >10 10 cm2. Measurements using Farmer are not performed for field sizes <5 5 cm2 because of the volume averaging effect. 2.3.2. Dose profiles The DPs are performed at a depth of 10 cm and an SAD of 100 cm. The detectors microLion, Diode, Semiflex, PinPoint and Farmer are used with the same range of field sizes as for the OFs. The Diode is selected as the reference detector for fields up to 10 10 cm2, and Semiflex is selected as the reference detector for the other fields. To evaluate the agreement among the detectors when the dose profiles are studied, we distinguish two regions as follows: a lowdose gradient region in the central part of the field and another region with a high-dose gradient in the field edge. The value of 80% in the dose profile is chosen to separate both regions. A point-bypoint comparison is conducted in the first region. To compare the results in second region (i.e., with the high-dose gradient), the penumbra 20e80% and the field width were calculated. The DPs from 5 5 cm2 to 40 40 cm2 are performed using a monitoring ionization chamber to minimize the effect of instant fluctuations in the beam. 2.3.3. Percent depth dose The PDDs are measured using the microLion, Diode, PinPoint and Roos. To compare measurements, the Diode is considered to be a reference detector for field sizes up to 10 10 cm2, and the Roos chamber is considered to be a reference detector for larger field sizes. The Diode is employed with the restriction in the field size, as mentioned above, and the Roos is dismissed for field sizes <5 5 cm2 because of the averaging effect. The monitoring ionization chamber is used in the same way as in DPs. 2.4. Uncertainty analysis
2.3.1. Output factors The OFs are obtained for each field size as detector responses at a water depth of 10 cm and an SAD of 100 cm. The measurements are normalized to the 10 10 cm2 field size. According to the set-up recommended by the TRS-398 code of practice (Andreo et al., 2000), for thimble chambers in radial irradiation, the positioning provided by TRUFIX should be corrected to the reference point. These measurements are made using the microLion, Diode, Semiflex, PinPoint and Farmer. Diode was chosen as a reference detector for field sizes 10 10 cm2 because of its reduced active volume (González-Castaño et al., 2011; Das et al., 2008). A Semiflex chamber was considered to be the reference for fields >10 10 cm2.
Although measurements in the present study were not performed under reference conditions, the TRS-398 code of practice has been followed to estimate the type B uncertainty of absorbed dose to water for the microLion chamber. This procedure has been followed previously, as described in a similar publication by Bucciolini et al. (2003). The three magnitudes analyzed in this paper are relative values, i.e., they are obtained as a quotient of two measurements. For this reason, uncertainties due to beam quality determination, long-term stability and other influence quantities, can be compensated. The first uncertainty source considered derives from the experimental set-up conditions, because their uncertainties can get different values for both measurements of every quotient. For this
Table 1 Technical specifications of the different detectors used in this work. Specification
microLion 31018 3
Sensitive volume (cm ) Operating voltage (V) Nominal response (nC/Gy) Field size range (cm2) Ion collection time (s) Approx. relative cost
0.0017 800 9.8 1 1.20 20 5.3 103 3.3
Diode 60012 6
2.5 10 0 175 1 1.10 10 w106 1.4
Semiflex 31010
PinPoint 31016
Farmer 30006
Roos 34001
0.125 400 3.3 2 2.40 40 0.1 103 1
0.016 400 0.4 2 2.30 30 60 106 1.3
0.6 300 20 5 5.40 40 0.18 103 1.2
0.35 100 12 4 4.40 40 0.3 103 2.4
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purpose the detector positioning variability and the field size influence are contemplated instead of using the 0.4% indicated in the TRS-398. With regard to the positioning, the product data specify that the MP3 water tank provides an accuracy of 0.1 mm, whereas the TRUFIX precision attachment offers 0.3 mm. To convert the distance uncertainty into the absorbed dose uncertainty, we use the information from the vicinity of the measurement point in DPs and PDDs. The combined result is estimated under the conservative assumption of an uncertainty rectangular distribution for both values. The field size influence is evaluated by resetting the jaw position before every measurement, repeating this procedure five times. An alternative way to avoid this influence is to state every field size by setting the collimator and subsequently measuring with every detector; however, this method greatly increases the measuring time. The second uncertainty source has as its origin the dosimeter reading relative to the beam monitor. Ionization readings for the OFs are not simultaneous; therefore, they are affected by the beam monitor. The 0.6% uncertainty from the TRS-398 is reported in this paper. With the aim of avoiding this effect, a monitoring chamber is employed for measurements of PDDs and DPs, except when the field size is < 5 5 cm2. Only if the monitoring chamber is present can this uncertainty component be compensated in the quotients. Because of the particular shape of DP curves, it seems adequate to express uncertainty in two different ways: in distance (mm) for the penumbra and the field width, where there is a high-dose gradient, or as a dose percent (%) for the plateau. Similarly for the DPs, the analysis of the global uncertainty is separately contemplated for two zones of the PDDs: the ascending portion (build-up region), which is shown in mm, and the descending portion, which is shown in dose percent. Concerning type A uncertainties, the absorbed dose statistical deviations are <0.1% in all of the studied cases. No averaging effect uncertainty has been contemplated in this evaluation. All of the uncertainty components involved in the present study are considered statistically independent. This assumption could indicate a global result that is slightly greater than the one obtained from the likely correlations between them. Table 2 summarizes the uncertainties in OFs, DPs and PDDs for the microLion chamber.
Relative response
1.02
1.00
0.98
0.96 0
10
20
30
40
Square field side (cm) Fig. 2. Field size dependence of the microLion chamber. The results are related to the Semiflex chamber.
As shown in Table 2, the combined standard uncertainty has been expanded with the recommended coverage factor k ¼ 2 (Lewis et al., 2003) that provides a level of confidence of approximately 95%. These expanded uncertainties are employed to evaluate the results. Before analyzing the relative quantities’ results, it is necessary to discuss the energy dependence. The microLion chamber presents energy dependence (Fig. 2). For the large field sizes, there is an increasing under-response, which reaches a 3% discrepancy in the 40 40 cm2 field. An over-response for field sizes <10 10 cm2 that reaches a 2% deviation is also observed. This behavior is similar to that shown for another isooctane chamber in a previous work by Pardo et al. (2005). Discrepancies could be explained by the photon scattering effect in the liquid chamber materials of the lateral wall, which has a significant thickness (Fig. 1a). In this way, the harder energy photon spectrum of small fields would generate extra electrons in this material, obtaining an over-response in the active volume. The lowenergy photons in large fields are attenuated in this material, and an under-response is thus achieved. An analogue reasoning has been followed previously in the study of a liquid chamber linear array with a frontal metal electrode (Martens et al., 2001). 3.1. Output factors
3. Results and discussion Fig. 3 shows the OFs at different field sizes. For the Semiflex and Farmer chambers, the OFs are in close agreement for all of the fields measured. This point confirms the reliability for both classical detectors. For field sizes <2 2 cm2, the Semiflex presents an
Although combined uncertainty has been calculated for the microLion chamber, this value has been assigned to the rest of the detectors to perform a more realistic comparison.
Table 2 Uncertainties of the three analyzed quantities for the microLion chamber. All values are expressed in %, except for the high dose gradient regions, where the cursive values indicate mm and the bold values are the expanded uncertainties. Square field side (cm)
OF 0.6
DP 1.2
2
Pl.a Component Type A Type B Beam monitor Field size Positioning Quadrature sum Expanded (k ¼ 2) a b c
0.14 0.9 1.6 0.8 2.0 4
Plateau. Build-up region. Electronic equilibrium region.
PDD 5
0.6 to 3 Edge
Pl.
0.6 Edge
0.14
0.3 0.2 1.0 2.0
<0.2 <0.1 0.9 1.8
0.9 <0.1 <0.1 0.9 1.8
B.U.b
5
1.2 to 3 Eq.c
B.U.
Eq.
B.U.
Eq.
0.9 <0.1 1.8 2.0 4
0.1 <0.1 0.6 0.6 1.2
0.9 <0.1 0.4 1.0 2.0
N/A <0.1 0.6 0.6 1.2
<0.1 0.3 0.3 0.6
0.14 0.1 0.2 0.6 0.6 1.2
N/A <0.1 <0.1 0.14 0.28
0.2 0.6 0.6 1.2
0.1 <0.1 0.6 0.6 1.2
E.M. Benítez et al. / Radiation Measurements 58 (2013) 79e86
1.25
Ouput factor
1.00 MicroLion
0.75
Diode PinPoint Semiflex
0.50
Farmer 0.25 0
10
20
30
40
Square field side (cm) Fig. 3. Output factors. The complete range of field sizes has been measured using the microLion, Semiflex and PinPoint. The Diode is not used for sizes >10 10 cm2 and the Farmer chamber for field sizes <5 5 cm2. The uncertainty bars are not shown because they are smaller than the symbols.
averaging effect due to its dimensions, which implies a lower signal regarding the reference detector. Although the PinPoint is considerably smaller than the Semiflex, the volume effect is also appreciated for narrow beams. This microchamber presents an over-response that increases with size and could be attributable to the cable/stem effect (Agostinelli et al., 2008), which involves a difference of 4% concerning a reference detector for the 40 40 cm2 field. Axial irradiation could be a solution to minimize this effect, but there is no holder for such detector orientation in the attachment system. The TG-106 report (Das et al., 2008) stated that “the chamber dimension must be small compared to the smallest field size, e.g.,
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less than 0.5 cm in any dimension (diameter or length) to avoid chamber averaging effects”. In this way, the microLion should have a suitable active volume for small fields, except for the 0.6 0.6 cm2 field, where the sensitive volume diameter plus 0.5 cm is greater than the field side. Following this reasoning, the liquid chamber could be used starting at a size of 0.8 0.8 cm2. For field sizes up to 10 10 cm2, the microLion OFs indicate an over-response that increases with size respect the OFs obtained with Diode, reaching a difference of 3% for a 5 5 cm2 field. For sizes >10 10 cm2, the liquid chamber has an under-response compared to Semiflex, achieving a difference of 3% for a 40 40 cm2 field. This behavior could be explained by the liquid detector energy dependence. Fig. 4 shows the agreement between the microLion chamber and the reference detectors, considering the uncertainties. 3.2. Dose profiles The details for penumbra analysis are shown in Table 3. The Diode presents the narrowest penumbras for the field sizes 10 10 cm2. This detector was not used for larger fields. The penumbras obtained with the microLion are slightly wider than those obtained with the Diode, possibly because the microLion’s sensitive volume is bigger than the Diode’s volume. However, the differences are lower than the uncertainty of the penumbra measurements. For field sizes >10 10 cm2, a comparison between the microLion and the reference detector (Semiflex) results in smaller penumbras for the liquid chamber. Obviously, the volume effect of Semiflex and Farmer chambers produces considerably wider penumbras. In this case, the differences are higher than the considered uncertainty. PinPoint shows penumbras that agree within uncertainty with the microLion for small and medium field sizes. The values are
Fig. 4. OFs of the microLion chamber versus the corresponding reference detector. Uncertainties for the Diode and the Semiflex are represented by a dashed line.
Table 3 Penumbras and field widths measured with the different detectors. Diode is not used for great fields due to its over-response. Farmer dimensions are not appropriate for small fields. Field side (cm)
0.6 1.2 2 3 5 10 20 30 40
Penumbra (mm)
Field width (cm)
microLion
Diode
Semiflex
PinPoint
3.2 3.9 4.2 4.4 4.8 5.4 6.1 6.6 7.0
3.0 3.6 3.8 4.1 4.5 5.2
4.1 5.0 5.6 5.8 6.3 7.1 8.2 9.0 9.8
3.5 4.3 4.6 4.9 5.3 6.1 7.6 9.6 12.9
Farmer
microLion
Diode
Semiflex
PinPoint
Farmer
6.9 7.8 8.9 9.6 10.2
0.70 1.21 2.01 2.99 4.98 9.96 19.95 29.99 40.16
0.67 1.20 2.00 3.01 4.99 9.98
0.84 1.24 2.01 3.00 5.00 10.00 20.02 30.08 40.27
0.74 1.22 2.00 2.98 5.00 9.99 20.00 30.06 40.25
5.01 10.01 20.03 30.10 40.29
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Fig. 5. DPs measured for the 0.6 0.6 cm2 field. To expose a realistic situation, no symmetrization was performed. For greater clarity in the curves, uncertainty bars are not represented.
effect is hardly appreciated because the average is taken on a region where the DPs have a constant slope. This effect can be appreciated only for the smallest field (Fig. 5). Although the results for field width are statistically compatible, it is interesting to note the comparison between the microLion and the Semiflex for 20 20 cm2: the results for microLion are systematically below those obtained using the Semiflex. This finding is in agreement with the under-response of the microLion to low-energy photons because these photons have a more important effect in the limits of large fields. Concerning the low-dose gradient region, the differences in the results for all of the fields are within uncertainties, except for the shoulder of the DPs, where the discrepancies are greater than the uncertainty. In Fig. 6, the details of the penumbra region measured using different detectors can be observed for a field size of 10 10 cm2. Although the response of the detectors in the plateau region is indistinguishable, the volume effect of the different detectors in the shoulder of the DPs is indicated. 3.3. Percent depth dose
Fig. 6. Partial detail of DPs acquired with the different detectors for the 10 10 cm2 field. For greater clarity in the curves, uncertainty bars are not represented.
higher for the PinPoint. This effect could be attributed to the larger sensitive volume of the PinPoint compared to the microLion. However, for the 30 30 cm2 and 40 40 cm2 fields, the PinPoint produces even wider penumbra than does the Semiflex. This behavior, also documented by Agostinelli et al. (2008), could be attributed to the cable/stem effect. The higher nominal response of the microLion (Table 1) makes this effect negligible. The results regarding field width are presented in Table 3. There are no clear discrepancies among the different detectors, considering the uncertainty in the measurements. In this case, the volume
The first millimeter in depth is not represented in the PDDs because the sensitive volume of detectors is totally or partially out of the water. In the build-up region, the comparison of PDDs is evaluated with the parameter distance to agreement (DTA), which is the distance between depths with the same PDD value for two detectors. The DTA results of the different detectors related to the reference detector are <0.5 mm for all of the field sizes, showing a good agreement within the uncertainties. The most unfavorable comparison is represented in Fig. 7a. The representation for the PDD curves of the microLion chamber and the Diode for the smallest field size is shown in Fig. 7b. In the build-up region, a clear concordance can be observed, whereas in the descending region, the microLion over-response that increases with the depth is appreciated. For this descending zone, the microLion results are in agreement with those from the reference detectors (Diode and Roos) except for the field sizes >20 20 cm2, as observed in Fig. 8 (for simplicity, only four fields are presented). Despite the results reported in the present study, it is necessary to note the clear behavior of the microLion PDDs for small and large field sizes. As shown, the liquid chamber gives an over-response to the reference detector for sizes <10 10 cm2 and an underresponse for the larger sizes. This behavior is in concordance with the energy dependence noted for this chamber.
100
90 80
MicroLion
70
Diode
Percent depth dose (%)
Percent depth dose (%)
100
60 50 40
MicroLion
80
Diode
60 40 20 0
30 0
2
4
6 8 Depth (mm)
(a)
10
12
0
40
80 120 Depth (mm)
160
200
(b)
Fig. 7. (a) PDD detail of the microLion and the Diode (reference detector) for the field size of 3 3 cm2 in the build-up region. (b) PDDs of the microLion and the reference detector for the field size of 0.6 0.6 cm2. For greater clarity in the curves, uncertainty bars are not represented.
E.M. Benítez et al. / Radiation Measurements 58 (2013) 79e86
5.0
0.6 x 0.6 cm 2
7.5
Relative deviation (%)
Relative deviation (%)
10.0 5.0 2.5 0.0 -2.5 MicroLion PinPoint
-5.0 -7.5
2 x 2 cm 2
2.5 0.0 MicroLion PinPoint
-2.5 -5.0
-10.0 20
60
100
3
140 180 Depth (mm)
220
260
20
300
2 1 0 -1 MicroLion PinPoint Roos
-2 -3 20
60
100
60
100
3
5 x 5 cm 2
Relative deviation (%)
Relative deviation (%)
85
140 180 Depth (mm)
220
260
300
220
260
300
30 x 30 cm 2
2 1 0 -1 MicroLion PinPoint
-2 -3
140
180
220
260
300
20
60
100
Depth (mm)
140 180 Depth (mm)
Fig. 8. Relative deviation of the PDD in the descending region of the different detectors compared to the reference detector (Diode up to 5 5 cm2 field and Roos for higher sizes). Graphics present a fringe with dashed line, which symbolizes the uncertainty interval.
The other detectors have an adequate response related to the reference detector, except PinPoint for fields >20 20 cm2, where this chamber presents an increasing over-response with the field size that is attributed to the cable/steam effect.
temperature and other relevant aspects with the aim of analyzing the microLion behavior for absolute dose measurements.
4. Conclusions
One of us (S G.-P.) acknowledges the partial support by the Junta de Andalucía (PQ09-FQM-5341 and FQM-0220) and the Spanish DGI (FPA2009-14091-C02-02).
The microLion isooctane ionization chamber has been evaluated for its use in relative dosimetry. It is compared to other common detectors by measuring OFs, PDDs and DPs in a wide range of field sizes of a 6 MV photon beam. The results confirm the suitability of the liquid chamber when high spatial resolution is required, as in DP measurements. The liquid chamber appears to be superior to air ionization chambers and is attributable to the high sensitivity of the liquid filling because it permits detectors with smaller detection volumes. Although Diode 60012 shows better spatial resolution than the microLion, it has a high-energy dependence that makes it inadvisable for relative dosimetry in field sizes >10 10 cm2. In this regard, the organic liquid filling seems more tissue-equivalent than silicon because it extends the range of sizes where the microLion presents a good behavior. In OFs and PDDs, some energy dependence for small and large fields can be appreciated. This energy dependence is more obvious for field sizes >20 20 cm2, where the differences in PDDs at great depths with other well-established detectors exceed the uncertainties estimated in this study. The liquid chamber has several drawbacks. First, applying high voltage for 15 min to reach stability before irradiation and requiring of an extra high voltage supply are factors that should be considered. The relative high acquiring cost, compared to other dosimeters that we have studied, could be another factor that should be taken into account. To complete the exhaustive evaluation of this detector, its suitability for absolute dosimetry should be studied. Future investigations should focus on long-term stability, the influence of
Acknowledgments
References Agostinelli, S., Garelli, S., Piergentili, M., Foppiano, F., 2008. Response to high-energy photons of PTW 31014 PinPoint ion chamber with a central aluminum electrode. Med. Phys. 35, 3293e3301. Alfonso, R., Andreo, P., Capote, R., Saiful Huq, M., Kilby, W., Kjäll, P., Mackie, T.R., Palmans, H., Rosser, K., Seuntjens, J., Ullrich, W., Vatnitsky, S., 2008. A new formalism for reference dosimetry of small and nonstandard fields. Med. Phys. 35, 5179e5186. Andersson, J., Tölli, H., 2011. Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams. Phys. Med. Biol. 56, 299e314. Andreo, P., Burns, D.T., Hohlfeld, K., Huq, M.S., Kanai, T., Laitano, F., Smyth, V., Vynckier, S., 2000. Absorbed dose determination in external beam radiotherapy, Vienna. IAEA Technical Reports Series No 398. Bucciolini, M., Banci Buonamici, F., Mazzocchi, S., De Angelis, C., Onori, S., Cirrone, G.A.P., 2003. Diamond detector versus silicon diode and ion chamber in photon beams of different energy and field size. Med. Phys. 30, 2149e2154. Chung, E., Soisson, E., Seuntjens, J., 2012. Dose homogeneity specification for reference dosimetry of nonstandard fields. Med. Phys. 39, 407e414. Das, I.J., Cheng, C.W., Watts, R.J., Ahnesjo, A., Gibbons, J., Li, X.A., Lowenstein, J., Mitra, R.K., Simon, W.E., Zhu, T.C., 2008. Accelerator beam data commissioning equipment and procedures: report of the TG-106 of the therapy physics committee of the AAPM. Med. Phys. 35, 4186e4215. Dasu, A., Löfroth, P.O., Wickman, G., 1998. Liquid ionization chamber measurements of dose distributions in small 6 MV photon beams. Phys. Med. Biol. 43, 21e36. Eberle, K., Engler, J., Hartmann, G., Hofmann, R., Hörandel, J.R., 2003. First tests of a liquid filled ionization chamber to monitor intensity modulated radiation beams. Phys. Med. Biol. 48, 3555e3564. Francescon, P., Cora, S., Satariano, N., 2011. Calculation of kfclin,fmsr Qclin,Qmsr for several small detectors and for two linear accelerators using Monte Carlo simulations. Med. Phys. 38, 6513e6527. Francescon, P., Kilby, W., Satariano, N., Cora, S., 2012. Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the CyberKnife system. Phys. Med. Biol. 57, 3741e3758.
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González-Castaño, D.M., Gómez, F., Brualla, L., Roselló, J.V., Planes, D., Sánchez, M., Pombar, M., 2011. A liquid-filled ionization chamber for high precision relative dosimetry. Phys. Med. 27, 89e96. Griessbach, I., Lapp, M., Bohsung, J., Gademann, G., Harder, D., 2005. Dosimetric characteristics of a new unshielded silicon diode and its application in clinical photon and electron beam. Med. Phys. 32, 3750e3754. Hrbacek, J., Lang, S., Klöck, S., 2011. Commissioning of photon beams of a flattening filter-free linear accelerator and the accuracy of beam modeling using an anisotropic analytical algorithm. Int. J. Radiat. Oncol. Biol. Phys. 80, 1228e1237. Leo, W.R., 1994. Techniques for Nuclear and Particle Physics Experiments: a How-to Approach. Springer, Berlin, p. 154. Lewis, V., Woods, M., Burgess, P., Green, S., Simpson, J., Wardle, J., 2003. Measurement Good Practice Guide No 49, The Assessment of Uncertainty in Radiological Calibration and Testing. National Physical Laboratory, United Kingdom, p. 3. Martens, C., De Wagter, C., De Neve, W., 2001. The value of the LA48 linear ion chamber array for characterization of intensity-modulated beams. Phys. Med. Biol. 46, 1131e1148.
Pardo, J., Franco, L., Gómez, F., Iglesias, A., Pazos, A., Pena, J., Lobato, R., Mosquera, J., Pombar, M., Sendón, J., 2005. Development and operation of a pixel segmented liquid-filled linear array for radiotherapy quality assurance. Phys. Med. Biol. 50, 1703e1716. Sterpin, E., Mackie, T.R., Vynckier, S., 2012. Monte Carlo computed machine-specific correction factors for reference dosimetry of TomoTherapy static beam for several ion chambers. Med. Phys. 39, 4066e4072. Stewart, K.J., Elliott, A., Seuntjens, J.P., 2007. Development of a guarded liquid ionization chamber for clinical dosimetry. Phys. Med. Biol. 52, 3089e3104. Tölli, H., Sjögren, R., Wendelsten, M., 2010. A two-dose-rate method for general recombination correction for liquid ionization chambers in pulsed beams. Phys. Med. Biol. 55, 4247e4260. Wickman, G., 1974. Radiation quality independent liquid ionization chamber for dosimetry of electron radiation from medical accelerators. Acta Radiol. Ther. Phys. Biol. 13, 37e48. Wickman, G., Nyström, H., 1992. The use of liquids ionization chambers for high precision radiotherapy dosimetry. Phys. Med. Biol. 37, 1789e1812.
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Evaluation of a liquid ionization chamber for relative dosimetry in small and large fields of radiotherapy photon beams E.M. Benítez*, F.J. Casado, S. García-Pareja, J.A. Martín-Viera, C. Moreno, V. Parra Servicio de Radiofísica Hospitalaria, Hospital Regional Universitario Carlos Haya, Av. Carlos Haya s/n, 29010 Málaga, Spain
h i g h l i g h t s When high spatial resolution is required the results confirm the suitability of the liquid chamber. Some energy dependence of the liquid detector can be appreciated in OFs and PDDs for small and large fields. For field sizes >20 20 cm2, the differences in PDDs at great depths exceed the uncertainties estimated. Some drawbacks should be considered: the time to reach stability, the high voltage supply required and the acquiring cost.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2012 Received in revised form 22 August 2013 Accepted 23 August 2013
Commissioning and quality assurance of radiotherapy linear accelerators require measurement of the absorbed dose to water, and a wide range of detectors are available for absolute and relative dosimetry in megavoltage beams. In this paper, the PTW microLion isooctane-filled ionization chamber has been tested to perform relative measurements in a 6 MV photon beam from a linear accelerator. Output factors, percent depth dose and dose profiles have been obtained for small and large fields. These quantities have been compared with those from usual detectors in the routine practice. In order to carry out a more realistic comparison, an uncertainty analysis has been developed, taking type A and B uncertainties into account. The results present microLion as a good option when high spatial resolution is needed, thanks to its reduced sensitive volume. The liquid filling also provides a high signal compared to other detectors, like that based on air filling. Furthermore, the relative response of microLion when field size is varied suggests that this detector has energy dependence, since it is appreciated an over-response for small fields and an under-response for the large ones. This effect is more obvious for field sizes wider than 20 20 cm2, where the differences in percent depth dose at great depths exceed the uncertainties estimated in this study. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Liquid ionization chamber Relative dosimetry Isooctane Uncertainty analysis
1. Introduction Commissioning and determining quality assurance for radiotherapy linear accelerators require measuring the absorbed dose to water. A wide range of detectors is available for absolute and relative dosimetry in megavoltage beams, such as air ionization chambers (in standard, mini and micro sizes), films and diamond detectors. Presently, liquid-filled ionization chambers are not the usual dosimeters used in routine practice.
* Corresponding author. Tel.: þ34 951291436; fax: þ34 951291464. E-mail addresses: [email protected], [email protected] (E.M. Benítez). 1350-4487/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2013.08.009
The first liquid ionization detectors were built in 1950s (Leo, 1994). Higher-density media than gases were studied to increase the number of ioneelectron pairs created and thus achieve a larger signal in the detection process. Additionally, the lower diffusion in liquids indicates that a lower active volume that obtained by reducing the ionization tracking and thereby better spatial resolution is obtained. In contrast to gases, transport phenomena were poorly understood because of an essential problem: The liquid contains electronegative impurities, such as oxygen, which can attach to drifting electrons and pose the technical problem of purification, which is not possible for any liquid substance. The previous situation reduces the number of liquids that are candidates for use in an ionization detector to a very select group that consists of liquefied noble gases, primarily argon and xenon,
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and a few hydrocarbons. To reduce ion recombination, the nonultra purified liquids need high electric fields. Regarding hydrocarbon filling, isooctane and tetramethylsilane (TMS) are commonly employed. The results of a comparative study of both liquids in parallel-plate ionization chambers showed that TMS has a sensitivity that is approximately 50% higher than that of isooctane. A general recombination in TMS is also slightly higher than in isooctane (Wickman and Nyström, 1992). Additionally, the ionization current in isooctane is 300 times higher than that in an air-filled ionization chamber of the same volume (Wickman, 1974). Several studies have described the development of liquid chamber prototypes for therapy beam dosimetry. In the article by Wickman and Nyström (1992), isooctane and TMS plane-parallel chambers were tested for measurements of the absorbed dose to water in radiotherapy fields. Dasu et al. (1998) obtained good results for relative dosimetry in narrow beams using a chamber filled with TMS. Eberle et al. (2003) analyzed another prototype in the online monitoring of Intensity Modulated Radiation Therapy (IMRT) fields, and Stewart et al. (2007) designed and constructed the isooctane-filled chamber named GLIC-03. More recently, González-Castaño et al. (2011) developed another isooctane chamber that presented an adequate behavior for high-precision dosimetry of photon beams. In 2009, PTW (Freiburg, Germany) launched into the market the isooctane-filled ionization chamber type 31018, with the commercial name of microLion. Several papers have been written about this chamber. One publication (Andersson and Tölli, 2011) introduced a procedure for estimating recombination losses in continuous beams, and another study described an approach for estimating the recombination losses in pulsed beams (Tölli et al., 2010). Hrbacek et al. (2011), in their work about the commissioning of a linear accelerator, used this detector to measure dose profiles for field sizes that ranged from 20 20 cm2 to 40 40 cm2. Chung et al. (2012) used the microLion chamber as a reference dose detector for non-standard fields to establish a plan-class-specific correction factor in the new formalism for reference dosimetry in such fields (Alfonso et al., 2008). In this way, the correction factors needed for small fields to convert ionization output factors into dose output factors have been calculated for the microLion chamber for Siemens and Elekta clinacs (Francescon et al., 2011), for CyberKnife (Francescon et al., 2012) and for TomoTherapy (Sterpin et al., 2012). At the time of this study, we have not found in a peerreview journal a complete test for the relative dosimetry of radiotherapy beams for the microLion chamber. Liquid chambers have a response that is considerably higher than air chambers. Thus, the manufacturer can reduce measuring active volume to achieve a high spatial resolution, which is useful in small field dosimetry, which in turn is necessary for radiosurgery and IMRT. The AAPM TG-106 report (Das et al., 2008) about accelerator-beam data commissioning equipment and procedures documented the following: “Small volume chambers tend to have different characteristics for large fields compared to small fields and should not be used for all field sizes unless it can be documented that accurate data can be acquired for all field sizes”. To test the behavior of this chamber for relative dosimetry in a water phantom, our group performed the following measurements using a 6 MV photon beam from a Varian DBX linear accelerator: percent depth dose (PDDs), output factors (OFs) and dose profiles (DPs). These quantities have been measured in the large and small fields that are used in conventional radiotherapy and special techniques, such as radiosurgery. For small fields, the correction factors should be incorporated, when available, for the Varian ClinacÒ and the set of detectors that have been considered. We compared the obtained results with those measured using the usual detectors in the routine practice. With the aim of
Fig. 1. (a) Front and (b) lateral schematic views of the liquid ionization chamber, microLion. The reference point is situated at the center of the sensitive volume.
quantifying the agreement of the comparison, we developed a procedure to estimate the measurement uncertainty, accounting for both type A and type B components. 2. Materials and methods 2.1. Experimental set-up For this study, we used 6 MV photon beams from a Varian DBX linear accelerator (Varian, Palo Alto, CA, USA). Measurements were performed from 0.6 0.6 cm2 to 40 40 cm2 by setting the secondary collimator jaws while the multileaf collimator was fully retracted. We set the dose rate at 300 MU min1, which is equivalent to 1.98 Gy min1 in reference conditions, i.e., at 100 cm SSD and 10 cm depth in water, for a field size of 10 10 cm2. We performed measurements in a water phantom included in the PTW MP3 Therapy Beam Analyzer. We positioned the detectors using the PTW TRUFIX precision attachment system, which served to accurately position the detector effective points by properly using holders and stop thimbles, thereby allowing an exchange of detectors without the need to set a new zero point. We used the electrometer, which is a dual-channel TANDEM type T10011 obtained from PTW. To acquire PDDs and DPs we employed the PTW MEPHYSTO 7.3 software. We performed all the measurements using positive polarity. We measured the temperature and pressure using a digital barometer (Tinycal Double Function T210, Gometrics, Barcelona, Spain) and a thermometer (HD 2307.0 RTD, Delta Ohm, Padova, Italy). 2.2. Detectors 2.2.1. MICROLION chamber The liquid ionization chamber is the PTW microLion, type 31018, which presents a sensitive volume of 0.0017 cm3 and has a radius and depth of 1.25 mm and 0.35 mm, respectively (Fig. 1). The electrode is made of graphite, and the front wall is composed of 0.02 mm varnish, 0.5 mm polystyrene and 0.28 mm graphite. The filling liquid is isooctane. The microLion chamber is polarized at 800 V, which is provided by the stand-alone PTW HV-SUPPLY T16036. Before irradiation, it is necessary to apply the high voltage for at least 15 min to reach the desired stability. A recommended pre-irradiation >6 Gy should be performed. The recombination factor for the microLion chamber has been estimated for several situations in the experimental set-up. A pulse
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repetition frequency at 377 Hz has been established by the manufacturer, which represents a value of 0.09 mGy per pulse, calculated under reference conditions. Variations in field size and detector positioning introduce modifications with respect to this result. Estimation on the experimental set-up in our measurements gives an overall interval from 0.01 to 0.14 mGy per pulse. A Swedish working group (Tölli et al., 2010) established the results for the microLion efficiency: In the most unfavorable condition, a recombination factor of 0.993 is obtained at 800 V compared to the recombination factor of 0.997 from the direct application of the Boag theory. The influence of this effect is neglected because the measurement uncertainty is greater, as we discuss below. With regard to the energy dependence, the microLion relative response has been analyzed by varying the field size because the scattered low-energy photon contribution increases with it. For this purpose, the Semiflex air ionization chamber has been employed as a reference detector. 2.2.2. Other detectors To establish a comparison, we performed a set of relative dosimetry measurements using the Diode 60012 and the air ionization chambers Semiflex 31010, PinPoint 31016, Farmer 30006 and Roos 34001, all of which are manufactured by PTW. Several characteristics of the detectors used in this study are presented in Table 1. Similar to the microLion chamber, the other detectors are exposed to a prior irradiation >6 Gy. We evaluated the leakage current pre- and post-irradiation and the charge loss and obtained a value <0.1 pA for all of the detectors, except for the PinPoint, where the value was <1 fA. In addition to the reference chambers, we performed several measurements using a PTW Semiflex 31002 monitoring chamber. 2.3. Relative dosimetric quantities Square fields with sides of 0.6, 1.2, 2, 3, 5, 10, 20, 30 and 40 cm were considered. Every relative dosimetric quantity analyzed in this paper has required a particular measurement set-up, as we detail below.
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The OFs are obtained for the complete range of field sizes using the microLion, Semiflex and PinPoint. The Diode presents an overresponse because of the scattered photons of low energy, whose effect is more pronounced in large photon beams (Griessbach et al., 2005). For this reason, the Diode is not used for sizes >10 10 cm2. Measurements using Farmer are not performed for field sizes <5 5 cm2 because of the volume averaging effect. 2.3.2. Dose profiles The DPs are performed at a depth of 10 cm and an SAD of 100 cm. The detectors microLion, Diode, Semiflex, PinPoint and Farmer are used with the same range of field sizes as for the OFs. The Diode is selected as the reference detector for fields up to 10 10 cm2, and Semiflex is selected as the reference detector for the other fields. To evaluate the agreement among the detectors when the dose profiles are studied, we distinguish two regions as follows: a lowdose gradient region in the central part of the field and another region with a high-dose gradient in the field edge. The value of 80% in the dose profile is chosen to separate both regions. A point-bypoint comparison is conducted in the first region. To compare the results in second region (i.e., with the high-dose gradient), the penumbra 20e80% and the field width were calculated. The DPs from 5 5 cm2 to 40 40 cm2 are performed using a monitoring ionization chamber to minimize the effect of instant fluctuations in the beam. 2.3.3. Percent depth dose The PDDs are measured using the microLion, Diode, PinPoint and Roos. To compare measurements, the Diode is considered to be a reference detector for field sizes up to 10 10 cm2, and the Roos chamber is considered to be a reference detector for larger field sizes. The Diode is employed with the restriction in the field size, as mentioned above, and the Roos is dismissed for field sizes <5 5 cm2 because of the averaging effect. The monitoring ionization chamber is used in the same way as in DPs. 2.4. Uncertainty analysis
2.3.1. Output factors The OFs are obtained for each field size as detector responses at a water depth of 10 cm and an SAD of 100 cm. The measurements are normalized to the 10 10 cm2 field size. According to the set-up recommended by the TRS-398 code of practice (Andreo et al., 2000), for thimble chambers in radial irradiation, the positioning provided by TRUFIX should be corrected to the reference point. These measurements are made using the microLion, Diode, Semiflex, PinPoint and Farmer. Diode was chosen as a reference detector for field sizes 10 10 cm2 because of its reduced active volume (González-Castaño et al., 2011; Das et al., 2008). A Semiflex chamber was considered to be the reference for fields >10 10 cm2.
Although measurements in the present study were not performed under reference conditions, the TRS-398 code of practice has been followed to estimate the type B uncertainty of absorbed dose to water for the microLion chamber. This procedure has been followed previously, as described in a similar publication by Bucciolini et al. (2003). The three magnitudes analyzed in this paper are relative values, i.e., they are obtained as a quotient of two measurements. For this reason, uncertainties due to beam quality determination, long-term stability and other influence quantities, can be compensated. The first uncertainty source considered derives from the experimental set-up conditions, because their uncertainties can get different values for both measurements of every quotient. For this
Table 1 Technical specifications of the different detectors used in this work. Specification
microLion 31018 3
Sensitive volume (cm ) Operating voltage (V) Nominal response (nC/Gy) Field size range (cm2) Ion collection time (s) Approx. relative cost
0.0017 800 9.8 1 1.20 20 5.3 103 3.3
Diode 60012 6
2.5 10 0 175 1 1.10 10 w106 1.4
Semiflex 31010
PinPoint 31016
Farmer 30006
Roos 34001
0.125 400 3.3 2 2.40 40 0.1 103 1
0.016 400 0.4 2 2.30 30 60 106 1.3
0.6 300 20 5 5.40 40 0.18 103 1.2
0.35 100 12 4 4.40 40 0.3 103 2.4
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purpose the detector positioning variability and the field size influence are contemplated instead of using the 0.4% indicated in the TRS-398. With regard to the positioning, the product data specify that the MP3 water tank provides an accuracy of 0.1 mm, whereas the TRUFIX precision attachment offers 0.3 mm. To convert the distance uncertainty into the absorbed dose uncertainty, we use the information from the vicinity of the measurement point in DPs and PDDs. The combined result is estimated under the conservative assumption of an uncertainty rectangular distribution for both values. The field size influence is evaluated by resetting the jaw position before every measurement, repeating this procedure five times. An alternative way to avoid this influence is to state every field size by setting the collimator and subsequently measuring with every detector; however, this method greatly increases the measuring time. The second uncertainty source has as its origin the dosimeter reading relative to the beam monitor. Ionization readings for the OFs are not simultaneous; therefore, they are affected by the beam monitor. The 0.6% uncertainty from the TRS-398 is reported in this paper. With the aim of avoiding this effect, a monitoring chamber is employed for measurements of PDDs and DPs, except when the field size is < 5 5 cm2. Only if the monitoring chamber is present can this uncertainty component be compensated in the quotients. Because of the particular shape of DP curves, it seems adequate to express uncertainty in two different ways: in distance (mm) for the penumbra and the field width, where there is a high-dose gradient, or as a dose percent (%) for the plateau. Similarly for the DPs, the analysis of the global uncertainty is separately contemplated for two zones of the PDDs: the ascending portion (build-up region), which is shown in mm, and the descending portion, which is shown in dose percent. Concerning type A uncertainties, the absorbed dose statistical deviations are <0.1% in all of the studied cases. No averaging effect uncertainty has been contemplated in this evaluation. All of the uncertainty components involved in the present study are considered statistically independent. This assumption could indicate a global result that is slightly greater than the one obtained from the likely correlations between them. Table 2 summarizes the uncertainties in OFs, DPs and PDDs for the microLion chamber.
Relative response
1.02
1.00
0.98
0.96 0
10
20
30
40
Square field side (cm) Fig. 2. Field size dependence of the microLion chamber. The results are related to the Semiflex chamber.
As shown in Table 2, the combined standard uncertainty has been expanded with the recommended coverage factor k ¼ 2 (Lewis et al., 2003) that provides a level of confidence of approximately 95%. These expanded uncertainties are employed to evaluate the results. Before analyzing the relative quantities’ results, it is necessary to discuss the energy dependence. The microLion chamber presents energy dependence (Fig. 2). For the large field sizes, there is an increasing under-response, which reaches a 3% discrepancy in the 40 40 cm2 field. An over-response for field sizes <10 10 cm2 that reaches a 2% deviation is also observed. This behavior is similar to that shown for another isooctane chamber in a previous work by Pardo et al. (2005). Discrepancies could be explained by the photon scattering effect in the liquid chamber materials of the lateral wall, which has a significant thickness (Fig. 1a). In this way, the harder energy photon spectrum of small fields would generate extra electrons in this material, obtaining an over-response in the active volume. The lowenergy photons in large fields are attenuated in this material, and an under-response is thus achieved. An analogue reasoning has been followed previously in the study of a liquid chamber linear array with a frontal metal electrode (Martens et al., 2001). 3.1. Output factors
3. Results and discussion Fig. 3 shows the OFs at different field sizes. For the Semiflex and Farmer chambers, the OFs are in close agreement for all of the fields measured. This point confirms the reliability for both classical detectors. For field sizes <2 2 cm2, the Semiflex presents an
Although combined uncertainty has been calculated for the microLion chamber, this value has been assigned to the rest of the detectors to perform a more realistic comparison.
Table 2 Uncertainties of the three analyzed quantities for the microLion chamber. All values are expressed in %, except for the high dose gradient regions, where the cursive values indicate mm and the bold values are the expanded uncertainties. Square field side (cm)
OF 0.6
DP 1.2
2
Pl.a Component Type A Type B Beam monitor Field size Positioning Quadrature sum Expanded (k ¼ 2) a b c
0.14 0.9 1.6 0.8 2.0 4
Plateau. Build-up region. Electronic equilibrium region.
PDD 5
0.6 to 3 Edge
Pl.
0.6 Edge
0.14
0.3 0.2 1.0 2.0
<0.2 <0.1 0.9 1.8
0.9 <0.1 <0.1 0.9 1.8
B.U.b
5
1.2 to 3 Eq.c
B.U.
Eq.
B.U.
Eq.
0.9 <0.1 1.8 2.0 4
0.1 <0.1 0.6 0.6 1.2
0.9 <0.1 0.4 1.0 2.0
N/A <0.1 0.6 0.6 1.2
<0.1 0.3 0.3 0.6
0.14 0.1 0.2 0.6 0.6 1.2
N/A <0.1 <0.1 0.14 0.28
0.2 0.6 0.6 1.2
0.1 <0.1 0.6 0.6 1.2
E.M. Benítez et al. / Radiation Measurements 58 (2013) 79e86
1.25
Ouput factor
1.00 MicroLion
0.75
Diode PinPoint Semiflex
0.50
Farmer 0.25 0
10
20
30
40
Square field side (cm) Fig. 3. Output factors. The complete range of field sizes has been measured using the microLion, Semiflex and PinPoint. The Diode is not used for sizes >10 10 cm2 and the Farmer chamber for field sizes <5 5 cm2. The uncertainty bars are not shown because they are smaller than the symbols.
averaging effect due to its dimensions, which implies a lower signal regarding the reference detector. Although the PinPoint is considerably smaller than the Semiflex, the volume effect is also appreciated for narrow beams. This microchamber presents an over-response that increases with size and could be attributable to the cable/stem effect (Agostinelli et al., 2008), which involves a difference of 4% concerning a reference detector for the 40 40 cm2 field. Axial irradiation could be a solution to minimize this effect, but there is no holder for such detector orientation in the attachment system. The TG-106 report (Das et al., 2008) stated that “the chamber dimension must be small compared to the smallest field size, e.g.,
83
less than 0.5 cm in any dimension (diameter or length) to avoid chamber averaging effects”. In this way, the microLion should have a suitable active volume for small fields, except for the 0.6 0.6 cm2 field, where the sensitive volume diameter plus 0.5 cm is greater than the field side. Following this reasoning, the liquid chamber could be used starting at a size of 0.8 0.8 cm2. For field sizes up to 10 10 cm2, the microLion OFs indicate an over-response that increases with size respect the OFs obtained with Diode, reaching a difference of 3% for a 5 5 cm2 field. For sizes >10 10 cm2, the liquid chamber has an under-response compared to Semiflex, achieving a difference of 3% for a 40 40 cm2 field. This behavior could be explained by the liquid detector energy dependence. Fig. 4 shows the agreement between the microLion chamber and the reference detectors, considering the uncertainties. 3.2. Dose profiles The details for penumbra analysis are shown in Table 3. The Diode presents the narrowest penumbras for the field sizes 10 10 cm2. This detector was not used for larger fields. The penumbras obtained with the microLion are slightly wider than those obtained with the Diode, possibly because the microLion’s sensitive volume is bigger than the Diode’s volume. However, the differences are lower than the uncertainty of the penumbra measurements. For field sizes >10 10 cm2, a comparison between the microLion and the reference detector (Semiflex) results in smaller penumbras for the liquid chamber. Obviously, the volume effect of Semiflex and Farmer chambers produces considerably wider penumbras. In this case, the differences are higher than the considered uncertainty. PinPoint shows penumbras that agree within uncertainty with the microLion for small and medium field sizes. The values are
Fig. 4. OFs of the microLion chamber versus the corresponding reference detector. Uncertainties for the Diode and the Semiflex are represented by a dashed line.
Table 3 Penumbras and field widths measured with the different detectors. Diode is not used for great fields due to its over-response. Farmer dimensions are not appropriate for small fields. Field side (cm)
0.6 1.2 2 3 5 10 20 30 40
Penumbra (mm)
Field width (cm)
microLion
Diode
Semiflex
PinPoint
3.2 3.9 4.2 4.4 4.8 5.4 6.1 6.6 7.0
3.0 3.6 3.8 4.1 4.5 5.2
4.1 5.0 5.6 5.8 6.3 7.1 8.2 9.0 9.8
3.5 4.3 4.6 4.9 5.3 6.1 7.6 9.6 12.9
Farmer
microLion
Diode
Semiflex
PinPoint
Farmer
6.9 7.8 8.9 9.6 10.2
0.70 1.21 2.01 2.99 4.98 9.96 19.95 29.99 40.16
0.67 1.20 2.00 3.01 4.99 9.98
0.84 1.24 2.01 3.00 5.00 10.00 20.02 30.08 40.27
0.74 1.22 2.00 2.98 5.00 9.99 20.00 30.06 40.25
5.01 10.01 20.03 30.10 40.29
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Fig. 5. DPs measured for the 0.6 0.6 cm2 field. To expose a realistic situation, no symmetrization was performed. For greater clarity in the curves, uncertainty bars are not represented.
effect is hardly appreciated because the average is taken on a region where the DPs have a constant slope. This effect can be appreciated only for the smallest field (Fig. 5). Although the results for field width are statistically compatible, it is interesting to note the comparison between the microLion and the Semiflex for 20 20 cm2: the results for microLion are systematically below those obtained using the Semiflex. This finding is in agreement with the under-response of the microLion to low-energy photons because these photons have a more important effect in the limits of large fields. Concerning the low-dose gradient region, the differences in the results for all of the fields are within uncertainties, except for the shoulder of the DPs, where the discrepancies are greater than the uncertainty. In Fig. 6, the details of the penumbra region measured using different detectors can be observed for a field size of 10 10 cm2. Although the response of the detectors in the plateau region is indistinguishable, the volume effect of the different detectors in the shoulder of the DPs is indicated. 3.3. Percent depth dose
Fig. 6. Partial detail of DPs acquired with the different detectors for the 10 10 cm2 field. For greater clarity in the curves, uncertainty bars are not represented.
higher for the PinPoint. This effect could be attributed to the larger sensitive volume of the PinPoint compared to the microLion. However, for the 30 30 cm2 and 40 40 cm2 fields, the PinPoint produces even wider penumbra than does the Semiflex. This behavior, also documented by Agostinelli et al. (2008), could be attributed to the cable/stem effect. The higher nominal response of the microLion (Table 1) makes this effect negligible. The results regarding field width are presented in Table 3. There are no clear discrepancies among the different detectors, considering the uncertainty in the measurements. In this case, the volume
The first millimeter in depth is not represented in the PDDs because the sensitive volume of detectors is totally or partially out of the water. In the build-up region, the comparison of PDDs is evaluated with the parameter distance to agreement (DTA), which is the distance between depths with the same PDD value for two detectors. The DTA results of the different detectors related to the reference detector are <0.5 mm for all of the field sizes, showing a good agreement within the uncertainties. The most unfavorable comparison is represented in Fig. 7a. The representation for the PDD curves of the microLion chamber and the Diode for the smallest field size is shown in Fig. 7b. In the build-up region, a clear concordance can be observed, whereas in the descending region, the microLion over-response that increases with the depth is appreciated. For this descending zone, the microLion results are in agreement with those from the reference detectors (Diode and Roos) except for the field sizes >20 20 cm2, as observed in Fig. 8 (for simplicity, only four fields are presented). Despite the results reported in the present study, it is necessary to note the clear behavior of the microLion PDDs for small and large field sizes. As shown, the liquid chamber gives an over-response to the reference detector for sizes <10 10 cm2 and an underresponse for the larger sizes. This behavior is in concordance with the energy dependence noted for this chamber.
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Fig. 7. (a) PDD detail of the microLion and the Diode (reference detector) for the field size of 3 3 cm2 in the build-up region. (b) PDDs of the microLion and the reference detector for the field size of 0.6 0.6 cm2. For greater clarity in the curves, uncertainty bars are not represented.
E.M. Benítez et al. / Radiation Measurements 58 (2013) 79e86
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Fig. 8. Relative deviation of the PDD in the descending region of the different detectors compared to the reference detector (Diode up to 5 5 cm2 field and Roos for higher sizes). Graphics present a fringe with dashed line, which symbolizes the uncertainty interval.
The other detectors have an adequate response related to the reference detector, except PinPoint for fields >20 20 cm2, where this chamber presents an increasing over-response with the field size that is attributed to the cable/steam effect.
temperature and other relevant aspects with the aim of analyzing the microLion behavior for absolute dose measurements.
4. Conclusions
One of us (S G.-P.) acknowledges the partial support by the Junta de Andalucía (PQ09-FQM-5341 and FQM-0220) and the Spanish DGI (FPA2009-14091-C02-02).
The microLion isooctane ionization chamber has been evaluated for its use in relative dosimetry. It is compared to other common detectors by measuring OFs, PDDs and DPs in a wide range of field sizes of a 6 MV photon beam. The results confirm the suitability of the liquid chamber when high spatial resolution is required, as in DP measurements. The liquid chamber appears to be superior to air ionization chambers and is attributable to the high sensitivity of the liquid filling because it permits detectors with smaller detection volumes. Although Diode 60012 shows better spatial resolution than the microLion, it has a high-energy dependence that makes it inadvisable for relative dosimetry in field sizes >10 10 cm2. In this regard, the organic liquid filling seems more tissue-equivalent than silicon because it extends the range of sizes where the microLion presents a good behavior. In OFs and PDDs, some energy dependence for small and large fields can be appreciated. This energy dependence is more obvious for field sizes >20 20 cm2, where the differences in PDDs at great depths with other well-established detectors exceed the uncertainties estimated in this study. The liquid chamber has several drawbacks. First, applying high voltage for 15 min to reach stability before irradiation and requiring of an extra high voltage supply are factors that should be considered. The relative high acquiring cost, compared to other dosimeters that we have studied, could be another factor that should be taken into account. To complete the exhaustive evaluation of this detector, its suitability for absolute dosimetry should be studied. Future investigations should focus on long-term stability, the influence of
Acknowledgments
References Agostinelli, S., Garelli, S., Piergentili, M., Foppiano, F., 2008. Response to high-energy photons of PTW 31014 PinPoint ion chamber with a central aluminum electrode. Med. Phys. 35, 3293e3301. Alfonso, R., Andreo, P., Capote, R., Saiful Huq, M., Kilby, W., Kjäll, P., Mackie, T.R., Palmans, H., Rosser, K., Seuntjens, J., Ullrich, W., Vatnitsky, S., 2008. A new formalism for reference dosimetry of small and nonstandard fields. Med. Phys. 35, 5179e5186. Andersson, J., Tölli, H., 2011. Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams. Phys. Med. Biol. 56, 299e314. Andreo, P., Burns, D.T., Hohlfeld, K., Huq, M.S., Kanai, T., Laitano, F., Smyth, V., Vynckier, S., 2000. Absorbed dose determination in external beam radiotherapy, Vienna. IAEA Technical Reports Series No 398. Bucciolini, M., Banci Buonamici, F., Mazzocchi, S., De Angelis, C., Onori, S., Cirrone, G.A.P., 2003. Diamond detector versus silicon diode and ion chamber in photon beams of different energy and field size. Med. Phys. 30, 2149e2154. Chung, E., Soisson, E., Seuntjens, J., 2012. Dose homogeneity specification for reference dosimetry of nonstandard fields. Med. Phys. 39, 407e414. Das, I.J., Cheng, C.W., Watts, R.J., Ahnesjo, A., Gibbons, J., Li, X.A., Lowenstein, J., Mitra, R.K., Simon, W.E., Zhu, T.C., 2008. Accelerator beam data commissioning equipment and procedures: report of the TG-106 of the therapy physics committee of the AAPM. Med. Phys. 35, 4186e4215. Dasu, A., Löfroth, P.O., Wickman, G., 1998. Liquid ionization chamber measurements of dose distributions in small 6 MV photon beams. Phys. Med. Biol. 43, 21e36. Eberle, K., Engler, J., Hartmann, G., Hofmann, R., Hörandel, J.R., 2003. First tests of a liquid filled ionization chamber to monitor intensity modulated radiation beams. Phys. Med. Biol. 48, 3555e3564. Francescon, P., Cora, S., Satariano, N., 2011. Calculation of kfclin,fmsr Qclin,Qmsr for several small detectors and for two linear accelerators using Monte Carlo simulations. Med. Phys. 38, 6513e6527. Francescon, P., Kilby, W., Satariano, N., Cora, S., 2012. Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the CyberKnife system. Phys. Med. Biol. 57, 3741e3758.
86
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González-Castaño, D.M., Gómez, F., Brualla, L., Roselló, J.V., Planes, D., Sánchez, M., Pombar, M., 2011. A liquid-filled ionization chamber for high precision relative dosimetry. Phys. Med. 27, 89e96. Griessbach, I., Lapp, M., Bohsung, J., Gademann, G., Harder, D., 2005. Dosimetric characteristics of a new unshielded silicon diode and its application in clinical photon and electron beam. Med. Phys. 32, 3750e3754. Hrbacek, J., Lang, S., Klöck, S., 2011. Commissioning of photon beams of a flattening filter-free linear accelerator and the accuracy of beam modeling using an anisotropic analytical algorithm. Int. J. Radiat. Oncol. Biol. Phys. 80, 1228e1237. Leo, W.R., 1994. Techniques for Nuclear and Particle Physics Experiments: a How-to Approach. Springer, Berlin, p. 154. Lewis, V., Woods, M., Burgess, P., Green, S., Simpson, J., Wardle, J., 2003. Measurement Good Practice Guide No 49, The Assessment of Uncertainty in Radiological Calibration and Testing. National Physical Laboratory, United Kingdom, p. 3. Martens, C., De Wagter, C., De Neve, W., 2001. The value of the LA48 linear ion chamber array for characterization of intensity-modulated beams. Phys. Med. Biol. 46, 1131e1148.
Pardo, J., Franco, L., Gómez, F., Iglesias, A., Pazos, A., Pena, J., Lobato, R., Mosquera, J., Pombar, M., Sendón, J., 2005. Development and operation of a pixel segmented liquid-filled linear array for radiotherapy quality assurance. Phys. Med. Biol. 50, 1703e1716. Sterpin, E., Mackie, T.R., Vynckier, S., 2012. Monte Carlo computed machine-specific correction factors for reference dosimetry of TomoTherapy static beam for several ion chambers. Med. Phys. 39, 4066e4072. Stewart, K.J., Elliott, A., Seuntjens, J.P., 2007. Development of a guarded liquid ionization chamber for clinical dosimetry. Phys. Med. Biol. 52, 3089e3104. Tölli, H., Sjögren, R., Wendelsten, M., 2010. A two-dose-rate method for general recombination correction for liquid ionization chambers in pulsed beams. Phys. Med. Biol. 55, 4247e4260. Wickman, G., 1974. Radiation quality independent liquid ionization chamber for dosimetry of electron radiation from medical accelerators. Acta Radiol. Ther. Phys. Biol. 13, 37e48. Wickman, G., Nyström, H., 1992. The use of liquids ionization chambers for high precision radiotherapy dosimetry. Phys. Med. Biol. 37, 1789e1812.