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Addendum to the determination of recombination and polarity correction factors, kS and kP, for small cylindrical ionization chambers in pulsed filtered and unfiltered beams: PTW 31023
ZEMEDI-10774; No. of Pages 4
ARTICLE IN PRESS
TECHNICAL COMMUNICATION
Addendum to the determination of recombination and polarity correction factors, kS and kP, for small cylindrical ionization chambers in pulsed filtered and unfiltered beams: PTW 31023 Gregor Bruggmoser 1,∗ , Rainer Saum 1 , Rafael Kranzer 2 1 2
Universitätsklinikum Freiburg, Klinik für Strahlenheilkunde, Robert Koch Str. 3, 79106 Freiburg, Germany PTW-Freiburg, Lörracher Str. 7, 79117 Freiburg, Germany
Received 7 August 2018; accepted 9 November 2018
Abstract This addendum provides correction factors for the recombination and the polarity effect for the new ionization chamber PTW PinPoint (type 31023). The measurements were made in filtered (WFF) and unfiltered (FFF) high-energy photon beams. It could be confirmed that both the initial and the general recombination effect of the chamber mainly depends on dose per pulse at the point of measurement and is independent of the filtration of the photon beam. Keywords: Ionization chamber, Saturation loss, Recombination, Polarity effect
1 Motivation
2.2 Measurements
In this technical communication we report on the determination of the coefficients used in the formula for the correction factor for ion collection efficiency as described in DIN 6800-2 [1] and the polarity effect for a ionization chamber recently introduced in the market. The study was performed in the same way like the previous one (Bruggmoser [2]).
The measurements were carried out using filtered (WFF) 6, 15, 18 MV as well as unfiltered (FFF) 6 and 10 MV photon beams. The procedure was identical to the previous study and is described there in more detail (Bruggmoser [2]).
2 Materials and methods 2.1 Ionization chamber The PTW PinPoint chamber (type 31023 will replace type 31014). Table 1 shows the relevant physical dimensions of the measuring volume. The outer dimensions remained unchanged.
∗ Corresponding
2.3 Evaluation of the recombination correction factor kS Jaffé plots have been created by plotting the reciprocal values of the readings 1/M versus the reciprocal chamber voltages 1/U for a variety of DP values. Jaffé plots are shown in Fig. 1. The polarizing voltage of the chambers was varied over the range of 50 V–400 V. In case of almost complete saturation the following Eq. (1) to determine the correction factor kS can be used. The coefficient γ represents a value which characterises the initial recombination whereas δ represents the volume
author: Gregor Bruggmoser, Universitätsklinikum Freiburg, Klinik für Strahlenheilkunde, Robert Koch Str. 3, 79106 Freiburg, Germany. E-mail: [email protected] (G. Bruggmoser).
Z Med Phys xxx (2018) xxx–xxx https://doi.org/10.1016/j.zemedi.2018.11.001 www.elsevier.com/locate/zemedi
ARTICLE IN PRESS 2
G. Bruggmoser et al. / Z Med Phys xxx (2018) xxx–xxx
Table 1 Ionization chamber studied. Ionization Chamber
Nominal measurement volume in cm3
Radius of the measurement volume in mm
Length of the measurement volume in mm
Diameter of the central electrode in mm
PTW 31023 PinPoint
0.015
1
5
0.6
Fig. 1. Jaffé plots for a range of doses per pulse for the PinPoint type 31023. Values are normalized at 50 V. The solid lines illustrate the linear fit to the data over a range of 50 V–200 V. Table 2 Experimentally determined values for ␥ and δ (see equation (2)) The values are valid within the DP range and for the chamber voltages stated, provided that the ion collection time is smaller than the time between two subsequent radiation pulses. Ionization chamber
γ in V
γ/U
δ in V mGy−1
δ/U in mGy−1
R2
Absorbed dose to water per pulse DP in mGy
Recommended voltage U in V
Ion collection time in ms
PTW PinPoint type 31023
0.333
1.66E−03
0.195
9.76E−04
0.904
0.2–3.0
200
0.03 at 200 V
recombination. The coefficients γ and δ in Eq. (1) were determined from linear plots of the kS values versus the absorbed dose to water per pulse DP at the point of measurement. kS = 1 +
γ + δ · DP U
(1)
The corresponding formula in the addendum to the AAPM’s TG-51 protocol (McEwen [3]) is
2.4 Determination of the correction factor for the polarization effect kp or rather Ppol According to (DIN6800-2 [1]) the correction factor kP as well as the corresponding PPol in TG-51 (McEwen [3]) was evaluated (Bruggmoser [2]).
3 Results and discussion 3.1 Jaffé plots
Pion = 1 + Cinit + Cgen · DPP
(2)
Pion is the correction factor for the recombination effect, DPP DP , Cint = γ/U and Cgen = δ/U.
Jaffé plots have been created for DP values ranging between about 0.19 and 2.75 mGy. In Fig. 1 an example of Jaffé plots is shown for the PinPoint 31023 measured for both polarities and corrected for polarity effect and air density. The
ARTICLE IN PRESS G. Bruggmoser et al. / Z Med Phys xxx (2018) xxx–xxx
3
Fig. 2. The recombination correction factor kS as a function of dose per pulse DP for the PinPoint type 31023 for filtered and unfiltered beams. The solid lines illustrate the linear fit to the data.
Fig. 3. Polarity correction kP and its variation with beam quality for the PinPoint type 31023 and a polarizing voltage of 200 V. Left image: polarity correction kP and its variation with beam quality Q. Q = 0.57 corresponds to the reference quality 60 Co. Right image: polarity correction PPol and its variation with beam quality %dd(10). %dd(10) = 0.57 corresponds to the reference quality 60 Co. Limits according to McEwen [3].
recommended maximum voltage for this chamber was found to be 200 V.
The chamber-to-chamber variation of experimentally determined kS values was within the statistical deviation of the measurements of less than 0.1%.
3.2 kS versus dose per pulse
3.3 Polarity effect
The coefficients γ and δ in Eq. (1) were determined from the linear plots and are given in Table 2. As can be seen from Fig. 2 there is no significant difference between WFF and FFF beams detectable.
Fig. 3 show a plot of the correction factors for the polarity effect kP according to DIN 6800-2, [1] (or Ppol according to TG-51, McEwen [3]) against beam quality. All values are well within the specified limits stated in the TG-51 Addendum [3].
ARTICLE IN PRESS 4
G. Bruggmoser et al. / Z Med Phys xxx (2018) xxx–xxx
4 Conclusions
References
The conclusions of our study are: The main impact on the correction factors is not the filtration of the beam (i.e. WFFor FFF-mode), but the dose per pulse DP . For the chamber investigated a linear relationship between kS and DP up to about 3 mGy/pulse was evaluated. The chamber fulfills the requirements of the TG-51 Addendum (McEwen [3]) with regard to the topics investigated in this study.
[1] DIN 6800, Dosismessverfahren nach der Sondenmethode für Photonenund Elektronenstrahlung, Part 2: Dosimetrie hochenergetischer Photonen-und Elektronenstrahlung mit Ionisationskammern; März 2008. [2] Bruggmoser G, Saum R, Kranzer R. Determination of the recombination and polarity correction factor kS and kp for small cylindrical ionization chambers PTW 31021 and PTW 31022 in pulsed filtered and unfiltered beams. Z Med Phys 2018;28(August (3)): 247–53. [3] McEwen MR, DeWerd L, Ibbott G, Followill D, Rogers WO, Seltzer S, et al. Addendum to the AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon beams. Med Phys 2014;41(April (4)), 041501-1–041501-20.
Acknowledgements Co-author R. Kranzer is employee of PTW-Freiburg.
Available online at www.sciencedirect.com
ScienceDirect
ARTICLE IN PRESS
TECHNICAL COMMUNICATION
Addendum to the determination of recombination and polarity correction factors, kS and kP, for small cylindrical ionization chambers in pulsed filtered and unfiltered beams: PTW 31023 Gregor Bruggmoser 1,∗ , Rainer Saum 1 , Rafael Kranzer 2 1 2
Universitätsklinikum Freiburg, Klinik für Strahlenheilkunde, Robert Koch Str. 3, 79106 Freiburg, Germany PTW-Freiburg, Lörracher Str. 7, 79117 Freiburg, Germany
Received 7 August 2018; accepted 9 November 2018
Abstract This addendum provides correction factors for the recombination and the polarity effect for the new ionization chamber PTW PinPoint (type 31023). The measurements were made in filtered (WFF) and unfiltered (FFF) high-energy photon beams. It could be confirmed that both the initial and the general recombination effect of the chamber mainly depends on dose per pulse at the point of measurement and is independent of the filtration of the photon beam. Keywords: Ionization chamber, Saturation loss, Recombination, Polarity effect
1 Motivation
2.2 Measurements
In this technical communication we report on the determination of the coefficients used in the formula for the correction factor for ion collection efficiency as described in DIN 6800-2 [1] and the polarity effect for a ionization chamber recently introduced in the market. The study was performed in the same way like the previous one (Bruggmoser [2]).
The measurements were carried out using filtered (WFF) 6, 15, 18 MV as well as unfiltered (FFF) 6 and 10 MV photon beams. The procedure was identical to the previous study and is described there in more detail (Bruggmoser [2]).
2 Materials and methods 2.1 Ionization chamber The PTW PinPoint chamber (type 31023 will replace type 31014). Table 1 shows the relevant physical dimensions of the measuring volume. The outer dimensions remained unchanged.
∗ Corresponding
2.3 Evaluation of the recombination correction factor kS Jaffé plots have been created by plotting the reciprocal values of the readings 1/M versus the reciprocal chamber voltages 1/U for a variety of DP values. Jaffé plots are shown in Fig. 1. The polarizing voltage of the chambers was varied over the range of 50 V–400 V. In case of almost complete saturation the following Eq. (1) to determine the correction factor kS can be used. The coefficient γ represents a value which characterises the initial recombination whereas δ represents the volume
author: Gregor Bruggmoser, Universitätsklinikum Freiburg, Klinik für Strahlenheilkunde, Robert Koch Str. 3, 79106 Freiburg, Germany. E-mail: [email protected] (G. Bruggmoser).
Z Med Phys xxx (2018) xxx–xxx https://doi.org/10.1016/j.zemedi.2018.11.001 www.elsevier.com/locate/zemedi
ARTICLE IN PRESS 2
G. Bruggmoser et al. / Z Med Phys xxx (2018) xxx–xxx
Table 1 Ionization chamber studied. Ionization Chamber
Nominal measurement volume in cm3
Radius of the measurement volume in mm
Length of the measurement volume in mm
Diameter of the central electrode in mm
PTW 31023 PinPoint
0.015
1
5
0.6
Fig. 1. Jaffé plots for a range of doses per pulse for the PinPoint type 31023. Values are normalized at 50 V. The solid lines illustrate the linear fit to the data over a range of 50 V–200 V. Table 2 Experimentally determined values for ␥ and δ (see equation (2)) The values are valid within the DP range and for the chamber voltages stated, provided that the ion collection time is smaller than the time between two subsequent radiation pulses. Ionization chamber
γ in V
γ/U
δ in V mGy−1
δ/U in mGy−1
R2
Absorbed dose to water per pulse DP in mGy
Recommended voltage U in V
Ion collection time in ms
PTW PinPoint type 31023
0.333
1.66E−03
0.195
9.76E−04
0.904
0.2–3.0
200
0.03 at 200 V
recombination. The coefficients γ and δ in Eq. (1) were determined from linear plots of the kS values versus the absorbed dose to water per pulse DP at the point of measurement. kS = 1 +
γ + δ · DP U
(1)
The corresponding formula in the addendum to the AAPM’s TG-51 protocol (McEwen [3]) is
2.4 Determination of the correction factor for the polarization effect kp or rather Ppol According to (DIN6800-2 [1]) the correction factor kP as well as the corresponding PPol in TG-51 (McEwen [3]) was evaluated (Bruggmoser [2]).
3 Results and discussion 3.1 Jaffé plots
Pion = 1 + Cinit + Cgen · DPP
(2)
Pion is the correction factor for the recombination effect, DPP DP , Cint = γ/U and Cgen = δ/U.
Jaffé plots have been created for DP values ranging between about 0.19 and 2.75 mGy. In Fig. 1 an example of Jaffé plots is shown for the PinPoint 31023 measured for both polarities and corrected for polarity effect and air density. The
ARTICLE IN PRESS G. Bruggmoser et al. / Z Med Phys xxx (2018) xxx–xxx
3
Fig. 2. The recombination correction factor kS as a function of dose per pulse DP for the PinPoint type 31023 for filtered and unfiltered beams. The solid lines illustrate the linear fit to the data.
Fig. 3. Polarity correction kP and its variation with beam quality for the PinPoint type 31023 and a polarizing voltage of 200 V. Left image: polarity correction kP and its variation with beam quality Q. Q = 0.57 corresponds to the reference quality 60 Co. Right image: polarity correction PPol and its variation with beam quality %dd(10). %dd(10) = 0.57 corresponds to the reference quality 60 Co. Limits according to McEwen [3].
recommended maximum voltage for this chamber was found to be 200 V.
The chamber-to-chamber variation of experimentally determined kS values was within the statistical deviation of the measurements of less than 0.1%.
3.2 kS versus dose per pulse
3.3 Polarity effect
The coefficients γ and δ in Eq. (1) were determined from the linear plots and are given in Table 2. As can be seen from Fig. 2 there is no significant difference between WFF and FFF beams detectable.
Fig. 3 show a plot of the correction factors for the polarity effect kP according to DIN 6800-2, [1] (or Ppol according to TG-51, McEwen [3]) against beam quality. All values are well within the specified limits stated in the TG-51 Addendum [3].
ARTICLE IN PRESS 4
G. Bruggmoser et al. / Z Med Phys xxx (2018) xxx–xxx
4 Conclusions
References
The conclusions of our study are: The main impact on the correction factors is not the filtration of the beam (i.e. WFFor FFF-mode), but the dose per pulse DP . For the chamber investigated a linear relationship between kS and DP up to about 3 mGy/pulse was evaluated. The chamber fulfills the requirements of the TG-51 Addendum (McEwen [3]) with regard to the topics investigated in this study.
[1] DIN 6800, Dosismessverfahren nach der Sondenmethode für Photonenund Elektronenstrahlung, Part 2: Dosimetrie hochenergetischer Photonen-und Elektronenstrahlung mit Ionisationskammern; März 2008. [2] Bruggmoser G, Saum R, Kranzer R. Determination of the recombination and polarity correction factor kS and kp for small cylindrical ionization chambers PTW 31021 and PTW 31022 in pulsed filtered and unfiltered beams. Z Med Phys 2018;28(August (3)): 247–53. [3] McEwen MR, DeWerd L, Ibbott G, Followill D, Rogers WO, Seltzer S, et al. Addendum to the AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon beams. Med Phys 2014;41(April (4)), 041501-1–041501-20.
Acknowledgements Co-author R. Kranzer is employee of PTW-Freiburg.
Available online at www.sciencedirect.com
ScienceDirect