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Effect of wedge filter and field size on photoneutron dose equivalent for an 18 MV photon beam of a medical linear accelerator
ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 84–89
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Effect of wedge filter and field size on photoneutron dose equivalent for an 18 MV photon beam of a medical linear accelerator Asghar Mesbahi a,b,, Ahmad Keshtkar a, Ehsan Mohammadi c, Mohammad Mohammadzadeh b a
Medical Physics Department, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran Radiation Therapy Department, Imam Hospital, Tabriz, Iran c Radiation Therapy Department, Imam Khomeini Hospital, Tehran, Iran b
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 May 2009 Received in revised form 15 August 2009 Accepted 17 August 2009
Photoneutrons produced during radiation therapy with high energy photons is the main source of unwanted out-of-field received doses of patients. To analyze the neutron dose equivalent (NDE) for wedged beams and its variation with field size, Monte Carlo (MC) modeling of an 18 MV photon beam was performed using MCNPX MC code. The results revealed that the NDE is on average 6.5 times higher for wedged beams. For open beams, the NDE decreased with increasing field size especially for field sizes 420 20 cm2. While, for wedged beams, the NDE increased with field size. It was suggested that the increase of NDE for wedged beams should be taken into account in radiation-induced secondary cancer risk estimations and radiation protection calculations. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Wedge filter Linear accelerators Neutron dose
1. Introduction Nowadays, high energy photons from electron linear accelerators (linacs) are used dominantly in radiation therapy centers. Photoneutrons are produced in high energy photon beams through the photonuclear giant dipole resonance reaction with linac head components (Barquero et al., 2005b; Becker et al., 2007; Facure et al., 2004; Golnik et al., 2007; Kralik et al., 2008; Pena et al., 2005; Rivera et al., 2008). Photons with energies 47 MeV can produce neutrons through interactions with the linac’s head components, treatment room and patient. Photoneutrons are generated in the head compartments which are located in the beam pathway and also in the head shielding materials. The major elements contributing to the photonuclear reaction are the target, beam collimation system including primary and secondary collimators and multi-leaf collimators. Studies show that the flattening filter can be a source of neutrons provided that it has been made from high atomic number materials such as tungsten and lead (Mesbahi, 2009; Pena et al., 2005). Studies have revealed that neutrons are emitted approximately isotropically from the accelerator head and are the most significant source of dose received by the patient during radiation therapy with high energy photon beams (Allen and
Corresponding author at: Medical Physics Department, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran. Tel./fax: +98 411 3364660. E-mail address: [email protected] (A. Mesbahi).
0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.08.008
Chaudhri, 1982, 1998; Chadwick, 1998; Kry et al., 2007; Mesbahi, 2009). Neutrons produced in linac head have an effective energy range of 1–2 MeV within the 10–18 MV X-ray beams (d’Errico et al., 2001) and are more effective than photons in producing radiation induced late effects. These neutrons typically have a radiation weighting factor of 20. Although, the out-of-field photon dose is larger than out-of-field neutron dose, neutrons are the dominant source of biological damage because of their high weighting factor. Peripheral dose received by various regions of body during radiation therapy with high energy photons is influenced significantly by the amount of photoneutrons produced in the linac’s head. As long as patient protection and occupational doses are concerned, photoneutrons are undesirable and should be controlled and minimized (Jaradat and Biggs, 2007; Rebello et al., 2008). The studies have shown that neutron fluence varies with field size for different medical linacs (Al-Ghamdi et al., 2008; Chibani and Ma, 2003; Hashemi et al., 2008; Reft et al., 2005). Chibani and Ma found that for the 18 MV photon beam of a Siemens Primus linac, the number of neutrons increased with increasing field size (Chibani and Ma, 2003). In an experimental study by Al-Ghamdi et al. it was shown that for fast neutrons the intensity does not vary significantly with field size for the measurements outside the primary beam and it decreases linearly with field size. The thermal neutron intensity at any location was found to be almost independent of field size (Al-Ghamdi et al., 2008). Another study by Hashemi et al. showed that neutron dose equivalent (NDE) increases with field size for both open and wedged beams
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(Hashemi et al., 2008). Reft et al. showed that for 18 MV Clinac 2100, the fast neutron dose was relatively independent of collimator setting (Reft et al., 2005). However, it is difficult to form conclusions based on these studies considering the effect of field size on the photoneutron dose. Wedge filters are made from lead or tungsten alloys and are usually used in conventional and three dimensional conformal treatments to modulate the photon intensity across the beam. They are located in beam pathway and cause substantial attenuation in the intensity of photon beams. Comparing with open beams, photon interactions with wedge filter materials can increase the photoneutron production considerably through (g, n) reactions. As far as we know, the effect of wedge filters on photoneutron dose has not been studied comprehensively yet. Although, neutron doses can be measured by different type of dosimeters, acquiring the results with the accuracy of better than 20% seems very difficult and cumbersome. MC methods were used frequently to estimate the neutron dose and fluence in several studies (Barquero et al., 2005a; Chibani and Ma, 2003; Cox et al., 2006; Enger et al., 2006; Facure et al., 2004, 2007; Falcao et al., 2007; Gur et al., 1978; Hashemi et al., 2008). In the current study, the head of an Elekta SL75/25 linac was simulated by the MCNPX MC code. The neutron dose for points located in in-field and out-offield regions was calculated for open and wedged beams, as well as different field sizes. The results were compared with the recently published measurement of Hashemi et al. (2008b) on the same linac and with other investigations.
2. Method and materials 2.1. MC simulation The 18 MeV photon beam of an Elekta SL75/25 linac was simulated based on information provided by the manufacturer
Bending magnet
Shielding (w+Pb)
Electron beam
85
(Fig. 1). The MCNPX MC code version 2.4.0 (LANL, 2002) was used for photoneutron calculations in the current study. The photon production from primary electrons in the target and as well as the photoneutron production in different head components have a very small cross section and MC calculations using full MC models are time consuming and associated with high statistical uncertainty in the results (Kry et al., 2007; Mesbahi, 2009; Solberg et al., 2001). This problem becomes more pronounced when photoneutron dose are planned to be calculated in out-offield regions where photons and neutrons have to have a sufficient probability to reach the detector cells. To overcome these problems we developed a point source model for photoneutron calculations in electron linear accelerators. A complete MC model based on the manufacturer data and direct measurements on the head shielding was developed in the MCNPX environment (Fig. 1). The open beam model was validated in a previous study (Mesbahi, 2009). The only difference between the current model and previous model was the head shielding. In order to accelerate the photoneutron dose calculations, a new model was built with multiple ring detectors located just under the target which were used to score the photons spectra and intensity at different angles relative to central axis of beam (DeMarco et al., 1998; Solberg et al., 2001). The rings were defined with an interval of 2 mm and the F5 tally was used. To score the neutrons produced in the target materials, a spherical surface was located around the target so that the point of interaction of electrons with target material was at the central of the sphere. Fluence and spectra were scored with a statistical uncertainty o1% for both neutrons and photons. Using these data, the second model was built in which the electron source was removed and the photon point source with the calculated photon spectra and fluence at different angels, was defined in an SDEF card. To validate this model, the calculated depth doses and beam profiles were compared with the measurements and the original model. Using the model the photoneutron doses at the isocenter and points with different distances from the isocenter, up to 100 cm were calculated (Fig. 2). Point A, at the distance of 100 from the central axis of beam and 80 cm height from the patient plane was also considered. The F6 tally was used to score the neutron dose in the detectors. The detectors were spherical cells with radius of 2 cm and filled with water. The neutron dose was calculated by running the photon source from the target. To decrease the run time in photoneutron calculations, the biased photonuclear production was enabled by setting the fourth entry on the Phys:P card to 1. The energy cut-off for photons and electrons was 7 MeV, because it was lower than
Target (w) Accelerator’s head
Primary Flattening filter
100 cm
collimator
Detector A
Wedge filter Automatic wedge filter
80 cm Shielding
SSD=100 cm Central axis
Secondary collimator (w+Pb)
Detectors with the radius of 2 cm 10 cm
Fig. 1. Simulated geometry of the accelerator’s head including components within the beam pathway and the head.
50 cm
100 cm
Fig. 2. The schematic representation of the Elekta SL 75/25 geometry and cells used as detectors in the simulations.
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the photon energy limit for photoneutron production (Chibani and Ma, 2003; Mesbahi, 2009; Pena et al., 2005). To take into account the neutrons produced from target in the first model, the neutron fluence was tallied using a sphere which included only the target. An isotropic neutron source was located in the target and the model was run in neutron mode and neutrons reaching the detectors were also scored. By scaling the contribution of these neutrons relative to photon fluence calculated from original (full Monte Carlo) model, the neutron
5x5 cm
100 Measurement MC
Percent depth dose
80
60
40
20
0 -60 -50 -40 -30 -20 -10 0 10 20 30 40 Distance from central axis (mm)
50
60
120 10x10 cm
Measurement
Percent depth dose
100
MC
doses in scoring cells were corrected for neutrons produced in the target. Using the new approach, the run time was decreased by a factor of 10. The statistical uncertainty of neutron dose calculations was lower than 3% for all field sizes. Both open and wedged beams were simulated and validated by comparing the calculated and measured percent depth dose and beam profile data. The Elekta automatic wedge filter was modeled according to the manufacturer drawings. For the wedged model, the 601 wedge filter made of lead alloy was added to the validated open-beam model. The comparison of MC calculated and measured beam profiles for the wedged beams are shown in Fig. 3. Field sizes of 5 5, 10 10, 20 20, 30 30, 30 40 and 40 40 cm2 were used for neutron dose calculations. Neutron dose was calculated in points locating at 0, 5, 10, 20,y,100 cm distance from central axis of beam using the F6 tally. The equivalent dose HT for neutrons were calculated by this P wR ðEn Þ DT ðEn Þ in which En denotes the neutron formula: HT ¼ energy of the ith energy bin, DT(En) is the absorbed dose of tissue and wR(En) is the weighting factor for neutrons. The weighting factors were calculated using ICRP-60 recommended formula (ICRP, 1991): wR ðEn Þ ¼ 5 þ 17 exp½lnð2En Þ2 =6 where En is the neutron energy in MeV. To calculate NDE per photon dose at dmax, the photon dose delivered at dmax per tally in terms of MeV was converted to dose per tally in terms of Joules. Then the mass of the scoring cell was calculated by the F6 tally and then used to convert the energy in the scoring cell at dmax to joules per kg per tally. The number of primary particles required to deliver 1 Gy at dmax was calculated. This number was multiplied by the F6 tally of the neutron of scoring cells in the patient plane to produce neutron dose in terms of Gy/X-ray Gy at dmax. This was converted to mSv/X-ray Gy at dmax using the weighting factor. The calculation was performed for all field sizes and both open and wedged cases.
80 3. Results
60 40 20
-100
-80
0 -60 -40 -20 0 20 40 60 Distance from central axis of beam (mm)
20x20 cm
80
100
160 Measurement 140
MC
Percent depth dose
120 100 80 60 40 20
-150
-100
0 -50 0 50 100 Distance from central axis (mm)
150
Fig. 3. Comparison of MC calculated and measured beam profiles for a 601 wedge filter at the depth of 10 cm in water phantom.
For both open and wedged beams, the NDEs for points at the patient plane and its variation with distance from central axis of beam are seen in Fig. 4. As can be seen in Fig. 4, for both open and wedged beams, the NDE is decreased with the distance from central axis for all field sizes. However, the neutron dose fall-off with distance is greater for closer points and gets smoother for points farther than 70 cm from central axis of beam. The effect of field size on neutron dose is completely different for open and wedged beams. The neutron dose decreases with field size for open beams while it increases with field size for wedged beams at the patient plane. For a point on the central axis, in open beams, the NDE decreases from 1 to 0.6 mSv/Gy X-ray when changing the field size from 5 5 cm2 to 30 40 cm2. But for wedged beam, it increases from 4 to 7 mSv/Gy X-ray for the same field sizes. A better representation of the effect of the field size on the NDE at the patient plane is shown Fig. 5. It can be seen that the neutron dose variation remains constant from 5 5 cm2 to 20 20 cm2 and begins to drop for larger field sizes. The effect gets more pronounced for the in-field region (Fig. 5A). For wedged beam, Fig. 5B shows that the neutron dose is increased with field size for all points except points at 80 and 100 cm from the central axis. The variation of NDE with field size is considerable for in-field points because the attenuation effect of secondary collimator jaws are decreased with increasing the field size. Table 1 shows the ratio of NDE for wedged and open beams. The NDE ratio is increased with field size for all points which were considered in the current study. Our results show that the NDE for
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Neutron dose equivalent (mSv/ X-ray Gy)
Neutron dose equivalent (mSv/ X-ray Gy)
1.2
Neutron dose equivalent (mSv/Gy X-ray)
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Open beam 5x5 cm 10x10 cm 20x20 cm 30x30 cm 30X40 cm 40x40 cm
1.0 0.8 0.6 0.4 0.2 0.0 10
20
Wedged beam 7
5x5 10x10 20x20 30x30 30x40
6 5 4 3 2 1 0 0
10
20
1.2
30 40 50 60 70 80 90 Distance from central axis (cm)
100 110
Fig. 4. Neutron dose equivalent variation with the distance from central axis for different field sizes. (A) Open beam and (B) wedged beam.
wedged beams is on average 6.5 times higher considering the data in Table 1 for all points and field sizes. Point A was used as a representative for points around the linac head shielding. Fig. 6 shows that for point A the NDE is also up to 6.5 times higher for wedged fields. For point A, in both cases the neutron dose is decreased with field size. But the rate of this decrease is lower for open than wedged beams. For wedged beam it changes from 1.5 to 0.94 mSv/Gy X-ray while it varies from 0.23 to 0.05 mSv/Gy X-ray for field sizes from 5 5 cm2 to 30 40 cm2.
4. Discussion In the current study, the NDEs at different points at the patient plane were calculated for an 18 MV photon beam considering the influence of the wedge filter. The results showed that the NDE is significantly higher for wedged beams. Additionally, the effect of field size on NDE was different for open and wedged beams. For the open beams, the NDE decreased with increasing field size. While for the wedged beams increasing the field size caused increase in the NDE. The studies on photoneutrons produced in medical linacs show controversial results concerning the effect of field size on neutron dose around medical linacs. Our results disagree with the MC study by Chibani and Ma (2003) in which they show that for the Primus 18 MV, Varian 15 and 18 MV photon beams, the neutron
0 cm
Open beam 1.0
5 cm 10 cm
0.8
20 cm 30 cm
0.6
40 cm 50 cm
0.4
60 cm 80 cm
0.2
100 cm 0.0 0
30 40 50 60 70 80 90 100 110 Distance from central axis (cm) Neutron dose equivalent (mSv/Gy X-ray)
0
87
10
20 30 40 Square field size (cm)
50
8 0 cm
Wedged beam
7
5 cm
6
10 cm
5
20 cm 30 cm
4
40 cm 3
50 cm
2
60 cm 80 cm
1
100 cm 0 0
10 20 30 Square field size (cm)
40
Fig. 5. Neutron dose equivalent variation with the field size for scoring cells with different distances from central axis in the patient plane. (A) Open beam and (B) wedged beam.
fluence increases with increasing field size for open beams. In their study, the field sizes 420 20 cm2 were not studied and the statistical uncertainty of their results was higher than 5% which has probably influenced their final conclusion because as it has been shown in the current study (Figs. 4 and 5) and other investigations, the NDE does not change significantly for field sizes o20 20 (Howell et al., 2005; Kim et al., 2007; Kry et al., 2007; Mao et al., 1997; Waller et al., 2003). It can be concluded that to differentiate the neutron dose variation between different field sizes, statistical uncertainty of o2% is required. The results of the current study are partly in agreement with the study of Kim et al. (2007) for photoneutrons of 10 and 15 MV photon beams for the Varian 2100C/2300C linac. In their study, the maximum equivalent dose was seen for the middle size of the irradiation fields, a 20 20 cm2, and the neutron dose decreased with field sizes higher than 20 20 cm2. Additionally, Mao et al. (1997) found that the neutron yield increases linearly as the field size decreases, is in good agreement with the results of the current study. Our reasoning for NDE decrease with increasing the field size in open beams is that closing the jaws causes more neutron attenuation, but the neutron yield increases simultaneously by increasing the number of photon–neutron interactions in secondary collimator jaws. So, the net effect is dominantly influenced by the neutron yield increase. According to our results, the NDE increases with field size for wedged beams, presumably
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Table 1 The ratio of neutron dose equivalent (NDE) for wedged to open beams for different field sizes and points. Field size
Detector at 0 cm
55 3.91 10 10 4.22 20 20 5.56 30 30 8.05 30 40 11.55 Mean
6.66
Detector at 5 cm
Detector at 10 cm
Detector at 20 cm
Detector at 30 cm
Detector at 40 cm
Detector at 50 cm
Detector at 60 cm
Detector at 70 cm
Detector at 80 cm
Detector at 90 cm
Detector at Detector 100 cm at A
3.62 4.04 5.56 8.27 11.95
3.97 4.38 5.73 8.41 12.57
3.81 4.10 5.33 8.75 13.46
4.04 4.19 5.29 8.34 13.94
4.33 4.20 5.33 7.59 12.49
4.36 4.22 4.68 7.26 11.14
4.57 4.18 4.51 6.54 10.40
4.74 4.31 4.59 5.49 8.93
5.01 4.66 4.79 4.79 7.39
5.20 5.05 4.91 5.79 8.78
5.12 4.99 4.73 5.48 7.99
6.42 6.38 6.37 8.39 11.24
6.69
7.01
7.09
7.16
6.79
6.33
6.04
5.61
5.33
5.94
5.66
7.76
Neutron dose equivalent (mSv/Gy X-ray)
1.6 Wedged beam
1.4
Open beam
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
5
10
15 20 25 30 Square field size (cm)
35
40
45
Fig. 6. Neutron dose equivalent for open and wedged beams at point A.
because the neutron production happens dominantly in wedge filter and the secondary collimator jaws act as neutron attenuators and their neutron producing role diminishes. Therefore, for wedged beams with increasing the field size more photoneutrons can reach the patient plane and the NDE is increased. In a similar experimental and MC studies by Hashemi et al. (2007, 2008), the NDE was measured using polycarbonate film dosimeters for an 18 MeV photon beam of Elekta SL75/25 for open and wedged beams. It was reported that the NDE was 4.8 times higher for wedged beams of the Elekta linac. Comparing to the current results, our estimated value is 25% higher. In both studies of Hashemi et al. (2007, 2008) the NDE increased with field size for both open and wedged beams. In another MC study on the Varian 2300C linac the increasing of neutron dose with field size was reported for open beams (Garnica-Garza, 2005). Our results for open beams are not consistent with both above mentioned studies. They suggested that the neutron contamination arises from above the secondary collimator and neutron contamination increases by opening the secondary collimator jaws and reducing the attenuation effect of secondary collimator jaws. We think that this reasoning does not seem completely reliable, because it is basically evident that both attenuation and photoneutron production take place in movable collimator jaws. A contribution of 12% in the photoneutron yield by secondary collimator jaws was reported by Pena et al. (2005). Therefore, although with increasing field size the attenuation effect of jaws is decreased, the
photoneutron production in jaws is reduced simultaneously and the contribution of the wedge filter to the neutron dose at the patient plane dominates (Al-Ghamdi et al., 2008; Allen and Chaudhri, 1982; Chibani and Ma, 2003; Difilippo et al., 2003; Facure et al., 2004; Garnica-Garza, 2005). Considering the NDE for open beams, the results of the current study are in good agreement with a similar study on an Elekta linac (Ongaro et al., 2000). On the other hand, our estimated NDE are 3 times lower than the reported values for both open and wedged beams by Hashemi et al. who used polycarbonate films as neutron dosimeters which are insensitive to energy range of neutrons produced in treatment room walls (Hasib and Piesch, 1978; Hashemi et al., 2007). In our study we simulated the same model of linac and ignored the effect of walls, but the results on the effect of field size were in contrast to each other. We think that these differences can be attributed to the large dosimetric uncertainty associated with polycarbonate dosimeters. Additionally, it seems reasonable to consider that the simple linac head geometry composed of a head shielding and an isotropic neutron source which was used in the above mentioned studies was not able to take into account the field size effect accurately. We only found one experimental study on the effect of wedge filter on photoneutron production in the literature (Hashemi et al., 2008). Our results are in agreement with that study considering the wedge filter effect. According to both studies it seems that by introducing the wedge filter composed of tungsten or lead into the pathway of high energy photons, the number of photoneutrons is increased through high energy photon interactions with high-z material of wedge filter. On the other hand, using the wedge filter, the photon fluence reaching the dmax is decreased by a rate which is equal to the wedge factor, for example 0.3 in the current study. So, to compensate for the attenuation effect of the wedge filter, the photon fluence and monitor units, required to produce a constant dose at the dmax, are increased more than 3 times and cause more photoneutron production for wedged beams. Another effect which adds to the neutron dose is the increase of backscattered photons interactions with head shielding and components. This effect may cause more photon and neutron leakage from linac head shielding.
5. Conclusion In the current study, the head of a medical linac was simulated by the MCNPX MC code to analyze the impact of the wedge filter on photoneutron production around the head and at the patient plane. The results showed higher photoneutrons for wedged beams. It was also shown that the NDE increases with field size for wedged beams and decreases with field size for open beams. The NDE was also reduced with distance from the central axis of beam for open and wedged beams. Considering the highest quality factor of 20 for neutrons produced in medical linacs, it is necessary to establish required procedures and guidelines to
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protect the patient from unwanted neutron doses in radiation therapy with high energy photons and wedged beams. It is recommended that the presence of increased photoneutrons in wedged beams should be taken in account when the patient received dose and secondary cancer risk from radiation therapy are calculated. It may also be necessary to establish more conservative design of accelerator room and door shielding for neutrons to meet radiation protection guidelines. References Al-Ghamdi, H., Fazal, U.r., Al-Jarallah, M.I., Maalej, N., 2008. Photoneutron intensity variation with field size around radiotherapy linear accelerator 18-MeV X-ray beam. Radiat. Meas. 43, S495–S499. Allen, P.D., Chaudhri, M.A., 1988. Photoneutron production in tissue during high energy bremsstrahlung radiotherapy. Phys. Med. Biol. 33, 1017–1036. Allen, P.D., Chaudhri, M.A., 1982. The dose contribution due to photonuclear reactions during radiotherapy. Med. Phys. 9, 904–906. Barquero, R., Edwards, T.M., Iniguez, M.P., Vega-Carrillo, H.R., 2005a. Monte Carlo simulation estimates of neutron doses to critical organs of a patient undergoing 18 MV X-ray LINAC-based radiotherapy. Med. Phys. 32, 3579–3588. Barquero, R., Mendez, R., Vega-Carrillo, HR., Iniguez, MP., Edwards, T.M., 2005b. Neutron spectra and dosimetric features around an 18 MV linac accelerator. Health Phys. 88, 48–58. Becker, J., Brunckhorst, E., Schmidt, R., 2007. Photoneutron production of a Siemens Primus linear accelerator studied by Monte Carlo methods and a paired magnesium and boron coated magnesium ionization chamber system. Phys. Med. Biol. 52, 6375–6387. Chadwick, M.B., 1998. Neutron, proton, and photonuclear cross-sections for radiation therapy and radiation protection. Radiat. Environ. Biophys. 37, 235–242. Chibani, O., Ma, C.M., 2003. Photonuclear dose calculations for high-energy photon beams from Siemens and Varian linacs. Med. Phys. 30, 1990–2000. Cox, M., Harris, P., Nam, G., Thomas, D., 2006. The use of a Monte Carlo method for uncertainty calculation, with an application to the measurement of neutron ambient dose equivalent rate. Radiat. Prot. Dosimetry 121, 12–23. d’Errico, F., Luszik-Bhadra, M., Nath, R., Siebert, B.R., Wolf, U., 2001. Depth doseequivalent and effective energies of photoneutrons generated by 6–18 MV X-ray beams for radiotherapy. Health Phys. 80, 4–11. DeMarco, J.J., Solberg, T.D., Smathers, J.B., 1998. CT-based Monte Carlo simulation tool for dosimetry planning and analysis. Medical Physics 25, 1–11. Difilippo, F., Papiez, L., Moskvin, V., Peplow, D., DesRosiers, C., Johnson, J., Timmerman, R., Randall, M., Lillie, R., 2003. Contamination dose from photoneutron processes in bodily tissues during therapeutic radiation delivery. Med. Phys. 30, 2849–2854. Enger, S.A., Munck af, R.P., Rezaei, A., Lundqvist, H., 2006. Monte Carlo calculations of thermal neutron capture in gadolinium: a comparison of GEANT4 and MCNP with measurements. Med. Phys. 33, 337–341. Facure, A., da Silva, A.X., Falcao, R.C., 2007. Monte Carlo simulation of scattered and thermal photoneutron fluences inside a radiotherapy room. Radiat. Prot. Dosimetry 123, 56–61. Facure, A., Falcao, R.C., da Silva, A.X., Crispim, V.R., 2004. Neutron dose rate evaluation for medical linear accelerators. Radiat. Prot. Dosimetry 111, 101–103. Falcao, R.C., Facure, A., Silva, A.X., 2007. Neutron dose calculation at the maze entrance of medical linear accelerator rooms. Radiat. Prot. Dosimetry 123, 283–287. Garnica-Garza, H.M., 2005. Characteristics of the photoneutron contamination present in a high-energy radiotherapy treatment room. Phys. Med. Biol. 50, 531–539.
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Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Effect of wedge filter and field size on photoneutron dose equivalent for an 18 MV photon beam of a medical linear accelerator Asghar Mesbahi a,b,, Ahmad Keshtkar a, Ehsan Mohammadi c, Mohammad Mohammadzadeh b a
Medical Physics Department, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran Radiation Therapy Department, Imam Hospital, Tabriz, Iran c Radiation Therapy Department, Imam Khomeini Hospital, Tehran, Iran b
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 May 2009 Received in revised form 15 August 2009 Accepted 17 August 2009
Photoneutrons produced during radiation therapy with high energy photons is the main source of unwanted out-of-field received doses of patients. To analyze the neutron dose equivalent (NDE) for wedged beams and its variation with field size, Monte Carlo (MC) modeling of an 18 MV photon beam was performed using MCNPX MC code. The results revealed that the NDE is on average 6.5 times higher for wedged beams. For open beams, the NDE decreased with increasing field size especially for field sizes 420 20 cm2. While, for wedged beams, the NDE increased with field size. It was suggested that the increase of NDE for wedged beams should be taken into account in radiation-induced secondary cancer risk estimations and radiation protection calculations. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Wedge filter Linear accelerators Neutron dose
1. Introduction Nowadays, high energy photons from electron linear accelerators (linacs) are used dominantly in radiation therapy centers. Photoneutrons are produced in high energy photon beams through the photonuclear giant dipole resonance reaction with linac head components (Barquero et al., 2005b; Becker et al., 2007; Facure et al., 2004; Golnik et al., 2007; Kralik et al., 2008; Pena et al., 2005; Rivera et al., 2008). Photons with energies 47 MeV can produce neutrons through interactions with the linac’s head components, treatment room and patient. Photoneutrons are generated in the head compartments which are located in the beam pathway and also in the head shielding materials. The major elements contributing to the photonuclear reaction are the target, beam collimation system including primary and secondary collimators and multi-leaf collimators. Studies show that the flattening filter can be a source of neutrons provided that it has been made from high atomic number materials such as tungsten and lead (Mesbahi, 2009; Pena et al., 2005). Studies have revealed that neutrons are emitted approximately isotropically from the accelerator head and are the most significant source of dose received by the patient during radiation therapy with high energy photon beams (Allen and
Corresponding author at: Medical Physics Department, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran. Tel./fax: +98 411 3364660. E-mail address: [email protected] (A. Mesbahi).
0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.08.008
Chaudhri, 1982, 1998; Chadwick, 1998; Kry et al., 2007; Mesbahi, 2009). Neutrons produced in linac head have an effective energy range of 1–2 MeV within the 10–18 MV X-ray beams (d’Errico et al., 2001) and are more effective than photons in producing radiation induced late effects. These neutrons typically have a radiation weighting factor of 20. Although, the out-of-field photon dose is larger than out-of-field neutron dose, neutrons are the dominant source of biological damage because of their high weighting factor. Peripheral dose received by various regions of body during radiation therapy with high energy photons is influenced significantly by the amount of photoneutrons produced in the linac’s head. As long as patient protection and occupational doses are concerned, photoneutrons are undesirable and should be controlled and minimized (Jaradat and Biggs, 2007; Rebello et al., 2008). The studies have shown that neutron fluence varies with field size for different medical linacs (Al-Ghamdi et al., 2008; Chibani and Ma, 2003; Hashemi et al., 2008; Reft et al., 2005). Chibani and Ma found that for the 18 MV photon beam of a Siemens Primus linac, the number of neutrons increased with increasing field size (Chibani and Ma, 2003). In an experimental study by Al-Ghamdi et al. it was shown that for fast neutrons the intensity does not vary significantly with field size for the measurements outside the primary beam and it decreases linearly with field size. The thermal neutron intensity at any location was found to be almost independent of field size (Al-Ghamdi et al., 2008). Another study by Hashemi et al. showed that neutron dose equivalent (NDE) increases with field size for both open and wedged beams
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(Hashemi et al., 2008). Reft et al. showed that for 18 MV Clinac 2100, the fast neutron dose was relatively independent of collimator setting (Reft et al., 2005). However, it is difficult to form conclusions based on these studies considering the effect of field size on the photoneutron dose. Wedge filters are made from lead or tungsten alloys and are usually used in conventional and three dimensional conformal treatments to modulate the photon intensity across the beam. They are located in beam pathway and cause substantial attenuation in the intensity of photon beams. Comparing with open beams, photon interactions with wedge filter materials can increase the photoneutron production considerably through (g, n) reactions. As far as we know, the effect of wedge filters on photoneutron dose has not been studied comprehensively yet. Although, neutron doses can be measured by different type of dosimeters, acquiring the results with the accuracy of better than 20% seems very difficult and cumbersome. MC methods were used frequently to estimate the neutron dose and fluence in several studies (Barquero et al., 2005a; Chibani and Ma, 2003; Cox et al., 2006; Enger et al., 2006; Facure et al., 2004, 2007; Falcao et al., 2007; Gur et al., 1978; Hashemi et al., 2008). In the current study, the head of an Elekta SL75/25 linac was simulated by the MCNPX MC code. The neutron dose for points located in in-field and out-offield regions was calculated for open and wedged beams, as well as different field sizes. The results were compared with the recently published measurement of Hashemi et al. (2008b) on the same linac and with other investigations.
2. Method and materials 2.1. MC simulation The 18 MeV photon beam of an Elekta SL75/25 linac was simulated based on information provided by the manufacturer
Bending magnet
Shielding (w+Pb)
Electron beam
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(Fig. 1). The MCNPX MC code version 2.4.0 (LANL, 2002) was used for photoneutron calculations in the current study. The photon production from primary electrons in the target and as well as the photoneutron production in different head components have a very small cross section and MC calculations using full MC models are time consuming and associated with high statistical uncertainty in the results (Kry et al., 2007; Mesbahi, 2009; Solberg et al., 2001). This problem becomes more pronounced when photoneutron dose are planned to be calculated in out-offield regions where photons and neutrons have to have a sufficient probability to reach the detector cells. To overcome these problems we developed a point source model for photoneutron calculations in electron linear accelerators. A complete MC model based on the manufacturer data and direct measurements on the head shielding was developed in the MCNPX environment (Fig. 1). The open beam model was validated in a previous study (Mesbahi, 2009). The only difference between the current model and previous model was the head shielding. In order to accelerate the photoneutron dose calculations, a new model was built with multiple ring detectors located just under the target which were used to score the photons spectra and intensity at different angles relative to central axis of beam (DeMarco et al., 1998; Solberg et al., 2001). The rings were defined with an interval of 2 mm and the F5 tally was used. To score the neutrons produced in the target materials, a spherical surface was located around the target so that the point of interaction of electrons with target material was at the central of the sphere. Fluence and spectra were scored with a statistical uncertainty o1% for both neutrons and photons. Using these data, the second model was built in which the electron source was removed and the photon point source with the calculated photon spectra and fluence at different angels, was defined in an SDEF card. To validate this model, the calculated depth doses and beam profiles were compared with the measurements and the original model. Using the model the photoneutron doses at the isocenter and points with different distances from the isocenter, up to 100 cm were calculated (Fig. 2). Point A, at the distance of 100 from the central axis of beam and 80 cm height from the patient plane was also considered. The F6 tally was used to score the neutron dose in the detectors. The detectors were spherical cells with radius of 2 cm and filled with water. The neutron dose was calculated by running the photon source from the target. To decrease the run time in photoneutron calculations, the biased photonuclear production was enabled by setting the fourth entry on the Phys:P card to 1. The energy cut-off for photons and electrons was 7 MeV, because it was lower than
Target (w) Accelerator’s head
Primary Flattening filter
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collimator
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Wedge filter Automatic wedge filter
80 cm Shielding
SSD=100 cm Central axis
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Detectors with the radius of 2 cm 10 cm
Fig. 1. Simulated geometry of the accelerator’s head including components within the beam pathway and the head.
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Fig. 2. The schematic representation of the Elekta SL 75/25 geometry and cells used as detectors in the simulations.
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the photon energy limit for photoneutron production (Chibani and Ma, 2003; Mesbahi, 2009; Pena et al., 2005). To take into account the neutrons produced from target in the first model, the neutron fluence was tallied using a sphere which included only the target. An isotropic neutron source was located in the target and the model was run in neutron mode and neutrons reaching the detectors were also scored. By scaling the contribution of these neutrons relative to photon fluence calculated from original (full Monte Carlo) model, the neutron
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doses in scoring cells were corrected for neutrons produced in the target. Using the new approach, the run time was decreased by a factor of 10. The statistical uncertainty of neutron dose calculations was lower than 3% for all field sizes. Both open and wedged beams were simulated and validated by comparing the calculated and measured percent depth dose and beam profile data. The Elekta automatic wedge filter was modeled according to the manufacturer drawings. For the wedged model, the 601 wedge filter made of lead alloy was added to the validated open-beam model. The comparison of MC calculated and measured beam profiles for the wedged beams are shown in Fig. 3. Field sizes of 5 5, 10 10, 20 20, 30 30, 30 40 and 40 40 cm2 were used for neutron dose calculations. Neutron dose was calculated in points locating at 0, 5, 10, 20,y,100 cm distance from central axis of beam using the F6 tally. The equivalent dose HT for neutrons were calculated by this P wR ðEn Þ DT ðEn Þ in which En denotes the neutron formula: HT ¼ energy of the ith energy bin, DT(En) is the absorbed dose of tissue and wR(En) is the weighting factor for neutrons. The weighting factors were calculated using ICRP-60 recommended formula (ICRP, 1991): wR ðEn Þ ¼ 5 þ 17 exp½lnð2En Þ2 =6 where En is the neutron energy in MeV. To calculate NDE per photon dose at dmax, the photon dose delivered at dmax per tally in terms of MeV was converted to dose per tally in terms of Joules. Then the mass of the scoring cell was calculated by the F6 tally and then used to convert the energy in the scoring cell at dmax to joules per kg per tally. The number of primary particles required to deliver 1 Gy at dmax was calculated. This number was multiplied by the F6 tally of the neutron of scoring cells in the patient plane to produce neutron dose in terms of Gy/X-ray Gy at dmax. This was converted to mSv/X-ray Gy at dmax using the weighting factor. The calculation was performed for all field sizes and both open and wedged cases.
80 3. Results
60 40 20
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Fig. 3. Comparison of MC calculated and measured beam profiles for a 601 wedge filter at the depth of 10 cm in water phantom.
For both open and wedged beams, the NDEs for points at the patient plane and its variation with distance from central axis of beam are seen in Fig. 4. As can be seen in Fig. 4, for both open and wedged beams, the NDE is decreased with the distance from central axis for all field sizes. However, the neutron dose fall-off with distance is greater for closer points and gets smoother for points farther than 70 cm from central axis of beam. The effect of field size on neutron dose is completely different for open and wedged beams. The neutron dose decreases with field size for open beams while it increases with field size for wedged beams at the patient plane. For a point on the central axis, in open beams, the NDE decreases from 1 to 0.6 mSv/Gy X-ray when changing the field size from 5 5 cm2 to 30 40 cm2. But for wedged beam, it increases from 4 to 7 mSv/Gy X-ray for the same field sizes. A better representation of the effect of the field size on the NDE at the patient plane is shown Fig. 5. It can be seen that the neutron dose variation remains constant from 5 5 cm2 to 20 20 cm2 and begins to drop for larger field sizes. The effect gets more pronounced for the in-field region (Fig. 5A). For wedged beam, Fig. 5B shows that the neutron dose is increased with field size for all points except points at 80 and 100 cm from the central axis. The variation of NDE with field size is considerable for in-field points because the attenuation effect of secondary collimator jaws are decreased with increasing the field size. Table 1 shows the ratio of NDE for wedged and open beams. The NDE ratio is increased with field size for all points which were considered in the current study. Our results show that the NDE for
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Open beam 5x5 cm 10x10 cm 20x20 cm 30x30 cm 30X40 cm 40x40 cm
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Fig. 4. Neutron dose equivalent variation with the distance from central axis for different field sizes. (A) Open beam and (B) wedged beam.
wedged beams is on average 6.5 times higher considering the data in Table 1 for all points and field sizes. Point A was used as a representative for points around the linac head shielding. Fig. 6 shows that for point A the NDE is also up to 6.5 times higher for wedged fields. For point A, in both cases the neutron dose is decreased with field size. But the rate of this decrease is lower for open than wedged beams. For wedged beam it changes from 1.5 to 0.94 mSv/Gy X-ray while it varies from 0.23 to 0.05 mSv/Gy X-ray for field sizes from 5 5 cm2 to 30 40 cm2.
4. Discussion In the current study, the NDEs at different points at the patient plane were calculated for an 18 MV photon beam considering the influence of the wedge filter. The results showed that the NDE is significantly higher for wedged beams. Additionally, the effect of field size on NDE was different for open and wedged beams. For the open beams, the NDE decreased with increasing field size. While for the wedged beams increasing the field size caused increase in the NDE. The studies on photoneutrons produced in medical linacs show controversial results concerning the effect of field size on neutron dose around medical linacs. Our results disagree with the MC study by Chibani and Ma (2003) in which they show that for the Primus 18 MV, Varian 15 and 18 MV photon beams, the neutron
0 cm
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Fig. 5. Neutron dose equivalent variation with the field size for scoring cells with different distances from central axis in the patient plane. (A) Open beam and (B) wedged beam.
fluence increases with increasing field size for open beams. In their study, the field sizes 420 20 cm2 were not studied and the statistical uncertainty of their results was higher than 5% which has probably influenced their final conclusion because as it has been shown in the current study (Figs. 4 and 5) and other investigations, the NDE does not change significantly for field sizes o20 20 (Howell et al., 2005; Kim et al., 2007; Kry et al., 2007; Mao et al., 1997; Waller et al., 2003). It can be concluded that to differentiate the neutron dose variation between different field sizes, statistical uncertainty of o2% is required. The results of the current study are partly in agreement with the study of Kim et al. (2007) for photoneutrons of 10 and 15 MV photon beams for the Varian 2100C/2300C linac. In their study, the maximum equivalent dose was seen for the middle size of the irradiation fields, a 20 20 cm2, and the neutron dose decreased with field sizes higher than 20 20 cm2. Additionally, Mao et al. (1997) found that the neutron yield increases linearly as the field size decreases, is in good agreement with the results of the current study. Our reasoning for NDE decrease with increasing the field size in open beams is that closing the jaws causes more neutron attenuation, but the neutron yield increases simultaneously by increasing the number of photon–neutron interactions in secondary collimator jaws. So, the net effect is dominantly influenced by the neutron yield increase. According to our results, the NDE increases with field size for wedged beams, presumably
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Table 1 The ratio of neutron dose equivalent (NDE) for wedged to open beams for different field sizes and points. Field size
Detector at 0 cm
55 3.91 10 10 4.22 20 20 5.56 30 30 8.05 30 40 11.55 Mean
6.66
Detector at 5 cm
Detector at 10 cm
Detector at 20 cm
Detector at 30 cm
Detector at 40 cm
Detector at 50 cm
Detector at 60 cm
Detector at 70 cm
Detector at 80 cm
Detector at 90 cm
Detector at Detector 100 cm at A
3.62 4.04 5.56 8.27 11.95
3.97 4.38 5.73 8.41 12.57
3.81 4.10 5.33 8.75 13.46
4.04 4.19 5.29 8.34 13.94
4.33 4.20 5.33 7.59 12.49
4.36 4.22 4.68 7.26 11.14
4.57 4.18 4.51 6.54 10.40
4.74 4.31 4.59 5.49 8.93
5.01 4.66 4.79 4.79 7.39
5.20 5.05 4.91 5.79 8.78
5.12 4.99 4.73 5.48 7.99
6.42 6.38 6.37 8.39 11.24
6.69
7.01
7.09
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6.79
6.33
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Neutron dose equivalent (mSv/Gy X-ray)
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5
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Fig. 6. Neutron dose equivalent for open and wedged beams at point A.
because the neutron production happens dominantly in wedge filter and the secondary collimator jaws act as neutron attenuators and their neutron producing role diminishes. Therefore, for wedged beams with increasing the field size more photoneutrons can reach the patient plane and the NDE is increased. In a similar experimental and MC studies by Hashemi et al. (2007, 2008), the NDE was measured using polycarbonate film dosimeters for an 18 MeV photon beam of Elekta SL75/25 for open and wedged beams. It was reported that the NDE was 4.8 times higher for wedged beams of the Elekta linac. Comparing to the current results, our estimated value is 25% higher. In both studies of Hashemi et al. (2007, 2008) the NDE increased with field size for both open and wedged beams. In another MC study on the Varian 2300C linac the increasing of neutron dose with field size was reported for open beams (Garnica-Garza, 2005). Our results for open beams are not consistent with both above mentioned studies. They suggested that the neutron contamination arises from above the secondary collimator and neutron contamination increases by opening the secondary collimator jaws and reducing the attenuation effect of secondary collimator jaws. We think that this reasoning does not seem completely reliable, because it is basically evident that both attenuation and photoneutron production take place in movable collimator jaws. A contribution of 12% in the photoneutron yield by secondary collimator jaws was reported by Pena et al. (2005). Therefore, although with increasing field size the attenuation effect of jaws is decreased, the
photoneutron production in jaws is reduced simultaneously and the contribution of the wedge filter to the neutron dose at the patient plane dominates (Al-Ghamdi et al., 2008; Allen and Chaudhri, 1982; Chibani and Ma, 2003; Difilippo et al., 2003; Facure et al., 2004; Garnica-Garza, 2005). Considering the NDE for open beams, the results of the current study are in good agreement with a similar study on an Elekta linac (Ongaro et al., 2000). On the other hand, our estimated NDE are 3 times lower than the reported values for both open and wedged beams by Hashemi et al. who used polycarbonate films as neutron dosimeters which are insensitive to energy range of neutrons produced in treatment room walls (Hasib and Piesch, 1978; Hashemi et al., 2007). In our study we simulated the same model of linac and ignored the effect of walls, but the results on the effect of field size were in contrast to each other. We think that these differences can be attributed to the large dosimetric uncertainty associated with polycarbonate dosimeters. Additionally, it seems reasonable to consider that the simple linac head geometry composed of a head shielding and an isotropic neutron source which was used in the above mentioned studies was not able to take into account the field size effect accurately. We only found one experimental study on the effect of wedge filter on photoneutron production in the literature (Hashemi et al., 2008). Our results are in agreement with that study considering the wedge filter effect. According to both studies it seems that by introducing the wedge filter composed of tungsten or lead into the pathway of high energy photons, the number of photoneutrons is increased through high energy photon interactions with high-z material of wedge filter. On the other hand, using the wedge filter, the photon fluence reaching the dmax is decreased by a rate which is equal to the wedge factor, for example 0.3 in the current study. So, to compensate for the attenuation effect of the wedge filter, the photon fluence and monitor units, required to produce a constant dose at the dmax, are increased more than 3 times and cause more photoneutron production for wedged beams. Another effect which adds to the neutron dose is the increase of backscattered photons interactions with head shielding and components. This effect may cause more photon and neutron leakage from linac head shielding.
5. Conclusion In the current study, the head of a medical linac was simulated by the MCNPX MC code to analyze the impact of the wedge filter on photoneutron production around the head and at the patient plane. The results showed higher photoneutrons for wedged beams. It was also shown that the NDE increases with field size for wedged beams and decreases with field size for open beams. The NDE was also reduced with distance from the central axis of beam for open and wedged beams. Considering the highest quality factor of 20 for neutrons produced in medical linacs, it is necessary to establish required procedures and guidelines to
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