- Email: [email protected]
Skin dose measurements using MOSFET and TLD for head and neck patients treated with tomotherapy
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 1683–1685
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
Skin dose measurements using MOSFET and TLD for head and neck patients treated with tomotherapy Rajesh A. Kinhikar a,, Vedang Murthy b, Vineeta Goel b, Chandrashekar M. Tambe a, Dipak S. Dhote c, Deepak D. Deshpande a a
Department of Medical Physics, Tata Memorial Centre, Parel, Mumbai 400012, India Department of Radiation Oncology, Tata Memorial Centre, Parel, Mumbai 400012, India c Department of Electronics, Brijlal Biyani Science College, Amravati, Maharashtra, India b
a r t i c l e in fo
abstract
Article history: Received 1 May 2008 Received in revised form 10 March 2009 Accepted 10 March 2009
The purpose of this work was to estimate skin dose for the patients treated with tomotherapy using metal oxide semiconductor field-effect transistors (MOSFETs) and thermoluminescent dosimeters (TLDs). In vivo measurements were performed for two head and neck patients treated with tomotherapy and compared to TLD measurements. The measurements were subsequently carried out for five days to estimate the inter-fraction deviations in MOSFET measurements. The variation between skin dose measured with MOSFET and TLD for first patient was 2.2%. Similarly, the variation of 2.3% was observed between skin dose measured with MOSFET and TLD for second patient. The tomotherapy treatment planning system overestimated the skin dose as much as by 10–12% when compared to both MOSFET and TLD. However, the MOSFET measured patient skin doses also had good reproducibility, with interfraction deviations ranging from 1% to 1.4%. MOSFETs may be used as a viable dosimeter for measuring skin dose in areas where the treatment planning system may not be accurate. & 2009 Published by Elsevier Ltd.
Keywords: Tomotherapy Skin dose MOSFET TLD
1. Introduction In vivo dosimetry has been used as a means for measuring and verifying the skin doses calculated by treatment planning systems (TPS) (Essers and Mijnheer, 1999; Higgins et al., 2003). Though thermoluminescent dosimeters (TLDs) are widely used for in vivo dosimetry, they are unable to permanently store dose information since the reading of the detectors erases the dosimetric information. They have been shown to provide dose readings with an accuracy of 2% (McGhee et al., 1993; Fairbanks and DeWerd, 1993), but their use requires careful and time-consuming handling and annealing procedures (Wood and Mayles, 1995; Kron et al., 1993, 1996). Metal oxide semiconductor field-effect transistors (MOSFETs) have been introduced as an alternative to the TLDs. MOSFETs are a quick and easy alternative as dose information can be obtained immediately after irradiation. The amount of radiation absorbed by a MOSFET is measured by the shift in threshold voltage before and after an irradiation. The MOSFET has additional advantages as better sensitivity, reproducibility, and stability with minimal temperature effects (Soubra et al., 1994; Butson et al., 1996; Cheung et al., 2004; Jornet et al., 2004; Scalchi et al.,
Corresponding author. Tel.: +91 22 24177000x4115; fax: +91 22 24146937.
E-mail address: [email protected] (R.A. Kinhikar). 0969-8043/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.apradiso.2009.03.008
2005; Ramaseshan et al., 1997, 2004; Scalchi and Francescon, 1998; Thomas and Hoole, 2004). The basic structure of the dual MOSFET dual bias detector is described elsewhere (Soubra et al., 1994). Many tumors in the head and neck are located close to the surface so skin sparing is limited in order to deliver an adequate dose to the target. This problem is amplified by the use of intensity-modulated radiation therapy (IMRT) and inverse planning, as skin doses in IMRT tend to be higher than with conventional therapy. This is due to the use of multiple beams that enter tangentially to the skin which are used to offset the build-up region of beams entering perpendicularly. The use of these tangential beams can cause an increase in skin dose of 19% and 27% with and without an immobilization mask, respectively (Lee et al., 2002). Therefore, a particularly useful application of MOSFET dosimetry is in IMRT (Marcie et al., 2005). There are also reports of inconsistencies between doses calculated by IMRT treatment planning systems and those measured through in vivo dosimetry on the surface and in buildup regions (Chung et al., 2005). An overestimation of surface dose by a treatment planning system of up to 18.5% of the prescribed dose has also been reported (Chung et al., 2005). The complexities introduced by inverse treatment planning make in vivo measurements on patients during treatment with IMRT a critical part of assuring accurate dose delivery (Bloemen-vanGurp et al., 2003).
ARTICLE IN PRESS 1684
R.A. Kinhikar et al. / Applied Radiation and Isotopes 67 (2009) 1683–1685
Recently a study performed does report on the skin dose measurements on the mask for the patients treated with tomotherapy (Amanda et al., 2008). However this study does not focus on the measurements carried out on the skin inside the mask. The objective of this study was to estimate the skin dose inside the mask for head and neck patients treated with IMRT using tomotherapy planning system and compare the measured skin dose with the planning system.
2. Materials and methods Two head and neck patients were selected for this study. Treatment planning for both these patients was carried out with tomotherapy treatment planning system (TomoPlan, V2.2). The first case was a 24 year lady with carcinoma of the nasopharynx with intracranial extension and multiple cervical adenopathy. In view of the intracranial and intra-orbital extension of the tumor and multiple lymph nodes extending into the lower neck, she was considered for radiotherapy with IMRT using tomotherapy (66 Gy/30 fractions with 2.2 Gy per fraction to planning target volume). She received concurrent chemotherapy with tomotherapy treatment. The field width of 2.5 cm, pitch of 0.3, and modulation factor of 2.8 was the treatment planning parameters. The prescription goal was 95% of the PTV receives at least 66 Gy. Second patient was diagnosed with carcinoma of buccal mucosa cavity. The prescription dose of 20 Gy was planned for 2.5 Gy per fraction. The field width of 2.5 cm, pitch of 0.3, and modulation factor of 2.0 was the treatment planning parameters for this patient. Standard MOSFET (TN-502RD sensors plus low sensitivity bias supply, Best Medical, Springfield, VA, USA) was used for calibration. The diagrammatic details of the unit are available at the manufacturer’s website. It is a software-controlled system (v 2.2) with semiconductor transistors (sensitive volume of 0:2 mm 0:2 mm 0:0005 mm). For all measurements, the bias supply was set at a standard sensitivity (1 mV/cGy). Before the MOSFETs were used for dose measurements they were first calibrated in a 6 MV tomotherapy static and rotational beams. Calibration factors were determined for both standard sensitivity MOSFETs. The MOSFETs were connected to a 5 V standard bias supply at least 1 h before calibrations were done. The bias supply was also connected to a reader, which provided a record of the threshold voltage of each MOSFET. One at a time, each detector was positioned at a depth of 1.5 cm in a solid water phantom. Five MOSFET detectors were individually calibrated. Ten centimeter slab of solid water was used for backscatter. The calibrations were done with a source-to-surface distance (SSD) of 85 cm at a field size of 5 40 cm2 . For ‘bubbleup’ calibrations, the MOSFET was placed with the epoxy bubble facing towards the beam and for ‘bubble-down’ calibrations the epoxy bubble was facing away from the beam. An Extradin A1SL ionization chamber (Standard Imaging, Middleton, WI) was positioned at a depth of 1.5 cm in the solid water phantom with SSD 85 cm for same field size mentioned above. After an initial reading of the threshold voltage was taken, a dose of 150 cGy was delivered to the detector. The threshold voltage was read again 2 min after the irradiation. This process was repeated three times for each calibration. Ion chamber readings were taken during each trial, and the dose was calculated using AAPM’s TG-51 (1999) parameters. The use of solid water rather than water was assumed to introduce a negligible uncertainty (Chuang et al., 2002; Mackie et al., 1993). Comparing the change in threshold voltage with the calculated dose delivered, a calibration factor was determined for each detector.
All five MOSFETs were calibrated periodically throughout their total lifetime, which is approximately 20,000 mV, to observe any change in sensitivity with total integrated dose. To test the accuracy of these calibration factors when used with the tomotherapy unit, same MOSFETs were calibrated on both the tomotherapy unit and the 6 MV linac beam for comparison. The MOSFET detectors were properly taped on the skin inside the mask. The TLDs were also placed just beside the MOSFET. An extreme caution was taken to maintain the same location of the detectors for every subsequent fraction in a week. The skin dose was measured for five consecutive fractions using both MOSFET and TLD. Prior to each irradiation, TLD-100 (LiF:Mg,Ti) powder (The Harshaw Chemical Co., Solon, OH, USA) was annealed using a thermal cycle: 400 C ð5 Þ for 1 h-cooling for 5 min2100 C for 2 h in a Programmable Muffle Furnace (Model-126, Fisher Scientific Co., Pittsburgh, PA, USA) and then cooled to normal room temperature. For annealing, the TL powder was placed inside a glass Petri dish with cover. Rexon UL-320 TLD Reader, (TLD systems Inc., USA) was used to record TL output at maximum acquisition temperature of 280 C using constant heating rate of 14 C=s: Constant time gap of 24 h was maintain between irradiation and read out. Dose response curve for the TLD-100 powder was generated in Co-60 gamma ray beam (Equinox 80, MDS Nordion, Canada) and was found linear in the range of 0.5–4.0 Gy. For measurements using TLD, about 40 mg of the freshly annealed TLD-100 powder was packed in square polyethylene pouch (approximately 1 cm 1 cm). The TL output of about 10 mg powders was recorded using Rexon TLD reader and this way four readings were obtained from each TL pouch. The mean value of net TL output per unit weight (nC/mg) of these four readings was used for calculation. The uncertainty in TLD-100 powder measurements was 2%.
3. Results and discussion All the MOSFETs were read within 15 min of the irradiation to reduce the fade effect. The MOSFET were validated as the surface detector by irradiating in a solid water phantom along with the TLDs so as to simulate the clinical conditions. Hence the MOSFET was used with very little build-up (only the immobilization mask). The calibration of the MOSFET revealed the calibration coefficient as 1.11 and 1.12 for static and rotational beam, respectively. This calibration coefficient was derived for both the jaw settings (2:5 cm 40 cm and 5 cm 40 cm). Thus the MOSFET has minimal field size dependence. The rotational measurements for MOSFET thus confirmed the negligible angular dependence for 360 . The reproducibility of the MOSFET was also checked and an excellent consistency was obtained for repetitive measurements. MOSFET measured patient skin doses also had good reproducibility, with inter-fraction deviations ranging from 1% to 1.4%. For first patient the MOSFET measured the skin dose as 90% while TLD read 92% of the prescription dose (2.2 Gy). The variation between skin dose measured with MOSFET and TLD was 2.2%. Similarly, for second patient the skin dose measured with MOSFET and TLD was 88% and 86% of the prescription dose, respectively. The variation of 2.3% was observed between skin dose measured with MOSFET and TLD. The treatment planning system estimated the skin dose as 100% for both the patients. Our findings with these two patients planned with tomotherapy treatment planning system supplement the previous literature (Amanda et al., 2008). It was observed that the planning system overestimates the skin dose by 10–12%. Cherpak et al. used TN502RDM (micro-MOSFET) for their study. They used both high and standard sensitivity MSOFET. The MSOFET was placed on the mask as well as inside the mask. The
ARTICLE IN PRESS R.A. Kinhikar et al. / Applied Radiation and Isotopes 67 (2009) 1683–1685
TLD chips were used for subsequent comparison of the dose. The TLD chips have the limitation of finite size and hence render some uncertainties in the skin dose estimation. In addition, chips cannot be used in a monolayer. TN502RD MOSFET was used in the current study. Standard sensitivity MOSFET only was used. The MSOFET was placed only inside the mask directly on the skin. The reproducibility was maintained daily. The TLD powder was used for subsequent comparison of the dose. The TLD powder can be used in a monolayer and hence is relatively more accurate method for skin dose estimation.
4. Conclusion The results show that MOSFETs can be used as accurate and reproducible detectors for tomotherapy skin dose measurements where high dose gradients are present and in areas where the treatment planning system may not be accurate. In vivo dosimetry measurements are a useful tool with tomotherapy for areas where a high skin dose is expected, as the treatment planning system may not give accurate dose values at the surface. References Amanda, C., Studinski, R.C.N., Cygler, J.E., 2008. MOSFET detectors in quality assurance of tomotherapy treatments. Radiother. Oncol. 86 (2), 242–250. Bloemen-vanGurp, E., du Bois, W., Bruinvis, I., 2003. Clinical dosimetry with MOSFET dosimeters to determine the dose along the field junction in a split beam technique. Radiother. Oncol. 67, 351–357. Butson, M.J., Rozenfeld, A., Mathus, J.N., 1996. A new radiotherapy surface dose detector: the MOSFET. Med. Phys. 23, 655–658. Cheung, T., Butson, M.J., Yu, P.K.N., 2004. Effects of temperature variation on MOSFET dosimetry. Phys. Med. Biol. 49, N191–196. Chuang, C.F., Verhey, L.J., Xia, P., 2002. Investigation of the use of MOSFET for clinical IMRT dosimetric verification. Med. Phys. 29, 1109–1115.
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Chung, H., Jin, H., Dempsey, J.F., 2005. Evaluation of surface and build-up region dose for intensity-modulated radiation therapy in head and neck cancer. Med. Phys. 32, 2682–2689. Essers, M., Mijnheer, B.J., 1999. In vivo dosimetry during external photon beam radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 43, 245–259. Fairbanks, E.J., DeWerd, L.A., 1993. Thermoluminescent characteristics of LiF: Mg, Ti from three manufacturers. Med. Phys. 20, 729–731. Higgins, P.D., Alaei, P., Gerbi, B.J., Dusenbery, K.E., 2003. In vivo diode dosimetry for routine quality assurance in IMRT. Med. Phys. 30, 3118–3123. Jornet, N., Carrasco, P., Jurado, D., 2004. Comparison study of MOSFET detectors and diodes for entrance in vivo dosimetry in 18 MV X-ray beams. Med. Phys. 31, 2534–2542. Kron, T., Buston, M., Wong, T., Metcalfe, P., 1993. Readout thermoluminescence dosimetry chips using contact planchet heater. Australas. Phys. Eng. Sci. Med. 16, 137–142. Kron, T., Butsnob, M., Huntb, F., Denhama, J., 1996. TLD extrapolation for skin dose determination in vivo. Radiother. Oncol. 41, 119–123. Lee, N., Chuang, C., Quivey, J.M., 2002. Skin toxicity due to intensity-modulated radiotherapy for head-and-neck carcinoma. Int. J. Radiat. Oncol. Biol. Phys. 53, 630–637. Mackie, T.R., Holmes, T.W., Swerdloff, S., 1993. Tomotherapy: a new concept in the delivery of dynamic conformal radiotherapy. Med. Phys. 20, 1709–1719. Marcie, S., Charpiot, E., Bensadoun, R.-J., 2005. In vivo measurements with MOSFET detectors in oropharynx and nasopharynx intensity-modulated radiation therapy. Int. J. Radiat. Oncol. Biol. Phys. 61, 1603–1606. McGhee, P.L., Humphreys, S., Dunscombe, P., 1993. An efficient approach to routine TL dosimetry. Med. Dosim. 18, 187–191. Ramaseshan, R., Russell, S., O’Brien, P., 1997. Clinical dosimetry using MOSFETs. Int. J. Radiat. Oncol. Biol. Phys. 37, 959–964. Ramaseshan, R., Kohli, K.S., Zhang, T.J., 2004. Performance characteristics of a microMOSFET as an in vivo dosimeter in radiation therapy. Phys. Med. Biol. 49, 4031–4048. Scalchi, P., Francescon, P., 1998. Calibration of a MOSFET detection system for 6-MV in vivo dosimetry. Int. J. Radiat. Oncol. Biol. Phys. 40, 987–993. Scalchi, P., Francescon, P., Rajaguru, P., 2005. Characterization of a new MOSFET detector configuration for in vivo skin dosimetry. Med. Phys. 32, 1571–1578. Soubra, M., Cygler, J., Mackay, G., 1994. Evaluation of dual bias dual metal oxide–silicon semiconductor field effect transistor detector as radiation dosimeter. Med. Phys. 21, 567–572. TG-51 AAPM, 1999. AAPM’s TG-51 protocol for clinical reference dosimetry of highenergy photon and electron beams. Med. Phys. 26, 1847–1870. Thomas, S.J., Hoole, A.C.F., 2004. The effect of optimization on surface dose in intensity modulated radiotherapy (IMRT). Phys. Med. Biol. 49, 4919–4928. Wood, J.J., Mayles, W.P., 1995. Factors affecting the precision of TLD dose measurements using an automatic TLD reader. Phys. Med. Biol. 40, 309–313.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
Skin dose measurements using MOSFET and TLD for head and neck patients treated with tomotherapy Rajesh A. Kinhikar a,, Vedang Murthy b, Vineeta Goel b, Chandrashekar M. Tambe a, Dipak S. Dhote c, Deepak D. Deshpande a a
Department of Medical Physics, Tata Memorial Centre, Parel, Mumbai 400012, India Department of Radiation Oncology, Tata Memorial Centre, Parel, Mumbai 400012, India c Department of Electronics, Brijlal Biyani Science College, Amravati, Maharashtra, India b
a r t i c l e in fo
abstract
Article history: Received 1 May 2008 Received in revised form 10 March 2009 Accepted 10 March 2009
The purpose of this work was to estimate skin dose for the patients treated with tomotherapy using metal oxide semiconductor field-effect transistors (MOSFETs) and thermoluminescent dosimeters (TLDs). In vivo measurements were performed for two head and neck patients treated with tomotherapy and compared to TLD measurements. The measurements were subsequently carried out for five days to estimate the inter-fraction deviations in MOSFET measurements. The variation between skin dose measured with MOSFET and TLD for first patient was 2.2%. Similarly, the variation of 2.3% was observed between skin dose measured with MOSFET and TLD for second patient. The tomotherapy treatment planning system overestimated the skin dose as much as by 10–12% when compared to both MOSFET and TLD. However, the MOSFET measured patient skin doses also had good reproducibility, with interfraction deviations ranging from 1% to 1.4%. MOSFETs may be used as a viable dosimeter for measuring skin dose in areas where the treatment planning system may not be accurate. & 2009 Published by Elsevier Ltd.
Keywords: Tomotherapy Skin dose MOSFET TLD
1. Introduction In vivo dosimetry has been used as a means for measuring and verifying the skin doses calculated by treatment planning systems (TPS) (Essers and Mijnheer, 1999; Higgins et al., 2003). Though thermoluminescent dosimeters (TLDs) are widely used for in vivo dosimetry, they are unable to permanently store dose information since the reading of the detectors erases the dosimetric information. They have been shown to provide dose readings with an accuracy of 2% (McGhee et al., 1993; Fairbanks and DeWerd, 1993), but their use requires careful and time-consuming handling and annealing procedures (Wood and Mayles, 1995; Kron et al., 1993, 1996). Metal oxide semiconductor field-effect transistors (MOSFETs) have been introduced as an alternative to the TLDs. MOSFETs are a quick and easy alternative as dose information can be obtained immediately after irradiation. The amount of radiation absorbed by a MOSFET is measured by the shift in threshold voltage before and after an irradiation. The MOSFET has additional advantages as better sensitivity, reproducibility, and stability with minimal temperature effects (Soubra et al., 1994; Butson et al., 1996; Cheung et al., 2004; Jornet et al., 2004; Scalchi et al.,
Corresponding author. Tel.: +91 22 24177000x4115; fax: +91 22 24146937.
E-mail address: [email protected] (R.A. Kinhikar). 0969-8043/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.apradiso.2009.03.008
2005; Ramaseshan et al., 1997, 2004; Scalchi and Francescon, 1998; Thomas and Hoole, 2004). The basic structure of the dual MOSFET dual bias detector is described elsewhere (Soubra et al., 1994). Many tumors in the head and neck are located close to the surface so skin sparing is limited in order to deliver an adequate dose to the target. This problem is amplified by the use of intensity-modulated radiation therapy (IMRT) and inverse planning, as skin doses in IMRT tend to be higher than with conventional therapy. This is due to the use of multiple beams that enter tangentially to the skin which are used to offset the build-up region of beams entering perpendicularly. The use of these tangential beams can cause an increase in skin dose of 19% and 27% with and without an immobilization mask, respectively (Lee et al., 2002). Therefore, a particularly useful application of MOSFET dosimetry is in IMRT (Marcie et al., 2005). There are also reports of inconsistencies between doses calculated by IMRT treatment planning systems and those measured through in vivo dosimetry on the surface and in buildup regions (Chung et al., 2005). An overestimation of surface dose by a treatment planning system of up to 18.5% of the prescribed dose has also been reported (Chung et al., 2005). The complexities introduced by inverse treatment planning make in vivo measurements on patients during treatment with IMRT a critical part of assuring accurate dose delivery (Bloemen-vanGurp et al., 2003).
ARTICLE IN PRESS 1684
R.A. Kinhikar et al. / Applied Radiation and Isotopes 67 (2009) 1683–1685
Recently a study performed does report on the skin dose measurements on the mask for the patients treated with tomotherapy (Amanda et al., 2008). However this study does not focus on the measurements carried out on the skin inside the mask. The objective of this study was to estimate the skin dose inside the mask for head and neck patients treated with IMRT using tomotherapy planning system and compare the measured skin dose with the planning system.
2. Materials and methods Two head and neck patients were selected for this study. Treatment planning for both these patients was carried out with tomotherapy treatment planning system (TomoPlan, V2.2). The first case was a 24 year lady with carcinoma of the nasopharynx with intracranial extension and multiple cervical adenopathy. In view of the intracranial and intra-orbital extension of the tumor and multiple lymph nodes extending into the lower neck, she was considered for radiotherapy with IMRT using tomotherapy (66 Gy/30 fractions with 2.2 Gy per fraction to planning target volume). She received concurrent chemotherapy with tomotherapy treatment. The field width of 2.5 cm, pitch of 0.3, and modulation factor of 2.8 was the treatment planning parameters. The prescription goal was 95% of the PTV receives at least 66 Gy. Second patient was diagnosed with carcinoma of buccal mucosa cavity. The prescription dose of 20 Gy was planned for 2.5 Gy per fraction. The field width of 2.5 cm, pitch of 0.3, and modulation factor of 2.0 was the treatment planning parameters for this patient. Standard MOSFET (TN-502RD sensors plus low sensitivity bias supply, Best Medical, Springfield, VA, USA) was used for calibration. The diagrammatic details of the unit are available at the manufacturer’s website. It is a software-controlled system (v 2.2) with semiconductor transistors (sensitive volume of 0:2 mm 0:2 mm 0:0005 mm). For all measurements, the bias supply was set at a standard sensitivity (1 mV/cGy). Before the MOSFETs were used for dose measurements they were first calibrated in a 6 MV tomotherapy static and rotational beams. Calibration factors were determined for both standard sensitivity MOSFETs. The MOSFETs were connected to a 5 V standard bias supply at least 1 h before calibrations were done. The bias supply was also connected to a reader, which provided a record of the threshold voltage of each MOSFET. One at a time, each detector was positioned at a depth of 1.5 cm in a solid water phantom. Five MOSFET detectors were individually calibrated. Ten centimeter slab of solid water was used for backscatter. The calibrations were done with a source-to-surface distance (SSD) of 85 cm at a field size of 5 40 cm2 . For ‘bubbleup’ calibrations, the MOSFET was placed with the epoxy bubble facing towards the beam and for ‘bubble-down’ calibrations the epoxy bubble was facing away from the beam. An Extradin A1SL ionization chamber (Standard Imaging, Middleton, WI) was positioned at a depth of 1.5 cm in the solid water phantom with SSD 85 cm for same field size mentioned above. After an initial reading of the threshold voltage was taken, a dose of 150 cGy was delivered to the detector. The threshold voltage was read again 2 min after the irradiation. This process was repeated three times for each calibration. Ion chamber readings were taken during each trial, and the dose was calculated using AAPM’s TG-51 (1999) parameters. The use of solid water rather than water was assumed to introduce a negligible uncertainty (Chuang et al., 2002; Mackie et al., 1993). Comparing the change in threshold voltage with the calculated dose delivered, a calibration factor was determined for each detector.
All five MOSFETs were calibrated periodically throughout their total lifetime, which is approximately 20,000 mV, to observe any change in sensitivity with total integrated dose. To test the accuracy of these calibration factors when used with the tomotherapy unit, same MOSFETs were calibrated on both the tomotherapy unit and the 6 MV linac beam for comparison. The MOSFET detectors were properly taped on the skin inside the mask. The TLDs were also placed just beside the MOSFET. An extreme caution was taken to maintain the same location of the detectors for every subsequent fraction in a week. The skin dose was measured for five consecutive fractions using both MOSFET and TLD. Prior to each irradiation, TLD-100 (LiF:Mg,Ti) powder (The Harshaw Chemical Co., Solon, OH, USA) was annealed using a thermal cycle: 400 C ð5 Þ for 1 h-cooling for 5 min2100 C for 2 h in a Programmable Muffle Furnace (Model-126, Fisher Scientific Co., Pittsburgh, PA, USA) and then cooled to normal room temperature. For annealing, the TL powder was placed inside a glass Petri dish with cover. Rexon UL-320 TLD Reader, (TLD systems Inc., USA) was used to record TL output at maximum acquisition temperature of 280 C using constant heating rate of 14 C=s: Constant time gap of 24 h was maintain between irradiation and read out. Dose response curve for the TLD-100 powder was generated in Co-60 gamma ray beam (Equinox 80, MDS Nordion, Canada) and was found linear in the range of 0.5–4.0 Gy. For measurements using TLD, about 40 mg of the freshly annealed TLD-100 powder was packed in square polyethylene pouch (approximately 1 cm 1 cm). The TL output of about 10 mg powders was recorded using Rexon TLD reader and this way four readings were obtained from each TL pouch. The mean value of net TL output per unit weight (nC/mg) of these four readings was used for calculation. The uncertainty in TLD-100 powder measurements was 2%.
3. Results and discussion All the MOSFETs were read within 15 min of the irradiation to reduce the fade effect. The MOSFET were validated as the surface detector by irradiating in a solid water phantom along with the TLDs so as to simulate the clinical conditions. Hence the MOSFET was used with very little build-up (only the immobilization mask). The calibration of the MOSFET revealed the calibration coefficient as 1.11 and 1.12 for static and rotational beam, respectively. This calibration coefficient was derived for both the jaw settings (2:5 cm 40 cm and 5 cm 40 cm). Thus the MOSFET has minimal field size dependence. The rotational measurements for MOSFET thus confirmed the negligible angular dependence for 360 . The reproducibility of the MOSFET was also checked and an excellent consistency was obtained for repetitive measurements. MOSFET measured patient skin doses also had good reproducibility, with inter-fraction deviations ranging from 1% to 1.4%. For first patient the MOSFET measured the skin dose as 90% while TLD read 92% of the prescription dose (2.2 Gy). The variation between skin dose measured with MOSFET and TLD was 2.2%. Similarly, for second patient the skin dose measured with MOSFET and TLD was 88% and 86% of the prescription dose, respectively. The variation of 2.3% was observed between skin dose measured with MOSFET and TLD. The treatment planning system estimated the skin dose as 100% for both the patients. Our findings with these two patients planned with tomotherapy treatment planning system supplement the previous literature (Amanda et al., 2008). It was observed that the planning system overestimates the skin dose by 10–12%. Cherpak et al. used TN502RDM (micro-MOSFET) for their study. They used both high and standard sensitivity MSOFET. The MSOFET was placed on the mask as well as inside the mask. The
ARTICLE IN PRESS R.A. Kinhikar et al. / Applied Radiation and Isotopes 67 (2009) 1683–1685
TLD chips were used for subsequent comparison of the dose. The TLD chips have the limitation of finite size and hence render some uncertainties in the skin dose estimation. In addition, chips cannot be used in a monolayer. TN502RD MOSFET was used in the current study. Standard sensitivity MOSFET only was used. The MSOFET was placed only inside the mask directly on the skin. The reproducibility was maintained daily. The TLD powder was used for subsequent comparison of the dose. The TLD powder can be used in a monolayer and hence is relatively more accurate method for skin dose estimation.
4. Conclusion The results show that MOSFETs can be used as accurate and reproducible detectors for tomotherapy skin dose measurements where high dose gradients are present and in areas where the treatment planning system may not be accurate. In vivo dosimetry measurements are a useful tool with tomotherapy for areas where a high skin dose is expected, as the treatment planning system may not give accurate dose values at the surface. References Amanda, C., Studinski, R.C.N., Cygler, J.E., 2008. MOSFET detectors in quality assurance of tomotherapy treatments. Radiother. Oncol. 86 (2), 242–250. Bloemen-vanGurp, E., du Bois, W., Bruinvis, I., 2003. Clinical dosimetry with MOSFET dosimeters to determine the dose along the field junction in a split beam technique. Radiother. Oncol. 67, 351–357. Butson, M.J., Rozenfeld, A., Mathus, J.N., 1996. A new radiotherapy surface dose detector: the MOSFET. Med. Phys. 23, 655–658. Cheung, T., Butson, M.J., Yu, P.K.N., 2004. Effects of temperature variation on MOSFET dosimetry. Phys. Med. Biol. 49, N191–196. Chuang, C.F., Verhey, L.J., Xia, P., 2002. Investigation of the use of MOSFET for clinical IMRT dosimetric verification. Med. Phys. 29, 1109–1115.
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