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Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors
Radiation Measurements xxx (2014) 1e5
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors F.F. Oliveira a, L.L. Amaral b, A.M. Costa a, *, T.G. Netto a a ~o Preto, Universidade de Sa ~o Paulo, Av. Bandeirantes 3900, 14040-901 Ribeira ~o Departamento de Física, Faculdade de Filosofia, Ci^ encias e Letras de Ribeira Preto, SP, Brazil b ~o Preto, Universidade de Sa ~o Paulo, Av. Bandeirantes 3900, 14048-900 Serviço de Radioterapia, Hospital das Clínicas da Faculdade de Medicina de Ribeira ~o Preto, SP, Brazil Ribeira
h i g h l i g h t s Characterization of a semiconductor dosimetric system. Characterization of a thermoluminescent dosimetric system. Application of the TLDs for treatment verification in total body irradiation treatments. Application of semiconductor detectors for in vivo dosimetry in total body irradiation treatments. Implementation of in vivo dosimetry as a part of a quality assurance program in radiotherapy.
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The objective of this work is the characterization of thermoluminescent and semiconductor detectors and their applications in treatment verification and in vivo dosimetry for total body irradiation (TBI) technique. Dose measurements of TBI treatment simulation performed with thermoluminescent detectors inserted in the holes of a “Rando anthropomorphic phantom” showed agreement with the prescribed dose. For regions of the upper and lower chest where thermoluminescent detectors received higher doses it was recommended the use of compensating dose in clinic. The results of in vivo entrance dose measurements for three patients are presented. The maximum percentual deviation between the measurements and the prescribed dose was 3.6%, which is consistent with the action level recommended by the International Commission on Radiation Units and Measurements (ICRU), i.e., ±5%. The present work to test the applicability of a thermoluminescent dosimetric system and of a semiconductor dosimetric system for performing treatment verification and in vivo dose measurements in TBI techniques demonstrated the value of these methods and the applicability as a part of a quality assurance program in TBI treatments. © 2014 Elsevier Ltd. All rights reserved.
Keywords: In vivo dosimetry Thermoluminescent dosimetry Semiconductor dosimetry Photon beam Radiotherapy
1. Introduction There are many steps in the chain of processes which determine the absorbed dose delivered to patients in radiotherapy and each of these steps may introduce uncertainties. The ultimate check of the actual dose delivered to a patient in radiotherapy can only be achieved by using in vivo dosimetry. In vivo dosimetry can be used to monitor the dose delivered in special treatment techniques such as the total body irradiation
* Corresponding author. Tel.: þ55 16 3602 4670; fax: þ55 16 3602 4887. E-mail address: [email protected] (A.M. Costa).
(TBI). Total body irradiation is considered a special procedure since it differs significantly from the standard treatment techniques used in radiotherapy. The differences are primarily due to the fact that the field of treatment for TBI exceed the size of the scattering volume (the entire body) in all directions, and that the irradiated volume is highly irregular in shape. TBI with megavoltage photon beams is used as a regimen of preparation for reconstitution of the bone marrow of patients with refractory malignancies, including leukemia, non-Hodgkin lymphoma and neuroblastoma. These regimens typically employ supraletais doses of chemotherapy and radiation and can produce greater toxicity. To avoid more severe damage, or even a possible mortality, it is important to ensure homogeneous dose distribution when the whole body is irradiated.
http://dx.doi.org/10.1016/j.radmeas.2014.08.008 1350-4487/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
2
F.F. Oliveira et al. / Radiation Measurements xxx (2014) 1e5
The purpose of TBI is to destroy cells in the bone marrow of the patient in order to immunosuppress it and prepare it to receive the new marrow. This preparation is usually accompanied by cycles of chemotherapy. Additionally, it is essential that methods of calculating dose for TBI treatments are reliable and follow a standard protocol. The principal techniques used for in vivo dosimetry are luminescent and semiconductor dosimetry (Kne zevi c et al., 2013; IAEA, 2013). Some other techniques have also been used for in vivo dosimetry, such metal oxide semiconductor field effect transistors dosimetry, alanine dosimetry, plastic scintillators dosimetry, radiochromic films dosimetry, conventional portal films or electronic portal imaging devices dosimetry, gel dosimetry and glass dosimetry (Evans et al., 2007; Rah et al., 2011). The choice between these techniques may depend on many factors such as availability, intrinsic characteristics of the detector type, measurement type, training of personnel, financial considerations and, of course, personal preference (Mijnheer et al., 2013). In this work we chose semiconductor diodes due to availability in our department and because diode measurements offer the advantage of giving immediate results. This work reports a study to test the applicability of a thermoluminescent dosimetric system for treatment verification and of a silicon diode dosimetric system for performing in vivo entrance dose measurements in TBI treatments. In vivo dosimetry was applied for patients undergoing TBI treatment at a radiotherapy department in a public hospital of Ribeir~ ao Preto, Brazil. The aim is the implementation of in vivo dosimetry as a part of a quality assurance program in TBI treatments. Quality assurance is an essential part of the radiotherapy process. It has become accepted that this is not just about ensuring that the treatment machines are correctly calibrated, but that it includes every part of the process. In vivo entrance dose measurements are a verification of the output and performance of the treatment unit and can also be used to check the accuracy of patient setup. Presently, treatment verification and in vivo dosimetry is considered a useful part of a quality assurance program in radiotherapy (Evans et al., 2007). However, in vivo dosimetry as routine verification is currently still applied in a small numbers of institutions in Brazil.
The calibration was performed in a TBI setup (393 cm sourcesurface-distance (SSD), 40 40 cm2 field size, gantry at 270 , collimator at 45 , dose rate of 100 monitor units (MU) per minute), using a solid water phantom (30 30 10 cm3) and the ionization chamber. To determine calibration factors, a total of 5 TLDs were
2. Materials and methods A total of 70 thermoluminescent dosimeters (TLD) were used. The thermoluminescent dosimeters are LiF:Mg,Ti (TLD 100) in the form of extruded square ribbons (about 3 3 0.9 mm3) manufactured by Harshaw. Thermoluminescent readouts were performed using an Harshaw model 2000B and 2000C manual TLD reader with a linear heating rate of 8 C/s. Nitrogen flux was used. Readouts were taken within 25 s and temperature between 50 C and 250 C. An oven and a furnace were used for annealing procedures of the LiF:Mg,Ti. The annealing procedure used consists of two subsequent annealings: 1 h at 400 C and 2 h at 100 C. The irradiations were carried out using a 6 MV Oncor-Siemens linear accelerator with polymethylmethacrylate serving as buildup material (1.3 cm thick). The reference standard system consists of a cylindrical ionization chamber (Farmer type) model FC065 (0.65 cm3) and an electrometer model Dose1, both from IBA. For the setup of the thermoluminescent dosimetric system all TLDs were annealed and irradiated to same dose. After readout, the procedure was repeated 2 times. A sensitivity factor was determined for each TLD. Supralinearity of response with dose of LiF:Mg,Ti after 1 Gy was investigated by determining the variation of TLD response with doses between 0.25 Gy and 3.0 Gy.
Fig. 1. Experimental setup mounted to the calibration of TLDs. a) and b) represent the placement on the right side of the “Rando anthropomorphic phantom” while c) represents the placement of left side.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
F.F. Oliveira et al. / Radiation Measurements xxx (2014) 1e5
placed inside the solid water phantom, next to the ionization chamber at a depth of 9.0 cm. During calibration of the TLDs for TBI setup a simulation of the treatment of TBI was performed with the aid of a “Rando” anthropomorphic phantom (male Alderson Radiation Therapy (ART) phantom, Radiology Support Devices). The phantom is divided into numbered sections that allow the insertion of TLDs in holes of approximately 0.5 cm in diameter, distributed at different depths. Three TLDs were inserted in each hole corresponding to the right, center and left of the following regions: head, neck, upper chest, lower chest, abdomen and pelvis. The “Rando” anthropomorphic phantom was irradiated under the conditions of TBI. A nominal dose of 0.75 Gy was administered at the first irradiation, at the right side, and after rotating the table 180 keeping the position of the isocenter, the left side was irradiated with more 0.75 Gy, as shown in Fig. 1aec. € fer, Silicon diode detectors model EDP-15, Scanditronix Wellho were characterized and well calibrated for in vivo dosimetry in TBI treatments. The diodes were also irradiated on a 6 MV OncorSiemens linear accelerator. The assembly formed by the ionization chamber and electrometer were also used as reference standard for the diode characterization and for the determination of the diode calibration factors. It was evaluated the diode response with temperature, dose rate, gantry angulation and field size. For diode response with temperature, the diodes were fixed on the inner wall of a water tub. The water inside the tank was heated (from 26 C to 38 C) and continuously monitored. The influence of dose rate on the sensitivity of the diodes was obtained by placing lead absorbers into a tray, for shielding in the radiation field, allowing variation in dose rate. The diodes were fixed on the surface of a homogeneous phantom with 100 cm source-to-surface-distance (SSD); the ionization chamber was fixed at depth of maximum dose (10 cm below the phantom surface) for a 10 10 cm2 field size. The absorbers thickness used were 1.5 cm, 3.0 cm and 4.5 cm. For diode directional dependence, the diodes were placed on a homogeneous phantom and the gantry position ranged in the following angles: 0 , 15 , 30 , 45 , 60 , 75 , 84 . For diode response with the field size, the diodes were positioned in the same setup for dose rate dependence and the measurements were performed varying the field of 3 3 cm2 to 40 40 cm2. A field of 10 10 cm2 was taken as reference and the field factor was calculated. The calibration was performed in the TBI setup, using a cubic water phantom (18 18 18 cm3) and the reference standard. To determine calibration factors, the diode was held at a fixed position (at phantom surface) and the ionization chamber depth was varied laterally inside the phantom, so the distance was varied in the chamber relative to the diode. The aim in this case was to simulate various thicknesses (latero-lateral distance), where the diode assumed different calibration factors, according to the dose recorded with the ionization chamber at each depth. Once the patient candidates for TBI treatments may be of any age, calibration factors for distances from 4.0 cm to 23.5 cm were obtained. Doses of 1.5 Gy were applied, being 0.75 Gy at the right side and other 0.75 Gy at the left side, keeping the position of isocenter. Temperature and pressure were monitored throughout the procedure. Subsequent to calibration, in vivo entrance dose measurements taken at abdominal midplane were performed involving 3 patients. The patients received a total dose of 12 Gy in a period of 4 days (3 Gy per day). A dose of 1.5 Gy was administered at the right side of the patient side and after the left side was irradiated with more 1.5 Gy. The diode was positioned always at abdominal midplane
3
entrance. The entrance dose D for a diode reading R was calculated according to the formula:
D ¼ RNW Cfield ;
(1)
where NW is the calibration factor and Cfield is the correction factor for the field size.
3. Results and discussion The TLD response ((integrated output current from TLD reader in microCoulombs) with dose was plotted versus the dose. The data are presented in Fig. 2. A formula proposed by Mayles et al. (2000) was applied to correct for the effect of supralinearity on the TLD response. Fig. 2 show a linear region up to about 1 Gy, from which the TLD response becomes supralinear, consistent with the literature (Mayles et al., 2000). The linear fit to the experimental data corrected by the formula proposed by Mayles et al. (2000) showed a correlation coefficient equal to 1, showing its applicability in clinical practice. The results of dose measurements of TBI treatment simulation performed with the TLDs inserted in the holes of a “Rando anthropomorphic phantom” are presented in Fig. 3 and, taking into account the estimated uncertainties, showed agreement with the prescribed dose of 1.5 Gy. However, regions such as the Upper and Lower Chest presented significantly higher dose values, since in this region the phantom simulates the thoracic cavity, where it lodges the lung and thus greater absorption of dose occurs by TLDs, due the difference in the phantom absorbing material (epoxy resin). It is noteworthy that in TBI treatments the region of the Chest should be shielded with a lead thick enough to absorb part of radiation, since the excess dosage can overwhelm the area and lead to the development of radio-induced pneumonitis. From the results, it can be concluded that, once the “Rando anthropomorphic phantom” has no arms, there is an increase in the dose in regions of the Upper and Lower Chest. However, in clinical practice the calculation of dose delivery planning taking into consideration the thickness of the patient's arm that is located in front of the beam of radiation and thus shields part of the incident energy. Although the patient's arms can compensate for part of the delivered dose, it would be appropriate that the clinic utilizes compensating dose in the lung region. A more detailed in vivo study
Fig. 2. Variation of TLD response with dose: +, raw TLD response; ,, corrected TLD response; d, linear fit. The slope of the linear region is 7.57 ± 0.16 mC/Gy.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
4
F.F. Oliveira et al. / Radiation Measurements xxx (2014) 1e5
Fig. 3. Dose measurements of TBI treatment simulation performed with the TLDs inserted in the holes of a “Rando anthropomorphic phantom”: Right Head (RH), Center Head (CH), Left Head (LH), Right Neck (RN), Center Neck (CN), Left Neck (LN), Right Upper Chest (RUC), Center Upper Chest (CUC), Left Upper Chest (LUC), Right Lower Chest (RLC), Center Lower Chest (CLC), Left Lower Chest (LLC), Right Abdomen (RA), Center Abdomen (CA), Left Abdomen (LA), Right Pelvis (RP), Center Pelvis (CP), Left Pelvis (LP). The horizontal straight line at value 1.5 Gy corresponds to the prescribed dose.
would be desirable to check how much radiation is shielded only with the patient's arms in front of the beam. The diode response with dose rate and the diode angular dependence was within 1.2%. The diode showed low dependence on dose rate and gantry angulation. The low dependence on the dose rate can be understood since, for diodes which were not previously irradiated, the low variation in response to the dose rate is checked. The dosimetric system had not previously been used in any dose measurement and therefore had their characterization and calibration performed as part of the objectives of this work. However, if the diodes receive various fractions of the dose over a long period of time, the sensitivity decreases and, thus, the variation in dose rate may occur in a more pronounced manner (Van Dam et al., 1990; Grusell and Rikner, 1993). The variation of the sensitivity of the diode relative to the gantry angle is caused partly by the construction of the detector (including
Fig. 5. In vivo entrance dose measurements with diode semiconductor detector for three patients undergoing TBI treatment. The horizontal straight line at value 3.0 Gy corresponds to the prescribed dose.
transmission through the thickness and the difference build-up of material on the sides of the junction pen) and partly by the patient backscattering or phantom. Still, we notice that the low angular dependence can be verified by the geometry of the diode. The diode provides cover build-up itself that guarantees a cylindrical geometry to the target volume, reducing variations in response to the angle. A maximum response variation of 2.2% was obtained for the diode response with temperature. Regarding the temperature dependence, although low, it was decided to use the diode within a hemisphere polystyrene so that it could be thermally insulated in relation to the patient. Since in vivo diode dosimetry is in direct contact with the surface of the patient, the heat insulator ensures that heat exchange between patient and dosimeter is less effective. The diode response with field size is shown in Fig. 4. The results of the diode response with field size showed an increase in diode response with increase in field size. Larger fields cause scattering of the radiation, causing an increase in diode reading. For fields above 800 cm2, the results show a trend of values remain constant, since for larger fields, the possibility of scattered radiation by the patien reaching the detector tends to not increase due to the absorption of the tissue itself. It can be concluded that there is no increase in the primary beam contribution with increasing field, but rather increase of scattered radiation which does not reach the central axis and does not contribute to the dose (Zhu, 2000). The results of in vivo entrance dose measurements for the three patients are presented in Fig. 5. The maximum percentual deviation between the measurements and the prescribed dose was 3.6%, which is consistent with the action level recommended by ICRU (1976), i.e., ±5%. The measurements presented a well calibrated dosimetric system. 4. Conclusion
Fig. 4. Diode response with field size.
This study showed that, for TBI treatments, when the patient is being prepared for a bone marrow transplant, and planning requires a great effect on the dose distribution, the methodology with semiconductor detectors presented a viable alternative, and has great importance for the dosimetric control. The study proved the applicability of diode semiconductors for quality control, for evaluation of the dose to be administered to the patient, at least
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
F.F. Oliveira et al. / Radiation Measurements xxx (2014) 1e5
throughout the first fraction of TBI treatment. Furthermore, it was demonstrated the applicability of TLD for control quality, demonstrating the value of thermoluminescent dosimetry as a treatment verification system and its effectiveness as part of a program of quality assurance in radiotherapy. Acknowledgments The authors acknowledge the partial financial support of the ~o de Amparo ~o Paulo e FAPESP e Fundaça a Pesquisa do Estado de Sa Brazil. References Evans, P., Marinello, G., Mayles, P., Nahun, A., Rosenwald, J.C. (Eds.), 2007. Handbook of Radiotherapy Physics: Theory and Practice. Taylor & Francis, pp. 867e895 chapter 40. Grusell, E., Rikner, G., 1993. Linearity with dose rate of low resistivity p-type silicon semiconductor detectors. Phys. Med. Biol. 38 (6), 785e792.
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IAEA, 2013. IAEA Human Health Report No. 8. Development of Procedures for in Vivo Dosimetry in Radiotherapy. IAEA. ICRU Report 24 ICRU, 1976. Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures. ICRU. Kne zevi c, Z., Stolarczyk, L., Bessieres, I., Bordy, J.M., Miljani c, S., Olko, P., 2013. Photon dosimetry methods outside the target volume in radiation therapy: optically stimulated luminescence (OSL), thermoluminescence (TL) and radiophotoluminescence (RPL) dosimetry. Radiat. Meas. 57, 9e18. Mayles, W.P.M., Heisig, S., Mayles, H.M.O., Williams, J.R., Thwaites, D.I. (Eds.), 2000. Radiotherapy Physics: in Practice. Oxford, pp. 220e246 chapter 11. Mijnheer, B., Beddar, S., Izewska, J., Reft, C., 2013. In vivo dosimetry in external beam radiotherapy. Med. Phys. 40 (7). Rah, J.-E., Hwang, U.-J., Jeong, H., Lee, S.-Y., Lee, D.-H., Shin, D.H., Yoon, M., Lee, S.B., Lee, R., Park, S.Y., 2011. Clinical application of glass dosimeter for in vivo dose measurements of total body irradiation treatment technique. Radiat. Meas. 46 (1), 40e45. Van Dam, J., Leunens, G., Dutreix, A., 1990. Correlation between temperature and dose rate dependence of semiconductor response; influence of accumulated dose. Radiother. Oncol. 19 (4), 345e351. Zhu, X.R., 2000. Entrance dose measurements for in-vivo diode dosimetry: comparison of correction factors for two types of commercial silicon diode detectors. J. Appl. Clin. Med. Phys. 1 (3), 100e107.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors F.F. Oliveira a, L.L. Amaral b, A.M. Costa a, *, T.G. Netto a a ~o Preto, Universidade de Sa ~o Paulo, Av. Bandeirantes 3900, 14040-901 Ribeira ~o Departamento de Física, Faculdade de Filosofia, Ci^ encias e Letras de Ribeira Preto, SP, Brazil b ~o Preto, Universidade de Sa ~o Paulo, Av. Bandeirantes 3900, 14048-900 Serviço de Radioterapia, Hospital das Clínicas da Faculdade de Medicina de Ribeira ~o Preto, SP, Brazil Ribeira
h i g h l i g h t s Characterization of a semiconductor dosimetric system. Characterization of a thermoluminescent dosimetric system. Application of the TLDs for treatment verification in total body irradiation treatments. Application of semiconductor detectors for in vivo dosimetry in total body irradiation treatments. Implementation of in vivo dosimetry as a part of a quality assurance program in radiotherapy.
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The objective of this work is the characterization of thermoluminescent and semiconductor detectors and their applications in treatment verification and in vivo dosimetry for total body irradiation (TBI) technique. Dose measurements of TBI treatment simulation performed with thermoluminescent detectors inserted in the holes of a “Rando anthropomorphic phantom” showed agreement with the prescribed dose. For regions of the upper and lower chest where thermoluminescent detectors received higher doses it was recommended the use of compensating dose in clinic. The results of in vivo entrance dose measurements for three patients are presented. The maximum percentual deviation between the measurements and the prescribed dose was 3.6%, which is consistent with the action level recommended by the International Commission on Radiation Units and Measurements (ICRU), i.e., ±5%. The present work to test the applicability of a thermoluminescent dosimetric system and of a semiconductor dosimetric system for performing treatment verification and in vivo dose measurements in TBI techniques demonstrated the value of these methods and the applicability as a part of a quality assurance program in TBI treatments. © 2014 Elsevier Ltd. All rights reserved.
Keywords: In vivo dosimetry Thermoluminescent dosimetry Semiconductor dosimetry Photon beam Radiotherapy
1. Introduction There are many steps in the chain of processes which determine the absorbed dose delivered to patients in radiotherapy and each of these steps may introduce uncertainties. The ultimate check of the actual dose delivered to a patient in radiotherapy can only be achieved by using in vivo dosimetry. In vivo dosimetry can be used to monitor the dose delivered in special treatment techniques such as the total body irradiation
* Corresponding author. Tel.: þ55 16 3602 4670; fax: þ55 16 3602 4887. E-mail address: [email protected] (A.M. Costa).
(TBI). Total body irradiation is considered a special procedure since it differs significantly from the standard treatment techniques used in radiotherapy. The differences are primarily due to the fact that the field of treatment for TBI exceed the size of the scattering volume (the entire body) in all directions, and that the irradiated volume is highly irregular in shape. TBI with megavoltage photon beams is used as a regimen of preparation for reconstitution of the bone marrow of patients with refractory malignancies, including leukemia, non-Hodgkin lymphoma and neuroblastoma. These regimens typically employ supraletais doses of chemotherapy and radiation and can produce greater toxicity. To avoid more severe damage, or even a possible mortality, it is important to ensure homogeneous dose distribution when the whole body is irradiated.
http://dx.doi.org/10.1016/j.radmeas.2014.08.008 1350-4487/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
2
F.F. Oliveira et al. / Radiation Measurements xxx (2014) 1e5
The purpose of TBI is to destroy cells in the bone marrow of the patient in order to immunosuppress it and prepare it to receive the new marrow. This preparation is usually accompanied by cycles of chemotherapy. Additionally, it is essential that methods of calculating dose for TBI treatments are reliable and follow a standard protocol. The principal techniques used for in vivo dosimetry are luminescent and semiconductor dosimetry (Kne zevi c et al., 2013; IAEA, 2013). Some other techniques have also been used for in vivo dosimetry, such metal oxide semiconductor field effect transistors dosimetry, alanine dosimetry, plastic scintillators dosimetry, radiochromic films dosimetry, conventional portal films or electronic portal imaging devices dosimetry, gel dosimetry and glass dosimetry (Evans et al., 2007; Rah et al., 2011). The choice between these techniques may depend on many factors such as availability, intrinsic characteristics of the detector type, measurement type, training of personnel, financial considerations and, of course, personal preference (Mijnheer et al., 2013). In this work we chose semiconductor diodes due to availability in our department and because diode measurements offer the advantage of giving immediate results. This work reports a study to test the applicability of a thermoluminescent dosimetric system for treatment verification and of a silicon diode dosimetric system for performing in vivo entrance dose measurements in TBI treatments. In vivo dosimetry was applied for patients undergoing TBI treatment at a radiotherapy department in a public hospital of Ribeir~ ao Preto, Brazil. The aim is the implementation of in vivo dosimetry as a part of a quality assurance program in TBI treatments. Quality assurance is an essential part of the radiotherapy process. It has become accepted that this is not just about ensuring that the treatment machines are correctly calibrated, but that it includes every part of the process. In vivo entrance dose measurements are a verification of the output and performance of the treatment unit and can also be used to check the accuracy of patient setup. Presently, treatment verification and in vivo dosimetry is considered a useful part of a quality assurance program in radiotherapy (Evans et al., 2007). However, in vivo dosimetry as routine verification is currently still applied in a small numbers of institutions in Brazil.
The calibration was performed in a TBI setup (393 cm sourcesurface-distance (SSD), 40 40 cm2 field size, gantry at 270 , collimator at 45 , dose rate of 100 monitor units (MU) per minute), using a solid water phantom (30 30 10 cm3) and the ionization chamber. To determine calibration factors, a total of 5 TLDs were
2. Materials and methods A total of 70 thermoluminescent dosimeters (TLD) were used. The thermoluminescent dosimeters are LiF:Mg,Ti (TLD 100) in the form of extruded square ribbons (about 3 3 0.9 mm3) manufactured by Harshaw. Thermoluminescent readouts were performed using an Harshaw model 2000B and 2000C manual TLD reader with a linear heating rate of 8 C/s. Nitrogen flux was used. Readouts were taken within 25 s and temperature between 50 C and 250 C. An oven and a furnace were used for annealing procedures of the LiF:Mg,Ti. The annealing procedure used consists of two subsequent annealings: 1 h at 400 C and 2 h at 100 C. The irradiations were carried out using a 6 MV Oncor-Siemens linear accelerator with polymethylmethacrylate serving as buildup material (1.3 cm thick). The reference standard system consists of a cylindrical ionization chamber (Farmer type) model FC065 (0.65 cm3) and an electrometer model Dose1, both from IBA. For the setup of the thermoluminescent dosimetric system all TLDs were annealed and irradiated to same dose. After readout, the procedure was repeated 2 times. A sensitivity factor was determined for each TLD. Supralinearity of response with dose of LiF:Mg,Ti after 1 Gy was investigated by determining the variation of TLD response with doses between 0.25 Gy and 3.0 Gy.
Fig. 1. Experimental setup mounted to the calibration of TLDs. a) and b) represent the placement on the right side of the “Rando anthropomorphic phantom” while c) represents the placement of left side.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
F.F. Oliveira et al. / Radiation Measurements xxx (2014) 1e5
placed inside the solid water phantom, next to the ionization chamber at a depth of 9.0 cm. During calibration of the TLDs for TBI setup a simulation of the treatment of TBI was performed with the aid of a “Rando” anthropomorphic phantom (male Alderson Radiation Therapy (ART) phantom, Radiology Support Devices). The phantom is divided into numbered sections that allow the insertion of TLDs in holes of approximately 0.5 cm in diameter, distributed at different depths. Three TLDs were inserted in each hole corresponding to the right, center and left of the following regions: head, neck, upper chest, lower chest, abdomen and pelvis. The “Rando” anthropomorphic phantom was irradiated under the conditions of TBI. A nominal dose of 0.75 Gy was administered at the first irradiation, at the right side, and after rotating the table 180 keeping the position of the isocenter, the left side was irradiated with more 0.75 Gy, as shown in Fig. 1aec. € fer, Silicon diode detectors model EDP-15, Scanditronix Wellho were characterized and well calibrated for in vivo dosimetry in TBI treatments. The diodes were also irradiated on a 6 MV OncorSiemens linear accelerator. The assembly formed by the ionization chamber and electrometer were also used as reference standard for the diode characterization and for the determination of the diode calibration factors. It was evaluated the diode response with temperature, dose rate, gantry angulation and field size. For diode response with temperature, the diodes were fixed on the inner wall of a water tub. The water inside the tank was heated (from 26 C to 38 C) and continuously monitored. The influence of dose rate on the sensitivity of the diodes was obtained by placing lead absorbers into a tray, for shielding in the radiation field, allowing variation in dose rate. The diodes were fixed on the surface of a homogeneous phantom with 100 cm source-to-surface-distance (SSD); the ionization chamber was fixed at depth of maximum dose (10 cm below the phantom surface) for a 10 10 cm2 field size. The absorbers thickness used were 1.5 cm, 3.0 cm and 4.5 cm. For diode directional dependence, the diodes were placed on a homogeneous phantom and the gantry position ranged in the following angles: 0 , 15 , 30 , 45 , 60 , 75 , 84 . For diode response with the field size, the diodes were positioned in the same setup for dose rate dependence and the measurements were performed varying the field of 3 3 cm2 to 40 40 cm2. A field of 10 10 cm2 was taken as reference and the field factor was calculated. The calibration was performed in the TBI setup, using a cubic water phantom (18 18 18 cm3) and the reference standard. To determine calibration factors, the diode was held at a fixed position (at phantom surface) and the ionization chamber depth was varied laterally inside the phantom, so the distance was varied in the chamber relative to the diode. The aim in this case was to simulate various thicknesses (latero-lateral distance), where the diode assumed different calibration factors, according to the dose recorded with the ionization chamber at each depth. Once the patient candidates for TBI treatments may be of any age, calibration factors for distances from 4.0 cm to 23.5 cm were obtained. Doses of 1.5 Gy were applied, being 0.75 Gy at the right side and other 0.75 Gy at the left side, keeping the position of isocenter. Temperature and pressure were monitored throughout the procedure. Subsequent to calibration, in vivo entrance dose measurements taken at abdominal midplane were performed involving 3 patients. The patients received a total dose of 12 Gy in a period of 4 days (3 Gy per day). A dose of 1.5 Gy was administered at the right side of the patient side and after the left side was irradiated with more 1.5 Gy. The diode was positioned always at abdominal midplane
3
entrance. The entrance dose D for a diode reading R was calculated according to the formula:
D ¼ RNW Cfield ;
(1)
where NW is the calibration factor and Cfield is the correction factor for the field size.
3. Results and discussion The TLD response ((integrated output current from TLD reader in microCoulombs) with dose was plotted versus the dose. The data are presented in Fig. 2. A formula proposed by Mayles et al. (2000) was applied to correct for the effect of supralinearity on the TLD response. Fig. 2 show a linear region up to about 1 Gy, from which the TLD response becomes supralinear, consistent with the literature (Mayles et al., 2000). The linear fit to the experimental data corrected by the formula proposed by Mayles et al. (2000) showed a correlation coefficient equal to 1, showing its applicability in clinical practice. The results of dose measurements of TBI treatment simulation performed with the TLDs inserted in the holes of a “Rando anthropomorphic phantom” are presented in Fig. 3 and, taking into account the estimated uncertainties, showed agreement with the prescribed dose of 1.5 Gy. However, regions such as the Upper and Lower Chest presented significantly higher dose values, since in this region the phantom simulates the thoracic cavity, where it lodges the lung and thus greater absorption of dose occurs by TLDs, due the difference in the phantom absorbing material (epoxy resin). It is noteworthy that in TBI treatments the region of the Chest should be shielded with a lead thick enough to absorb part of radiation, since the excess dosage can overwhelm the area and lead to the development of radio-induced pneumonitis. From the results, it can be concluded that, once the “Rando anthropomorphic phantom” has no arms, there is an increase in the dose in regions of the Upper and Lower Chest. However, in clinical practice the calculation of dose delivery planning taking into consideration the thickness of the patient's arm that is located in front of the beam of radiation and thus shields part of the incident energy. Although the patient's arms can compensate for part of the delivered dose, it would be appropriate that the clinic utilizes compensating dose in the lung region. A more detailed in vivo study
Fig. 2. Variation of TLD response with dose: +, raw TLD response; ,, corrected TLD response; d, linear fit. The slope of the linear region is 7.57 ± 0.16 mC/Gy.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
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Fig. 3. Dose measurements of TBI treatment simulation performed with the TLDs inserted in the holes of a “Rando anthropomorphic phantom”: Right Head (RH), Center Head (CH), Left Head (LH), Right Neck (RN), Center Neck (CN), Left Neck (LN), Right Upper Chest (RUC), Center Upper Chest (CUC), Left Upper Chest (LUC), Right Lower Chest (RLC), Center Lower Chest (CLC), Left Lower Chest (LLC), Right Abdomen (RA), Center Abdomen (CA), Left Abdomen (LA), Right Pelvis (RP), Center Pelvis (CP), Left Pelvis (LP). The horizontal straight line at value 1.5 Gy corresponds to the prescribed dose.
would be desirable to check how much radiation is shielded only with the patient's arms in front of the beam. The diode response with dose rate and the diode angular dependence was within 1.2%. The diode showed low dependence on dose rate and gantry angulation. The low dependence on the dose rate can be understood since, for diodes which were not previously irradiated, the low variation in response to the dose rate is checked. The dosimetric system had not previously been used in any dose measurement and therefore had their characterization and calibration performed as part of the objectives of this work. However, if the diodes receive various fractions of the dose over a long period of time, the sensitivity decreases and, thus, the variation in dose rate may occur in a more pronounced manner (Van Dam et al., 1990; Grusell and Rikner, 1993). The variation of the sensitivity of the diode relative to the gantry angle is caused partly by the construction of the detector (including
Fig. 5. In vivo entrance dose measurements with diode semiconductor detector for three patients undergoing TBI treatment. The horizontal straight line at value 3.0 Gy corresponds to the prescribed dose.
transmission through the thickness and the difference build-up of material on the sides of the junction pen) and partly by the patient backscattering or phantom. Still, we notice that the low angular dependence can be verified by the geometry of the diode. The diode provides cover build-up itself that guarantees a cylindrical geometry to the target volume, reducing variations in response to the angle. A maximum response variation of 2.2% was obtained for the diode response with temperature. Regarding the temperature dependence, although low, it was decided to use the diode within a hemisphere polystyrene so that it could be thermally insulated in relation to the patient. Since in vivo diode dosimetry is in direct contact with the surface of the patient, the heat insulator ensures that heat exchange between patient and dosimeter is less effective. The diode response with field size is shown in Fig. 4. The results of the diode response with field size showed an increase in diode response with increase in field size. Larger fields cause scattering of the radiation, causing an increase in diode reading. For fields above 800 cm2, the results show a trend of values remain constant, since for larger fields, the possibility of scattered radiation by the patien reaching the detector tends to not increase due to the absorption of the tissue itself. It can be concluded that there is no increase in the primary beam contribution with increasing field, but rather increase of scattered radiation which does not reach the central axis and does not contribute to the dose (Zhu, 2000). The results of in vivo entrance dose measurements for the three patients are presented in Fig. 5. The maximum percentual deviation between the measurements and the prescribed dose was 3.6%, which is consistent with the action level recommended by ICRU (1976), i.e., ±5%. The measurements presented a well calibrated dosimetric system. 4. Conclusion
Fig. 4. Diode response with field size.
This study showed that, for TBI treatments, when the patient is being prepared for a bone marrow transplant, and planning requires a great effect on the dose distribution, the methodology with semiconductor detectors presented a viable alternative, and has great importance for the dosimetric control. The study proved the applicability of diode semiconductors for quality control, for evaluation of the dose to be administered to the patient, at least
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008
F.F. Oliveira et al. / Radiation Measurements xxx (2014) 1e5
throughout the first fraction of TBI treatment. Furthermore, it was demonstrated the applicability of TLD for control quality, demonstrating the value of thermoluminescent dosimetry as a treatment verification system and its effectiveness as part of a program of quality assurance in radiotherapy. Acknowledgments The authors acknowledge the partial financial support of the ~o de Amparo ~o Paulo e FAPESP e Fundaça a Pesquisa do Estado de Sa Brazil. References Evans, P., Marinello, G., Mayles, P., Nahun, A., Rosenwald, J.C. (Eds.), 2007. Handbook of Radiotherapy Physics: Theory and Practice. Taylor & Francis, pp. 867e895 chapter 40. Grusell, E., Rikner, G., 1993. Linearity with dose rate of low resistivity p-type silicon semiconductor detectors. Phys. Med. Biol. 38 (6), 785e792.
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IAEA, 2013. IAEA Human Health Report No. 8. Development of Procedures for in Vivo Dosimetry in Radiotherapy. IAEA. ICRU Report 24 ICRU, 1976. Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures. ICRU. Kne zevi c, Z., Stolarczyk, L., Bessieres, I., Bordy, J.M., Miljani c, S., Olko, P., 2013. Photon dosimetry methods outside the target volume in radiation therapy: optically stimulated luminescence (OSL), thermoluminescence (TL) and radiophotoluminescence (RPL) dosimetry. Radiat. Meas. 57, 9e18. Mayles, W.P.M., Heisig, S., Mayles, H.M.O., Williams, J.R., Thwaites, D.I. (Eds.), 2000. Radiotherapy Physics: in Practice. Oxford, pp. 220e246 chapter 11. Mijnheer, B., Beddar, S., Izewska, J., Reft, C., 2013. In vivo dosimetry in external beam radiotherapy. Med. Phys. 40 (7). Rah, J.-E., Hwang, U.-J., Jeong, H., Lee, S.-Y., Lee, D.-H., Shin, D.H., Yoon, M., Lee, S.B., Lee, R., Park, S.Y., 2011. Clinical application of glass dosimeter for in vivo dose measurements of total body irradiation treatment technique. Radiat. Meas. 46 (1), 40e45. Van Dam, J., Leunens, G., Dutreix, A., 1990. Correlation between temperature and dose rate dependence of semiconductor response; influence of accumulated dose. Radiother. Oncol. 19 (4), 345e351. Zhu, X.R., 2000. Entrance dose measurements for in-vivo diode dosimetry: comparison of correction factors for two types of commercial silicon diode detectors. J. Appl. Clin. Med. Phys. 1 (3), 100e107.
Please cite this article in press as: Oliveira, F.F., et al., Treatment verification and in vivo dosimetry for total body irradiation using thermoluminescent and semiconductor detectors, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.08.008