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OC-0517 SMALL FIELD DOSIMETRY USING OPTICAL-FIBER RADIOLUMINESCENCE AND RADPOS DOSIMETRY SYSTEMS
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Results: The relative difference in response of all detectors with reference values is presented in the included figure for all cones used in this work. All the silicon diodes show an important over-response (≥1.7%) for the 5 mm and 7.5 mm cones due to the water nonequivalence of the silicon but are in good agreement for the 20 mm cone and higher. Of the three diodes, the PTW 60008 shows the highest discrepancy (up to 5.4%) compared to the Monte Carlo simulation results because of the metallic shielding. The microLion chamber shows good agreement for all cones except at 5 mm where the response is 2.5% below the reference data because of volume averaging effect. Both PSDs agree with the reference data within 1% with the exception of the 1.0 mm diameter detector in the 5 mm cone. The 0.5 mm diameter PSD gives the best agreement with Monte Carlo having a mean relative difference of 0.4% over all the cones. The reproducibility of the PTW diodes is within 0.15%, 0.3% for the SFD diode, 0.12% for the microLion chamber and 0.39% for the PSDs.
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proposed by Francescon et al. (J Appl Clin Med Phys, 10 (1), 147-52, 2009) were applied. Since FWHM of our Cyberknife source is 2.4mm, the μMOSFET correction factors based on interpolation of Francescon et al. values are 0.956 and 0.992 for 5 and 7.5 mm cones, respectively. Results: The μMOSFET/RADPOS measurements (corrections applied) yielded ROF of 0.650+/- 1.8% and 0.811+/- 0.8% for 5 and 7.5 mm cones, respectively, and were in excellent agreement with GafChromic film values (averaged for EBT1 and EBT2) of 0.645+/-2% and 0.807+/-2%. Assuming EBT film as a 'golden standard' (no correction required), the RL system overestimated the ROF for 5 mm cone by 5.5% and 3.9% for 5 and 7.5 mm cones, respectively. This is in agreement with Francescon et al. hypothesis that small solid state detectors require a correction factor lower than 1 applied to their readings to correct for excessive scatter due to relative high atomic number (10.2 for Al2O3) compared to water. For cone sizes 10-60 mm all detectors gave results in excellent agreement, with differences well within the measurement uncertainty for each detector. Conclusions: Our study suggests that the μMOSFET/RADPOS and fibercoupled RL dosimetry systems are well suited for Cyberknife cone ROF measurements, provided that appropriate correction factors are applied for cone sizes 5 and 7.5 mm. OC-0518 APPLICATION OF THE NEW IAEA-AAPM DOSIMETRY FORMALISM TO CLINICAL IMRT QUALITY ASSURANCE E. Chung1, R. Ruo2, J. Seuntjens1 1 McGill University, Medical Physics Unit, Montreal, Canada 2 McGill University Health Centre, Medical Physics, Montreal, Canada
Conclusions: All detectors gave accurate results without any corrections for the 20 mm cone and higher. The only dosimeter needing no correction factor for all CyberKnife cones used is the 0.5 mm diameter PSD. Therefore, to evaluate accurately the dose in water for smaller fields, the sensitive volume should be smaller than 1 mm perpendicularly to the field to neglect the volume averaging effect and the materials of the detector should be as water equivalent as possible. These two characteristics can be achieved with PSDs. OC-0517 SMALL FIELD DOSIMETRY USING OPTICAL-FIBER RADIOLUMINESCENCE AND RADPOS DOSIMETRY SYSTEMS N. Ploquin1, G. Kertzscher2, E. Vandervoort1, C.E. Andersen2, J. Cygler1 1 The Ottawa Hospital Cancer Centre, Department of Medical Physics, Ottawa, Canada 2 Center for Nuclear Technologies, Technical University of Denmark, Roskilde, Denmark Purpose/Objective: We have investigated the use of two new dosimetry systems for small field dosimetry. The first system is based on Al2O3:C radioluminescence (RL) (Radiat Meas, 46 (10), 1090-98, 2011). The main part of the RL dosimetry system is a small (2x0.5x0.5 mm3) Al2O3:C crystal (Landauer Inc, USA). The RL signal generated in the crystal by ionizing radiation can be read remotely via thin optical fiber cables. The system was originally developed for in vivo dose verification during external beam radiotherapy and brachytherapy (Radiother Oncol, 100 (3), 456-62, 2011). However, due to the small dimensions of the Al2O3 crystal, the system may have applications in small field dosimetry. The second system is the RADPOS system (Med Phys, 36, 1672-79, 2009), a novel 4D dosimetry system available from BEST Medical Canada. RADPOS probe consists of 2 sensors: a small antenna as an electromagnetic positioning sensor and a μMOSFET for dose measurement. Materials and Methods: Relative output factors (ROF) for Cyberknife cones ranging from 5 to 60 mm were measured using RL and RADPOS systems. For comparison, measurements were also carried out using a mobileMOSFET system (BEST Medical Canada) and GafChromic films EBT1 and EBT2 (ISP, USA). The MOSFET detectors in both mobileMOSFET and RADPOS systems were standard sensitivity μMOSFETs (TN-502RDM), with a standard bias applied during irradiation. The measurements were performed in a solid water phantom at the depth of 1.5 cm and SSD = 78.5 cm. Detector readings for each cone were normalized to those for 60 mm cone. For MOSFET detectors in both mobileMOSFET and RADPOS systems, the corrections
Purpose/Objective To investigate the sensitivity of the clinical correction factor to dose homogeneity in IMRT field deliveries. Materials and Methods: 20 different linear accelerator (Varian® ClinacTM 21 EX)-based clinical IMRT fields were transferred to the CT images of a 30×30×17 Solid WaterTM phantom to create IMRT QA fields. Air-filled ionization chambers were calibrated against a reference detector, the PTW micro liquid ion chamber (microLion), at the center of the phantom. The phantom position was adjusted for each IMRT QA field to place the detector or chamber at the lowest dose gradient region in a PTV. Based on the new dosimetry formalism, the clinical correction factor in each IMRT delivery was measured for two 0.6 cm3 Farmer-type chambers, Exradin A12 and NE2571, and a smaller (0.057 cm3) Exradin A1SL air-filled ionization chamber in a fully-rotated delivery and a delivery at a single gantry angle, collapsed delivery. The dependence of the measured clinical correction factors to dose homogeneity at the collecting volume of the air-filled ionization chamber was analyzed. Results: Homogeneity index (HI) of the 20 IMRT QA fields varied between 1.6% and 8.9% in the fully-rotated delivery, while it was in a range of 5.6% and 15.5% in the collapsed delivery. The overall clinical correction factor measurement uncertainties were 0.39% and 0.56% in the fully-rotated and collapsed deliveries, respectively. In the fullyrotated delivery, the clinical correction factor was not different from unity by more than 0.7% for an IMRT QA field with the HI less than 4%. For a more heterogeneous field, the correction factor differed from unity by up to 2.4%. In the collapsed delivery, the correction factor deviated from unity by up to 3.7% with increasing dose heterogeneity at the chamber collecting volume. For the Exradin A1SL, the correction factor both in the fully-rotated and collapsed deliveries was generally more close to unity compared to that for the Farmertype chambers due to the reduced dose gradient on the collecting volume of the smaller chamber.
Results: The relative difference in response of all detectors with reference values is presented in the included figure for all cones used in this work. All the silicon diodes show an important over-response (≥1.7%) for the 5 mm and 7.5 mm cones due to the water nonequivalence of the silicon but are in good agreement for the 20 mm cone and higher. Of the three diodes, the PTW 60008 shows the highest discrepancy (up to 5.4%) compared to the Monte Carlo simulation results because of the metallic shielding. The microLion chamber shows good agreement for all cones except at 5 mm where the response is 2.5% below the reference data because of volume averaging effect. Both PSDs agree with the reference data within 1% with the exception of the 1.0 mm diameter detector in the 5 mm cone. The 0.5 mm diameter PSD gives the best agreement with Monte Carlo having a mean relative difference of 0.4% over all the cones. The reproducibility of the PTW diodes is within 0.15%, 0.3% for the SFD diode, 0.12% for the microLion chamber and 0.39% for the PSDs.
ESTRO 31
proposed by Francescon et al. (J Appl Clin Med Phys, 10 (1), 147-52, 2009) were applied. Since FWHM of our Cyberknife source is 2.4mm, the μMOSFET correction factors based on interpolation of Francescon et al. values are 0.956 and 0.992 for 5 and 7.5 mm cones, respectively. Results: The μMOSFET/RADPOS measurements (corrections applied) yielded ROF of 0.650+/- 1.8% and 0.811+/- 0.8% for 5 and 7.5 mm cones, respectively, and were in excellent agreement with GafChromic film values (averaged for EBT1 and EBT2) of 0.645+/-2% and 0.807+/-2%. Assuming EBT film as a 'golden standard' (no correction required), the RL system overestimated the ROF for 5 mm cone by 5.5% and 3.9% for 5 and 7.5 mm cones, respectively. This is in agreement with Francescon et al. hypothesis that small solid state detectors require a correction factor lower than 1 applied to their readings to correct for excessive scatter due to relative high atomic number (10.2 for Al2O3) compared to water. For cone sizes 10-60 mm all detectors gave results in excellent agreement, with differences well within the measurement uncertainty for each detector. Conclusions: Our study suggests that the μMOSFET/RADPOS and fibercoupled RL dosimetry systems are well suited for Cyberknife cone ROF measurements, provided that appropriate correction factors are applied for cone sizes 5 and 7.5 mm. OC-0518 APPLICATION OF THE NEW IAEA-AAPM DOSIMETRY FORMALISM TO CLINICAL IMRT QUALITY ASSURANCE E. Chung1, R. Ruo2, J. Seuntjens1 1 McGill University, Medical Physics Unit, Montreal, Canada 2 McGill University Health Centre, Medical Physics, Montreal, Canada
Conclusions: All detectors gave accurate results without any corrections for the 20 mm cone and higher. The only dosimeter needing no correction factor for all CyberKnife cones used is the 0.5 mm diameter PSD. Therefore, to evaluate accurately the dose in water for smaller fields, the sensitive volume should be smaller than 1 mm perpendicularly to the field to neglect the volume averaging effect and the materials of the detector should be as water equivalent as possible. These two characteristics can be achieved with PSDs. OC-0517 SMALL FIELD DOSIMETRY USING OPTICAL-FIBER RADIOLUMINESCENCE AND RADPOS DOSIMETRY SYSTEMS N. Ploquin1, G. Kertzscher2, E. Vandervoort1, C.E. Andersen2, J. Cygler1 1 The Ottawa Hospital Cancer Centre, Department of Medical Physics, Ottawa, Canada 2 Center for Nuclear Technologies, Technical University of Denmark, Roskilde, Denmark Purpose/Objective: We have investigated the use of two new dosimetry systems for small field dosimetry. The first system is based on Al2O3:C radioluminescence (RL) (Radiat Meas, 46 (10), 1090-98, 2011). The main part of the RL dosimetry system is a small (2x0.5x0.5 mm3) Al2O3:C crystal (Landauer Inc, USA). The RL signal generated in the crystal by ionizing radiation can be read remotely via thin optical fiber cables. The system was originally developed for in vivo dose verification during external beam radiotherapy and brachytherapy (Radiother Oncol, 100 (3), 456-62, 2011). However, due to the small dimensions of the Al2O3 crystal, the system may have applications in small field dosimetry. The second system is the RADPOS system (Med Phys, 36, 1672-79, 2009), a novel 4D dosimetry system available from BEST Medical Canada. RADPOS probe consists of 2 sensors: a small antenna as an electromagnetic positioning sensor and a μMOSFET for dose measurement. Materials and Methods: Relative output factors (ROF) for Cyberknife cones ranging from 5 to 60 mm were measured using RL and RADPOS systems. For comparison, measurements were also carried out using a mobileMOSFET system (BEST Medical Canada) and GafChromic films EBT1 and EBT2 (ISP, USA). The MOSFET detectors in both mobileMOSFET and RADPOS systems were standard sensitivity μMOSFETs (TN-502RDM), with a standard bias applied during irradiation. The measurements were performed in a solid water phantom at the depth of 1.5 cm and SSD = 78.5 cm. Detector readings for each cone were normalized to those for 60 mm cone. For MOSFET detectors in both mobileMOSFET and RADPOS systems, the corrections
Purpose/Objective To investigate the sensitivity of the clinical correction factor to dose homogeneity in IMRT field deliveries. Materials and Methods: 20 different linear accelerator (Varian® ClinacTM 21 EX)-based clinical IMRT fields were transferred to the CT images of a 30×30×17 Solid WaterTM phantom to create IMRT QA fields. Air-filled ionization chambers were calibrated against a reference detector, the PTW micro liquid ion chamber (microLion), at the center of the phantom. The phantom position was adjusted for each IMRT QA field to place the detector or chamber at the lowest dose gradient region in a PTV. Based on the new dosimetry formalism, the clinical correction factor in each IMRT delivery was measured for two 0.6 cm3 Farmer-type chambers, Exradin A12 and NE2571, and a smaller (0.057 cm3) Exradin A1SL air-filled ionization chamber in a fully-rotated delivery and a delivery at a single gantry angle, collapsed delivery. The dependence of the measured clinical correction factors to dose homogeneity at the collecting volume of the air-filled ionization chamber was analyzed. Results: Homogeneity index (HI) of the 20 IMRT QA fields varied between 1.6% and 8.9% in the fully-rotated delivery, while it was in a range of 5.6% and 15.5% in the collapsed delivery. The overall clinical correction factor measurement uncertainties were 0.39% and 0.56% in the fully-rotated and collapsed deliveries, respectively. In the fullyrotated delivery, the clinical correction factor was not different from unity by more than 0.7% for an IMRT QA field with the HI less than 4%. For a more heterogeneous field, the correction factor differed from unity by up to 2.4%. In the collapsed delivery, the correction factor deviated from unity by up to 3.7% with increasing dose heterogeneity at the chamber collecting volume. For the Exradin A1SL, the correction factor both in the fully-rotated and collapsed deliveries was generally more close to unity compared to that for the Farmertype chambers due to the reduced dose gradient on the collecting volume of the smaller chamber.