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Adapting Gafchromic Film for Measuring the Tg-43 Parameters of HDR Brachytherapy Sources
S88
Abstracts / Brachytherapy 14 (2015) S11eS106
this effect appeared to be maximal for the 3.0 cm cylinder size (1.0 cc dose for 3 cm treatment length versus 5 cm treatment length 362 cGy vs 434 cGy) and minimal for the 3.5 cm cylinder size (1.0 cc dose for 3 cm treatment length vs 5 cm treatment length 415 cGy vs 444 cGy). There was a more marked effect on these doses when switching from surface to 5 mm depth prescriptions; for example, in the 2.5 cm cylinder size 5 cm treatment length condition, bladder 2 cc dose increased from 378 cGy to 532 cGy when changing from surface to 5 mm depth. With regards to the rectum doses, the trends were similar (figure 1). Conclusions: Our results indicated that changing the prescription from surface to 5 mm depth has the largest effect among treatment parameters in increasing dose to the bladder and rectum. However, other factors, including treatment length and cylinder size, also play a significant role in altering dose to OARs. Further work is planned to correlate these dosimetric results with acute and long-term toxicities to the bladder and rectum in HDR vaginal cuff brachytherapy. PHYSICS POSTERS Wednesday e Friday PO24 Adapting Gafchromic Film for Measuring the Tg-43 Parameters of HDR Brachytherapy Sources Nicholas Borges, BS Physics1, David Medich, PhD Physics1, John Munro, PhD Physics2. 1Worcester Polytechnical Institute, Worcester, MA, USA; 2 Source Production Equipment Company, St Rose, LA, USA. Thermoluminescent dosimeters have been demonstrated to have significant energy response deviations from tissue when used to characterize mid-energy brachytherapy sources such as Yb-169 and diagnostic x-ray sources (Medich, Med. Phys. 2010); this is because the electron-density of the dosimeter differs from that of tissue resulting in an energy-dependent dosimetric response. Gafchromic EBT2 and EBT3 film is a self-developing radiation dosimetry film developed to behave dosimetrically similar to tissue to within 10% for photons between 20 keV and 100 keV and within 2-3% for photons of greater energy. Additionally, it is resistant to temperature and light effects and it features ease of calibration to known fields when read by a scanner (the analog to digital converter initially gives a deviation of 0.1% - 0.5%). The film’s response of optical density change to dose also is linear up to about 600 cGy. Combined with energy independence, the film should give an accurate tissue correction factor with any given angle from the source. To that end, we present our efforts for adapting EBT2 and EBT3 Gafchromic dosimetry for the clinical dosimetric characterization of a middle-energy brachytherapy source. In-phantom measurements were taken at various distances with a calibrated air ionization chamber to define a characteristic absorbed dose response-curve for a Cs-137 calibration source. Gafchromic films were next placed in a solidwater phantom and maintained at a fixed distance from our source for a suitable duration to achieve the desired total absorbed dose as predicted by both the calibration source’s output and by our ion chamber measurements. Next, films were placed into a water phantom to obtain the Task Group 43 dosimetric parameters of a SPEC Ir-192 Model M-15 brachytherapy source. Films were digitized into an RGB 48 Bit image using an Epson 10000XL professional scanner to obtain an optical density to absorbed dose calibration curve using a MatLab program. The program takes the film and breaks the image into three colors (red, blue, green) and consistently finds the highest dose near the center within a 2 pixel matrix. By using a radial averaging technique, the orientation of the seed and the uniformity of the field can be ascertained. Thus we can evaluate the films that more accurately than current equipment that is associated with high dosimetric gradients created by a brachytherapy source that would cause large dosimetric-response perturbations. These data are presently being analyzed for presentation and will be compared against published Monte Carlo and TLD results. PO25 Commissioning of the Acuros BV GBBS Algorithm Ileana Iftimia, PhD, Per H. Halvorsen, MS. Radiation Oncology, Lahey Clinic, Burlington, MA, USA.
Purpose: To complete a series of calculations and measurements in order to validate the Acuros BV GBBS algorithm for dose computation with the intent to use it for heterogeneity corrected dose calculations in HDR planning. Materials and Methods: After license installation, a functionality test was performed followed by a simple acceptance test designed to compare the Acuros dose values against vendor data for a few points placed in a water phantom at given distances from an HDR Ir 192 source. The agreement was within the þ/- 2% tolerance. Three more thorough tests were designed for commissioning, as described in the following. The first test was a plan with a single HDR source located in a water phantom. Various calculation points (p1-p6) were inserted at 1, 2, and 4 cm from the center of the source in longitudinal and radial directions. Dose was computed using the TG 43 formalism, Acuros (resolution 0.25 cm), and Acuros (resolution 0.05 cm). The results were tabulated and compared. The effect of resolution used for dose computation was evaluated, especially for the high gradient and/or low dose regions. For the second test, a solid water phantom was CT scanned. Plans were generated and computed using the TG 43 formalism considering an HDR source located outside of the phantom at close and farther-away distances, placed: a) vertical; and b) horizontal relative to the phantom surface. Dose was then computed with Acuros in solid water using a resolution of 0.25 and 0.05 cm. Also, dose was computed with Acuros assuming the medium is ‘‘water’’ (resolution 0.05 cm). A calculation point was inserted at 5 mm depth. The results were tabulated and compared. For the last test, three phantoms were CT scanned for end-to-end testing: a solid water (SW) phantom, a SW phantom with a lung insert, and a SW phantom with a bone insert. A holder for a vaginal cylinder insert was incorporated in these phantoms, with a vaginal insert placed inside and with a GYN dummy wire advanced to the distal end. A slab to accommodate an Exradin A16 cylindrical ion microchamber was also incorporated. The chamber was placed in for the CT scanning. The CT external lasers were used to center the ion chamber over the vaginal insert. Plans were generated assuming a single HDR source placed inside of the vaginal insert (at the distal end). A calculation point was placed approximately at the center of the ion chamber active volume. The dose calculation was performed with the TG 43 formalism, Acuros (assuming the medium is ‘‘water’’), and Acuros (using the real mass density of the medium). The resolution was set to 0.05 cm for the Acuros calculations. Measurements were performed in phantom using the A16 ion microchamber connected to a Keithley electrometer delivering the same dwell time as set for the calculations. The chamber readings in the ‘‘lung’’ and ‘‘bone’’ phantoms were scaled to dose using the data from the SW phantom (measured versus calculated) and also using the ADCL chamber factor. The calculated dose was compared with the measured values for the ‘‘lung’’ and ‘‘bone’’ phantoms. Results: The first test showed that the resolution used for dose computation has a significant effect, especially in the high gradient and/or low dose regions. We recommend using a resolution of 0.05 cm for all treatment planning. Also, for the same regions, we noticed significant differences (up to 13%) between dose values computed with Acuros versus TG 43 formalism, especially in very high-gradient areas. The second test demonstrated an overall good agreement (within 4%) between dose values calculated with Acuros (when medium was assumed as being ‘‘water’’) versus the TG 43 formalism, for both horizontal and vertical source orientation relative to the phantom surface. The maximum difference (~4%) was encountered in the high gradient regions when the source was vertically placed. The end-to-end tests showed good agreement (within ~5%) between calculated versus measured dose to points beyond a lung or a bone heterogeneity insert. Conclusions: The tests described here have demonstrated acceptable performance of the Acuros BV GBBS algorithm. Preliminary clinical plans were generated for the Varian surface applicators (vertical source geometry), assuming water medium, and a resolution of 0.05 cm. The percent depth dose agreement with vendor data when the source was located at -15 mm was within 1 mm. Based on the findings described herein, the Acuros BV GBBS algorithm was released for clinical use.
Abstracts / Brachytherapy 14 (2015) S11eS106
this effect appeared to be maximal for the 3.0 cm cylinder size (1.0 cc dose for 3 cm treatment length versus 5 cm treatment length 362 cGy vs 434 cGy) and minimal for the 3.5 cm cylinder size (1.0 cc dose for 3 cm treatment length vs 5 cm treatment length 415 cGy vs 444 cGy). There was a more marked effect on these doses when switching from surface to 5 mm depth prescriptions; for example, in the 2.5 cm cylinder size 5 cm treatment length condition, bladder 2 cc dose increased from 378 cGy to 532 cGy when changing from surface to 5 mm depth. With regards to the rectum doses, the trends were similar (figure 1). Conclusions: Our results indicated that changing the prescription from surface to 5 mm depth has the largest effect among treatment parameters in increasing dose to the bladder and rectum. However, other factors, including treatment length and cylinder size, also play a significant role in altering dose to OARs. Further work is planned to correlate these dosimetric results with acute and long-term toxicities to the bladder and rectum in HDR vaginal cuff brachytherapy. PHYSICS POSTERS Wednesday e Friday PO24 Adapting Gafchromic Film for Measuring the Tg-43 Parameters of HDR Brachytherapy Sources Nicholas Borges, BS Physics1, David Medich, PhD Physics1, John Munro, PhD Physics2. 1Worcester Polytechnical Institute, Worcester, MA, USA; 2 Source Production Equipment Company, St Rose, LA, USA. Thermoluminescent dosimeters have been demonstrated to have significant energy response deviations from tissue when used to characterize mid-energy brachytherapy sources such as Yb-169 and diagnostic x-ray sources (Medich, Med. Phys. 2010); this is because the electron-density of the dosimeter differs from that of tissue resulting in an energy-dependent dosimetric response. Gafchromic EBT2 and EBT3 film is a self-developing radiation dosimetry film developed to behave dosimetrically similar to tissue to within 10% for photons between 20 keV and 100 keV and within 2-3% for photons of greater energy. Additionally, it is resistant to temperature and light effects and it features ease of calibration to known fields when read by a scanner (the analog to digital converter initially gives a deviation of 0.1% - 0.5%). The film’s response of optical density change to dose also is linear up to about 600 cGy. Combined with energy independence, the film should give an accurate tissue correction factor with any given angle from the source. To that end, we present our efforts for adapting EBT2 and EBT3 Gafchromic dosimetry for the clinical dosimetric characterization of a middle-energy brachytherapy source. In-phantom measurements were taken at various distances with a calibrated air ionization chamber to define a characteristic absorbed dose response-curve for a Cs-137 calibration source. Gafchromic films were next placed in a solidwater phantom and maintained at a fixed distance from our source for a suitable duration to achieve the desired total absorbed dose as predicted by both the calibration source’s output and by our ion chamber measurements. Next, films were placed into a water phantom to obtain the Task Group 43 dosimetric parameters of a SPEC Ir-192 Model M-15 brachytherapy source. Films were digitized into an RGB 48 Bit image using an Epson 10000XL professional scanner to obtain an optical density to absorbed dose calibration curve using a MatLab program. The program takes the film and breaks the image into three colors (red, blue, green) and consistently finds the highest dose near the center within a 2 pixel matrix. By using a radial averaging technique, the orientation of the seed and the uniformity of the field can be ascertained. Thus we can evaluate the films that more accurately than current equipment that is associated with high dosimetric gradients created by a brachytherapy source that would cause large dosimetric-response perturbations. These data are presently being analyzed for presentation and will be compared against published Monte Carlo and TLD results. PO25 Commissioning of the Acuros BV GBBS Algorithm Ileana Iftimia, PhD, Per H. Halvorsen, MS. Radiation Oncology, Lahey Clinic, Burlington, MA, USA.
Purpose: To complete a series of calculations and measurements in order to validate the Acuros BV GBBS algorithm for dose computation with the intent to use it for heterogeneity corrected dose calculations in HDR planning. Materials and Methods: After license installation, a functionality test was performed followed by a simple acceptance test designed to compare the Acuros dose values against vendor data for a few points placed in a water phantom at given distances from an HDR Ir 192 source. The agreement was within the þ/- 2% tolerance. Three more thorough tests were designed for commissioning, as described in the following. The first test was a plan with a single HDR source located in a water phantom. Various calculation points (p1-p6) were inserted at 1, 2, and 4 cm from the center of the source in longitudinal and radial directions. Dose was computed using the TG 43 formalism, Acuros (resolution 0.25 cm), and Acuros (resolution 0.05 cm). The results were tabulated and compared. The effect of resolution used for dose computation was evaluated, especially for the high gradient and/or low dose regions. For the second test, a solid water phantom was CT scanned. Plans were generated and computed using the TG 43 formalism considering an HDR source located outside of the phantom at close and farther-away distances, placed: a) vertical; and b) horizontal relative to the phantom surface. Dose was then computed with Acuros in solid water using a resolution of 0.25 and 0.05 cm. Also, dose was computed with Acuros assuming the medium is ‘‘water’’ (resolution 0.05 cm). A calculation point was inserted at 5 mm depth. The results were tabulated and compared. For the last test, three phantoms were CT scanned for end-to-end testing: a solid water (SW) phantom, a SW phantom with a lung insert, and a SW phantom with a bone insert. A holder for a vaginal cylinder insert was incorporated in these phantoms, with a vaginal insert placed inside and with a GYN dummy wire advanced to the distal end. A slab to accommodate an Exradin A16 cylindrical ion microchamber was also incorporated. The chamber was placed in for the CT scanning. The CT external lasers were used to center the ion chamber over the vaginal insert. Plans were generated assuming a single HDR source placed inside of the vaginal insert (at the distal end). A calculation point was placed approximately at the center of the ion chamber active volume. The dose calculation was performed with the TG 43 formalism, Acuros (assuming the medium is ‘‘water’’), and Acuros (using the real mass density of the medium). The resolution was set to 0.05 cm for the Acuros calculations. Measurements were performed in phantom using the A16 ion microchamber connected to a Keithley electrometer delivering the same dwell time as set for the calculations. The chamber readings in the ‘‘lung’’ and ‘‘bone’’ phantoms were scaled to dose using the data from the SW phantom (measured versus calculated) and also using the ADCL chamber factor. The calculated dose was compared with the measured values for the ‘‘lung’’ and ‘‘bone’’ phantoms. Results: The first test showed that the resolution used for dose computation has a significant effect, especially in the high gradient and/or low dose regions. We recommend using a resolution of 0.05 cm for all treatment planning. Also, for the same regions, we noticed significant differences (up to 13%) between dose values computed with Acuros versus TG 43 formalism, especially in very high-gradient areas. The second test demonstrated an overall good agreement (within 4%) between dose values calculated with Acuros (when medium was assumed as being ‘‘water’’) versus the TG 43 formalism, for both horizontal and vertical source orientation relative to the phantom surface. The maximum difference (~4%) was encountered in the high gradient regions when the source was vertically placed. The end-to-end tests showed good agreement (within ~5%) between calculated versus measured dose to points beyond a lung or a bone heterogeneity insert. Conclusions: The tests described here have demonstrated acceptable performance of the Acuros BV GBBS algorithm. Preliminary clinical plans were generated for the Varian surface applicators (vertical source geometry), assuming water medium, and a resolution of 0.05 cm. The percent depth dose agreement with vendor data when the source was located at -15 mm was within 1 mm. Based on the findings described herein, the Acuros BV GBBS algorithm was released for clinical use.