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The Dosimetric Impact of Supplementing Pre-Planned Prostate Implants With Discretionary 125I Seeds
S66
Abstracts / Brachytherapy 10 (2011) S14eS101
PD63 The Dosimetric Impact of Supplementing Pre-Planned Prostate Implants With Discretionary 125I Seeds Neil McParland, BSc, MRT(T)1, Nick Chng, MSc2, Mira Keyes, MD3, James Morris, MD3, Tom Pickles, MD3, Michael McKenzie, MD3, John Wu, MD3, Ingrid Spadinger, PhD2. 1Radiation Therapy, BCCA, Vancouver, BC, Canada; 2Medical Physics, BCCA, Vancouver, BC, Canada; 3 Radiation Oncology, BCCA, Vancouver, BC, Canada. Purpose: The British Columbia Cancer Agency (BCCA) Provincial Prostate Brachytherapy program was established in 1998. Over 3000 implants have been done to date. Implants are performed using a pre-planned, real time ultrasound-guided transperineal technique, with stranded seeds and modified peripheral loading to deliver an mPD of 144 Gy to the prostate plus margins. For each implant, 5 extra seeds (2 stranded and 3 loose) are provided to be used at the discretion of the physician. The aim of this research was to investigate the dosimetric impact of these extra seeds, and the circumstances under which they are most commonly used. Materials and Methods: Post-implant questionnaires were completed by 5 experienced physicians to prospectively collect information on 70 consecutive implants performed over a 4 month period. After each implant, the location and rationale for using any extra seeds was recorded by each physician. All study patients underwent day-0 post implant dosimetry. A previously developed plan reconstruction algorithm was used to distinguish the extra seeds from those which were planned. The dose distributions with and without the extra seeds were compared for the whole prostate, anterior-superior (ASQ), anterior-inferior (AIQ), posteriorsuperior (PSQ), and posterior-inferior (PIQ) prostate quadrants, urethra and rectum. The Conformity Index (CI) and External Index (EI) were computed to assess collateral dose outside the target. Results: Extra seeds were used in 83% of the cohort with a median of 5 extra seeds/implant. The majority of the extra seeds were deposited in the ASQ (64%) and less frequently in other quadrants; PSQ (24%), AIQ (7%) and PIQ (5%). The most commonly reported reasons for the use of extra seeds was to improve coverage of the anterior base (42% of responses), and target regions of biopsy confirmed cancer (26%). The use of extra seeds resulted in a mean increase in whole prostate V100, V150 and V200 of 3.7% (mean V100 90.5% to 93.8%), 13% (mean V150 45% to 50.6%) and 19% (mean V200 13.5% to 16%), respectively. Mean whole prostate D90 increased from 147Gy to 156Gy. The use of extra seeds increased V100 over the 90% threshold in 15 (26%) patients. Five patients (9%) who received dose supplementation would otherwise have been classified as having suboptimal coverage (mean V100 of 83.2% improved to 91.1% after using extra seeds). Quadrant analysis demonstrated that extra seeds had the most impact on ASQ coverage, in line with the stated goals of their use, with a mean V100 increase of 13.6% (mean V100 74% to 83%) and a mean D90 increase of 9.1% (mean D90 123Gy to 134Gy). Extra seeds increased the rectal dose (VR100) by a mean of 5.9% (mean VR100 0.543 cc to 0.579cc). The mean urethral dose increased from 134.8Gy to 140.6Gy with a mean increase of 4.5% (range 1.16Gy to 5.8 Gy). The CI increased from a mean of 1.89 to 1.92 while the mean EI increased from 0.809 to 0.869. Conclusions: Bolstering coverage of the anterior prostate base is the main reason extra seeds were used in this cohort, and this aim was consistently achieved with only minor impact on the urethral, rectal and extraprostatic tissues. However, there was a considerable increase in prostate V150 and V200. The accuracy with which dose was boosted in the biopsy positive regions is under investigation.
PD64 Comparison of TG43 to Acuros Dose Calculations for High-DoseRate Gynecological Brachytherapy Bruce Libby, PhD, Ruben Ter-Antonyan, PhD, Bernard F. Schneider, PhD, MD. Radiation Oncology, University of Virginia, Charlottesville, VA. Purpose: In order to assess the effect of tissue and applicator heterogeneities on the dose distributions and ICRU38 point doses for
gynecological HDR treatments, a retrospective analysis of HDR tandem and ovoid treatments for cervical cancer using the AcurosÔ dose calculation algorithm was performed. Materials and Methods: Patients undergoing HDR T&O treatments for cervical cancer had applicator placement weekly concurrent with their external beam radiation. The patients’ CT scans were transferred to the planning system (BrachyVision v.8.9) and dwell times in the tandem and ovoids were manually optimized to deliver 6 Gy to Point A with the normal pear-shaped dose distributions, while keeping the bladder and rectal ICRU38 point doses within tolerance. All calculations for patient treatment were based on TG43 dose calculation formalism. Retrospectively, the dose for each scan was recalculated using the AcurosÔ dose calculation algorithm, which accounts for heterogeneities in the tissues and the applicator. Dose volume histograms for the bladder and rectum were created, and the doses to Point A, bladder, and rectal points were also determined to compare to those calculated using TG43 formalism. Results: Use of the AcurosÔ dose algorithm to correct for heterogeneities in 15 treatment plans shows that the average doses to the bladder and two separate rectal points are 5.9% and 9.3e9.8% lower than those calculated with TG43 formalism. Renormalization of the dose to Point A to the prescribed dose still yields bladder and rectal doses that are 1.9 and 6% lower than those calculated using TG43 formalism. The mean bladder and rectal doses were also shown to be lower using the more accurate dose calculation algorithm. Structure
AcurosÔ
ICRU38 Bladder ICRU38 Rectum 1 ICRU38 Rectum 2 Bladder Mean dose Rectum Mean dose
393 234 273 243 151
89 57 88 55 30
TG43 cGy cGy cGy cGy cGy
417 259 300 260 161
Difference 91 61 92 55 30
cGy cGy cGy cGy cGy
5.9 9.8 9.3 6.9 5.8
1.1% 1.6% 1.5% 2.9% 1.1%
Conclusions: Because brachytherapy doses had never been calculated incorporating heterogeneity corrections, it is not possible to make any clinical conclusions regarding the use of AcurosÔ for HDR treatments of gynecological malignancies. Knowing that the reported dose, using TG43 formalism, is greater than that with AcurosÔ can help guide further research as the true critical structure limits. At this point, as with RTOG protocols, both the TG43 and AcurosÔ calculations should be performed, so that clinical outcomes can be correlated with better knowledge of the doses that the rectum and bladder are receiving.
PD65 Estimating Seed Orientation in Strands for Anisotropy Sensitive Post-implant Dosimetry of the Prostate Nick Chng, MSc1, Rosey Rasoda2, Ingrid Spadinger, PhD1, W. James Morris, MD3, Tim Salcudean, PhD2. 1Medical Physics, British Columbia Cancer Agency, Vancouver, BC, Canada; 2Electrical and Computer Engineering, University of British Columbia, Vancouver, BC, Canada; 3 Radiation Oncology, British Columbia Cancer Agency, Vancouver, BC, Canada. Purpose: In post-implant dosimetry for prostate brachytherapy, seeds are commonly treated using the TG-43 1D formalism because seed orientations are difficult to determine from CT images, the current standard for the procedure. However, implants using stranded seeds tend to maintain the relative spacing and orientation of each seed along the implant trajectory. The aim of this study was to develop a method for determining seed orientations from reconstructed strand paths in postimplant CT, and to investigate the dosimetric impact of applying the TG43 2D formalism to clinical post-implant analysis. Materials and Methods: Using in-house software, the pre-plan to postimplant seed correspondence was determined for a cohort of 30 patients during routine CT-based post-implant dosimetry. All patients were implanted with stranded seed trains. Spline curves were fit to each set of
Abstracts / Brachytherapy 10 (2011) S14eS101
PD63 The Dosimetric Impact of Supplementing Pre-Planned Prostate Implants With Discretionary 125I Seeds Neil McParland, BSc, MRT(T)1, Nick Chng, MSc2, Mira Keyes, MD3, James Morris, MD3, Tom Pickles, MD3, Michael McKenzie, MD3, John Wu, MD3, Ingrid Spadinger, PhD2. 1Radiation Therapy, BCCA, Vancouver, BC, Canada; 2Medical Physics, BCCA, Vancouver, BC, Canada; 3 Radiation Oncology, BCCA, Vancouver, BC, Canada. Purpose: The British Columbia Cancer Agency (BCCA) Provincial Prostate Brachytherapy program was established in 1998. Over 3000 implants have been done to date. Implants are performed using a pre-planned, real time ultrasound-guided transperineal technique, with stranded seeds and modified peripheral loading to deliver an mPD of 144 Gy to the prostate plus margins. For each implant, 5 extra seeds (2 stranded and 3 loose) are provided to be used at the discretion of the physician. The aim of this research was to investigate the dosimetric impact of these extra seeds, and the circumstances under which they are most commonly used. Materials and Methods: Post-implant questionnaires were completed by 5 experienced physicians to prospectively collect information on 70 consecutive implants performed over a 4 month period. After each implant, the location and rationale for using any extra seeds was recorded by each physician. All study patients underwent day-0 post implant dosimetry. A previously developed plan reconstruction algorithm was used to distinguish the extra seeds from those which were planned. The dose distributions with and without the extra seeds were compared for the whole prostate, anterior-superior (ASQ), anterior-inferior (AIQ), posteriorsuperior (PSQ), and posterior-inferior (PIQ) prostate quadrants, urethra and rectum. The Conformity Index (CI) and External Index (EI) were computed to assess collateral dose outside the target. Results: Extra seeds were used in 83% of the cohort with a median of 5 extra seeds/implant. The majority of the extra seeds were deposited in the ASQ (64%) and less frequently in other quadrants; PSQ (24%), AIQ (7%) and PIQ (5%). The most commonly reported reasons for the use of extra seeds was to improve coverage of the anterior base (42% of responses), and target regions of biopsy confirmed cancer (26%). The use of extra seeds resulted in a mean increase in whole prostate V100, V150 and V200 of 3.7% (mean V100 90.5% to 93.8%), 13% (mean V150 45% to 50.6%) and 19% (mean V200 13.5% to 16%), respectively. Mean whole prostate D90 increased from 147Gy to 156Gy. The use of extra seeds increased V100 over the 90% threshold in 15 (26%) patients. Five patients (9%) who received dose supplementation would otherwise have been classified as having suboptimal coverage (mean V100 of 83.2% improved to 91.1% after using extra seeds). Quadrant analysis demonstrated that extra seeds had the most impact on ASQ coverage, in line with the stated goals of their use, with a mean V100 increase of 13.6% (mean V100 74% to 83%) and a mean D90 increase of 9.1% (mean D90 123Gy to 134Gy). Extra seeds increased the rectal dose (VR100) by a mean of 5.9% (mean VR100 0.543 cc to 0.579cc). The mean urethral dose increased from 134.8Gy to 140.6Gy with a mean increase of 4.5% (range 1.16Gy to 5.8 Gy). The CI increased from a mean of 1.89 to 1.92 while the mean EI increased from 0.809 to 0.869. Conclusions: Bolstering coverage of the anterior prostate base is the main reason extra seeds were used in this cohort, and this aim was consistently achieved with only minor impact on the urethral, rectal and extraprostatic tissues. However, there was a considerable increase in prostate V150 and V200. The accuracy with which dose was boosted in the biopsy positive regions is under investigation.
PD64 Comparison of TG43 to Acuros Dose Calculations for High-DoseRate Gynecological Brachytherapy Bruce Libby, PhD, Ruben Ter-Antonyan, PhD, Bernard F. Schneider, PhD, MD. Radiation Oncology, University of Virginia, Charlottesville, VA. Purpose: In order to assess the effect of tissue and applicator heterogeneities on the dose distributions and ICRU38 point doses for
gynecological HDR treatments, a retrospective analysis of HDR tandem and ovoid treatments for cervical cancer using the AcurosÔ dose calculation algorithm was performed. Materials and Methods: Patients undergoing HDR T&O treatments for cervical cancer had applicator placement weekly concurrent with their external beam radiation. The patients’ CT scans were transferred to the planning system (BrachyVision v.8.9) and dwell times in the tandem and ovoids were manually optimized to deliver 6 Gy to Point A with the normal pear-shaped dose distributions, while keeping the bladder and rectal ICRU38 point doses within tolerance. All calculations for patient treatment were based on TG43 dose calculation formalism. Retrospectively, the dose for each scan was recalculated using the AcurosÔ dose calculation algorithm, which accounts for heterogeneities in the tissues and the applicator. Dose volume histograms for the bladder and rectum were created, and the doses to Point A, bladder, and rectal points were also determined to compare to those calculated using TG43 formalism. Results: Use of the AcurosÔ dose algorithm to correct for heterogeneities in 15 treatment plans shows that the average doses to the bladder and two separate rectal points are 5.9% and 9.3e9.8% lower than those calculated with TG43 formalism. Renormalization of the dose to Point A to the prescribed dose still yields bladder and rectal doses that are 1.9 and 6% lower than those calculated using TG43 formalism. The mean bladder and rectal doses were also shown to be lower using the more accurate dose calculation algorithm. Structure
AcurosÔ
ICRU38 Bladder ICRU38 Rectum 1 ICRU38 Rectum 2 Bladder Mean dose Rectum Mean dose
393 234 273 243 151
89 57 88 55 30
TG43 cGy cGy cGy cGy cGy
417 259 300 260 161
Difference 91 61 92 55 30
cGy cGy cGy cGy cGy
5.9 9.8 9.3 6.9 5.8
1.1% 1.6% 1.5% 2.9% 1.1%
Conclusions: Because brachytherapy doses had never been calculated incorporating heterogeneity corrections, it is not possible to make any clinical conclusions regarding the use of AcurosÔ for HDR treatments of gynecological malignancies. Knowing that the reported dose, using TG43 formalism, is greater than that with AcurosÔ can help guide further research as the true critical structure limits. At this point, as with RTOG protocols, both the TG43 and AcurosÔ calculations should be performed, so that clinical outcomes can be correlated with better knowledge of the doses that the rectum and bladder are receiving.
PD65 Estimating Seed Orientation in Strands for Anisotropy Sensitive Post-implant Dosimetry of the Prostate Nick Chng, MSc1, Rosey Rasoda2, Ingrid Spadinger, PhD1, W. James Morris, MD3, Tim Salcudean, PhD2. 1Medical Physics, British Columbia Cancer Agency, Vancouver, BC, Canada; 2Electrical and Computer Engineering, University of British Columbia, Vancouver, BC, Canada; 3 Radiation Oncology, British Columbia Cancer Agency, Vancouver, BC, Canada. Purpose: In post-implant dosimetry for prostate brachytherapy, seeds are commonly treated using the TG-43 1D formalism because seed orientations are difficult to determine from CT images, the current standard for the procedure. However, implants using stranded seeds tend to maintain the relative spacing and orientation of each seed along the implant trajectory. The aim of this study was to develop a method for determining seed orientations from reconstructed strand paths in postimplant CT, and to investigate the dosimetric impact of applying the TG43 2D formalism to clinical post-implant analysis. Materials and Methods: Using in-house software, the pre-plan to postimplant seed correspondence was determined for a cohort of 30 patients during routine CT-based post-implant dosimetry. All patients were implanted with stranded seed trains. Spline curves were fit to each set of