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Predicting IQ and the Risk of Hearing Loss Following Proton Versus Photon Radiation Therapy for Pediatric Brain Tumor Patients
E684
International Journal of Radiation Oncology Biology Physics
For the left and right optic nerves, the average mean doses for SI vs MI plans in Gy were, respectively, 2.31.0 vs 0.70.8 (pZ0.009) and 2.32.1 vs 0.50.4 (pZ0.062). The average maximum left and right optic nerve doses were 4.82.0 vs 1.21.2 (pZ0.001) and 3.72.9 vs 1.01.3 (pZ0.031), respectively. The estimated mean total treatment times were 2.3 and 3.1 times longer for 3 isocenter and 4 isocenter plans than for single isocenter plans, respectively. Conclusion: While multiple isocenter plans take longer to deliver than single isocenter plans, they are capable of higher dose conformity, lower doses to critical structures, and lower doses to normal brain tissue. Author Disclosure: E. Kendall: None. O. Algan: None. I. Ali: None. S. Ahmad: None. J. Arntzen: None. S. Hossain: None.
3676
3675 Plan Robustness Analysis for Target With Large Interfraction Motion in Proton Pencil Beaming Scanning (PBS): Patient-Specific Plan Evaluation of Prostate Treatment S. Tang,1,2 L. Song,1 C.C. Chen,1 C. Chang,2 and D. Mah1; 1ProCure Proton Therapy Center, Somerset, NJ, 2Texas Center for Proton Therapy, Irving, TX Purpose/Objective(s): Prostates generally have random interfraction motion up to 10 mm, which can be mitigated by imaging implanted fiducial markers. A bilateral beam arrangement for PBS patients may be not robust because daily targeting results in variable path lengths through bones along the beam path. We investigate 2 planning strategies to generate robust proton plans using PBS and provide a quantitative method of evaluating plan robustness for individual patients. Materials/Methods: A group of 10 patients with low-risk prostate cancer were planned in a commercially available treatment planning system (TPS). The plans were performed using a deterministic optimization method following the Proton Collaborative Group protocol with bilateral opposing beams. The clinical target volume (CTV) was defined as prostate. A planning target volume (PTV) was defined by expanding the CTV 2 mm posteriorly and 3 mm elsewhere in 3D; a PTV EVAL was created based on the PTV plus 5-mm expansion in lateral (beam) directions to accommodate range uncertainties. The primary planning objectives were: (1) at least 99.9% of CTV receives full prescription dose; (2) 98% of the prescription dose covers at least 98% of the PTV EVAL; and (3) organs at risk meet the criteria specified in the protocol. Using the robustness optimization planning capability in the TPS, which incorporates 3.5% range uncertainties and setup uncertainties comparable to PTV expansion during the optimization process, all patients were retrospectively replanned directly to CTV without using the PTV to the same clinical goals. Robustness Evaluation: The dosimetric analysis was performed based on shifting the isocenter and CTV contour together to mimic the possible prostate interfraction motion. With combinations of isocenter shifts in superior-inferior and anterior-posterior directions (up to 15-mm in 3-mm intervals) and 3.5% range uncertainty, a total of 363 evaluation plans were created for each patient. Results: Both deterministic and robustness optimization methods provide reliable PBS plans with respect possible prostate interfraction motion, but the deterministic optimization can produce a good clinical plan with <30% computation time of robustness optimization. Robustness optimization cannot self-consistently consider the dosimetric variation due to interfraction motion. In this cohort, the CTV D98%>98% or CTV V98%>98% are achievable for all patients if the offset is <5 mm in any direction despite range uncertainties. However, for large interfraction motion the plan robustness is significantly affected by individual patient anatomy and must be patient specific. Conclusion: We developed a method to evaluate the plan robustness for proton PBS prostate treatment. It can provide patient specific setup tolerance for bony structure offset for a given prostate interfraction displacement. This approach can be generalized and applied to other targets with large interfraction motion. Author Disclosure: S. Tang: None. L. Song: None. C. Chen: None. C. Chang: None. D. Mah: None.
Considerations in Bilateral Lung Treatment Planning With Volumetric Modulated Arc Therapy Y. Song,1 B.A. Mueller,1 J. Laguna,1 C.H. Obcemea,1 Z. Saleh,2 X. Tang,3 S. Both,4 and B.R. Mychalczak1; 1Memorial Sloan Kettering Cancer Center, New York, NY, 2Memorial Sloan Kettering Cancer Center, New York, NY, United States, 3Memorial Sloan Kettering, West Harrison, NY, 4 Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY Purpose/Objective(s): The treatment of large bilateral lung tumors with extensive mediastinal and bilateral hilar involvement is challenging due to the increased risk of radiation-induced pneumonitis. 3D-CRT can achieve some degree of success by reducing irradiated lung volume. However, its limited beam angular samples and lack of intensity modulation can result in poor target dose conformity and elevated organs-at-risk (OARs) doses. Intensity modulated radiation therapy provides necessary intensity modulation, but its beamlet-based approach tends to produce a large number of small-MU and small-aperture segments, resulting in plans with high MU, long treatment time, and poor target dose homogeneity. Volumetric modulated arc therapy (VMAT) uses an aperture-based optimization technique, which minimizes both MU and radiation leakage. Though VMAT yields lower intensity levels, this intrinsic deficiency is largely compensated for by increasing beam angular samples and using dose rate and gantry angular speed modulations. In this report, we present our clinical approach to bilateral lung planning with VMAT. Materials/Methods: Patients are simulated in supine with Alpha Cradle mold. A 4DCT scan with bellows is performed from the thoracic inlet to the costophrenic angles. The MIP is reconstructed and the ITV is delineated. The PTV is created by adding an appropriate margin to the ITV. VMAT plans are computed with a commercially available treatment planning system. The isocenter is placed at the geometric center of the PTV to minimize the field size and MLC dose leakage. The plan contains 4 to 6 partial and full arcs. For cases with extensive bilateral hilar and SCV adenopathy, 2 to 3 anterior partial arcs in the range of 65 to 295 are used to cover the superior portion of the PTV. The inferior portion of the PTV is covered by 2 to 3 full arcs. The gantry angles in control point sequences are interleaved among arcs to avoid reproducing identical intensity patterns. The collimator angles are set to 0 as all these cases have an elongated PTV in the Y-direction. To avoid interleaf end dose leakage through the parked MLC leaves, X-jaws are set asymmetrically. In addition to NTO, a series of concentric dose tuning structures with a fine step size and high priority scores are also created. An artificial skin structure of 1 cm is used to drive the dose inward. Results: With VMAT, the MSKCC lung dose tolerance (V20Gy 37%) is easily met without compromising target coverage, as compared to IMRT. Dose tolerances for other OARs can also be met if they are not inside the PTV. Our VMAT plans generally exhibit a higher conformity index and a steeper dose gradient with significant reduction in MU and treatment time. Conclusion: Several approaches to the treatment of large bilateral lung tumors have been proposed, including a combination of IMRT and VMAT, with IMRT as a base plan. Our experience shows that with a well-designed planning strategy, a single VMAT plan can meet all clinical criteria. Author Disclosure: Y. Song: None. B.A. Mueller: None. J. Laguna: None. C.H. Obcemea: None. Z. Saleh: None. X. Tang: None. S. Both: None. B.R. Mychalczak: None.
3677 Predicting IQ and the Risk of Hearing Loss Following Proton Versus Photon Radiation Therapy for Pediatric Brain Tumor Patients D. Fortin,1,2 A. Ng,1 D.S.C. Tsang,1,2 M.B. Sharpe,1,2 N.J. Laperriere,1,2 and D. Hodgson1,2; 1Princess Margaret Cancer Centre, Toronto, ON, Canada, 2Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada Purpose/Objective(s): Proton therapy allows for the delivery of ionizing radiation over a well-defined range with minimal exit dose compared to
Volume 96 Number 2S Supplement 2016 photons. Intensity modulated proton therapy (IMPT) may further improve dose conformality with respect to 3-dimensional proton therapy. The increased sparing of normal tissues should translate into improved neurocognitive outcomes. Models were used to estimate the intelligence quotient (IQ) and the risk of hearing loss 5 years after radiation therapy and to compare outcomes of proton against photon in pediatric brain tumors. Materials/Methods: Pediatric patients who had received radical photon intensity modulated radiation therapy (IMRT) were randomly selected from our retrospective database: 10 cases each of craniopharyngioma, ependymoma, and medulloblastoma and 20 cases of glioma. The existing planning CT and contoured structures were used to generate IMPT plans employing a robust optimization procedure. The RBE-corrected dose to brain structures and the cochleas were calculated for both IMPT and IMRT. A dose-dependent IQ model was applied to estimate IQ using a Markov chain Monte Carlo technique. Cumulative probability distributions (CDF) were calculated to perform a statistical interpretation and to compare proton versus photon outcomes. The reported incidence of hearing loss as a function of cochlear dose in the literature was used to estimate the probability of occurrence. Results: The average dose to the brain was less in all IMPT plans compared to IMRT, ranging from a 6.7% reduction (PZ0.003) in the case of medulloblastoma to 38% (PZ0.007) for craniopharyngioma. This dose reduction translated into a gain in IQ of 1.9 points on average for protons vs photons for the whole cohort at 5 years posttreatment (PZ0.011). In terms of specific diseases, the gains in IQ points were 0.8, 1.6, 2.3, and 2.7 for medulloblastoma, ependymoma, glioma, and craniopharyngioma, respectively. When estimating the IQ using dose to the temporal lobes, these gains increased to 3.1 to 6.0 IQ points. Overall, the probability for IQ deficits 7.5 points was estimated to be 32% for IMPT compared to 48% for IMRT. Hearing loss probability was evaluated on a per-ear-basis and was found to be systematically less for proton versus photon: 2.9% versus 7.2% (P<10-7). Conclusion: Tools to calculate IQ and hearing loss probabilities following radiation therapy have been developed to help guide the choice of treatment modality for pediatric brain tumor patients on a case-by-case basis. IQ predictions post-IMPT and IMRT were found to be very similar, but a modest gain was systematically observed for proton therapy in all patients. Given the uncertainties within the IQ model used and our reinterpretation, the predicted gains may be underestimated. Additional long-term clinical studies are needed to improve our understanding of radiation on the developing brain. Author Disclosure: D. Fortin: None. A. Ng: None. D.S. Tsang: None. M.B. Sharpe: None. N.J. Laperriere: None. D. Hodgson: None.
3678 A Novel Method to Build Intraoperative Flexibility Into Prefabricated or Fixed-Length Low-Dose-Rate Prostate Brachytherapy A.R. Powers,1 R.D. Sheu,1 H.M. McGee,1 and R.G. Stock2; 1Icahn School of Medicine at Mount Sinai, New York, NY, 2Ichan School of Medicine at Mount Sinai, New York, NY Purpose/Objective(s): A common difficulty with preplanning LDR seed implants for prostate cancer is ensuring reproducibility at the time of the implant. Seed implants from one commercial source that offers a homogenous linear source of radioactive material along with other prefabricated brachytherapy products present a unique problem due to their fixed length. Despite accurate preplanning, there are still changes in prostate size, shape, and position that occur on the day of the brachytherapy procedure. Slight variations in the angle and axis of the prostate need to be accounted for intraoperatively. We sought to develop a combined preplan and intraoperative approach for LDR brachytherapy with fixed-length strands. Materials/Methods: We aim to identify a small number and length of extra brachytherapy seeds that can be ordered in addition to the preplanned strands in order to allow for optimal real-time flexibility. We developed an
Poster Viewing E685 in-house software program that requires the prostate contour and preplan needle position to analyze the effect that small variations in needle placement and prostate rotation have on the appropriate length of string. The first deviation analyzed in our program is needle position (2 and 4 mm in Ant-Post and Right-Left direction from the preplan needle position). Because the strands are ordered in centimeter intervals, we considered a deviation of >0.66 cm to be significant. The second deviation analyzed in the program was prostate rotation of 5 and 10 degrees in the sagittal plane above or below the preplan axis. Results: When both rotational and positional deviations were applied to the interior strings, there were no significant changes in string length. When the AP or RL deviations were applied to the peripheral strings, we saw a significant change in string length. Using our software program, we determined that <30% extra activity will allow for sufficient flexibility at the time of implant. Conclusion: This software program helped us overcome the limitations of using prefabricated fixed-length sources by predicting for a number of extra radioactive strands that allow for real-time flexibility during the implant procedure. Author Disclosure: A.R. Powers: None. R. Sheu: None. H.M. McGee: None. R.G. Stock: None.
3679 Breathing-Induced Prostate Motion: Real-Time Data Show SecondOrder Effects C. Panaino,1 T. Giandini,1 M. Carrara,1 S. Frasca,1 B. Avuzzi,2 S. Morlino,1 D. Bosetti,2 N. Bedini,1 S. Villa,1 T. Rancati,2 R. Valdagni,1 and E. Pignoli2; 1Fondazione IRCCS Instituto Nazionale dei Tumori, Milano, Italy, 2Fondazione IRCCS Instituto Nazionale dei Tumori, Milan, Italy Purpose/Objective(s): An electromagnetic (ELM) system able to localize 3 transponders permanently implanted in the prostate with submillimeter accuracy is currently used in our department for continuous real-time tracking of the tumor. In the last year, we focused our attention on the development of a tool to selectively filter the signal produced by the transponders and to deconvolve it in its main components. In particular, the aim of this study was to validate and apply the developed tool to quantify the amplitude of prostate motion induced only by the patient’s breathing. Materials/Methods: Software was developed to selectively filter the signal of the ELM transponders. Based on a patient-specific analysis of the power density spectrum of the recorded tracks, the developed software removes noise and automatically isolates their main components, i.e., the low-frequency motion (long-term motion) and the high-frequency harmonic motion (breathing-induced motion). The software was validated on data recorded with a moving phantom, provided with a home-made insert with 3 transponders. Simulated breathing frequencies (BFs) of 10, 12, 14, 16, 18, 20, 22, and 24 cycles/min were recorded for at least 1 minute with the ELM system and analyzed. After validation, the tracks of 15 prostate patients who underwent external beam radiation therapy were analyzed for a total of 506 treatment sessions (TSs). For each TS, the amplitude of prostate motion along the 3 main directions was obtained. Average amplitudes, standard deviations, and overall amplitude ranges were reported. Results: For the in-phantom validation, the developed software automatically computed the correct cycles per minute within a 0.6% uncertainty. The average BF was 18.41.2 cycles/min (range: 13.6O26.2 cycles/min). The resulting prostate excursions induced only by patient’s breathing, averaged over all TSs, were 0.20.1 mm, 0.40.1 mm, and 0.50.1 mm for L-R, A-P, and C-C directions, respectively (ranges: 0.1O0.5 mm, 0.2O0.6 mm, and 0.2O0.7 mm). 3D prostate excursions ranged between 0.4 mm and 1 mm. A very limited intrapatient variation over all TSs was observed for both BF and prostate maximum excursions, showing that patients tend to behave in the same way over all TSs. No significant correlation was found between the 2 parameters. Conclusion: The obtained results show that prostate harmonic motion induced by patient’s breathing represents a reduced source of intrasession
International Journal of Radiation Oncology Biology Physics
For the left and right optic nerves, the average mean doses for SI vs MI plans in Gy were, respectively, 2.31.0 vs 0.70.8 (pZ0.009) and 2.32.1 vs 0.50.4 (pZ0.062). The average maximum left and right optic nerve doses were 4.82.0 vs 1.21.2 (pZ0.001) and 3.72.9 vs 1.01.3 (pZ0.031), respectively. The estimated mean total treatment times were 2.3 and 3.1 times longer for 3 isocenter and 4 isocenter plans than for single isocenter plans, respectively. Conclusion: While multiple isocenter plans take longer to deliver than single isocenter plans, they are capable of higher dose conformity, lower doses to critical structures, and lower doses to normal brain tissue. Author Disclosure: E. Kendall: None. O. Algan: None. I. Ali: None. S. Ahmad: None. J. Arntzen: None. S. Hossain: None.
3676
3675 Plan Robustness Analysis for Target With Large Interfraction Motion in Proton Pencil Beaming Scanning (PBS): Patient-Specific Plan Evaluation of Prostate Treatment S. Tang,1,2 L. Song,1 C.C. Chen,1 C. Chang,2 and D. Mah1; 1ProCure Proton Therapy Center, Somerset, NJ, 2Texas Center for Proton Therapy, Irving, TX Purpose/Objective(s): Prostates generally have random interfraction motion up to 10 mm, which can be mitigated by imaging implanted fiducial markers. A bilateral beam arrangement for PBS patients may be not robust because daily targeting results in variable path lengths through bones along the beam path. We investigate 2 planning strategies to generate robust proton plans using PBS and provide a quantitative method of evaluating plan robustness for individual patients. Materials/Methods: A group of 10 patients with low-risk prostate cancer were planned in a commercially available treatment planning system (TPS). The plans were performed using a deterministic optimization method following the Proton Collaborative Group protocol with bilateral opposing beams. The clinical target volume (CTV) was defined as prostate. A planning target volume (PTV) was defined by expanding the CTV 2 mm posteriorly and 3 mm elsewhere in 3D; a PTV EVAL was created based on the PTV plus 5-mm expansion in lateral (beam) directions to accommodate range uncertainties. The primary planning objectives were: (1) at least 99.9% of CTV receives full prescription dose; (2) 98% of the prescription dose covers at least 98% of the PTV EVAL; and (3) organs at risk meet the criteria specified in the protocol. Using the robustness optimization planning capability in the TPS, which incorporates 3.5% range uncertainties and setup uncertainties comparable to PTV expansion during the optimization process, all patients were retrospectively replanned directly to CTV without using the PTV to the same clinical goals. Robustness Evaluation: The dosimetric analysis was performed based on shifting the isocenter and CTV contour together to mimic the possible prostate interfraction motion. With combinations of isocenter shifts in superior-inferior and anterior-posterior directions (up to 15-mm in 3-mm intervals) and 3.5% range uncertainty, a total of 363 evaluation plans were created for each patient. Results: Both deterministic and robustness optimization methods provide reliable PBS plans with respect possible prostate interfraction motion, but the deterministic optimization can produce a good clinical plan with <30% computation time of robustness optimization. Robustness optimization cannot self-consistently consider the dosimetric variation due to interfraction motion. In this cohort, the CTV D98%>98% or CTV V98%>98% are achievable for all patients if the offset is <5 mm in any direction despite range uncertainties. However, for large interfraction motion the plan robustness is significantly affected by individual patient anatomy and must be patient specific. Conclusion: We developed a method to evaluate the plan robustness for proton PBS prostate treatment. It can provide patient specific setup tolerance for bony structure offset for a given prostate interfraction displacement. This approach can be generalized and applied to other targets with large interfraction motion. Author Disclosure: S. Tang: None. L. Song: None. C. Chen: None. C. Chang: None. D. Mah: None.
Considerations in Bilateral Lung Treatment Planning With Volumetric Modulated Arc Therapy Y. Song,1 B.A. Mueller,1 J. Laguna,1 C.H. Obcemea,1 Z. Saleh,2 X. Tang,3 S. Both,4 and B.R. Mychalczak1; 1Memorial Sloan Kettering Cancer Center, New York, NY, 2Memorial Sloan Kettering Cancer Center, New York, NY, United States, 3Memorial Sloan Kettering, West Harrison, NY, 4 Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY Purpose/Objective(s): The treatment of large bilateral lung tumors with extensive mediastinal and bilateral hilar involvement is challenging due to the increased risk of radiation-induced pneumonitis. 3D-CRT can achieve some degree of success by reducing irradiated lung volume. However, its limited beam angular samples and lack of intensity modulation can result in poor target dose conformity and elevated organs-at-risk (OARs) doses. Intensity modulated radiation therapy provides necessary intensity modulation, but its beamlet-based approach tends to produce a large number of small-MU and small-aperture segments, resulting in plans with high MU, long treatment time, and poor target dose homogeneity. Volumetric modulated arc therapy (VMAT) uses an aperture-based optimization technique, which minimizes both MU and radiation leakage. Though VMAT yields lower intensity levels, this intrinsic deficiency is largely compensated for by increasing beam angular samples and using dose rate and gantry angular speed modulations. In this report, we present our clinical approach to bilateral lung planning with VMAT. Materials/Methods: Patients are simulated in supine with Alpha Cradle mold. A 4DCT scan with bellows is performed from the thoracic inlet to the costophrenic angles. The MIP is reconstructed and the ITV is delineated. The PTV is created by adding an appropriate margin to the ITV. VMAT plans are computed with a commercially available treatment planning system. The isocenter is placed at the geometric center of the PTV to minimize the field size and MLC dose leakage. The plan contains 4 to 6 partial and full arcs. For cases with extensive bilateral hilar and SCV adenopathy, 2 to 3 anterior partial arcs in the range of 65 to 295 are used to cover the superior portion of the PTV. The inferior portion of the PTV is covered by 2 to 3 full arcs. The gantry angles in control point sequences are interleaved among arcs to avoid reproducing identical intensity patterns. The collimator angles are set to 0 as all these cases have an elongated PTV in the Y-direction. To avoid interleaf end dose leakage through the parked MLC leaves, X-jaws are set asymmetrically. In addition to NTO, a series of concentric dose tuning structures with a fine step size and high priority scores are also created. An artificial skin structure of 1 cm is used to drive the dose inward. Results: With VMAT, the MSKCC lung dose tolerance (V20Gy 37%) is easily met without compromising target coverage, as compared to IMRT. Dose tolerances for other OARs can also be met if they are not inside the PTV. Our VMAT plans generally exhibit a higher conformity index and a steeper dose gradient with significant reduction in MU and treatment time. Conclusion: Several approaches to the treatment of large bilateral lung tumors have been proposed, including a combination of IMRT and VMAT, with IMRT as a base plan. Our experience shows that with a well-designed planning strategy, a single VMAT plan can meet all clinical criteria. Author Disclosure: Y. Song: None. B.A. Mueller: None. J. Laguna: None. C.H. Obcemea: None. Z. Saleh: None. X. Tang: None. S. Both: None. B.R. Mychalczak: None.
3677 Predicting IQ and the Risk of Hearing Loss Following Proton Versus Photon Radiation Therapy for Pediatric Brain Tumor Patients D. Fortin,1,2 A. Ng,1 D.S.C. Tsang,1,2 M.B. Sharpe,1,2 N.J. Laperriere,1,2 and D. Hodgson1,2; 1Princess Margaret Cancer Centre, Toronto, ON, Canada, 2Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada Purpose/Objective(s): Proton therapy allows for the delivery of ionizing radiation over a well-defined range with minimal exit dose compared to
Volume 96 Number 2S Supplement 2016 photons. Intensity modulated proton therapy (IMPT) may further improve dose conformality with respect to 3-dimensional proton therapy. The increased sparing of normal tissues should translate into improved neurocognitive outcomes. Models were used to estimate the intelligence quotient (IQ) and the risk of hearing loss 5 years after radiation therapy and to compare outcomes of proton against photon in pediatric brain tumors. Materials/Methods: Pediatric patients who had received radical photon intensity modulated radiation therapy (IMRT) were randomly selected from our retrospective database: 10 cases each of craniopharyngioma, ependymoma, and medulloblastoma and 20 cases of glioma. The existing planning CT and contoured structures were used to generate IMPT plans employing a robust optimization procedure. The RBE-corrected dose to brain structures and the cochleas were calculated for both IMPT and IMRT. A dose-dependent IQ model was applied to estimate IQ using a Markov chain Monte Carlo technique. Cumulative probability distributions (CDF) were calculated to perform a statistical interpretation and to compare proton versus photon outcomes. The reported incidence of hearing loss as a function of cochlear dose in the literature was used to estimate the probability of occurrence. Results: The average dose to the brain was less in all IMPT plans compared to IMRT, ranging from a 6.7% reduction (PZ0.003) in the case of medulloblastoma to 38% (PZ0.007) for craniopharyngioma. This dose reduction translated into a gain in IQ of 1.9 points on average for protons vs photons for the whole cohort at 5 years posttreatment (PZ0.011). In terms of specific diseases, the gains in IQ points were 0.8, 1.6, 2.3, and 2.7 for medulloblastoma, ependymoma, glioma, and craniopharyngioma, respectively. When estimating the IQ using dose to the temporal lobes, these gains increased to 3.1 to 6.0 IQ points. Overall, the probability for IQ deficits 7.5 points was estimated to be 32% for IMPT compared to 48% for IMRT. Hearing loss probability was evaluated on a per-ear-basis and was found to be systematically less for proton versus photon: 2.9% versus 7.2% (P<10-7). Conclusion: Tools to calculate IQ and hearing loss probabilities following radiation therapy have been developed to help guide the choice of treatment modality for pediatric brain tumor patients on a case-by-case basis. IQ predictions post-IMPT and IMRT were found to be very similar, but a modest gain was systematically observed for proton therapy in all patients. Given the uncertainties within the IQ model used and our reinterpretation, the predicted gains may be underestimated. Additional long-term clinical studies are needed to improve our understanding of radiation on the developing brain. Author Disclosure: D. Fortin: None. A. Ng: None. D.S. Tsang: None. M.B. Sharpe: None. N.J. Laperriere: None. D. Hodgson: None.
3678 A Novel Method to Build Intraoperative Flexibility Into Prefabricated or Fixed-Length Low-Dose-Rate Prostate Brachytherapy A.R. Powers,1 R.D. Sheu,1 H.M. McGee,1 and R.G. Stock2; 1Icahn School of Medicine at Mount Sinai, New York, NY, 2Ichan School of Medicine at Mount Sinai, New York, NY Purpose/Objective(s): A common difficulty with preplanning LDR seed implants for prostate cancer is ensuring reproducibility at the time of the implant. Seed implants from one commercial source that offers a homogenous linear source of radioactive material along with other prefabricated brachytherapy products present a unique problem due to their fixed length. Despite accurate preplanning, there are still changes in prostate size, shape, and position that occur on the day of the brachytherapy procedure. Slight variations in the angle and axis of the prostate need to be accounted for intraoperatively. We sought to develop a combined preplan and intraoperative approach for LDR brachytherapy with fixed-length strands. Materials/Methods: We aim to identify a small number and length of extra brachytherapy seeds that can be ordered in addition to the preplanned strands in order to allow for optimal real-time flexibility. We developed an
Poster Viewing E685 in-house software program that requires the prostate contour and preplan needle position to analyze the effect that small variations in needle placement and prostate rotation have on the appropriate length of string. The first deviation analyzed in our program is needle position (2 and 4 mm in Ant-Post and Right-Left direction from the preplan needle position). Because the strands are ordered in centimeter intervals, we considered a deviation of >0.66 cm to be significant. The second deviation analyzed in the program was prostate rotation of 5 and 10 degrees in the sagittal plane above or below the preplan axis. Results: When both rotational and positional deviations were applied to the interior strings, there were no significant changes in string length. When the AP or RL deviations were applied to the peripheral strings, we saw a significant change in string length. Using our software program, we determined that <30% extra activity will allow for sufficient flexibility at the time of implant. Conclusion: This software program helped us overcome the limitations of using prefabricated fixed-length sources by predicting for a number of extra radioactive strands that allow for real-time flexibility during the implant procedure. Author Disclosure: A.R. Powers: None. R. Sheu: None. H.M. McGee: None. R.G. Stock: None.
3679 Breathing-Induced Prostate Motion: Real-Time Data Show SecondOrder Effects C. Panaino,1 T. Giandini,1 M. Carrara,1 S. Frasca,1 B. Avuzzi,2 S. Morlino,1 D. Bosetti,2 N. Bedini,1 S. Villa,1 T. Rancati,2 R. Valdagni,1 and E. Pignoli2; 1Fondazione IRCCS Instituto Nazionale dei Tumori, Milano, Italy, 2Fondazione IRCCS Instituto Nazionale dei Tumori, Milan, Italy Purpose/Objective(s): An electromagnetic (ELM) system able to localize 3 transponders permanently implanted in the prostate with submillimeter accuracy is currently used in our department for continuous real-time tracking of the tumor. In the last year, we focused our attention on the development of a tool to selectively filter the signal produced by the transponders and to deconvolve it in its main components. In particular, the aim of this study was to validate and apply the developed tool to quantify the amplitude of prostate motion induced only by the patient’s breathing. Materials/Methods: Software was developed to selectively filter the signal of the ELM transponders. Based on a patient-specific analysis of the power density spectrum of the recorded tracks, the developed software removes noise and automatically isolates their main components, i.e., the low-frequency motion (long-term motion) and the high-frequency harmonic motion (breathing-induced motion). The software was validated on data recorded with a moving phantom, provided with a home-made insert with 3 transponders. Simulated breathing frequencies (BFs) of 10, 12, 14, 16, 18, 20, 22, and 24 cycles/min were recorded for at least 1 minute with the ELM system and analyzed. After validation, the tracks of 15 prostate patients who underwent external beam radiation therapy were analyzed for a total of 506 treatment sessions (TSs). For each TS, the amplitude of prostate motion along the 3 main directions was obtained. Average amplitudes, standard deviations, and overall amplitude ranges were reported. Results: For the in-phantom validation, the developed software automatically computed the correct cycles per minute within a 0.6% uncertainty. The average BF was 18.41.2 cycles/min (range: 13.6O26.2 cycles/min). The resulting prostate excursions induced only by patient’s breathing, averaged over all TSs, were 0.20.1 mm, 0.40.1 mm, and 0.50.1 mm for L-R, A-P, and C-C directions, respectively (ranges: 0.1O0.5 mm, 0.2O0.6 mm, and 0.2O0.7 mm). 3D prostate excursions ranged between 0.4 mm and 1 mm. A very limited intrapatient variation over all TSs was observed for both BF and prostate maximum excursions, showing that patients tend to behave in the same way over all TSs. No significant correlation was found between the 2 parameters. Conclusion: The obtained results show that prostate harmonic motion induced by patient’s breathing represents a reduced source of intrasession