- Email: [email protected]
A Dosimetric Comparison Between Volumetric Modulated Arc and 3D Treatment Plans for Synchronous Bilateral Breast Patients
S750
International Journal of Radiation Oncology Biology Physics
diameter eyeball were calculated using MCNPX2.6 Monte Carlo simulation. The eye applicator was designed to contour the eye and to be in contact with the eye surface during the treatment. A total physical dose of 85 Gy was used as reference dose in the treated tumor volume, based on the American Brachytherapy Society (ABS) recommendations for brachytherapy of uveal melanomas. Re-188 has the highest parent isotope cross-section (73 b), followed by 11.4 b for Pr-142 and 1.28 b for Y-90. The half-life of Re-188 was the shortest (17.0 h), compared to Y-90 (64.0 h) and Pr-142 (19.12 h). Results: The simulated dose per decay at 1 mm from the eye surface at the central axis for Re-188, Pr-142 and Y-90 were 3.85 10-11, 4.16 10-11 and 6.57 10-11 Gy respectively. For 85 Gy dose at the eye surface, the observed dose at 1 mm for Re-188, Pr-142 and Y-90 were 50.5, 52.1 and 55.3 Gy, respectively. Activation of Re-188 and Pr-142 glass eye applicator can be achieved in a common low fluence neutron research reactor, making these nuclides accessible and lowering production costs. For all nuclides studied, the beta dose profile showed that this device can provide very high doses at short ranges within the eyeball, being suitable for the treatment of superficial eye lesions. Y-90 showed the highest dose per decay at the point studied, 70.6% higher than Re-188 and 57.9% higher than Pr-142. Dose penetration for Y-90 was also higher than Re-188 (9.5%) and Pr-142 (6.1%). Conclusions: Glass eye plaque using Re-188, Pr-142, and Y-90 are viable for the treatment of superficial eye cancers, due to the high doses achieved in short distances, sparing healthy surrounding issues. Glass eye plaques could also be further molded according to the tumor shape and size to achieve the desired dose distribution. The feasibility and costs of each of these radionuclides should be taken into account for each clinical setup configuration, time from the nuclide activation to dose delivery, and desired dose penetration in tissue. Author Disclosure: M. Ferreira: None. J. Jung: None.
VMAT by up to 30%, which leads to less leakage/scatter dose to the patient and can potentially translate into faster dose delivery. Conclusions: We have proposed a rotational arc strategy to optimally implement SPORT. The resultant dose distribution is improved as compared with the 1- and 2-arc VMAT for clinically complicated cases. The delivery can still be accomplished by a single arc, leading to a simplified and efficient delivery process. Author Disclosure: R. Li: None. L. Xing: None.
3430 An Adaptive Planning Strategy for Station Parameter Optimized Radiation Therapy (SPORT): Segmentally Boosted VMAT R. Li and L. Xing; Stanford University, Stanford, CA Purpose/Objective(s): The concepts of station parameter optimized radiation therapy (SPORT) and dense angularly sampled and sparse intensity modulated radiation therapy (DASSIM-RT) were recently introduced to maximally utilize the technical capacity of a digital LINAC. Here we present a rotational arc implementation of SPORT in which the traditional 1-arc VMAT is differentially boosted to improve target coverage while sparing critical structures. Materials/Methods: The demand for intensity modulation varies from angle to angle and a rotational arc therapy should fully take this into account in the planning process. The essence of the new segmentally boosted treatment scheme is to provide the necessary beam intensity modulation in those directions that need it, by inserting additional modulated apertures on an on-demand basis. In this implementation, we utilize a “demand metric” based on a modulation index at each station point to decide which station or control points need intensity modulation. To boost the modulation at selected stations, additional segments are added in the vicinity of the selected stations. The added segments are then optimized together with the original set of station control points as a whole. We apply the segmentally boosted planning technique to four previously treated clinical cases: two head and neck (H&N) cases, one prostate case, and one liver case. The proposed planning technique is compared with conventional 1-arc and 2-arc VMAT. Results: In general, the proposed segmentally boosted VMAT technique achieves better critical structure sparing than 1-arc VMAT, with similar or better target coverage in all four clinical cases. Compared with 2-arc VMAT, segmentally boosted VMAT achieves similar critical structure sparing for the liver and prostate case, and outperforms 2-arc VMAT in the more complicated H&N cases. Yet, segmentally boosted VMAT achieves such plan quality generally with fewer machine monitor units than 2-arc
3431 Are Photons in the Energy Range of 1-6 MV A More Optimum Choice Than Higher Energy Photons for Delivering a Conformal Dose Distribution in the Target Volume With Better Quality in Portal Images? Y. Zhang,1,2 Y. Feng,1 X. Ming,1,2 R. Nath,2 and J. Deng2; 1Tianjin University, Tianjin, China, 2Yale University, New Haven, CT Purpose/Objective(s): To investigate the energy dependence of tumor coverage, integral dose, and image quality of an amorphous silicon electronic portal imaging device (EPID) for radiation therapy treatments using photons beams in the energy range of 1-6 MV with the goal of finding an optimum photon energy which maximizes tumor coverage, minimizes integral dose and maximizes image quality. Materials/Methods: Using Monte Carlo transport code EGS4/BEAM, photon beams with energy in the range of 1 to 6 MV from a linac were simulated. Phase space data were scored above the jaws with full particle transport through the linac components specified by the manufacturer. Based on beam characterization, multiple source models were generated to represent the photon beams of various energies. These data were then used as beam input for subsequent Monte Carlo dose calculations in the patient anatomy with a benchmarked EGS4 user code, MCSIM. The CT and contours of a brain tumor patient were converted into a 3D patient phantom. The EPID was modeled as an external structure comprised of a series of slabs located at 120 cm source-to-detector-distance. A single open beam incident along AP direction was simulated for each photon energy with 3D dose distributions scored in patient anatomy and EPID. To reduce the intrinsic variance in beam fluence, the EPID image quality was assessed after subtracting the image without patient anatomy from the one with patient anatomy. The integral dose, the PTV dose distribution index, and the contrast-to-noise ratio (CNR) of the EPID images were analyzed to perform a quantitative comparison at different photon energies. Results: For this brain tumor case, the integral dose was found to be higher for both 1 MV and 6 MV photon beams with a minimum value at 4 MV. However, the largest difference in integral dose was small; up to 5.5% for the beam energies ranging from 1 to 6 MV. The PTV dose distribution index increased linearly with beam energy from 0.98 at 1 MV to 1.53 at 6 MV, indicating a more conformal dose distribution in PTV with higher energy beams. The CNR values of EPID images were found to decrease monotonically with the increase of beam energy, up to 54% difference was observed between 1 MV and 6 MV photon beams. Conclusions: It has been shown that current Monte Carlo codes using realistic linac designs and actual patient data, can be used to perform a quantitative analysis of dosimetric and imaging parameters to search for more optimum photon energy for radiation therapy treatments. With lower energy photon beams (< 6 MV) the EPID image quality can be improved considerably. Further investigation on the optimal beam energy selection correlated with tumor location and volume is in progress. Author Disclosure: Y. Zhang: None. Y. Feng: None. X. Ming: None. R. Nath: None. J. Deng: None.
3432 A Dosimetric Comparison Between Volumetric Modulated Arc and 3D Treatment Plans for Synchronous Bilateral Breast Patients D. Levin, E. Shekel, D. Epstein, Y. Tova, S. Zalmanov-Faermann, and R. Pfeffer; Assuta Medical Centers, Tel Aviv, Israel
Volume 87 Number 2S Supplement 2013 Purpose/Objective(s): The purpose of the current study is to compare standard tangential and volumetric modulated arc (VMAT) plans for synchronous bilateral breast patients in terms of target coverage and doses to organs at risk (OARs). Materials/Methods: For 10 bilateral breast patients we created separate left and right tangential plans and a single VMAT plan encompassing both breasts as target volume. Patients were prescribed to 50 Gy in 2 Gy fractions. Breast PTV was contracted 3 mm inside the body contour. Lungs, heart, and spine were contoured. For 3D plans each breast was planned independently, aiming to achieve adequate coverage. Where a significant portion of the heart was in the left breast fields, we used a multileaf collimator to block the heart. The hotspot was kept below 107%. A plan sum of the independent plans was generated to ensure no dose overlaps, and for comparison with the VMAT plan. VMAT plans were normalized to 95% dose covering 95% PTV volume, with maximum dose below 107%. OAR optimization objectives were based on QUANTEC data: Both lungs: mean lung dose (MLD) <20 Gy, V20<30%,V5<70%; Heart: V25<10%, Maximal dose ALARA; Spine: maximal dose < 50 Gy. The conformity index (CI) was calculated for all plans. We used the MannWhitney Rank-Sum test to determine statistical significance between 3D and VMAT plans. Results: PTV coverage was similar for 3D and VMAT plans. However, CI for VMAT was 0.91 0.04, and for 3D plans 0.56 0.07. This difference was statistically significant (p < 0.001). All lung metrics were also significantly different (p < 0.001), in favor of 3D plans: MLD was 14.2 2.9 (RA) vs 5.74 0.8 (3D), V20 was 22.4 10.7 (RA) vs 9.1 1.7 (3D), V5 was 86.5 7.5 (RA) vs 20.3 3.2 (3D). Other statistically significant parameters (p < 0.001) were: D2%, the measure used for maximal heart dose: 22.5 6.5 (RA) vs 5.4 2.5 (3D), and D2% for the spinal cord: 14.4 2.9 (RA) vs 0.5 0.3 (3D). Conclusions: Conformity in the VMAT plans was much better than in 3D plans. In addition, VMAT requires a single isocenter setup, which is much simpler than the standard setup, where treatment is given sequentially to each breast, necessitating separate setups for each breast. Thus, treatment time is shorter, and treatment is easier on the patients. However, 3D plans are overwhelmingly superior to VMAT in terms of doses to OARs. These advantages, in our clinic, outweigh the drawbacks of a more complex setup. Thus, the breast is a unique site, where organs at risk are better spared by a conventional, “simple” tangential plan rather than a more sophisticated VMAT plan. Author Disclosure: D. Levin: None. E. Shekel: None. D. Epstein: None. Y. Tova: None. S. Zalmanov-Faermann: None. R. Pfeffer: None.
3433 Efficient VMAT Treatment Plan Optimization Using Non-Uniformly Distributed Control Points Y. Na,1 R. Li,1 T. Suh,2 and L. Xing1; 1Stanford University, Stanford, CA, 2 The Catholic University of Korea, Seoul, Korea, Republic of Korea Purpose/Objective(s): This study proposes a new method to add control points in non-uniformly distributed scheme improving the dose distribution and delivery efficiency for single-arc volumetric modulated arc therapy (VMAT) plans. Materials/Methods: The algorithm generates piecewise constant fluence maps using total-variation regulation (TVR) for each beam direction. An initial arc spacing of 6 creates 60 beams directions for a single arc, 360 . The optimized fluence maps of one fluence profile per the arc spacing are distributed as an initial coarse sampling of control points over the single arc. From the initial control points having an aperture sequence (A0, . AN), each association (A0-A1, ., AN-1-AN) of apertures at the control points is assigned a mutual information(MI) score. A higher score represents that the association has a stronger similarity and vice-versa between the shapes of two adjacent apertures. A new control point (B0) is added to the mid-point of two apertures (A0-B0-A1) in which the A0-A1 association has the lowest MI score having the biggest aperture shape difference. A new shape of aperture at B0 is created by the interpolation of the adjacent apertures. The aperture shape is then rectified to be a sequencing of arc
Poster Viewing Abstracts S751 using the manufacture constraints. The entire process is iteratively repeated until the overall score meets the given criterion and/or to reach the minimum gantry spacing 2 . The implemented plan optimization is evaluated using three clinical cases of head and neck, lung, and prostate previously treated with a linac. The plan results of the proposed optimization method are compared with conventional plan optimization in terms of dose distribution and delivery efficiency. Results: The target dose coverage and critical structure sparing of plans obtained using the proposed method with non-uniformly distributed 120 control points are almost identical to those obtained using the conventional method with 180 control points uniformly distributed. The propose method reduced dose delivery time approximately 32%, 26%, and 30% for head neck, lung, and prostate plans, respectively. Conclusions: The propose method adding new control points in nonuniformly distributed scheme reduced the dose delivery time and improved the dose conformity for single-arc VMAT plans. The results show that this new algorithm can substantially provide future station mediated planning parameters. Author Disclosure: Y. Na: None. R. Li: None. T. Suh: None. L. Xing: None.
3434 Permanent Interstitial Radiation With Cs-131 Implants for Gynecologic Malignancies: Analysis and Results of an Initial 13 Applications C.E. Wooten, M.E. Randall, P. Aryal, and J. Feddock; University of Kentucky, Lexington, KY Purpose/Objective(s): Interstitial brachytherapy is an ideal, yet underutilized, treatment modality for small volume gynecological malignancies, especially in circumstances where higher doses of radiation are needed while respecting normal tissue toxicity. Our institution has extensive experience using Au-198 radioactive sources for interstitial brachytherapy. We recently performed the first interstitial implant for a gynecologic malignancy using a newer isotope, Cs-131, which has a more favorable dosimetric profile. This report reviews our initial experience with the Cs131 radioactive isotope for gynecological patients. Materials/Methods: A retrospective review was performed evaluating all patients treated with Cs-131 permanent interstitial radiation at our institution from July 2011 through February 2013. Patients had biopsy-proven recurrent endometrial adenocarcinoma (n Z 4), recurrent endometrial serous papillary carcinoma (n Z 1), recurrent fallopian tube adenocarcinoma (n Z 1), primary cervical squamous cell carcinoma (n Z 1), recurrent cervical squamous cell carcinoma (n Z 1), recurrent vaginal squamous cell (n Z 1) and melanoma (n Z 1). Doses were prescribed to a depth of 5 mm using Patterson Parker planar implant rules, and the total activity in Au-198 equivalent was calculated using a conversion factor of 1.1 for Cs-131. Results: A total of 13 Cs-131 interstitial implantations were performed on ten patients. Eight patients underwent implantation of a single lesion, 2 patients underwent implantation of 2 separate lesions, and 1 patient underwent a second implantation of the same lesion. Four implants were delivered as definitive treatment for recurrence with an average dose of 52 Gy (range, 33-75 Gy); 8 were given as boost with average dose of 29.0 Gy (range, 22-49.5 Gy); 1 patient underwent upfront brachytherapy prior to external radiation. The median area of the prescription isodose was 9.28 cm2 (range, 2-17.5 cm2). The median number of Cs-131 sources used was 16, with median activity of 1.43 U (range, 0.49-2.34 U). After a median follow-up of 7 months (range, 1-19), the actuarial local control rate is 91%. The expected toxicities of grade 1-2 acute mucositis were observed without any other significant grade 3 toxicities noted. Conclusions: Our series is the first and only to report on clinical outcomes following Cs-131 permanent interstitial implants for gynecologic malignancies. We conclude that Cs-131 seeds are effective in achieving durable local control for small volume gynecologic malignancies. Author Disclosure: C.E. Wooten: None. M.E. Randall: None. P. Aryal: None. J. Feddock: None.
International Journal of Radiation Oncology Biology Physics
diameter eyeball were calculated using MCNPX2.6 Monte Carlo simulation. The eye applicator was designed to contour the eye and to be in contact with the eye surface during the treatment. A total physical dose of 85 Gy was used as reference dose in the treated tumor volume, based on the American Brachytherapy Society (ABS) recommendations for brachytherapy of uveal melanomas. Re-188 has the highest parent isotope cross-section (73 b), followed by 11.4 b for Pr-142 and 1.28 b for Y-90. The half-life of Re-188 was the shortest (17.0 h), compared to Y-90 (64.0 h) and Pr-142 (19.12 h). Results: The simulated dose per decay at 1 mm from the eye surface at the central axis for Re-188, Pr-142 and Y-90 were 3.85 10-11, 4.16 10-11 and 6.57 10-11 Gy respectively. For 85 Gy dose at the eye surface, the observed dose at 1 mm for Re-188, Pr-142 and Y-90 were 50.5, 52.1 and 55.3 Gy, respectively. Activation of Re-188 and Pr-142 glass eye applicator can be achieved in a common low fluence neutron research reactor, making these nuclides accessible and lowering production costs. For all nuclides studied, the beta dose profile showed that this device can provide very high doses at short ranges within the eyeball, being suitable for the treatment of superficial eye lesions. Y-90 showed the highest dose per decay at the point studied, 70.6% higher than Re-188 and 57.9% higher than Pr-142. Dose penetration for Y-90 was also higher than Re-188 (9.5%) and Pr-142 (6.1%). Conclusions: Glass eye plaque using Re-188, Pr-142, and Y-90 are viable for the treatment of superficial eye cancers, due to the high doses achieved in short distances, sparing healthy surrounding issues. Glass eye plaques could also be further molded according to the tumor shape and size to achieve the desired dose distribution. The feasibility and costs of each of these radionuclides should be taken into account for each clinical setup configuration, time from the nuclide activation to dose delivery, and desired dose penetration in tissue. Author Disclosure: M. Ferreira: None. J. Jung: None.
VMAT by up to 30%, which leads to less leakage/scatter dose to the patient and can potentially translate into faster dose delivery. Conclusions: We have proposed a rotational arc strategy to optimally implement SPORT. The resultant dose distribution is improved as compared with the 1- and 2-arc VMAT for clinically complicated cases. The delivery can still be accomplished by a single arc, leading to a simplified and efficient delivery process. Author Disclosure: R. Li: None. L. Xing: None.
3430 An Adaptive Planning Strategy for Station Parameter Optimized Radiation Therapy (SPORT): Segmentally Boosted VMAT R. Li and L. Xing; Stanford University, Stanford, CA Purpose/Objective(s): The concepts of station parameter optimized radiation therapy (SPORT) and dense angularly sampled and sparse intensity modulated radiation therapy (DASSIM-RT) were recently introduced to maximally utilize the technical capacity of a digital LINAC. Here we present a rotational arc implementation of SPORT in which the traditional 1-arc VMAT is differentially boosted to improve target coverage while sparing critical structures. Materials/Methods: The demand for intensity modulation varies from angle to angle and a rotational arc therapy should fully take this into account in the planning process. The essence of the new segmentally boosted treatment scheme is to provide the necessary beam intensity modulation in those directions that need it, by inserting additional modulated apertures on an on-demand basis. In this implementation, we utilize a “demand metric” based on a modulation index at each station point to decide which station or control points need intensity modulation. To boost the modulation at selected stations, additional segments are added in the vicinity of the selected stations. The added segments are then optimized together with the original set of station control points as a whole. We apply the segmentally boosted planning technique to four previously treated clinical cases: two head and neck (H&N) cases, one prostate case, and one liver case. The proposed planning technique is compared with conventional 1-arc and 2-arc VMAT. Results: In general, the proposed segmentally boosted VMAT technique achieves better critical structure sparing than 1-arc VMAT, with similar or better target coverage in all four clinical cases. Compared with 2-arc VMAT, segmentally boosted VMAT achieves similar critical structure sparing for the liver and prostate case, and outperforms 2-arc VMAT in the more complicated H&N cases. Yet, segmentally boosted VMAT achieves such plan quality generally with fewer machine monitor units than 2-arc
3431 Are Photons in the Energy Range of 1-6 MV A More Optimum Choice Than Higher Energy Photons for Delivering a Conformal Dose Distribution in the Target Volume With Better Quality in Portal Images? Y. Zhang,1,2 Y. Feng,1 X. Ming,1,2 R. Nath,2 and J. Deng2; 1Tianjin University, Tianjin, China, 2Yale University, New Haven, CT Purpose/Objective(s): To investigate the energy dependence of tumor coverage, integral dose, and image quality of an amorphous silicon electronic portal imaging device (EPID) for radiation therapy treatments using photons beams in the energy range of 1-6 MV with the goal of finding an optimum photon energy which maximizes tumor coverage, minimizes integral dose and maximizes image quality. Materials/Methods: Using Monte Carlo transport code EGS4/BEAM, photon beams with energy in the range of 1 to 6 MV from a linac were simulated. Phase space data were scored above the jaws with full particle transport through the linac components specified by the manufacturer. Based on beam characterization, multiple source models were generated to represent the photon beams of various energies. These data were then used as beam input for subsequent Monte Carlo dose calculations in the patient anatomy with a benchmarked EGS4 user code, MCSIM. The CT and contours of a brain tumor patient were converted into a 3D patient phantom. The EPID was modeled as an external structure comprised of a series of slabs located at 120 cm source-to-detector-distance. A single open beam incident along AP direction was simulated for each photon energy with 3D dose distributions scored in patient anatomy and EPID. To reduce the intrinsic variance in beam fluence, the EPID image quality was assessed after subtracting the image without patient anatomy from the one with patient anatomy. The integral dose, the PTV dose distribution index, and the contrast-to-noise ratio (CNR) of the EPID images were analyzed to perform a quantitative comparison at different photon energies. Results: For this brain tumor case, the integral dose was found to be higher for both 1 MV and 6 MV photon beams with a minimum value at 4 MV. However, the largest difference in integral dose was small; up to 5.5% for the beam energies ranging from 1 to 6 MV. The PTV dose distribution index increased linearly with beam energy from 0.98 at 1 MV to 1.53 at 6 MV, indicating a more conformal dose distribution in PTV with higher energy beams. The CNR values of EPID images were found to decrease monotonically with the increase of beam energy, up to 54% difference was observed between 1 MV and 6 MV photon beams. Conclusions: It has been shown that current Monte Carlo codes using realistic linac designs and actual patient data, can be used to perform a quantitative analysis of dosimetric and imaging parameters to search for more optimum photon energy for radiation therapy treatments. With lower energy photon beams (< 6 MV) the EPID image quality can be improved considerably. Further investigation on the optimal beam energy selection correlated with tumor location and volume is in progress. Author Disclosure: Y. Zhang: None. Y. Feng: None. X. Ming: None. R. Nath: None. J. Deng: None.
3432 A Dosimetric Comparison Between Volumetric Modulated Arc and 3D Treatment Plans for Synchronous Bilateral Breast Patients D. Levin, E. Shekel, D. Epstein, Y. Tova, S. Zalmanov-Faermann, and R. Pfeffer; Assuta Medical Centers, Tel Aviv, Israel
Volume 87 Number 2S Supplement 2013 Purpose/Objective(s): The purpose of the current study is to compare standard tangential and volumetric modulated arc (VMAT) plans for synchronous bilateral breast patients in terms of target coverage and doses to organs at risk (OARs). Materials/Methods: For 10 bilateral breast patients we created separate left and right tangential plans and a single VMAT plan encompassing both breasts as target volume. Patients were prescribed to 50 Gy in 2 Gy fractions. Breast PTV was contracted 3 mm inside the body contour. Lungs, heart, and spine were contoured. For 3D plans each breast was planned independently, aiming to achieve adequate coverage. Where a significant portion of the heart was in the left breast fields, we used a multileaf collimator to block the heart. The hotspot was kept below 107%. A plan sum of the independent plans was generated to ensure no dose overlaps, and for comparison with the VMAT plan. VMAT plans were normalized to 95% dose covering 95% PTV volume, with maximum dose below 107%. OAR optimization objectives were based on QUANTEC data: Both lungs: mean lung dose (MLD) <20 Gy, V20<30%,V5<70%; Heart: V25<10%, Maximal dose ALARA; Spine: maximal dose < 50 Gy. The conformity index (CI) was calculated for all plans. We used the MannWhitney Rank-Sum test to determine statistical significance between 3D and VMAT plans. Results: PTV coverage was similar for 3D and VMAT plans. However, CI for VMAT was 0.91 0.04, and for 3D plans 0.56 0.07. This difference was statistically significant (p < 0.001). All lung metrics were also significantly different (p < 0.001), in favor of 3D plans: MLD was 14.2 2.9 (RA) vs 5.74 0.8 (3D), V20 was 22.4 10.7 (RA) vs 9.1 1.7 (3D), V5 was 86.5 7.5 (RA) vs 20.3 3.2 (3D). Other statistically significant parameters (p < 0.001) were: D2%, the measure used for maximal heart dose: 22.5 6.5 (RA) vs 5.4 2.5 (3D), and D2% for the spinal cord: 14.4 2.9 (RA) vs 0.5 0.3 (3D). Conclusions: Conformity in the VMAT plans was much better than in 3D plans. In addition, VMAT requires a single isocenter setup, which is much simpler than the standard setup, where treatment is given sequentially to each breast, necessitating separate setups for each breast. Thus, treatment time is shorter, and treatment is easier on the patients. However, 3D plans are overwhelmingly superior to VMAT in terms of doses to OARs. These advantages, in our clinic, outweigh the drawbacks of a more complex setup. Thus, the breast is a unique site, where organs at risk are better spared by a conventional, “simple” tangential plan rather than a more sophisticated VMAT plan. Author Disclosure: D. Levin: None. E. Shekel: None. D. Epstein: None. Y. Tova: None. S. Zalmanov-Faermann: None. R. Pfeffer: None.
3433 Efficient VMAT Treatment Plan Optimization Using Non-Uniformly Distributed Control Points Y. Na,1 R. Li,1 T. Suh,2 and L. Xing1; 1Stanford University, Stanford, CA, 2 The Catholic University of Korea, Seoul, Korea, Republic of Korea Purpose/Objective(s): This study proposes a new method to add control points in non-uniformly distributed scheme improving the dose distribution and delivery efficiency for single-arc volumetric modulated arc therapy (VMAT) plans. Materials/Methods: The algorithm generates piecewise constant fluence maps using total-variation regulation (TVR) for each beam direction. An initial arc spacing of 6 creates 60 beams directions for a single arc, 360 . The optimized fluence maps of one fluence profile per the arc spacing are distributed as an initial coarse sampling of control points over the single arc. From the initial control points having an aperture sequence (A0, . AN), each association (A0-A1, ., AN-1-AN) of apertures at the control points is assigned a mutual information(MI) score. A higher score represents that the association has a stronger similarity and vice-versa between the shapes of two adjacent apertures. A new control point (B0) is added to the mid-point of two apertures (A0-B0-A1) in which the A0-A1 association has the lowest MI score having the biggest aperture shape difference. A new shape of aperture at B0 is created by the interpolation of the adjacent apertures. The aperture shape is then rectified to be a sequencing of arc
Poster Viewing Abstracts S751 using the manufacture constraints. The entire process is iteratively repeated until the overall score meets the given criterion and/or to reach the minimum gantry spacing 2 . The implemented plan optimization is evaluated using three clinical cases of head and neck, lung, and prostate previously treated with a linac. The plan results of the proposed optimization method are compared with conventional plan optimization in terms of dose distribution and delivery efficiency. Results: The target dose coverage and critical structure sparing of plans obtained using the proposed method with non-uniformly distributed 120 control points are almost identical to those obtained using the conventional method with 180 control points uniformly distributed. The propose method reduced dose delivery time approximately 32%, 26%, and 30% for head neck, lung, and prostate plans, respectively. Conclusions: The propose method adding new control points in nonuniformly distributed scheme reduced the dose delivery time and improved the dose conformity for single-arc VMAT plans. The results show that this new algorithm can substantially provide future station mediated planning parameters. Author Disclosure: Y. Na: None. R. Li: None. T. Suh: None. L. Xing: None.
3434 Permanent Interstitial Radiation With Cs-131 Implants for Gynecologic Malignancies: Analysis and Results of an Initial 13 Applications C.E. Wooten, M.E. Randall, P. Aryal, and J. Feddock; University of Kentucky, Lexington, KY Purpose/Objective(s): Interstitial brachytherapy is an ideal, yet underutilized, treatment modality for small volume gynecological malignancies, especially in circumstances where higher doses of radiation are needed while respecting normal tissue toxicity. Our institution has extensive experience using Au-198 radioactive sources for interstitial brachytherapy. We recently performed the first interstitial implant for a gynecologic malignancy using a newer isotope, Cs-131, which has a more favorable dosimetric profile. This report reviews our initial experience with the Cs131 radioactive isotope for gynecological patients. Materials/Methods: A retrospective review was performed evaluating all patients treated with Cs-131 permanent interstitial radiation at our institution from July 2011 through February 2013. Patients had biopsy-proven recurrent endometrial adenocarcinoma (n Z 4), recurrent endometrial serous papillary carcinoma (n Z 1), recurrent fallopian tube adenocarcinoma (n Z 1), primary cervical squamous cell carcinoma (n Z 1), recurrent cervical squamous cell carcinoma (n Z 1), recurrent vaginal squamous cell (n Z 1) and melanoma (n Z 1). Doses were prescribed to a depth of 5 mm using Patterson Parker planar implant rules, and the total activity in Au-198 equivalent was calculated using a conversion factor of 1.1 for Cs-131. Results: A total of 13 Cs-131 interstitial implantations were performed on ten patients. Eight patients underwent implantation of a single lesion, 2 patients underwent implantation of 2 separate lesions, and 1 patient underwent a second implantation of the same lesion. Four implants were delivered as definitive treatment for recurrence with an average dose of 52 Gy (range, 33-75 Gy); 8 were given as boost with average dose of 29.0 Gy (range, 22-49.5 Gy); 1 patient underwent upfront brachytherapy prior to external radiation. The median area of the prescription isodose was 9.28 cm2 (range, 2-17.5 cm2). The median number of Cs-131 sources used was 16, with median activity of 1.43 U (range, 0.49-2.34 U). After a median follow-up of 7 months (range, 1-19), the actuarial local control rate is 91%. The expected toxicities of grade 1-2 acute mucositis were observed without any other significant grade 3 toxicities noted. Conclusions: Our series is the first and only to report on clinical outcomes following Cs-131 permanent interstitial implants for gynecologic malignancies. We conclude that Cs-131 seeds are effective in achieving durable local control for small volume gynecologic malignancies. Author Disclosure: C.E. Wooten: None. M.E. Randall: None. P. Aryal: None. J. Feddock: None.