ABSTRACT Purpose: The purpose of this work is to develop an inverse planning approach for Gamma K... more ABSTRACT Purpose: The purpose of this work is to develop an inverse planning approach for Gamma Knife Perfexion that continuously delivers the dose over a path in the target. Methods: Our approach consists of two steps: finding a path in the target, and optimizing shot shapes for that path. To obtain the path, a set of well‐positioned isocentres are selected in the target, and then a Hamiltonian path that visits all the isocentres exactly once is found by graph‐theory and spiral‐based approaches. Then, a linear optimization model is solved to obtain the optimal shot shapes and intensities by minimizing dose spillage from the target. The dose restrictions for the target and the organs‐at‐risk are constraints in the optimization. We additionally consider several criteria specific to continuous path, including machine speed constraints, delivery accuracy, preference for single/multiple paths, and smoothness of movement. Results: We tested our approach on seven clinical cases and compared against inverse step‐and‐shoot and manually‐generated forward plans. The mean difference in Paddick CI compared to forward and inverse step‐and‐shoot was 0.04 and −0.04, respectively, while the Classic CI mean difference was −0.05 and 0.05, respectively. The mean dose difference to 1mm^3 brainstem was −0.5Gy (range: −1.6Gy to 0.1 Gy) and −0.24Gy (range: −1.9Gy to 0.9Gy) compared to forward and inverse step‐and‐shoot plans. However, the average beam‐on time improved by 30min (range: −82.9min to −0.62min) and 103min (range: −304min to −9min) over forward and inverse step‐and‐shoot plans, respectively. The mean computational time for continuous path was 19.5min, a 198min improvement over inverse step‐and‐shoot plans. Conclusion: The continuous path treatment plans showed comparable plan quality with forward and inverse step‐and‐shoot plans, while achieving better beam‐on times. The computational time was also improved compared to the inverse step‐and‐shoot approach. This work was by part funded by Elekta, Stockholm, Sweden.
ABSTRACT Purpose: The purpose of this work is to develop an automated inverse planning approach t... more ABSTRACT Purpose: The purpose of this work is to develop an automated inverse planning approach to generate singe-fraction and fractionated stereotactic radiosurgery (SRS) treatment plans for Gamma Knife Perfexion. Methods: Our automated approach consists of two steps: 1) a grassfire-based algorithm to carefully determine the isocentre locations; 2) a penalty-based optimization to find the optimal shot shapes and their intensities to minimize the deviation of the delivered dose from the objective dose in all structures. For single-fraction SRS, a margin-less approach was taken: conformity of dose to the gross tumor volume (GTV) with a steep dose fall-off was prioritized. For fractionated radiosurgery, dose homogeneity was given a higher priority since planning target volumes (PTV) were applied to account for daily setup variation, and these PTVs could overlap with organs-at-risk (OARs). The two-step approach was tested on seven clinical cases with PTV sizes of 0.5cm^3-56.5cm^3. In the tested cases, the PTV had 0%-38% overlap with OARs. Results: For single-fraction SRS, the dose to 1mm^3 brainstem was on average 0.24Gy (range: -2.4Gy to +2.0Gy) lower compared to manually-generated plans. Beam-on time varied with the number of isocentres, but on average was 33min longer than manually- generated plans. The optimization algorithm took 215min on average, while isocentre selection performed in <10s.For fractionated SRS, the average PTV coverage was V95=94.9% (range: 92.7%-97.6%) and the mean dose to 1 mm^3 brainstem was 87.8% of the prescription dose (range: 35.4%- 108.8%). The mean beam-on time per fraction per dose-per-fraction was 4.8min/Gy (range: 0.9min/Gy-10.3min/Gy). We observed a tradeoff between conformity and OARs-sparing in both plans, and added sensitivity to isocentre locations in fractionated plans. In all the cases, GTV received the full prescription dose. Conclusions: The results indicated that automated inverse planning yields improved conformity and OAR-sparing for single- fraction SRS and is capable of generating homogeneous fractionated SRS. This work is partially funded by Elekta Instrument, AB, Stockholm, Sweden.
Purpose: The purpose of this work is to develop a framework to the inverse problem for radiosurge... more Purpose: The purpose of this work is to develop a framework to the inverse problem for radiosurgery treatment planning on the Gamma Knife V R Perfexion TM (PFX) for intracranial targets. Methods: The approach taken in the present study consists of two parts. First, a hybrid grassfire and sphere-packing algorithm is used to obtain shot positions (isocenters) based on the geometry of the target to be treated. For the selected isocenters, a sector duration optimization (SDO) model is used to optimize the duration of radiation delivery from each collimator size from each individual source bank. The SDO model is solved using a projected gradient algorithm. This approach has been retrospectively tested on seven manually planned clinical cases (comprising 11 lesions) including acoustic neuromas and brain metastases. Results: In terms of conformity and organ-at-risk (OAR) sparing, the quality of plans achieved with the inverse planning approach were, on average, improved compared to the manually generated plans. The mean difference in conformity index between inverse and forward plans was À0.12 (range: À0.27 to þ0.03) and þ0.08 (range: 0.00-0.17) for classic and Paddick definitions, respectively, favoring the inverse plans. The mean difference in volume receiving the prescribed dose (V 100 ) between forward and inverse plans was 0.2% (range: À2.4% to þ2.0%). After plan renormalization for equivalent coverage (i.e., V 100 ), the mean difference in dose to 1 mm 3 of brainstem between forward and inverse plans was À0.24 Gy (range: À2.40 to þ2.02 Gy) favoring the inverse plans. Beam-on time varied with the number of isocenters but for the most optimal plans was on average 33 min longer than manual plans (range: À17 to þ91 min) when normalized to a calibration dose rate of 3.5 Gy=min. In terms of algorithm performance, the isocenter selection for all the presented plans was performed in less than 3 s, while the SDO was performed in an average of 215 min. Conclusions: PFX inverse planning can be performed using geometric isocenter selection and mathematical modeling and optimization techniques. The obtained treatment plans all meet or exceed clinical guidelines while displaying high conformity. V C 2012 American Association of Physicists in Medicine. [http://dx.
ABSTRACT Purpose: The purpose of this work is to develop an inverse planning approach for Gamma K... more ABSTRACT Purpose: The purpose of this work is to develop an inverse planning approach for Gamma Knife Perfexion that continuously delivers the dose over a path in the target. Methods: Our approach consists of two steps: finding a path in the target, and optimizing shot shapes for that path. To obtain the path, a set of well‐positioned isocentres are selected in the target, and then a Hamiltonian path that visits all the isocentres exactly once is found by graph‐theory and spiral‐based approaches. Then, a linear optimization model is solved to obtain the optimal shot shapes and intensities by minimizing dose spillage from the target. The dose restrictions for the target and the organs‐at‐risk are constraints in the optimization. We additionally consider several criteria specific to continuous path, including machine speed constraints, delivery accuracy, preference for single/multiple paths, and smoothness of movement. Results: We tested our approach on seven clinical cases and compared against inverse step‐and‐shoot and manually‐generated forward plans. The mean difference in Paddick CI compared to forward and inverse step‐and‐shoot was 0.04 and −0.04, respectively, while the Classic CI mean difference was −0.05 and 0.05, respectively. The mean dose difference to 1mm^3 brainstem was −0.5Gy (range: −1.6Gy to 0.1 Gy) and −0.24Gy (range: −1.9Gy to 0.9Gy) compared to forward and inverse step‐and‐shoot plans. However, the average beam‐on time improved by 30min (range: −82.9min to −0.62min) and 103min (range: −304min to −9min) over forward and inverse step‐and‐shoot plans, respectively. The mean computational time for continuous path was 19.5min, a 198min improvement over inverse step‐and‐shoot plans. Conclusion: The continuous path treatment plans showed comparable plan quality with forward and inverse step‐and‐shoot plans, while achieving better beam‐on times. The computational time was also improved compared to the inverse step‐and‐shoot approach. This work was by part funded by Elekta, Stockholm, Sweden.
ABSTRACT Purpose: The purpose of this work is to develop an automated inverse planning approach t... more ABSTRACT Purpose: The purpose of this work is to develop an automated inverse planning approach to generate singe-fraction and fractionated stereotactic radiosurgery (SRS) treatment plans for Gamma Knife Perfexion. Methods: Our automated approach consists of two steps: 1) a grassfire-based algorithm to carefully determine the isocentre locations; 2) a penalty-based optimization to find the optimal shot shapes and their intensities to minimize the deviation of the delivered dose from the objective dose in all structures. For single-fraction SRS, a margin-less approach was taken: conformity of dose to the gross tumor volume (GTV) with a steep dose fall-off was prioritized. For fractionated radiosurgery, dose homogeneity was given a higher priority since planning target volumes (PTV) were applied to account for daily setup variation, and these PTVs could overlap with organs-at-risk (OARs). The two-step approach was tested on seven clinical cases with PTV sizes of 0.5cm^3-56.5cm^3. In the tested cases, the PTV had 0%-38% overlap with OARs. Results: For single-fraction SRS, the dose to 1mm^3 brainstem was on average 0.24Gy (range: -2.4Gy to +2.0Gy) lower compared to manually-generated plans. Beam-on time varied with the number of isocentres, but on average was 33min longer than manually- generated plans. The optimization algorithm took 215min on average, while isocentre selection performed in <10s.For fractionated SRS, the average PTV coverage was V95=94.9% (range: 92.7%-97.6%) and the mean dose to 1 mm^3 brainstem was 87.8% of the prescription dose (range: 35.4%- 108.8%). The mean beam-on time per fraction per dose-per-fraction was 4.8min/Gy (range: 0.9min/Gy-10.3min/Gy). We observed a tradeoff between conformity and OARs-sparing in both plans, and added sensitivity to isocentre locations in fractionated plans. In all the cases, GTV received the full prescription dose. Conclusions: The results indicated that automated inverse planning yields improved conformity and OAR-sparing for single- fraction SRS and is capable of generating homogeneous fractionated SRS. This work is partially funded by Elekta Instrument, AB, Stockholm, Sweden.
Purpose: The purpose of this work is to develop a framework to the inverse problem for radiosurge... more Purpose: The purpose of this work is to develop a framework to the inverse problem for radiosurgery treatment planning on the Gamma Knife V R Perfexion TM (PFX) for intracranial targets. Methods: The approach taken in the present study consists of two parts. First, a hybrid grassfire and sphere-packing algorithm is used to obtain shot positions (isocenters) based on the geometry of the target to be treated. For the selected isocenters, a sector duration optimization (SDO) model is used to optimize the duration of radiation delivery from each collimator size from each individual source bank. The SDO model is solved using a projected gradient algorithm. This approach has been retrospectively tested on seven manually planned clinical cases (comprising 11 lesions) including acoustic neuromas and brain metastases. Results: In terms of conformity and organ-at-risk (OAR) sparing, the quality of plans achieved with the inverse planning approach were, on average, improved compared to the manually generated plans. The mean difference in conformity index between inverse and forward plans was À0.12 (range: À0.27 to þ0.03) and þ0.08 (range: 0.00-0.17) for classic and Paddick definitions, respectively, favoring the inverse plans. The mean difference in volume receiving the prescribed dose (V 100 ) between forward and inverse plans was 0.2% (range: À2.4% to þ2.0%). After plan renormalization for equivalent coverage (i.e., V 100 ), the mean difference in dose to 1 mm 3 of brainstem between forward and inverse plans was À0.24 Gy (range: À2.40 to þ2.02 Gy) favoring the inverse plans. Beam-on time varied with the number of isocenters but for the most optimal plans was on average 33 min longer than manual plans (range: À17 to þ91 min) when normalized to a calibration dose rate of 3.5 Gy=min. In terms of algorithm performance, the isocenter selection for all the presented plans was performed in less than 3 s, while the SDO was performed in an average of 215 min. Conclusions: PFX inverse planning can be performed using geometric isocenter selection and mathematical modeling and optimization techniques. The obtained treatment plans all meet or exceed clinical guidelines while displaying high conformity. V C 2012 American Association of Physicists in Medicine. [http://dx.
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Papers by Kimia Ghobadi