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An Automated Inverse Planning Optimization Approach for Single-Fraction and Fractionated Radiosurgery Using Gamma Knife Perfexion

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K Ghobadi

K Ghobadi1*, H Ghaffari1, D Aleman1, M Ruschin2, D Jaffray3, (1) University of Toronto, Toronto, Ontario, Canada (2) Odette Cancer Centre, Toronto, Canada (3) Princess Margaret Hospital, Toronto, Ontario, Canada

SU-D-211-3 Sunday 2:15:00 PM - 3:00:00 PM Room: 211

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 1mm^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.

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