A knowledge-based planning model to identify fraction-reduction opportunities in brain stereotactic radiotherapy

Objective: To develop and validate a HyperArc-based RapidPlan (HARP) model for three-fraction brain stereotactic radiotherapy (SRT) plans to then use to replan previously treated five-fraction SRT plans. Demonstrating the possibility of reducing the number of fractions while achieving acceptable organs-at-risk (OAR) doses with improved target biological effective dose (BED) to brain lesions. Methods: Thirty-nine high-quality clinical three-fraction HyperArc brain SRT plans (24-27 Gy) were used to train the HARP model, with a separate 10 plans used to validate its effectiveness. Fifty-eight five-fraction HyperArc brain SRT plans (30-40 Gy) attempted to be retrospectively replanned for three fractions scheme using the HARP model. All planning was done within the Eclipse treatment planning system for a TrueBeam LINAC with a 6 MV-FFF beam and Millenium 120 MLCs and dosimetric parameters were analyzed per brain SRT protocol. Results: The HyperArc RapidPlan model was successfully trained and tested, with the validation set demonstrating a statistically significant (p = 0.01) increase in GTV D-100% from 28.5 +/- 0.7 Gy to 29.4 +/- 0.6 Gy from the original to RapidPlan plans. No statistically significant differences were found for the OAR metrics (p > 0.05). The five-fraction replans were successful for 20 of the 58 five-fraction brain SRT plans. For those 20 successful brain SRT plans, the maximum doses to OAR were clinically acceptable with a three-fraction scheme including an average V-18Gy to Brain-PTV of 9.9 +/- 5.9 cc. Additionally, the replanned five-fraction brain SRT plans achieved a higher BED to the tumors, increasing from a GTV D-100% of 52.9 +/- 4.5 Gy for the original five-fraction plans to 57.3 +/- 3.1 Gy for the three-fraction RapidPlan plans. All RapidPlan plans were generated automatically, without manual input, in under 20 min. Conclusions: The HARP model developed in this research was used to successfully identify select five-fraction plans that were able to be reduced to three-fraction SRT treatments while achieving clinically acceptable OAR doses and improved target BED. This tool encourages a fast and standardized way to provide physicians with more options when choosing the necessary fractionation scheme(s) for HyperArc SRT to single- and multiple brain lesions.