Purpose: Stereotactic body radiation therapy (SBRT) planning based on a static dataset of the simulation day anatomy is historically created over at least a week. We hypothesize that the quality of daily on-table adaptive plans is similar to that of the baseline plan when delivering stereotactic magnetic resonance image guided adaptive radiation therapy (SMART) for pancreas cancer (PCa) on a 0.35T MR Linac.
Methods: Fifty (n=50) PCa patients received 50 Gy/5-fraction SMART. Baseline planning included: 3 mm gastrointestinal (GI) PRV, 50 Gy/100% and 33 Gy/66% optimization targets based on GI PRV, high-dose and low-dose conformality rings, and a contracted GTV for ablative 120%-140% region. For each adaptive fraction, GI anatomy was re-contoured, followed by propagation of planning structures and optimization. If conformality, coverage, or organ at risk (OAR) goals were not achieved, optimization was repeated until satisfactory. Plan quality was evaluated for target coverage (TC=PTV V100%/PTV vol), PTV D90% and D80%, homogeneity index (HI=PTV D2%/ D98%), high-dose conformity (PITV), low-dose conformity (D2cm), and gradient (R50%).
Results: OAR constraints were met on all baseline (n=50) and adaptive (n=250) plans. Mean±SD PTV coverages for baseline and adaptive plans were 41.7±6.3 Gy and 40.5±6.3 Gy (D90%), 45.6±5.6 Gy and 44.4±5.5 Gy (D80%), and 76%±20% and 70%±20% (TC), respectively, all not statistically significant (NSS). Mean homogeneity and conformity indices were 0.84, 0.76 (PITV), 3.37, 3.43 (R50%), 1.54, 1.57 (HI), and 58%, 60% (D2cm), respectively, all NSS. Median number of adaptive re-optimizations was 2.
Conclusion: For a site with GI interfraction motion causing daily changes in the spatial position of the dosimetric gradient, similar target coverage quality was observed between baseline and adaptive plans due to robustness of GI PRV technique subdividing the target. Planning rings enabled robustness in conformity. This work supports the feasibility towards utilizing a patient-independent template baseline plan for daily adaptation.
Funding Support, Disclosures, and Conflict of Interest: Dr. Mittauer reports personal fees from ViewRay Inc, other from MR Guidance LLC, grants from ViewRay Inc. Dr. Mehta reports personal fees from Zap, Mevion, Karyopharm, Tocagen, AstraZeneca, Oncoceutics; Dr. Chuong reports personal fees from ViewRay Inc, Sirtex, Advanced Accelerator Applications, grants from ViewRay Inc, AstraZeneca, Novocure.