A Novel Pre-plan Technique for On-table Adaptation of Pancreatic Stereotactic MR-guided Online Adaptive Radiotherapy

Abstract

Currently on-table adaptive radiotherapy (ART) is performed by daily re-optimization of initial planning objectives developed from the initial simulation anatomy. Eliminating the initial plan creation may reduce start time, simulation and planning resources, and improve clinical efficacy. We hypothesize that for the treatment of pancreas SBRT using ART, adapting a generic pre-plan to the anatomy of the day results in dosimetrically equivalent plan quality of adapting a patient-specific initial plan to the anatomy of the day. Thirty-eight (n = 38) patients were treated for pancreatic cancer with MR-guided online adaptive radiotherapy (MRgoART) to 50 Gy/5 fractions (fx). An initial plan was developed over 8 days. For on-table adaptation, the initial plan was adapted to the anatomy of the day for each fx and delivered. Retrospectively, we developed a single template pre-plan technique based on optimization objectives from simulation anatomy across 38 patients. To assess feasibility of the pre-plan technique, we adapted the template pre-plan to the on-table fx 3 anatomy of the day (ADAPTtemplate-to-fx3) and compared the plan quality to the patient-specific initial plan adapted to fx 3 anatomy of the day (ADAPTsim-to-fx3) for all patients. Adapted plans were single-optimized and then normalized to greatest violated OAR. RTOG plan quality metrics were evaluated for target coverage (TC) (PTV V100%/PTV vol), homogeneity index (HI) (PTV D2%/ D98%), high dose conformity (PITV), low dose conformity (D2cm), and gradient (R50%). PTV coverage of V100%, D80% were evaluated. Seventy-six (n = 76) adaptions were performed for ADAPTtemplate-to-fx3 and ADAPTsim-to-fx3. Due to plan-normalization, no OARs were violated. PTV coverage was improved for respective ADAPTtemplate-to-fx3 compared to ADAPTsim-to-fx3 for 58% of cases for D80% (45.8±5.9 vs 45.3±9.1, p = 0.8), 61% of cases for TC (0.66±0.30 vs 0.55±0.30, p = 0.1), and 66% of cases for V100% (63±27% vs 55±32% p = 0.2). Homogeneity and conformity indices between respective ADAPTtemplate-to-fx3 and ADAPTsim-to-fx3 were 0.72±0.39 vs 0.60±0.41, p = 0.2 (PITV), 1.94±0.71 vs 1.52±0.22, p<0.001 (HI), 3.14±1.45 vs 3.52±1.80, p = 0.3 (R50%), and 0.61±0.12 vs 0.58±0.22, p = 0.5 (D2cm). For online adaptation in pancreas SBRT, 66% of the single-optimized template plan had improved coverage (PTV V100) with no OAR violation compared to the patient-specific single-optimized initial plan to the anatomy of the day. Given that the plan quality was not significantly different, this works supports AI-plan automation for patients receiving online ART, potentially eliminating the need for initial plan creation and even simulation. Future investigation of the feasibility and efficacy will be launched in a Phase I/II randomized clinical trial between template plan and initial plan for MRgoART.