Identifying predictors of on-table adaptation for pancreas stereotactic body radiotherapy (SBRT).

Abstract

702 Background: There is growing interest in the use of stereotactic body radiotherapy (SBRT) to treat pancreatic cancer. The purpose of this study was to identify any clinical or dosimetric parameters that predict which individuals may benefit from on-table adaptation during pancreas SBRT with MRI-guided radiotherapy. Methods: This was a single center retrospective study of patients undergoing MRI-guided SBRT to the pancreas from 2016 to 2022. An online daily adaptive workflow was in place for all fractions according to pre-specified criteria. Pre-treatment clinical variables and dosimetric parameters on the patient’s simulation scan were recorded for each SBRT course, and their ability to predict for on-table adaptation was analyzed using ordinal logistic regression. The outcome measure was number of fractions adapted. Results: Seventy-three SBRT courses consisting of 365 fractions were included in this study. Median prescription dose was 40 Gy in five fractions (range, 33-50 Gy); 56% and 44% of courses were prescribed ≤ 40 Gy and > 40 Gy, respectively. Median gross and planning target volume (GTV and PTV) coverage were 94.7% and 84.3%, respectively. Median number of fractions adapted per course was three, with overall 58% (211 out of 365) total fractions adapted. On univariable analysis, the prescription dose ( > 40 Gy vs ≤ 40 Gy), GTV size, stomach V20 and V25, duodenum V20 and Dmax, large bowel V33 and V35, heterogeneity index, and gradient index were significant determinants of the use of on-table adaptation (all p < 0.05). On multivariable analysis, only prescription dose remained significant (adjusted odds ratio 7.66, p = 0.003). Conclusions: Prescription dose in MRI-guided pancreas SBRT is the only reliable predictor of on-table adaptation. Providers should strongly consider utilization of adaptive technology when delivering ablative doses to the pancreas.[Table: see text]