1132 - CT-Guided Online Adaptive Stereotactic Body Radiotherapy for Pancreas Ductal Adenocarcinoma: Early Outcomes and Toxicity Data

Purpose/Objective(s): Dose escalation to biologically effective doses (BED) of 70 Gy or more may improve local control and overall survival in patients with pancreatic ductal adenocarcinoma (PDAC). We have previously demonstrated that daily CT-guided online adaptive radiotherapy (OART) provided superior coverage and decreased dose to organs at risk (OARs). We now presents our data regarding toxicity, local control (LC), and overall survival (OS).

Materials/Methods: This is a single institution retrospective analysis of patients with PDAC treated with OART on the Ethos platform (Varian Medical Systems, a technology company) at a single institution. All patients were treated to 40 Gy in 5 fraction (BED = 72 Gy). CTCAEv5.0 toxicities were recorded. PTV_4000 V95%, PTV_2500 V95%, and D0.03 cc to stomach, duodenum and bowel were reported for scheduled (S) and adapted (A) plans. Toxicities within 3 months of start of RT were considered acute, and toxicities after 3 months of RT completion were considered late. OS and LC at one year from RT were estimated with the Kaplan-Meier method. Statistical analysis was performed using a two-sided T test and a of 0.05.

Results: Forty patients with PDAC treated from October 2022 – May 2024 were included in the analysis. A majority of patients were locally advanced (n=26, 65%), but patients with recurrent disease (n=10, 25%) and stable oligometastatic disease (n=4, 10%). 35 (87.5%) patients received chemotherapy prior to RT, 11 (27.5%) of patients went on to surgery, and 20 (50%) received adjuvant chemotherapy. Median follow from OART was 12 months. The adapted plan was selected for 99% of treatments. Adapted plans improved PTV_4000 and PTV_2500 coverage as well as decreasing dose to the duodenum and small bowel. Table 1. Treatment was well tolerated with only 6 patients experiencing acute grade 3+ toxicities and only 1 directly attributable to OART. Six patients developed late grade 3+ toxicities with only 1 directly attributable to OART. Median LC from OART was 24.7 months and 1 year LC from OART was 77.9% (95%CI: 64.5%-94.1%). Median OS from OART was 16.9 months and 1 year OS from OART was 90.1% (95%CI: 0.8-1).

Conclusion: This data suggests that the dosimetric improvements seen with CT-based OART allows for delivery of high BED with minimal toxicity. Our initial experience demonstrated favorable 1 year LC and OS with CT-based OART. Prospective study of CT-based OART warrants further investigation for PDAC.