Improved PTV coverage and OAR sparing with stereotactic MRI-guided online adaptive radiotherapy with elective fields...

Ablative stereotactic radiotherapy for pancreatic ductal adenocarcinoma (PDAC) is limited by adjacent radiosensitive organs at risk (OAR) that exhibit significant daily positional variability. This study aims to investigate the benefit of stereotactic MRI-guided online adaptive radiotherapy (SMART) with elective fields on target coverage and OAR dose sparing in patients with PDAC.

We retrospectively analysed 62 consecutive PDAC patients treated with SMART between 2020-2024, totaling 310 fractions. For each fraction, a non-adapted plan (PLAN_PREDICT) was compared to its online-adapted counterpart (PLAN_REOPTIMIZED). We assessed planning target volume (PTV) coverage and OAR constraints. Plan-adaptation was classified as "useful" when the PLAN_REOPTIMIZED met all objectives or improved PTV coverage or OAR sparing by ≥ 10 %, as "futile" when PLAN_REOPTIMIZED failed to meet or markedly improve objectives, and as "not necessary" when PLAN_PREDICT already met all objectives.

Prescribed doses were 40 Gy (71 %) and 33 Gy (29 %) over five fractions. A simultaneous integrated boost approach was used in 47 % of patients. The PLAN_PREDICT met all planning goals in 1 % of fractions, which increased to 72 % with PLAN_REOPTIMIZED. Adaptation yielded significant improvements in PTV coverage (p < 0.001) and reductions in V33Gy for stomach (median 0.26 cc vs. 0.01 cc), duodenum (median 1.08 cc vs. 0.07 cc) and bowel (median 2.04 cc vs. 0.06 cc), all p < 0.001. Adaptation was deemed useful in 305 (98 %), not necessary in 4 (1 %) and futile in 1 fraction (0.3 %).

For the vast majority of PDAC patients treated with stereotactic radiotherapy with elective fields daily online adaptation enhances target coverage and OAR sparing.