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preliminary mrgrt SABR

Dose accumulation was feasible for toxicity assessment in MR-guided abdominal SABR

Accumulated gastrointestinal doses were generally lower than planned, while higher small-bowel dose showed exploratory associations with diarrhoea after MR-guided SABR.

RadOncToday · ·1 min read

KEY POINTS

  • This retrospective single-institution study included 41 consecutive patients treated on an Elekta Unity MR-Linac with 5-fraction SABR between 2022 and 2025. Targets were pancreatic in 30 patients and hepatic in 11, with a median prescribed dose of 40 Gy and a range of 30-60 Gy in 5 alternate-day fractions.
  • Daily treatment used an adapt-to-shape workflow, with organs at risk re-contoured and each adapted fraction dose deformably mapped back to the planning MRI using structure-specific deformable registration before summation.
  • Accumulated gastrointestinal doses were usually lower than reference-plan doses. Median large-bowel D0.5cc decreased from 19.2 to 16.9 Gy, small-bowel D0.5cc from 22.5 to 18.5 Gy, and duodenal D0.5cc from 32.2 to 30.0 Gy, all p < 0.001.
  • Similar reductions were seen for larger-volume bowel metrics. Large-bowel D10cc decreased from 16.0 to 14.5 Gy and duodenal D10cc from 20.3 to 19.4 Gy, both p < 0.001; small-bowel D10cc decreased from 15.4 to 14.7 Gy, p = 0.017.
  • Target dose was broadly maintained. Accumulated GTV D95 was 33.9 Gy versus 35.2 Gy in the reference plan, with no significant difference (p = 0.471), while accumulated GTV minimum dose was slightly higher.
  • Higher accumulated small-bowel dose correlated with worsening diarrhoea. At 3 months, D0.5cc, D5cc and D10cc each showed rₛ = 0.4, p = 0.040, 0.029 and 0.029, respectively; at 6 months, correlations persisted at rₛ = 0.4-0.5, p = 0.014-0.019.
  • Treatment was well tolerated, with no acute or subacute post-treatment toxicity above grade 2. However, the dose-toxicity analyses were univariable, follow-up was limited to 6 months, and deformable dose accumulation was not validated using voxel-level landmarks.

CLINICAL TAKEAWAY

DIR-based accumulated dose can be linked to prospective toxicity data after MR-guided abdominal SABR, offering a potential route toward delivered-dose rather than planned-dose toxicity modelling. The small-bowel diarrhoea signal is interesting but hypothesis-generating, and the study does not establish new bowel constraints or justify treatment adaptation based on accumulated dose.

SOURCE

Clinical and Translational Radiation Oncology