Do Rectal Proton Therapy Plans Require Adaptation for Rectal Gas Variability?
Abstract
Purpose
Rectal gas variability introduces density heterogeneities that may compromise dose robustness in rectal proton therapy and affect bladder and bowel dosimetry. This study evaluates the impact of rectal gas variability on target coverage and critical organ-at-risk (OAR) dose in robustly optimized pencil beam scanning (PBS) plans.
Methods
Ten rectal cancer patients with a single planning CT were retrospectively analyzed. Initial treatment plans were prescribed to 4500 cGy in 25 fractions (180 cGy per fraction) to the clinical target volume (CTV) using intensity-modulated proton therapy (IMPT) with two posterior-oblique beams and robust multifield optimization incorporating setup and range uncertainty. In the absence of longitudinal CBCT data, interfraction anatomical variability was simulated by generating ten synthetic CT scenarios per patient through deformable mapping of rectal gas, small bowel gas and bladder filling anatomies across the cohort while constraining bony anatomy. Dosimetric indicators were used to characterize target coverage, dose homogeneity, and small bowel and bladder dose behavior.
Results
Across the ten-patient cohort, average CTV_4500 D98% decreased from 4435.8 cGy under nominal conditions to 4429.5 cGy (−0.14%) and 4348.0 cGy (−1.98%) with increasing rectal gas extent, while CTV_4500 D2% increased from 4727.8 cGy to 5263.5 cGy (+11.33%), indicating reduced dose homogeneity. Small bowel dosimetry demonstrated marked variability and redistribution, with D5% ranging from 2016–3388 cGy, V1500 cGy ranging from 97–209 cc, and high-dose regions (D0.03 cc) spanning 4665–5256 cGy. Bladder intermediate-dose metrics also varied widely (D50%: 250–3590 cGy; D35%: 3033–4049 cGy), whereas high-dose bladder regions remained relatively stable.
Conclusion
While robust IMPT planning mitigates modest rectal gas–induced perturbations, interfraction rectal gas variability can degrade target dose homogeneity and drive substantial redistribution of small bowel and bladder dose. These findings highlight limitations of static robustness assumptions and support gas-aware evaluation and selective adaptive strategies in rectal proton therapy.