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Impact of two immobilization systems on geometry, dosimetry, and treatment-related toxicities in rectal radiotherapy

Objectives This study aimed to compare the prone position on a belly board device (BBD) immobilization device versus a vacuum bag cushion (VBC) in rectal cancer patients receiving radiotherapy. Background In radiotherapy, it is ideal to adapt to the most favorable, least time-consuming and most cost-effective technique for patient positioning and immobilization with maximum reproducibility. Patients and methods Fifty-five rectal cancer patients scheduled for irradiation were equally randomized to be immobilized with one of the studied immobilization systems and scanned in the prone position. The dose–volume histograms for organs at risk were compared for both immobilization methods and setup accuracy was analyzed by an electronic portal imaging device; treatment-related toxicity was reported. Results There was no significant difference between the studied groups in the volume of planning target volume receiving 95% of the dose (P = 0.42). Volume of the small bowel receiving 45 Gy was higher in the VBC arm than the BBD arm (P < 0.001), but there was no significant difference between the studied groups in the incidence of acute small-bowel toxicities of diarrhea and abdominal pain (P = 0.94 and 0.66, respectively), onset and severity. Setup errors were significantly higher in the belly board arm in the X, Y and Z directions (P = 0.004, P < 0.001, and P < 0.001, respectively). Magnitude was significantly higher in the belly board arm (P < 0.001). Planning target volume margins according to Van-Herck's equation and Stroom's equation in the X, Y and Z directions were larger in the belly board arm, compared to the VBC arm. Conclusion The VBC system was more reproducible than BBD, with comparable acute treatment-related toxicities.