The role of image-guided radiotherapy in the treatment of anorectal cancer using prone belly-board positioning

Aim: To evaluate Radiation Therapy Oncology Group planning target volume margins of 7-10 mm for radiation therapy in anorectal cancer using prone bellyboard positioning without image guidance. Patients and Methods: 375 kV cone beam computed tomography imageguided radiotherapy (IGRT) images from 20 patients treated for anorectal cancer were retrospectively analyzed for setup shifts. We calculated the total translational shift for each patient and the frequency with which setup shifts exceeded 7 mm and 10 mm. Results: A total of 42.7% of treatments required shifts >7 mm and 20.8% >10 mm. The mean translational shift was 7.1 mm. 70% of patients experienced shifts 7 mm in 20% or more of their treatments and 25% of 10 mm in 20% or more of their treatments; 15% experienced shifts 10 mm in over half of their treatments. van Herk calculations suggest margins of 12.8 mm are necessary for accuracy without IGRT. Conclusion: IGRT using a prone belly board and 7-10 mm margins requires daily image-guidance to prevent planning target volume misses and ensure optimal dose delivery.