The Use of DRR Designed Fields Is Unnecessarily Toxic and Geographically Inadequate, Analysis of Drr Designed Fields in Preoperative Radiotherapy in Rectal Cancer

Abstract

Background:The DRR images and skeletal landmarks are widely used to design treatment portals in preoperative treatment of cancer rectum followed by assessment of isodose lines evaluation, however this practice lacks real volumetric assessment of delivered dose to target volumes and risk organs. Aim:To assess the dose delivered to target volumes and normal organs with and without contouring.Methods: 10 cases of treated cancer rectum CTs were selected, previously treated by conventional technique using DRR and skeletal landmarks, same cases were contoured using RTOG atlas to create CTV 45 Gy (Rectum + tumor, mesorectum and LN (presacral, internal iliacs and obturator LN) and then 1 cm was added for final PTV design, urinary bladder as well as bowel bag were also contoured. Two volumes were designed and named PTV contouring and PTV DRR (opposite to DRR isodosline 95%).Results: PTV DRR was nearly double PTV contouring with mean volumes 2886 cm3 and 1405 cm3 respectively, p value of 0.005. Coverage was not optimum with mean volume received therapeutic dose 93.5 %. Doses to urinary bladder and bowel was similar to others conventional 2D and may be little bit higher with mean results for urinary bladder V20 100%,30 89%, V40 89%, and bowel V20 51%, V30 32.5, V40 26%. Conclusion:the use of DRR designed fields, guided by skeletal landmarks, results in over irradiation of normal pelvic structures, and may miss the treatment volume especially for low rectal tumours, visual assessment of isodose lines on non-contoured cuts is significantly misleading.