Setup and target volume shape variation in rectal cancer radiotherapy: a systematic literature review

INTRODUCTION: This systematic literature review aimed to evaluate treatment uncertainties associated with modern rectal cancer radiotherapy, focusing on systematic and random errors in setup and target volume (TV) shape variation, with the aim of supporting clinics in optimizing treatment accuracy and consistency in daily practice. METHODS: A systematic literature review was conducted to quantify geometrical uncertainties with respect to setup and TV shape variations in the treatment of rectal cancer. It included studies reporting uncertainties for setup, CTV (Clinical Target Volume) shape variations (rectum and mesorectum), GTVp (Gross Tumor Volume - Primary Tumor), GTVn (Gross Tumor Volume - Nodes), and/or elective pelvic lymph nodes; excluding non-English language publications. RESULTS: 196 reports were assessed for full text screening and 32 publications were selected for final reporting. Most authors utilized on board imaging to calculate setup errors and TV shape variation, and all reported substantial uncertainties. Setup uncertainty was reported to be very different for prone (w/o belly board) vs supine position, in favour of supine position. TV shape variation showed large systematic and random errors, especially for GTVp and the upper anterior part of the mesorectum. A subset of publications analyzed the positional uncertainties of elective draining lymph nodes, which are typically not removed during surgery, showing their small positional variations in relation to bone structures. CONCLUSION: Setup and TV shape variation are significant and non-negligible geometric uncertainties. Setup errors can largely be minimized with daily (2D or 3D) IGRT, whereas TV shape variations-particularly in the mesorectum-needs to be assessed daily with 3D imaging and requires anisotropic margins.