Potential dosimetric benefit of an adaptive plan selection strategy for short-course radiotherapy in rectal cancer patients

An adaptive plan selection strategy can account for daily target volume variations for radiotherapy in rectal cancer patients. The aim was to quantify the daily dosimetric consequences of plan selection compared to a non-adaptive approach. Ten patients with rectal cancer, treated with 25Gy in five fractions to the mesorectum and pelvic lymph nodes, were selected. The adaptive strategy was simulated by creating three plans per patient, with varying upper ventral PTV margins, and selecting the smallest PTV covering the entire mesorectum on every daily CBCT scan. Subsequently, mesorectum, bladder, and bowel cavity were delineated on these scans. Daily dose-volume histograms were calculated for both the adaptive and non-adaptive plan, with a ventral PTV margin of 20mm. Coverage of the mesorectum, defined as V95%>99%, was calculated, as well as bladder and bowel cavity V95% and V15Gy. In one patient, mesorectum coverage improved. A reduction in bladder V95% and bowel cavity V15Gy was found, of 6.9% and 18.4cm(3) (p <0.01), respectively. Plan selection for radiotherapy in rectal cancer can improve coverage of the target volume. Overall dosimetric sparing of bladder and bowel cavity was limited but could be beneficial for individual patients