Short-course preoperative radiotherapy increases pelvic fracture risk in rectal cancer

Introduction: Pelvic insufficiency fractures (PIFs) are adverse events associated with chemoradiotherapy (CRT) administered preoperatively in rectal cancer, with incidences of 0-33.6% reported in the literature. Data on PIFs after 5 x 5 Gy fractionated short-course radiotherapy (SCRT) using highly conformal radiotherapy techniques such as volumetric modulated arc therapy (VMAT) is limited.Methods: The Turku University Hospital colorectal cancer database was searched for patients operated on for stage I-III rectal cancer during the years 2014-2018. The hospital's routine follow-up includes a 2-year computed tomography (CT) scan, which was systemically re-evaluated to detect PIFs. Only radiotherapy delivered using VMAT and image-guided approaches was included. Baseline demographics, tumor data, and dose-volume data were collected to identify risk factors for PIFs.Results: Median time to CT scan was 24 months. Among the 164 patients analyzed, the 2-year PIF incidence was 22.2% for SCRT (n = 12/54, OR 9.1 (CI95% 1.9-42.9), p = 0.004), 9.1% for CRT (n = 4/44, OR 3.2 (CI95% 0.6-18.3), p = 0.13) and 3.0% (n = 2/66, reference) for those operated on without radiotherapy. The PIF incidence was not explained by differences in dose-volume data in either the SCRT or CRT groups. Fracture risk was higher in women, up to 50% after SCRT. Conclusions: Every fifth patient treated with SCRT and rectal surgery presented with a PIF. Critical bony structures to be avoided during radiotherapy contouring could not be identified. Clinicians, especially those involved with the follow-up of rectal cancer, should be aware of this potentially debilitating and surprisingly common adverse event.