Reduction of Tumor Length by >50%, Post Neoadjuvant Chemoradiation as a Predictor for Complete Response and Organ Preservation in Rectal Cancer

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<h3>Purpose/Objective(s)</h3> 20-30% of the locally advanced rectal cancer patients undergoing neoadjuvant chemoradiation (NACRT) have complete response (CR) and are suitable for wait and watch approach. This tumor response can be enhanced by radiotherapy dose escalation such as brachytherapy which is typically given at the end of NACRT. However, there is no data so as to which patients are likely to benefit by brachytherapy to achieve CR. We aimed to identify decrease in tumor length following CTRT, as predictor of CR, and eventual organ preservation. <h3>Materials/Methods</h3> Patients diagnosed with T2-T4, N0-N2 distal rectal cancer treated from December 2017 to March 2021 were retrospectively reviewed. All were treated with radiation dose of 50 Gray (Gy) in 25 fractions (#) with concurrent capecitabine. All patients received escalated dose of radiation by high dose rate endorectal brachytherapy of 8-12 Gy in 2-3 #s within 2 to 3 weeks post CTRT. Baseline tumor length (MRI) and residual tumor length after CTRT during brachytherapy (assessed with planning MRI) were compared for response. Statistical analysis was done to determine decrease in tumor length as predictor of CR. <h3>Results</h3> One hundred patients records were reviewed. Disease and treatment characteristics are provided in table 1. Median length of tumor at baseline was 4.9 cm (3.8-6 cm). 39 patients had >50% decrement in tumor length (median length 3 cm) from baseline post CTRT, whereas 61 patients had <50% response. Out of 39 patients with>50% response, 26 patients achieved CR (66.7%) while 13 patients had partial or no response (33.3%), while out of 61 patients with <50% decrease in tumor length, 27 achieved CR (44.2%) and 34 had partial or no response (55.8%). The patients who had >50% reduction in tumor length had higher probability of achieving CR than the patients with <50% reduction (66% vs 44% p=0.029). <h3>Conclusion</h3> Response assessment at the end of CTRT with respect to decrease in length of tumor may predict CR and thus can lead to better selection of patients for escalated radiation dose via endorectal brachytherapy leading to higher rate of organ preservation.