ESTRO 2024 - Abstract Book

S1463

Clinical - Lower GI

ESTRO 2024

80

Digital Poster

Overview of tumour sterilization rates in rectal cancers among treatment modalities; is more better?

Ian J. Gerard 1 , Kim Ma 2 , Carol-Ann Vasilevsky 3 , Marylise Boutros 4 , Shirin Enger 5 , Slobodan Devic 5 , Té Vuong 6

1 McGill University Health Centre, Radiation Oncology, Montreal, Canada. 2 Jewish General Hospital, Medical Oncology, Montreal, Canada. 3 Jewish General Hospital, Colorectal Surgery, Montreal, Canada. 4 Cleveland Clinic Florida, Colorectal Surgery, Parkland, USA. 5 Jewish General Hospital, Medical Physics, Montreal, Canada. 6 Jewish General Hospital, Radiation Oncology, Montreal, Canada

Purpose/Objective:

Optimal treatment strategies for rectal cancers that minimize toxicity and maximize clinical/pathological complete response (cCR/pCR) is a topic of active research. With growing interest in organ preservation and non-operative management, and many treatment options with similar outcomes, shared decision making with patients on which treatment to pursue continues to evolve. For most patients, the avoidance of a permanent ostomy remains a very important outcome and dedicated up-front strategies are needed to increase the proportion of patients who can avoid these interventions while maximizing their chance at disease control. Previous work by Appelt et al. [1] suggests a radiation dose-response relationship with cCR. In this work we perform an exploratory analysis on published prospective studies using different treatment regimens to evaluate the relationships between sterilization rates (pCR or cCR) and radiation dose, treatment times, and rates of grade 3+ adverse events.

Material/Methods:

We searched PUBMED from inception until August 2023 for studies reporting on cCR or pCR using the search string:

"rectal cancer" (or variations) AND ("radiotherapy" or "chemoradiotherapy" or "chemotherapy") (or variations)

Papers included had to be full text English language with information on chemotherapy regimens, timing, information available for estimation of tumour dose in EQD2 a/b=10 Gy , cCR and/or pCR rates, and rates of grade 3+ adverse events. 611 papers were screened, and 21 papers met criteria for inclusion in the analysis. Using the cohorts from each included paper, we estimated the relationship between EQD2 and sterilization rates at TME (pCR) or after treatment completion (cCR) using bounded logistic regression weighted on cohort size. The dose resulting in 50% response (D50), and normalized dose-response gradient at D50 (γ50) were also calculated. The rates of grade 3+ adverse events for each arm of each study and the correlation between total treatment time and sterilization rates was also evaluated using linear regression and evaluated using the coefficient of determination.

Results:

21 non-overlapping cohorts were identified meeting all inclusion criteria for analysis. Treatment strategies varied significantly and included chemotherapy (CT) alone, chemoradiotherapy (CRT) alone, total neoadjuvant therapy (TNT), brachytherapy alone (BT), CT and BT, and CRT and BT. EQD2 to primary tumours varied from 0Gy to > 160 Gy. A dose-response relationship between the EQD2 and pCR at total mesorectal excision (Figure 1) and cCR after