Early prediction of pathological complete response (pCR) of rectal cancer after 1 week of preoperative radiochemotherapy (RCT) using positron emission computererized tomography (PET-CT) imaging

572 Background: Preoperative radiochemotherapy (RCT) is the standard treatment of locally advanced rectal cancer (LARC), obtaining pathological complete response (pCR) in 15%-30% of cases. Post-RCT reduction of 18F-fluorodeoxyglucose (FDG) uptake within the tumor compared with the baseline, i.e. the tumor's metabolic response, correlates with pCR. However, an earlier prediction of pCR could enable tailored modifications of the treatment. We hence evaluated the correlation between the metabolic response after only one week of RCT for LARC and the actual pCR at the post-RCT surgery. Methods: Patients (pts) were eligible for this prospective study if they had LARC, defined as T3-4NX or TxN+ tumors by pre-treatment PET-CT and endoscopic ultrasound. Pts received standard RCT regimen, consisting of 50.4Gy radiotherapy concurrently with a fluoropyrimidine-based chemotherapy, followed by surgery. Pts underwent baseline FDG-PET-CT imaging within 2 weeks prior to the initiation of RCT and a second one on day 8 of RCT. Maximum standardized uptake value (SUV-max) was measured in both scans and changes in FDG- uptake were recorded. Man-Whitney test was used to evaluate differences in the SUV-max between baseline and day 8 in pts obtaining pCR and those who did not. Results: Twenty pts participated in the study. Half were males and the median age was 64 years. Ten pts had T3N0 tumors and 10 had T3N+ disease. Radical surgery was done in 19 pts and local excision in one. Considering the entire group, there was a borderline-significant difference between the metabolic response of pts with pCR and those without pCR (Chi-square = 3.429, p = 0.064). Yet, the changes in FGD-uptake were able to identify pts who achieved pCR and those who did not: only pts with a decrease of more than 33% in SUV-max had pCR while none of the pts who had less than 8.9% decrease in SUV-max had pCR. Conclusions: A decrease in SUV-max between baseline-PET-CT scans and scans done after only one week of RCT for LARC may be able to predict the achievement of pCR in the post-RCT surgical specimen. Validation in a larger independent cohort is planned. No significant financial relationships to disclose.