18F-FDG PET/CT-based treatment response evaluation in locally advanced rectal cancer: a prospective validation of long-term outcomes

To prospectively evaluate the usefulness of F-18-FDG PET/CT) imaging for predicting histopathological response and long-term clinical outcomes in locally advanced rectal cancer (LARC). This prospective study included 38 patients with a confirmed diagnosis of LARC (cT3-4 or cN+) who underwent F-18-FDG PET/CT before and after neoadjuvant therapy (NAT). Total mesorectal excision was scheduled 6 weeks after NAT and was followed by an expert histopathological analysis of the surgical specimen. Baseline variables and previously identified maximum FDG standardized uptake value (SUVmax) cut-off values before NAT (SUVmax(PRE) a parts per thousand yen6) and after NAT (SUVmax(POST) a parts per thousand yen2), and the absolute and percentage reductions from baseline SUVmax (a dagger SUVmax < 4 and a dagger SUVmax% < 65 %, respectively) were applied to differentiate patients showing a metabolic tumour response from nonresponders. These features were correlated with tumour regression grade (TRG), disease-free survival (DFS) and overall survival (OS). Significantly higher 5-year DFS and OS were seen in 19 responders (TRG 3 or 4) than in 19 nonresponders (TRG 0-2; 94.4 vs. 48.8 %, p = 0.001; 94.7 vs. 63.2 %, p = 0.02, respectively). In multivariate analysis the only PET/CT SUVmax-based parameter significantly correlated with the likelihood of recurrence and survival was a dagger SUV% < 65 % (HR = 5.95, p = 0.02, for DFS; HR = 5.26, p = 0.04, for OS) This prospective study proved that F-18-FDG PET/CT is a valuable imaging tool for assessing rectal cancer TRG and long-term prognosis, and could potentially serve as an intermediate endpoint in treatment optimization research and rectal cancer patient care.