The role of sequential PET/CT imaging to predict complete response after definitive chemoradiotherapy for locally advanced squamous cell carcinoma of the anal canal.

12 Background: The standard treatment for locally advanced squamous cell carcinoma of the anal canal (SCCAC) is definitive radiotherapy (RT) with concurrent chemotherapy (CRT), with excellent outcomes. Nevertheless, rates of locoregional failure of 10% to 30% have been reported. Due to the narrow therapeutic window between time-dependent response to RT and prompt salvage surgery, the definition of a surveillance strategy after radical CRT is challenging. In particular the contribution of 18-FDG PET/CT is not clearly established. The purpose of this study was to evaluate the effectiveness of follow-up MR and 18-FDG PET/CT to predict response to CRT in patients with SCCAC. Methods: We analyzed a cohort of consecutive patients who received definitive RT/CRT from January 2019 to February 2024. After completion of therapy, radiologic (rCR) and metabolic complete response (mCR) was assessed with pelvic MR (RECIST v.1.0) and 18-FDG PET/CT at 3 ad 6 months respectively. Local Control (LC) was defined as time from the end of CRT to local failure or last follow-up. Univariate analysis (UVA) was performed to identify variables associated with outcome using the log rank test. Sensitivity and Specificity analysis was performed to assess the correlation between rCR/mCR and LC. Results: A total of 67 patients (median age 67 years, range 44-83) were included. All patients completed RT, and 52 received concurrent CRT. Baseline and surveillance MR and 18-FDG PET/CT were available for 49 patients. After a median follow-up of 28 months, 12 patients experienced disease progression (PD) (10 local; 2 distant), resulting to salvage surgery in 10 cases after biopsy. At 3 years, LC rate was 78%. At UVA, only mCR was correlated with LC (3-year LC 90% versus 59%, p=0,0137; HR 0,01660, IC 95% 0,03-0,84). For prediction of LC, accuracy of mCR at 6 months was 83.7% versus 62.5% for rCR. Specificity/Sensitivity analysis is summarized in the table. Conclusions: 18FDG PET/CT at 6 months may identify patients with a higher likelihood of developing complete tumor regression following CRT. Further data is needed to confirm the appropriate surveillance strategy for patient undergoing CRT in clinical practice. Specificity and sensitivity analysis. Sensitivity Specificity Positive Predictive Value Negative Predictive Value mCR 83.3% 84.0% 92.6% 68.7% rCR 25.0% 70.0% 14.0% 82%