Response Assessment with 18F-FDG PET/CT Scan in Patients with Anal Cancer Treated with Radical Radiotherapy

Abstract

Functional imaging like positron emission tomography (PET) in combination with computed tomography (CT) has been found to be an important tool in post-treatment response assessment in several malignancies. The objective of this study was to identify the utility of early post-treatment response assessment with 18F-FDG PET/CT scan in patients with squamous cell carcinoma (SCC) of anal canal primarily treated with radical radiotherapy. We reviewed the provincial population database for anal cancer patients who were treated with radical radiotherapy (2005-2015) and subsequently underwent post-treatment 18F-FDG PET/CT scans (majority within 12-17 weeks after treatment completion) for response assessment. Information related to patient and tumor characteristics, treatment regimen were collected. Disease free survival (DFS) and cancer specific survival (CSS) were estimated using Kaplan-Meier product limit method. Cox multivariable proportional hazard model was used to determine the association of post-treatment maximum standardized uptake value (SUVmax) as a continuous variable with DFS and CSS after adjustment for age at diagnosis, stage, radiotherapy dose, use of concomitant chemotherapy, use of salvage surgery, sex and performance status. A total of 287 patients were treated with radical radiotherapy over the time period of the study and 56 had post-treatment response assessment with PET/CT. Median age of the study cohort was 60.5 years (IQR, 53-66 years). Median radiotherapy dose was 54 Gy (IQR, 53.6-54 Gy). Among a total of 56 patients, concurrent chemotherapy was offered in 53 (95%) patients. Mean post-treatment SUVmax for the primary tumor and node were 2.6(±3.9) and 4.3(±6.7). On multivariable analysis (MVA), post-treatment SUVmax for the primary tumor had a significant association with CSS (hazard ratio (HR): 1.39, 95% confidence interval (CI): 1.13-1.70, p=0.002). The HR for post-treatment nodal SUVmax was 1.09 (95% CI: 0.88-1.36, p=0.4). The association of post-treatment SUVmax of tumor (HR: 1.01, 95% CI: 0.86-1.18, p=0.88) or node (HR: 1.10, 95% CI: 0.92-1.31, p=0.31) with DFS was not statistically significant. The current population-based study shows significant correlation of presence of FDG avid residual tumor with cancer specific survival. Every 1 unit increase in post-treatment SUVmax of the primary tumor was associated with 39% increase in the relative risk of death. Although post-treatment residual FDG avid node showed similar direction of association, the overall association was not statistically significant which could be attributed to the limited sample size. The study alludes to the importance of post-treatment PET based response assessment in patients with anal SCC and early institution of salvage treatment in presence of FDG avid residual tumor to avoid potentially fatal consequences. Further prospective validation with randomized clinical trial is worthwhile to prove this hypothesis.