697 Background: A major challenge in identifying candidates for nonoperative management of locally advanced rectal cancer is predicting pathological complete response (pCR) following chemoradiation therapy (CRT). We evaluated the ability of pre- and post-CRT PET imaging to predict pCR and long-term prognosis. Methods: We retrospectively identified patients at our institution from 2002–2015 with locally advanced rectal cancer who underwent CRT, pre- and post-CRT PET imaging, and surgical resection. Logistic regression and Kaplan-Meier estimates were used to analyze the association of PET variables with tumor pCR and survival outcomes. Receiver operator characteristic curves were generated to define optimal cutoff points for predictive PET variables. Results: 140 patients matched the inclusion criteria. The pCR rate was 28%, and median follow-up time was 48 months. On multivariable analysis, pCR was associated with lower median post-CRT SUVmax(3.2 vs 5.2, p=0.009) and higher median SUV percent decrease (72 vs 58%, p=0.009). ROC curves were generated for SUV percent decrease (AUC=0.70) and post-CRT SUV (AUC=0.69) to estimate cutoff points for maximum specificity and sensitivity to predict pCR. Post-CRT SUV <4.3 and SUV percent decrease of >66% were equally predictive of pCR with a sensitivity of 65%, specificity of 72%, PPV of 44%, and NPV of 86%. Median 5-year OS and RFS were significantly improved for patients with post-CRT SUV <4.3 (OS, 86 vs 66%, p=0.01; RFS, 75 vs 52%, p=0.01) or SUV percent decrease of >66% (OS, 82 vs 66%, p=0.05; RFS, 75 vs 54%, p=0.01). Patients with stage III disease and a post-CRT SUV <4.3 were in effect downstaged, with a median 5-year OS equivalent to that of patients with stage II disease (Table 1; 86 vs 86%). Conclusions: PET/CT may be a useful modality in predicting pCR and overall survival in patients undergoing CRT for rectal cancer. Inclusion of pre-CRT PET does not appear to add prognostic value for pCR compared with post-CRT PET alone. Patients with a post-CRT SUV of >4.3 should not be considered for nonoperative management, as an estimated 86% of these patients will not have a pCR.
Nonoperative management of rectal cancer
