Background: Patients with ER after surgery and prior to postoperative radiation (RT) for SCC of the OC have aggressive biology and poor prognosis. After the introduction of a PET/CT simulator in our department, we incorporated post-operative PET/CT as part of RT planning. We hypothesized PET/CT would improve detection of macroscopic disease before postoperative RT. Methods: We reviewed the medical records of patients treated with postoperative radiotherapy between 2005 and 2019 for OC SCC. Clinicopathologic risk factors were recorded. Intermediate risk factors (IRFs) included pT3-4 disease, nodal disease, perineural invasion (PNI), lymphovascular invasion (LVI), and close ( < 5mm) surgical margins (SM); extranodal extension (ENE) and positive SM were considered high-risk factors (HRF). Patients were stratified into risk groups based upon the number and type of risk factors: 0-1 IRFs, 2 IRFs, ≥3 IRFs, and any HRF. Patients were considered to have ER if they had biopsy confirmed recurrence, or if the imaging or exam was sufficiently suspicious, after discussion with the head and neck team, to warrant treatment to definitive doses of RT (70 Gy). Results: Our cohort included 391 patients with SCC of the OCC who were treated with postoperative radiotherapy. 61% of patients were male, 35% had pT3-4 disease, 36% had pN2a-3 disease, 53% had PNI, 20% had LVI, 30% had ENE, and 14% had positive SM. The most common sites were oral tongue (46%), alveolar ridge (18%), and buccal mucosa (13%). 237 (61%) patients underwent postoperative PET/CT planning, and 165 patients (41%) were planned with CT only. Patients screened with post-operative PET/CT were more likely to be diagnosed with ER (46/237, 19.4%) than those simulated with CT only (6/154, 3.9%, p < 0.0001). Among patients simulated with PET/CT, 7%, 9%, 14%, and 35% of patients were diagnosed with ER for patients with 0-1 IRFs, 2 IRFs, ≥3 IRFs, and any HRF, respectively. Median follow-up was 4.1 years (95% CI 3.6 – 4.5). Among 52 patients with ER, 24 (49.0%) had local, 41 (83.7%) had regional, and 5 (10.2%) had distant recurrence. 17 (33%) of ER were biopsy proven. For patients with ER, 3-year freedom from locoregional recurrence, distant-metastasis free survival, and overall survival were 45.2% (95% CI 32% - 64%), 55% (95% CI 42% – 72%), and 43% (95% CI 30% - 61%), respectively. For patients without ER, use of postoperative PET/CT was associated with improved disease-free survival (HR 0.68, 95% CI 0.46 – 0.98, p = 0.041) and overall survival (HR 0.59, 95% CI 0.38 – 0.91, p = 0.019). Conclusions: Postoperative PET/CT may increase detection ER compared to CT simulation alone and improve risk stratification. Patients with ER are at high risk of locoregional failure, distant metastases, and mortality, despite salvage therapy. A prospective trial is underway at our institution to systemically study the role of PET/CT for detection of ER.