LAUSR

e17590 Background: Papillary thyroid carcinoma (PTC) with hobnail morphology (HM) is an aggressive variant associated with poorer outcomes compared to classical PTC, with a little over 100 cases reported in the literature. We aim to identify prognostic factors potentially associated with recurrence and death in the largest single institutional experience of PTC with HM. Methods: From an IRB approved registry, we identified all PTC HM cases with patient, diagnostic and treatment data and outcomes. Hobnail variant (HV) PTC had ≥30% HM; pure hobnail features (PHF) had < 30% HM without tall cell morphology (TCM) & multi variant features (MVF) had both HM & TCM. Demographic and clinical features at time of HM diagnosis that were potentially associated with 3-yr event free (death or recurrence) survival (EFS) were evaluated using Kaplan-Meir method (KM). Results: Forty-five (median age 55 yo, range 19-86 yo; 66.7% female) HM pts (35 HM at initial PTC, 10 at recurrence) were evaluable. Majority were ECOG 0 (84.1%). HM: 44.4% MVF, 37.8% PHF, & 17.8% HV; 68.9% pT3/T4; 64.4% node positive; & 15.6% metastatic. Positive surgical margins (+SM) with HM in 31.8%. RAI at HM given to 73%. Overall EFS was 83.5% at 1-yr & 69.6% at 2-yr. Of 13 events at 3 yrs, 4 were deaths (3 from PTC) & 9 recurrences. All events were in those > 55 yo (p < 0.001) with pT3/T4 disease (p = 0.01). Non-0 ECOG (KM 2-yr EFS: 28.6% vs 77.7%, p = 0.002), cT3/T4 (49.5% vs 85.5%p = 0.004), and +SM (32.1% vs 92.1%, p = 0.001) were also associated with EFS compared to their counterpart. HM grouping, RAI, initial HM vs HM at recurrence, sex, and tumor size were not statistically significant. Conclusions: To our knowledge, this is the largest reported cohort of PTC with HM. We identified several factors potentially associated with recurrence and death. There was no difference in outcomes between HV versus PHF versus MVF, which raises question to validity of 30% cutoff in defining “variant” versus “features of variant”. Age > 55 yo, T3/T4 disease, and +SM exhibited significantly inferior EFS. Further evaluation in a multi-institutional cohort will help in validating these findings and help identify patients who warrant more aggressive initial treatment.