Association between the primary site of disease and the stage at diagnosis for merkel cell carcinoma: A population-based analysis

e21600 Background: Merkel cell carcinoma [MCC] is a rare, aggressive skin cancer with a high rate of recurrence, metastasis, and poor prognosis. It commonly involves the head and neck (HN) region (41% of diagnosed cases), followed by the upper and lower extremities and trunk in decreasing frequency. We used the Survival, Epidemiology, and End Results [SEER] November 2021 database to assess the incidence, cause-specific survival, and the association between the primary site of disease and the stage at diagnosis for primary MCC. Methods: Using the SEER-November 2021 (17 registry) database, a retrospective analysis was performed on the 4120 cases of pathologically confirmed MCC (ICD-O-3 code 8247), diagnosed between 2004-2015. Baseline characteristics were assessed using the Chi-squared test and the Kruskal-Wallis test. Multivariate multinomial logistic regression was performed to evaluate the association, with alpha at 0.05. Analysis was performed using SEERStat and SAS 9.4. Results: The overall age-adjusted incidence rate (IR) of MCC was 0.4 cases per 100,000 with an observed 5-year survival [OS] of 47.8% and cause-specific 5-year survival [CSS] of 68.7%. The median age at diagnosis was 79 years (interquartile range - 66,83) with a male preponderance (60.4%). At baseline, males had higher odds of being diagnosed at stages III or IV (p < 0.001). The odds of being diagnosed at stages II, III, or IV were significantly higher in patients with the trunk as the primary site compared to the HN region with an OR 3.8 (p < 0.001), 2.4 (p < 0.001), and 2.2 (p = 0.001), respectively. For patients with the upper limb as the primary site, the odds of being diagnosed at stage II were 1.4 times higher than the HN region (p = 0.0095), and stage IV was 0.4 times (p = 0.03) higher. For the lower limb and hip, the odds of being diagnosed at stages II, III, or IV were significantly higher than the HN region with an OR 2.6 (p < 0.001), 1.6 (p < 0.0005), and 1.5 (p = 0.04), respectively. Conclusions: While less prevalent than the MCC of the HN region, patients with MCC of the trunk, upper or lower extremities had a higher probability of being diagnosed with an advanced-stage malignancy compared to the former. Additional prospective studies are required to investigate this association further and explore its significance in site-specific treatment approaches and survival factors. [Table: see text]