(P 289) MERKEL CELL CARCINOMA OF UNKNOWN PRIMARY: CLINICAL PRESENTATION AND OUTCOMES

Objective: Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous malignancy that usually occurs in the head/neck or extremities. However, there are reports of MCC developing in the lymph nodes or parotid gland without evidence of a primary cutaneous lesion. The objective of this study was to examine the clinical presentation and outcomes of MCC of unknown primary (MCCUP)

Methods: We reviewed 415 patients with biopsy-proven MCC. Patients with MCC of unknown primary (n=37, 9%, MCCUP) made up the study cohort. The primary endpoints of the study were rate of recurrence, recurrence-free survival, and overall survival.

Results: Patients with MCCUP presented with tumors in lymph nodes (n = 34) or parotid gland (n = 3). Nodal disease was most commonly detected in the inguinal/external iliac (n = 15) or axillary (n = 14) regions. Mean age at diagnosis was 70 years and 24% were female. Patients presented with distant metastases in 24.3% of cases. Patients with stage IIIA disease treated with regional lymph node dissection (RLND) had a lower risk of disease recurrence (HR 0.26, p=0.046). Recurrence-free survival was 59.3% at 5 years (Figure 1). Thirteen of the 14 cases of disease recurrence occurred within 2 years of diagnosis in, with no recurrence events observed beyond 2.5 years. Disease-specific survival was 63.3% at 5 years (Figure 2).

Conclusion: Patients with MCCUP have a high risk of recurrence and mortality. Although recurrence is frequent, it almost always occurs early and thus enhanced follow-up focused on the first 2-3 years after diagnosis appears preferred. The optimal treatment for MCCUP has yet to be elucidated, although therapeutic RLND appears beneficial for these patients.