Merkel cell carcinoma is a highly aggressive cutaneous malignancy with a high rate of lymph node and distant metastatic disease. Approximately one third of patients present with stage IIIB (nodal) disease.

This cohort study was performed to analyze the outcome of patients with stage IIIB disease with or without an occult primary.

The details of 91 patients with stage IIIB (nodal) Merkel cell carcinoma treated curatively between 1985 and 2010 at 3 tertiary referral hospitals in Australia were reviewed. Kaplan-Meier plots were used with the primary end point being overall survival. Secondary end points were disease-free survival and relapse-free survival. A multivariate Cox regression analysis was performed for known prognostic factors.

Of 91 patients with stage IIIB (nodal) disease, 36 (40%) had an occult primary. A total of 78 patients (86%) had surgery and 79 patients (87%) had definitive or adjuvant radiotherapy. With a median follow-up of 4.3 years, those with an occult primary did significantly better in terms of overall survival, disease-free survival, and relapse-free survival. On multivariate analysis, occult primary and patient age were the only factors predicting survival with hazard ratios of 0.30 (95% confidence interval 0.13-0.67) and 1.64 (95% confidence interval 1.13-2.38), respectively.

This is a retrospective study over several decades with patients treated using various modalities.

This study indicates that for patients with stage IIIB (nodal) Merkel cell carcinoma, the presence of an occult primary confers a significantly better prognosis that may have implications in the future staging and treatment of patients with stage III disease.