Pericardial Oligometastasis From Merkel Cell Carcinoma Treated With Stereotactic Ablative Radiotherapy.

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine skin cancer that often has a poor prognosis due to its propensity for distant metastases. A 74-year-old man with a history of MCC of the left forearm, previously treated with surgery and adjuvant radiotherapy, was found to have an asymptomatic pericardial nodule associated with pericardial effusion on routine follow-up imaging. Subsequent biopsy confirmed metastatic MCC. Restaging demonstrated no other sites of metastases. The pericardial metastasis was treated with stereotactic ablative radiotherapy (SABR) to a dose of 40 Gray (Gy) in five daily fractions. A significant decrease in the size of the pericardial nodule was observed 2.3 months post-SABR, and a clinical complete response was achieved 8.9 months post-SABR. Apart from an asymptomatic mildly increased pericardial effusion, no other acute or late adverse effects were observed. At 40.1 months post-SABR, the patient continued to show a durable response to treatment with no evidence of recurrence. To the best of our knowledge, this case of pericardial oligometastases from MCC treated with SABR is the longest reported duration of response.