Metastatic breast involvement from Merkel cell tumour: a case report

Background: Merkel cell carcinoma (MCC) is an uncommon and aggressive skin cancer, very rarely metastasising to the breast. MCC metastases to the breast with known primary tumour have earlier been reported in the literature, but never as seen in this case, where a metastasis is found in an intramammary lymph node and where primary tumour could not be localised. This case adds to the existing literature with new insights to MCC presentation into a male patient. Case Description: We present a case of a 64-year-old man with a small palpable tumour in his right breast persistent for 2 weeks and no other symptoms associated with cancer. The patient had a history of aorto-coronary bypass grafting and replacement of the aortic valve, hypertension, and a dilated aorta, but was asymptomatic and working his job daily. The patient was examined by a breast surgeon and referred for mammography, ultrasound, and biopsy. Ultrasound-guided biopsy was carried out and final pathology report revealed a MCC metastasis in an intramammary lymph node. Positron emission tomography/computed tomography (PET/CT) scan identified an area in the gastric ventricle with thickened wall, but could not identify any primary tumour, since biopsies from the gastric ventricle was without cancer. The patient was followed up by the plastic surgeons with no skin signs of MCC and followed up continuously with PET/CT every 3 months. Conclusions: MCC metastasis to the breast has been reported casuistically before, but this case presents a MCC metastasis in an intramammary lymph node with an unknown primary tumour which has not been reported before. This case adds to the many and unpredictable ways a MCC metastasis can present itself clinically, suggesting that diagnosis may be difficult. With this case, we also emphasize the importance of a breast surgeon’s clinical skills in distinguishing between tumours thought benign and tumours that should be biopsied, since a small soft lump in the male breast or chest region can easily be mistaken for gynecomastia or lipoma by an untrained eye, and therefore further interventions dismissed. The takeaway message from this case report is, that breast examination should be considered part of the routine follow-up not only for women but also for men with MCC.