820 Atypical Presentation of Muscular Metastasis of Melanoma in the Contralateral Limb

Case Presentation

A gentleman in his 50s presented to his GP with an asymptomatic, soft, enlarging mass on his left upper arm. He had previously been diagnosed with a 4.5mm cutaneous malignant melanoma (MM) on his right arm, which was treated with an excision biopsy and a further 2cm wide local excision. He further had a sentinel lymph node biopsy (SLNB), which confirmed no residual melanoma. He was subsequently followed up routinely by the skin cancer multidisciplinary team (MDT) for five years with no concern.

The patient subsequently underwent an ultrasound scan, a Magnetic Resonance Imaging (MRI) scan, and a final staging Computed Tomography (CT). Imaging was suspicious for an intramuscular soft tissue malignancy. Core biopsy of this mass suggested a diagnosis of metastatic MM. Excision was performed within three weeks of his core biopsy. The lesion was completely excised, and formal pathology confirmed the diagnosis of metastatic MM. He has been referred to the oncology services for the commencement of Nivolumab (anti PD-1 antibody) immunotherapy according to the genetic analysis.

Summary

This case highlights a rare example of muscular metastasis of MM, presenting in a distant site, despite a negative SLNB, over five years from the original diagnosis. Such cases highlight the unpredictable nature of MM, reminding clinicians of the need for a low threshold for investigation of soft tissue masses in patients with a history of cutaneous malignancy.