Isolated melanoma metastasis in a patient with large congenital nevus without detectable primary melanoma: a case report and review of literature.

Giant congenital pigmented nevi constitute an extremely diverse group of skin lesions with varying morphologies. These nevi are often associated with many clinical implications, such as increased risk of melanoma and the presence of neurocutaneous melanosis, with melanoma being the primary concern. We present a rare case of a 62-year-old patient with a giant congenital birthmark who reported to the oncology department due to a tumor in the lower abdomen detected during an ultrasound examination. A biopsy of the lesion showed the presence of melanoma metastasis. Four independent dermatologists performed a dermoscopic examination of the patient's skin and mucous membranes. In the PET/CT examination, apart from the previously described change in the lower abdomen, no metabolically active foci with features of malignant growth were found. The patient underwent surgical removal of the lesion in the lower abdomen. The postoperative histopathological examination confirmed the presence of metastasis of melanoma in the subcutaneous tissue of the abdomen with no connection to the epidermis. The mutation was not found in the molecular test. For stage IV R0 melanoma with distant metastasis, with stage T0N0M1a, the only adjuvant treatment option following radical resection is nivolumab. After a rheumatological consultation, the patient was qualified for adjuvant treatment with nivolumab.