Rapid onset and unpredicted findings of a giant nodular melanoma arising on a congenital nevus: a case report.

Melanoma is a malignancy that develops from pigment-producing cells known as melanocytes. Although it is considered one of the most fatal tumors in the world, its early diagnosis is associated with a better prognosis and overall survival. A 49-year-old man was admitted to the dermatology department with a 15 cm lesion on the left arm. It was growing on a congenital nevus to reach an enormous size. Surgical excision was not possible and the treatment was controversial as the patient showed unpredicted resistance to immunotherapy. Further evaluations revealed false-negative BRAF mutation, which completely changed the course of treatment. Cutaneous melanoma is a rare malignancy, accounting only for 1% of skin cancer cases, and having it arising on a pre-existing congenital nevus is even much rarer. Although there is no decisive definition of giant melanoma, some authors define it as a lesion of more than 10 cm in diameter. Through the literature, only a few cases of giant melanoma on the arm have been reported. Through our paper, we are revealing the importance of early diagnosis and treatment of melanoma and confirming the significant role of regular follow-up for patients with a congenital melanocytic nevus. Moreover, we are showing the importance of having alternative methods for detecting BRAF mutations to avoid false-negative results and have better outcomes.