There are some explanations for this unfavorable evolution. The fact that cutaneous melanoma associated with GCMN typically grows in the dermis makes it more difficult to detect in clinical exams. Unlike the malignant transformation that occurs in small or medium CMN, starting at the dermal-epidermal junction, and thus rapidly changing the appearance of a nevus, in the case of GCMN it is often necessary that a large nodule develops or ulceration occurs before the diagnosis of cancer is made. Moreover, the often rough or nodular surface of GCMN further complicates the early observation of the tumor. Another factor that may contribute to the severity of the disease is the great extent of nevic lesions (commonly crossing the midline), which causes their lymphatic drainage to be performed by multiple channels. Moreover, the presence of malignant cells in the deeper layers of the nevus facilitates tumoral spread through the lymph and blood vessels of greater caliber, favoring the occurrence of early metastases. Besides, the occurrence of the disease in extracutaneous sites may contribute to the delay in its recognition. The difficult clinical detection of the tumor contributes to the fact that in 24% of cases, melanoma will already be metastatic at the time of diagnosis without the primary site being identified.