Background: Melanoma is an aggressive malignancy that tends to form metastases in the brain. The nature of metastatic melanoma with unknown primary is poorly understood and has only theoretical assumptions. Less than 10% of melanoma cases present with an unknown primary tumor location.
Case presentation: A 68-year-old Caucasian male patient, Canadian citizen, presented to the hospital with left-sided weakness and deviation of right side of angle of mouth for three days. Pertinent positive physical signs were motor weakness of left upper and lower extremities. Labs were unremarkable. CT-scan of the brain showed right temporal lobe lobulated masses associated with vasogenic edema, mass effect, and leftward midline shift, confirmed with MRI, which showed a large 5.4 x 4.2 cm complex right temporal lobe mass. Intravenous dexamethasone and intravenous levetiracetam were started. CT-scan of the chest, abdomen, and pelvis was negative for any suspicious lesion. A positron emission tomography scan showed no suspicious hypermetabolic activity.
The patient underwent stereotactic right temporal craniotomy with volumetric resection of the tumor and magnetic resonance image-guided, computer-assisted stereotactic volumetric resection of the right temporal intra-axial lesion and duraplasty by the neurosurgical team, without any complications. Intraoperatively the lesion was noted to be violaceous, whitish, granular, and encapsulated. Later, he also underwent revision of craniotomy and gross total tumor resection under neuronavigation.
Histopathology described the tissue from resection as a malignant melanoma, with tumor cells staining positive for vimentin, SOX-10, HMB-45 and MART-1.
Despite an extensive evaluation by oncology and dermatology, no primary cutaneous lesion could be found.
The patient received palliative radiation therapy with a total dose of 3000 cGy delivered over ten fractions. During his therapy, no significant side effects were observed. He was started on intravenous nivolumab. As the patient was a Canadian citizen, he moved back to Canada for further treatment.
Conclusion: Our patient represents an example of malignant melanoma with an unknown primary. The current literature suggests that about 2–3% of all malignant melanoma patients present with a metastasis without a detectable primary tumor. Spontaneous regression of the primary lesion remains a plausible explanation of malignant melanoma with unknown primary
