Primary Intracranial Malignant Melanoma (PIMM) is a very rare neoplasm and account for 1% of all melanomas. It account for 0.1% of all intracranial tumors. PIMMs are derived from melanocytes that are normally present in leptomeninges. It is more common in males and have a poor survival rates over all.
In humans melanocytes are found in skin, mucous membrane, uvea, etc. There are many theories for the origin of primary intracranial melanomas. Endodermal theory states that some aberrant embryonic ectodermal cells in central nervous system produces melanin pigment. Neurogenic theory states that neural crest gives rise to pigment cells. Neural crest later develops into mesodermal and neural elements. Mesodermal theory proposes that the mesodermal pigment cells reach the central nervous system through pial vessels.
Hayward proposed few features of a primary intracranial melanoma. He suggested that for a PIMM, there should no malignant melanoma outside the CNS, evidence of leptomeningeal involvement, hydrocephalus, intramedullary spinal lesions, tumor located in pituitary or pineal region and a single intracerebral lesion.
Gibson et al divided intracranial melanomas into two pathological types. Diffuse type which infiltrates pia-mater and the subarachnoid space (SAS); and the solitary (discrete) type which is present as nodular mass. Diffuse type was more prevalent in younger patients (mean age 26-years). It presented with clinical features of intracranial hypertension, cranial nerve deficit or meningism. Solitary type is more common in adults (mean age 44-years). Most of the solitary tumors are supratentorial. Leptomeningeal involvement is frequently seen in solitary melanoma. Our patient has a solitary lesion.
Symptoms included headache, vomiting (especially due to raised intracranial pressure), hydrocephalus (43%), focal neurological deficit (due to mass effect, 35%) and convulsion or subarachnoid hemorrhage (16%). Headache and focal neurological deficit were present in our case. Intracranial hypertension and hydrocephalus is more commonly seen in diffuse type. Presence of congenital melanocytic nevus increases risk of malignant melanoma. 25% of patients of primary intracranial melanomas have congenital nevus. This was not the case in our patient. Extracranial metastasis of PIMM is rare.
It is difficult to differentiate primary central nervous system (CNS) melanomas from metastatic melanomas on the basis of imaging alone. Melanin cells in cerebrospinal fluid (CSF) analysis helps in the diagnosis. 86-97% melanocyte tumors are positive for HMB-45 Antigen. Other commonly used tumor markers are S100 and Melan A. All three were positive in our patient.
Treatment of choice for intraparenchymal melanocytic tumor is complete resection and postoperative chemoradiotherapy. Survival in adults can be up to 17 years and 8 months in pediatric patients. Mean survival was found to be better (19.6 months) in patients in whom complete resection was done as compared with those where partial resection was done (9.3 months). Also prognosis of secondary melanoma was poorer than PIMM. There is 95% mortality in secondary intracranial melanomas.
Initially, radiotherapy (RT) was not offered to patients of PIMM as melanomas were radioresistant tumors but recent studies showed that it was effective in controlling tumor growth. Stereotactic Radiosurgery (SRS) along with whole brain radiotherapy (WBRT) has been shown to reduce recurrence and improve survival. High dose of 5000 centigray as SRS in small lesions (size less than 3cm) gave better tumor control in almost 80% of cases. Hydrocephalus caused by leptomeningeal spread of the disease has been shown to respond well to WBRT.
Dacarbazine is commonly used in the treatment of PIMMs after surgery or radiotherapy. It has 16-20% effectiveness. Another agents are temozolomide and Dimethyl-triazeno-imidazole-carboxamide which is common in use.
Despite optimum treatment, median survival is less than a year. Leptomeningeal spread have a median survival of 10 weeks.