Balloon Cell Melanoma
Balloon cell melanoma (BCM) is a rare variant of melanoma in which the tumor cells have abundant clear or eosinophilic granular cytoplasm causing ballooning of cells. The tumor cells contain PAS+ diastase-resistant granules which are composed of ribonucleoprotein. They result either from abnormal metabolism of melanosomes or due to melanosome degeneration. HMB-45 and S-100 protein are generally positive. The poor prognosis usually seen with BCM is related to tumor depth at the time of presentation. Differential Diagnosis: Balloon cell change is more common in nevi than in melanomas. Balloon cell nevi lack the nuclear pleomorphism and mitotic activity seen with melanomas. Amelanotic variants of balloon cell melanoma may be mistaken for sebaceous or xanthomatous tumors. Sebaceous tumors are positive for EMA and negative for HMB-45 and S-100 protein. Xanthomatous lesions are CD68+ and S-100 protein negative. BCMs with clear cytoplasm may mimic clear cell carcinomas (squamous, hidradenocarcinoma, metastatic renal cell carcinoma).