Cystic brain metastases are relatively rare 1).
The pathogenesis of cyst formation is not clear. According to the literature the exudative fluid, often seen as massive edema surrounding the metastatic tumor, tends to collect inside the tumor and expand as a cystic mass depending on its local conditions 2).
Cumings suggested that cyst formation is from tumor degeneration followed by transudation of fluid from nearby blood vessels 3).
Alternatively, Gardner et al. advocate that the cystic components in brain tumors are merely interstitial fluid without its normal drainage route due to the lack of lymphatics in the surrounding brain 4).