Castan was diagnosed with the problem after reporting he was feeling dizzy following Roma's 1-0 victory over Empoli in September.
Tests revealed that 28-year-old Brazilian was suffering from swelling on the brain caused by the accumulation of fluid, and Castan required a three-and-a-half hour operation to remove it.
It remains unclear how long Castan will be sidelined for as he completes his rehabilitation, with the Serie A club saying more will become clear in the coming days.
"The cavernoma itself, which measured 3 cm, was completely removed," a statement read. "This was confirmed by an MRI scan carried out immediately after the operation.
"Castan has now woken up and is breathing unaided, and has been informed of the operation’s successful outcome.
"He will remain in intensive care for the next 24 hours, after which a prognosis will be formed."
Castan was an integral part of the Roma side the challenged for the Serie A title last season, starting 36 league matches for Rudi Garcia's men.
