The story
Either (1) an outdated coinage from the days when word production with the suffix -oma wasn't yet restricted semantically to neoplasms (19th century), or (2) back-formed via slightly misguided rebracketing from various words for tumors (such as astrocytoma, plasmacytoma, histiocytoma, and pheochromocytoma); possibly generated via both routes, if reinvented independently.
Etymology from Wiktionary, CC BY-SA 4.0.