Either (1) an outdated coinage from the days when word production with the suffix -oma wasn't yet restricted semantically to neoplasms (19th century), or (2) back-formed via slightly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.
cytoma (plural cytomas or cytomata)
This is a rare and outdated word that has no use in present-day histopathology. It looks like the tail end of various words for tumors (such as astrocytoma, plasmacytoma, histiocytoma, and pheochromocytoma), but such words are not formed from any putative pattern of X- + *cytoma; rather, they are formed from the pattern of X-cyte + -oma, "a tumor composed of X-cytes, that is, X type of cells".