▁cancer . ▁It ▁is ▁currently ▁class ified ▁as ▁a ▁my el op rol ifer ative ▁ne op las m , ▁in ▁which ▁the ▁pro l ifer ation ▁of ▁an ▁ab normal ▁clone ▁of ▁h emat opo iet ic ▁stem ▁cells ▁in ▁the ▁b one ▁mar row ▁and ▁other ▁sites ▁results ▁in ▁fib ros is , ▁or ▁the ▁replacement ▁of ▁the ▁mar row ▁with ▁scar ▁t issue . ▁ ▁The ▁term ▁my el of ib ros is ▁alone ▁usually ▁refers ▁to ▁primary ▁my el of ib ros is ▁( PM F ), ▁also ▁known ▁as ▁chron ic ▁id iop ath ic ▁my el of ib ros is ▁( c IM F ); ▁the ▁terms ▁id iop ath ic ▁and ▁primary ▁mean ▁that ▁in ▁these ▁cases ▁the ▁disease ▁is ▁of ▁unknown ▁or ▁sp ont aneous ▁origin . ▁This ▁is ▁in ▁contrast ▁with ▁my el of ib ros is ▁that ▁develop s ▁secondary ▁to ▁poly cy th emia ▁ver a ▁or ▁essential ▁th rom b oc yth a emia . ▁My el of ib ros is ▁is ▁a ▁form ▁of ▁my el oid ▁met ap las ia , ▁which ▁refers ▁to ▁a ▁change ▁in ▁cell ▁type ▁in ▁the ▁blood - form ing ▁t issue ▁of ▁the ▁b one ▁mar row , ▁and ▁often ▁the ▁two ▁terms ▁are ▁used ▁syn onym ously . ▁The ▁terms ▁ag n ogen ic ▁my el oid ▁met ap las ia ▁and ▁my el of ib ros is ▁with ▁my el oid ▁met ap las ia ▁( MM M ) ▁were ▁also