. ▁This ▁behavior ▁could ▁be ▁a ▁factor ▁in ▁why ▁C TC ▁clusters ▁have ▁such ▁a ▁significant ▁met ast atic ▁potential . ▁ ▁Fre quency ▁ ▁The ▁detection ▁of ▁C TC s ▁may ▁have ▁important ▁pro gn ost ic ▁and ▁th era pe ut ic ▁imp lications ▁but ▁because ▁their ▁numbers ▁can ▁be ▁very ▁small , ▁these ▁cells ▁are ▁not ▁easily ▁detected . ▁It ▁is ▁estimated ▁that ▁among ▁the ▁cells ▁that ▁have ▁det ached ▁from ▁the ▁primary ▁tum or , ▁only ▁ 0 . 0 1 % ▁can ▁form ▁met ast ases . ▁Cir cul ating ▁tum or ▁cells ▁are ▁found ▁in ▁frequencies ▁on ▁the ▁order ▁of ▁ 1 - 1 0 ▁C TC ▁per ▁m L ▁of ▁whole ▁blood ▁in ▁patients ▁with ▁met ast atic ▁disease . ▁ ▁For ▁comparison , ▁a ▁m L ▁of ▁blood ▁contains ▁a ▁few ▁million ▁white ▁blood ▁cells ▁and ▁a ▁billion ▁red ▁blood ▁cells . ▁This ▁low ▁frequency , ▁associated ▁to ▁difficulty ▁of ▁ident ifying ▁cancer ous ▁cells , ▁means ▁that ▁a ▁key ▁component ▁of ▁understanding ▁C TC s ▁bi ological ▁properties ▁require ▁techn ologies ▁and ▁approaches ▁capable ▁of ▁isol ating ▁ 1 ▁C TC ▁per ▁m L ▁of ▁blood , ▁either ▁by ▁en rich ment , ▁or ▁better ▁yet ▁with ▁en rich ment - free ▁ass ays ▁that ▁identify ▁all ▁C TC ▁sub types ▁in ▁sufficiently ▁high ▁definition ▁to ▁satisfy ▁di agnostic ▁path ology ▁image - quantity ▁requirements ▁in ▁patients ▁with ▁a ▁variety ▁of ▁cancer ▁types . ▁To ▁date ▁C TC s ▁have ▁been ▁detected ▁in ▁several