▁which ▁may ▁also ▁apply ▁different ▁morph ological ▁criteria . ▁A ▁recent ▁study ▁in ▁pro state ▁cancer ▁showed ▁that ▁many ▁different ▁morph ological ▁definitions ▁of ▁circul ating ▁tum or ▁cells ▁have ▁similar ▁pro gn ost ic ▁value , ▁even ▁though ▁the ▁absolute ▁number ▁of ▁cells ▁found ▁in ▁patients ▁and ▁normal ▁don ors ▁varied ▁by ▁more ▁than ▁a ▁de cade ▁between ▁different ▁morph ological ▁definitions . ▁ ▁History ▁C TC s ▁were ▁observed ▁for ▁the ▁first ▁time ▁in ▁ 1 8 6 9 ▁in ▁the ▁blood ▁of ▁a ▁man ▁with ▁met ast atic ▁cancer ▁by ▁Thomas ▁Ash worth , ▁who ▁post ulated ▁that ▁" cell s ▁identical ▁with ▁those ▁of ▁the ▁cancer ▁itself ▁being ▁seen ▁in ▁the ▁blood ▁may ▁tend ▁to ▁throw ▁some ▁light ▁upon ▁the ▁mode ▁of ▁origin ▁of ▁multiple ▁tum ours ▁existing ▁in ▁the ▁same ▁person ". ▁ ▁A ▁thorough ▁comparison ▁of ▁the ▁morph ology ▁of ▁the ▁circul ating ▁cells ▁to ▁tum or ▁cells ▁from ▁different ▁les ions ▁led ▁Ash worth ▁to ▁conclude ▁that ▁" One ▁thing ▁is ▁certain , ▁that ▁if ▁they ▁[ CT C ] ▁came ▁from ▁an ▁existing ▁cancer ▁structure , ▁they ▁must ▁have ▁passed ▁through ▁the ▁greater ▁part ▁of ▁the ▁circul atory ▁system ▁to ▁have ▁arrived ▁at ▁the ▁internal ▁s aph ena ▁ve in ▁of ▁the ▁sound ▁leg ". ▁ ▁The ▁importance ▁of ▁C TC s ▁in ▁modern ▁cancer ▁research ▁began ▁in ▁the ▁mid ▁ 1 9 9 0 s ▁with ▁the ▁demonstr ation ▁that ▁C TC s ▁exist ▁early ▁on ▁in ▁the ▁course ▁of ▁the ▁disease . ▁Those