The term poorly differentiated neoplasm is used when the pathologist cannot distinguish between carcinoma and other cancers, such as lymphoma, melanoma, ...PoorlydifferentiatedneoplasmsofunknownprimarysiteINTRODUCTIONCancerofunknownprimarysite(CUP)isacommonclinicalentity,accountingfor2percentofallcancerdiagnosesintheSurveillance,Epidemiology,andEndResults(SEER)registriesbetween1973and1987 .Withinthiscategory,tumorsfrommanyprimarysiteswithvaryingbiologiesarerepresented;thisheterogeneityhasmadethedesignoftherapeuticstudiesdifficult.SubstantialimprovementshavebeenmadeinthemanagementandtreatmentofsomepatientswithCUP.Theidentificationofspecificsubgroupsoftreatablepatientshasbeenmadepossiblebythedevelopmentofspecializedimmunohistologictechniquesthatcanaidintumorcharacterization andbytherecognitionofseveraldistinctclinicalsyndromesthatpermitpredictionofchemotherapyresponsiveness.ThetypicalpatientwithCUPpresentswithsymptomsreferabletoametastaticsite.Theinitialwork-up,includingphysicalexamination,laboratoryandradiographicstudy,failstoidentifytheprimarysite.Lightmicroscopicevaluationofbiopsymaterialplacesthetumorintooneoffivehistologiccategories,whichthenguidesfurtherevaluation:Adenocarcinoma approximately70percentPoorlydifferentiatedcarcinoma 15to20percent;anadditional10percentrepresentpoorlydifferentiatedadenocarcinomaSquamouscellcarcinoma 5percentNeuroendocrinecarcinoma lessthan5percentPoorlydifferentiatedneoplasm(nodefiningcharacteristics)lessthan5percentThecategoryofpoorlydifferentiatedneoplasmofunknownprimarysitewillbereviewedhere.Thetermpoorlydifferentiatedneoplasmisusedwhenthepathologistcannotdistinguishbetweencarcinomaandothercancers,suchaslymphoma,melanoma,orsarcoma.However,amoreprecisediagnosisisessentialsincetherapyforthisgroupoftumorsisquitedifferent,andmaybepotentiallycurativeinsomepatients.Asanexample,non-Hodgkin'slymphomas,whichareoftencurablewithcombinationchemotherapy,accountfor34to66percentofthepoorlydifferentiatedneoplasmsofunknownprimarysiteinseveralserie