Thymomas are tumors of thymic epithelial cells. Several classification systems for thymoma based on cytologic and biologic criteria have been proposed. One simple and clinically useful classification is as follows:
Benign or encapsulated thymoma: cytologically and biologically benign
Type I: cytologically benign but infiltrative and locally aggressive
Type II (thymic carcinoma): cytologically and biologically malignant
MORPHOLOGYMacroscopically,thymomasarelobulated,firm,gray-whitemassesupto15to20cmindimension.Mostappearencapsulated,but20%to25%penetratethecapsuleandinfiltrateperithymictissues.Microscopically,virtuallyallthymomasarecomposedofamixtureofepithelialtumorcellsandnonneoplasticthymocytes(immatureTcells).Inbenign thymomas, theepithelialcellsarespindledorelongatedandresemblethosethatnormallypopulatethemedulla.Asaresult,thesearesometimesreferredtoasmedullary thymomas. Inothertumors,thereisanadmixtureoftheplumper,rounder,cortical-typeepithelialcells;thispatternissometimesreferredtoasamixedthymoma.Themedullaryandmixedpatternsaccountfor60%to70%ofallthymomas.
Malignant thymoma type I iscytologicallyblandbutlocallyinvasive;itaccountsfor20%to25%ofallthymomas.Thesetumorsalsooccasionally(andunpredictably)metastasize.Theyarecomposedofvaryingproportionsofepithelialcellsandreactivethymocytes.Theepithelialcellsusuallyhaveabundantcytoplasmandroundedvesicularnuclei,anappearancesimilartonormalthymiccorticalepithelialcells;spindledepithelialcellsaresometimespresentaswell.Theepithelialcellsoftenpalisadearoundbloodvessels.Thecriticaldistinguishingfeatureisthepenetrationofthecapsulewiththeinvasionofsurroundingstructures.
Malignant thymoma type II isperhapsbetterthoughtofasaformofthymiccarcinoma. Thesetumorsaccountforabout5%ofthymomas.Macroscopically,theyusuallyarefleshy,obviouslyinvasivemassesthatoftenmetastasizetosuchsitesasthelungs.Microscopically,mostresemblesquamouscellcarcinoma.
Thymomas are rare. They may arise at any age, but most occur in middle-aged adults. In a large series about 30% were asymptomatic; 30% to 40% produced local manifestations such as cough, dyspnea, and superior vena cava syndrome; and the remainder were associated with a systemic disease, most commonly myasthenia gravis, in which a concomitant thymoma is discovered in 15% to 20% of patients. Removal of the tumor often leads to improvement of the neuromuscular disorder. In addition to myasthenia gravis , thymomas may be associated with several other pareneoplastic syndromes. These include in rough order of frequency , pure red cell aplasia, hypogammaglobulinemia, and multi organ autoimmunity. The latter bears resemblance with graft versus host disease.
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