title: Microthymoma: definition of the entity and distinction from nodular hyperplasia of the thymic epithelium (so-called microscopic thymoma).
abstract: We report 2 cases of microscopic-sized thymoma, which probably represents the earliest phase of thymoma development. The 2 patients presented with pure red cell aplasia and myasthenia gravis, respectively. The thymectomy specimens did not reveal tumor on gross examination, but histologically each contained small thymomas measuring 5 mm and 7 mm in largest dimension, respectively. One of the tumors was unencapsulated and involved a single lobule only, and the other was encapsulated and comprised two lobules. The tumors consisted of ovoid epithelial cells with pale nuclei and distinct nucleoli, scattered in a background of small lymphocytes. Foci of medullary differentiation and perivascular space were identified in the 2 cases, respectively. The lymphocytes were confirmed to be immature T cells on immunohistochemical studies (CD3+, TdT+). Except for the microscopic size, the morphology of the two tumors conforms to conventional type B1/B2 and type B2 thymoma, respectively. We propose calling such incidental small tumor "microthymoma" to distinguish it from the so-called microscopic thymoma, which is composed of small thymic epithelial nests and probably more appropriately termed "nodular hyperplasia" of the thymic epithelium.
authors.affiliation: Department of Pathology, Queen Elizabeth Hospital, Hong Kong, SAR China. cheuk_wah@hotmail.com
authors.forename: Wah
authors.initials: W
authors.lastname: Cheuk
authors.forename: William Y W
authors.initials: WY
authors.lastname: Tsang
authors.forename: John K C
authors.initials: JK
authors.lastname: Chan
mesh_terms: D000328:Adult; D014408:Biomarkers, Tumor / Q000378:metabolism; D017252:CD3 Complex / Q000378:metabolism; D004253:DNA Nucleotidylexotransferase / Q000378:metabolism; D006801:Humans; D006965:Hyperplasia; D007150:Immunohistochemistry; D008297:Male; D008875:Middle Aged; D013601:T-Lymphocytes / Q000378:metabolism / Q000473:pathology; D013945:Thymoma / Q000378:metabolism / Q000473:pathology* / Q000601:surgery; D013950:Thymus Gland / Q000378:metabolism / Q000473:pathology* / Q000601:surgery; D013953:Thymus Neoplasms / Q000378:metabolism / Q000473:pathology* / Q000601:surgery; D016896:Treatment Outcome
chemical_list: D014408:Biomarkers, Tumor; D017252:CD3 Complex; D004253:DNA Nucleotidylexotransferase