形态学不典型的化生型胸腺瘤1例并文献复习OACSTPCD

Metaplastic thymoma(MT)is a rare type of thymoma.Currently,most studies on MT,both domestically and internationally,are based on small-scale research and case reports summarizing the classical histological features of MT.Here,we report a case of morphologically atypical MT.The patient,a 44-year-old male,was admitted due to the discovery of a mediastinal mass during a health examination one month prior.A computed tomography scan showed a mediastinal mass in his chest.The patient underwent mediastinal mass resection at the First Affiliated Hospital of Zhejiang University School of Medicine in October 2022.Gross examination showed a piece of gray-yellow,adipose-like tissue with a slightly hard,gray-white mass,with relatively clear boundaries.Microscopic examination showed a nodular tumor without a distinct fibrous capsule or lobulated structure.Perivascular spaces were not prominent,with some areas showing a few collagen fibers.At high magnification,densely stained areas consisted of abundant epithelial-like cells and a few plump spindle cells,with only a few lymphocytes in the background.Some tumor cells exhibited a whorled structure with squamous differentiation.Immunohistochemistry showed positivity for P40,P63,cytokeratin 19(CK19),and cytokeratin-pan(CK-pan),with limited positivity for mucin-1(MUC-1)and vimentin.A few T lymphocytes were positive for CD3 and CD5,with partial weak to moderate positivity for P53 and membrane positivity for β-catenin.CD20,CD117,paired box protein-8(PAX-8),chromogranin A(CgA),synapsin(Syn),terminal transferase(TdT),signal transducer and activator of transcription 6(STAT6),and nuclear protein of the testis(NUT)were all negative,with a low Ki-67 proliferation index.Fluorescence in situ hybridization revealed a positive result for the Yes associated protein 1-mastermind like transcriptional coactivator 2(YAP1-MAML2)fusion gene.Postoperative pathological diagnosis confirmed MT,Masaoka stage II.The patient received six sessions of radiotherapy postoperatively and was disease-free after 14 months of follow-up.