Micronodular Thymoma with Lymphoid Stroma Diagnosed with Core Needle Biopsy. A Case Report

Micronodular thymoma with lymphoid stroma (MNT) is an unusual type of thymoma characterized by multiple, discrete, small epithelial nodules with cytological features of type A thymoma that are separated by abundant reactive lymphoid tissue. Diagnosis of MNT usually relies on surgical specimens and could be challenging with needle biopsies. We present a case diagnosed via a core needle biopsy since the patient was unfit for and refused surgery.

The patient was a 79-year-old man with an incidentally found mediastinal mass. Subsequent computed tomography scans showed that the tumor measured 10.7 x 10.0 x 9.0 cm in size and invaded the pericardium. Needle core biopsies revealed small nodules of bland-looking epithelial cells in a background of prominent lymphoid stroma with germinal center formation. The lymphoid stroma was composed mainly of B-cells admixed with scattered T-cells. The patient was unfit for and refused surgery and was discharged under stable condition with no evidence of metastasis 6 months later.

Our case demonstrates that the diagnosis of MNT could readily be made even with a core needle biopsy specimen when the characteristic morphologic and immunohistochemical features are present. Our literature review shows some ethnic and/or geographic differences in MNTs between Asian and Western populations.