The spectrum of micronodular thymic epithelial tumours with lymphoid B-cell hyperplasia

Aims: A rare type of thymoma, micronodular thymoma with lymphoid B-cell hyp erplasia, was recently reported by Suster and Moran. Thymic epithelial tumo urs with a similar pattern but with varied cytological features of the tumo ur cells are analysed. Methods and results: A total of 11 cases of thymic epithelial tumours chara cterized by micronodular proliferation of tumour cells separated by abundan t lymphoid stroma with prominent germinal centres were reviewed clinicopath ologically and examined immunohistochemically. The presence of Epstein-Barr virus (EBV) genome was also examined by in-situ hybridization. Based on th e morphology of tumour epithelial cells, cases were subdivided into four gr oups: group 1 (two cases) having spindle epithelial cells; group 2 (two cas es) showing an admixture of spindle and polygonal epithelial cells, with mi ld to moderate cytological atypia in four cases, and group 4 (two cases) re presenting lymphoepithelioma-like carcinoma. The degree of cytological atyp ia and the number of tumour cells positive for MIB-1 and p53 gradually incr eased towards group 4. The abundant lymphoid stroma in all cases contained many CD20-positive B-cells and CD3 and CD45RO-positi-rre T-cells. CD99-posi tive immature T-cells were present in all cases of groups 1 and 2 and in mo st cases of group 3, but not in both cases of group 4 tumours. IgG, IgM and IgD-positive plasma cells and lymphocytes were also IgD-positive plasma ce lls and lymphocytes were also present in all cases, more prominent in those of groups 3 and 4. The EBV genome was detected in only a few lymphocytes i n five cases. Conclusions: The tumours in this series belong to a distinct category of th ymic epithelial tumours and each of the above groups may constitute a spect rum in the continuum of cytological atypia. The aetiological relationship o f EBV with these tumours could not be proved. The lymphoid B-cell hyperplas ia may result from a host immune response and map suggest a favourable clin ical course of this type of tumour.