PMBL exhibits diffuse growth and is composed of large or intermediate-sized lymphoid cells with a varying amount of sclerosis, which may surround groups of cells and cause a compartmentalizing or alveolar pattern. The cytologic features include round to ovoid cells with transformed nuclei akin to other centroblastic lymphomas, with varying degrees of pleomorphism, including cells with lobulated nuclei and eosinophilic nucleoli. Immunoblasts and anaplastic cytologic features can be observed. Some PMBLs harbor Reed-Sternberg–like cells and variants in some microscopic fields.[30,31] Many cases have abundant clear or faintly eosinophilic cytoplasm that is not an artifact of formalin fixation.[15,31] Zonal necrosis has been correlated with large mediastinal tumors (≥10 cm), and poor survival in 1 study.
The medullary thymic B cell is regarded as the putative cell of origin of PMBL and gives rise to a tumor that is largely confined to the mediastinum by definition. Within the lymphoma, thymic structures and epithelial cells can be seen, although organoid lobular thymic architecture is rarely sampled in smaller biopsy specimens. A cytokeratin immunostain can highlight components of remnant thymus. The thymic epithelial meshwork can be highly distorted and effaced or can proliferate and result in cystic changes.[4,30,31] Lymph node involvement by PMBL takes the form of contiguous spread, often through marginal sinuses, to quickly infiltrate the interfollicular areas resulting in a diffusely effaced lymph node with slight capsular fibrosis.
Am J Clin Pathol. 2021;156(4):497-512. © 2021 American Society for Clinical Pathology