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Diagnostic utility of programmed cell death ligand 1 (clone SP142) in mediastinal composite lymphoma: A report of two cases

Composite lymphoma is a well‐known diagnostic entity exhibiting the synchronous occurrence of two or more distinct types of lymphomas in the same specimen. Here we report two patients, a 14‐year‐old female (Case 1) and a 45‐year‐old male (Case 2), with mediastinal composite lymphoma, comprising nodular sclerosis classic Hodgkin lymphoma (NSCHL) and primary mediastinal large B‐cell lymphoma (PMBL). Both patients had a mediastinal mass, and manifested two different histologic components in the same biopsy, one characteristic of NSCHL and the other PMBL. The NSCHL areas included Hodgkin and Reed–Sternberg (HRS) cells with typical immunophenotypic features (CD30‐positive and CD20‐negative), whereas the sheets of large tumor cells characteristic of PMBL were strongly and uniformly CD20‐positive. Interestingly, although both cases showed neoplastic PD‐L1 (nPD‐L1) positivity on the HRS cells of NSCHL, they differed regarding nPD‐L1 expression on the PMBL tumor cells. In Case 1, the nPD‐L1‐negative PMBL component was anatomically situated outside the NSCHL lesion. On the other hand, in Case 2, the nPD‐L1‐positive PMBL component was characterized by transitional or continuous areas with the NSCHL component. These findings suggested that nPD‐L1 expression may define two subtypes of PMBL that are more similar to or distinct from classic Hodgkin lymphoma.