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A 47-year-old woman, seronegative for anti–HTLV-1 antibodies, was incidentally found to have a solitary colonic mass and pericolic lymphadenopathy on computed tomography. Grossly, a yellowish 50 mm × 45 mm × 25 mm mass involved all layers of the colonic wall (panels A-B). The tumor histologically exhibited scattered lymphoid follicles. In the interfollicular area, medium-sized lymphocytes with clear cytoplasm and small round-to-cleaved nuclei proliferated within reactive inflammatory infiltrates with prominent high endothelial venules (panels C-D; hematoxylin and eosin stain, original magnification ×100 and ×400). The atypical lymphocytes expressed CD3, CD4, and multiple T-follicular helper (TFH) cell markers PD-1, ICOS (panels E [PD1], F [ICOS], original magnification ×400), and BCL6, whereas they were negative for CD8, CD20, and EBER-ISH. CD21 immunostaining showed no expansion of the follicular dendritic cell meshwork. In addition, the tumor with identical histological and immunophenotypic features involved the enlarged lymph node (panels G-K, original magnification ×40 to ×400). Polymerase chain reaction analysis demonstrated a clonal T-cell receptor γ gene rearrangement (panel L).

These findings are most compatible with extranodal TFH-cell lymphoma, angioimmunoblastic type (TFHL-AI), pattern 2, primarily arising in the colon with secondary nodal involvement. Representative gene mutations in nodal TFHL-AI, including RHOA, TET2, IDH2, and DNMT3A mutations, were not detected, suggesting nodal-extranodal biological differences. The patient remains asymptomatic and is undergoing CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) chemotherapy. This could be the first report of primary colonic TFHL-AI.

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