Aberrant immunophenotypes break classification rules, cause diagnostic uncertainties and may contribute to therapeutic dilemmas. An unusual case of cyclin D1-positive follicular lymphoma published recently in this journal showed a three-way translocation involving IgH, BCL2 and CCND1 genes.1 We present a case of a true low-grade follicular lymphoma expressing cyclin D1, without evidence of the CCND1 translocation. Confusingly, an incidental unrelated Cyclin D1-positive plasma cell clone was discovered on bone marrow staging.

A 77-year-old male patient presented with dysphagia due to large right tonsillary enlargement. He was well without B symptoms. His peripheral blood counts, serum calcium and renal function were normal. CT of the neck, chest, abdomen and pelvis showed no lymphadenopathy or other organomegaly. An IgG κ paraprotein of 10.8 g/l was demonstrated, with mild immune paresis (IgA 0.72 g/l, IgM 0.38 g/l) and abnormal serum-free light chains (κ 49.93 mg/l, λ 6.53 mg/l). Skeletal survey showed no lytic lesions.

The excised right tonsil was packed by lymphoid follicles dominated by centrocytes mixed with few centroblasts, expressing CD20, PAX5, CD10, BCL6, BCL2 and λ and negative for CD3, CD5 and CD43 and κ (figure 1A–D). Surprisingly, most centrocytes and centroblasts were also positive for cyclin D1 (figure 1E,F). Expression of cyclin D1 followed patterns seen for CD10 and BCL6, including focal spillage of positive cells outside confines of follicles. Additionally, there were a few neoplastic follicles at the margin of the tonsil, which had features reminiscent of the in situ pattern of follicular lymphoma, featuring better preserved mantle zones, surrounding smaller germinal centres composed …