Subtle CD20 positivity in the bone marrow of a patient who has a mucosa-associated lymphoid tissue lymphoma should not be regarded as evidence of involvement in the bone marrow

Bone marrow (BM) biopsies of some mucosa-associated lymphoid tissue (MALT) lymphoma patients show scattered or small clusters of CD20+ cells without definite lesions (subtle CD20 positivity). The aim of this study was to evaluate the clinical significance of BM involvement and subtle CD20 positivity in 122 patients diagnosed with MALT lymphoma. Methods and results: Patients were divided into three categories: BM involvement [BM(+)], subtle CD20 positivity, and no BM involvement [BM(-)]. Eleven (9%) showed BM involvement, and 17 (14%) showed subtle CD20 positivity. BM(+) patients had significantly worse progression-free survival (PFS) than BM(-) patients [hazard ratio (HR) 6.25, P=0.01], but there was no significant difference between subtle CD20 positivity and BM(-) patients. Patients with >30 CD3+ cells among 100 nucleated cells in the areas with increased numbers of CD3+ cells had significantly worse PFS than those with <15 CD3+ cells (HR 5.49, P=0.02). BM(+) patients with >30 CD3+ cells had worse PFS than those with ≤30 CD3+ cells (P=0.029), with an extent of BM(+) involvement of >10% positively correlating with >30 CD3+ cells (P=0.015). Conclusions: Patients with BM(+) MALT lymphoma showed significantly worse PFS than those with subtle CD20 positivity and BM(-) MALT lymphoma, but the PFS of patients with subtle CD20 positivity MALT lymphoma was not significantly different from that of those with BM(-) MALT lymphoma. Increased numbers of BM T cells in MALT lymphoma patients might be suggestive of a worse prognosis.