Monotypic plasma cell proliferation of uncertain clinical significance mimicking interstitial cystitis: an early lesion of MALT lymphoma?

We prospectively studied our institutional experience of bladder extranodal marginal zone (mucosa-associated lymphoid tissue [MALT]) lymphoma including bladder biopsies in which the possibility of MALT lymphoma was considered. We identified a subset of cases primary to the urinary bladder, presenting with prominent plasma cell infiltrates and symptoms mimicking bladder pain syndrome/interstitial cystitis. These proliferations were designated for this study as “monotypic plasma cell proliferation of uncertain clinical significance” (MPCP-US), as the features were insufficient for diagnosis of MALT lymphoma. We identified 33 patients, consisting of 22 cases of MPCP-US (6 of which were associated with amyloid deposition) and 11 cases of MALT lymphoma. MPCP-US was more prevalent in men (73%), a mass lesion was not identified at cystoscopy, and only 1 case had an accompanying urinary tract infection (4.5%). Histologically, MPCP-US presented as monotypic plasma cells arranged in a superficial band-like distribution in the lamina propria, predominantly kappa restricted (68%) and IgA⁺ or IgM⁺ (64% and 23%, respectively) and without a histologic mass of atypical B cells or plasma cells, not diagnostic for established MALT lymphoma or plasmacytoma. Secondary involvement of the bladder by other lymphoproliferative disorders was excluded and there was no evidence of progressive disease. MALT lymphomas are presented for comparison and our analysis demonstrated that MPCP-US represent a different clinicopathologic entity compared with classic MALT lymphoma. We present the first series of cases of MPCP-US. The recognition of this entity is fundamental to the development of management protocols to relieve intractable symptoms mimicking bladder pain syndrome/ interstitial cystitis in these patients.