Marginal zone lymphoma is a non-Hodgkin lymphoma arising from postgerminal centre marginal zone B cells. According to the 2016 World Health Organization (WHO) classification, MZL is subdivided into three types: extranodal MZL or MALT lymphoma, nodal MZL and splenic MZL [11].MALT lymphoma is the most typical type, but primary CNS MALT lymphoma is an extremely rare entity, especially in the brain parenchyma. Initial studies showed that the most common location was the dura [12]. Only 7 cases with brain parenchyma involvement have been reported, including our patient. The site of origin was the midbrain in our patient. The lesion location and clinical characteristics of the other 6 patients are shown in Table 1. Clinical symptoms are not specific, depending on the site of the lesion.
The CNS has no mucosa or lymphoid tissue, and dural-based MALT lymphoma can be explained by the embryological analogy that meningothelial cells of the arachnoid membrane could be analogous to epithelial cells, where MALT lymphomas arise [16]. However, non-dural-based MALT lymphoma is questionably explained by this theory. It is currently believed that the aetiology of MALT lymphoma is related to chronic immune stimulation caused by infection or inflammation. For instance, gastric MALT lymphoma is associated with Helicobacter pylori, Sjögren syndrome or Hashimoto thyroiditis and carries a significant risk for the development of MZL [17]. Interestingly, our patient had a 1-year history of tuberculosis and received standardized anti-tuberculosis treatment. After admission, the Helicobacter pylori examination was positive, and the patient also underwent Helicobacter pylori eradication therapy. The pathogenesis may be explained by the inflammation-based theory. However, we have no direct evidence that primary CNS MALT lymphoma is associated with Mycobacterium tuberculosis or Helicobacter pylori infection.
The diagnosis of MALT lymphoma should be confirmed by histopathological and immunohistochemical features. Differential diagnoses include lymphoplasmacytic lymphoma (LPL) and follicular lymphoma. The immunohistochemistry results of follicular lymphoma usually indicate positivity for CD10 and Bcl-2 [18]. LPL and MALT lymphoma have similar morphological and immunohistochemical profiles, but relative to MALT lymphoma, LPL typically involves the bone marrow and is associated with Waldenstrom’s macroglobulinemia [19]. Our patient’s immunohistochemical findings indicated CD20+ and CD79a+ results, without bone marrow involvement, and no clinical history of Waldenstrom’s macroglobulinemia. At the same time, clonal rearrangement of IgH was detected by PCR. According to these findings, the diagnosis was most consistent with MALT lymphoma.
MALT lymphoma tends to be indolent. There is no standard treatment for CNS MALT lymphoma. The treatment modalities reported in the existing literature include surgery, radiotherapy, and chemotherapy. As shown in Table 1, among patients with lesions arising from the brain parenchyma, 3 of the 6 patients received chemotherapy: two patients had stable disease, one patient showed tumour remission, the other 3 patients received radiotherapy and had a complete response. MALT lymphoma is radiosensitive. In 2011, a randomized phase III trial reported that there was no difference in clinical efficacy between the radiotherapy dose of 24 Gy and 40-45Gy for indolent NHL [20]. Currently, reduced-dose (24-30 Gy) radiotherapy is preferred for indolent lymphoma. Unlike high-grade CNS lymphoma, the role of intrathecal chemotherapy or systemic chemotherapy currently remains unclear in low-grade CNS lymphoma [21]. Because of the particularity of the lesion location, our patient could not be totally resected by surgery and achieved complete remission by radiotherapy alone. Involved-site radiation therapy (ISRT) is an effective initial treatment for extranodal marginal zone lymphoma [22]. The radiation field in our case included only the primary lesion demonstrated on MRI and PET, not as reported in the prior literature [7,8,13]. Reexamination during treatment showed residual disease, so we believed that one month after the end of radiotherapy might be the best time to evaluate the effect.