Imaging findings of primary bladder MALT lymphoma: A case report

Abstract

We report the case of a 64-year-old woman diagnosed with primary bladder MALT lymphoma. She was referred to our hospital after body CT during a medical checkup revealed bladder tumors. She had no significant symptoms. Blood tests and urinalysis conducted at presentation showed no abnormalities. Contrast-enhanced CT revealed two tumors protruding into the bladder lumen with smooth margins and broad bases on the posterior wall, exhibiting homogeneous contrast enhancement. On MRI, these tumors showed mild hyperintensity on T2-weighted imaging and marked diffusion restriction on diffusion-weighted imaging, and one tumor was considered to have a stalk. Cystoscopy suggested that the tumors were submucosal. Transurethral resection of the bladder tumor (TUR-BT) was performed with the preoperative diagnosis of bladder cancer due to marked diffusion restriction on MRI. However, the pathological diagnosis was MALT lymphoma. FDG-PET/CT for staging of the lymphoma showed intense FDG uptake in lesions (SUVmax of 20.4 and 24.4) with no significant uptake in other organs indicative of lymphoma involvement. Thus, a final diagnosis of primary bladder MALT lymphoma was made. Distinguishing MALT lymphoma from bladder cancer based solely on CT and MRI findings is challenging. The cystoscopic appearance of a submucosal tumor, along with high SUVmax on FDG-PET and tumor multiplicity, may contribute to the diagnosis of primary bladder MALT lymphoma. If FDG uptake in the tumor bed after TUR-BT is due to a residual tumor, FDG-PET could serve as a useful diagnostic adjunct for an accurate diagnosis. Further studies are warranted to evaluate the utility of FDG-PET for patients with primary bladder MALT lymphoma.

Translated Abstract

膀胱原発MALTリンパ腫と診断された64歳女性の症例を報告する．健康診断での腹部CTで膀胱腫瘍を指摘され当院を受診した．自覚症状はなく，血液，尿検査でも特記すべき所見は認めなかった．造影CTでは膀胱後壁に均一な造影効果のある辺縁平滑で広基性に内腔に突出する腫瘍を2つ認め，MRIではそれぞれT2WIで軽度高信号，強い拡散制限を認めた．膀胱鏡では粘膜下腫瘍を疑う所見であったが，MRIの矢状断では腫瘍の1つは有茎性にも見え，粘膜下腫瘍との判断は困難であった．MRIで強い拡散制限を呈していたことから膀胱癌を疑いtransurethral resection of the bladder tumorが施行されたが，病理結果はMALTリンパ腫であった．膀胱外の病変確認目的に撮像されたFDG-PET/CTでは，膀胱以外にリンパ腫病変を疑う有意な集積は認めなかったが，術後の病変部位には残存病変を疑う尿よりも強いFDGの集積を認めた．CT, MRIでは膀胱癌との鑑別は容易ではない．が，もし本症例の病変部位に認めた強いFDGの集積が残存病変によるものであれば，膀胱鏡とFDG-PET/CT所見の組み合わせ，また病変が複数存在することは本疾患の診断の一助になる可能性がある．膀胱原発MALTリンパ腫におけるFDG-PETの有用性についてはさらなる研究が望まれる．

INTRODUCTION

Primary bladder malignant lymphoma is rare because the bladder lacks lymphoid tissue, accounting for less than 1% of all primary bladder tumors.^{1, 2} The associated symptoms are nonspecific, including hematuria, recurrent urinary tract infections, and dysuria. Similarly, common risk factors are nonspecific and include chronic inflammation, urinary tract infection, and autoimmune diseases.^3-5 Therefore, distinguishing primary bladder lymphoma from bladder cancer, the most common primary bladder tumor, is clinically challenging. We report a case of primary bladder MALT lymphoma, emphasizing its imaging features.

CASE REPORT

A 64-year-old female with no significant symptoms was referred to our clinic after body CT during a medical checkup revealed bladder tumors. Her medical history included Hashimoto's disease, which was regularly monitored. She was not taking any medication and had never smoked. Blood tests conducted at presentation showed no atypical lymphocytes, and no abnormal laboratory results were noted, including lactate dehydrogenase levels. Soluble interleukin-2 receptor was not assessed at that time. Additionally, urinalysis showed no hematuria or occult blood, and urine cytology was negative.

Contrast-enhanced CT showed two tumors measuring 15 × 14 × 5 mm and 17 × 18 × 8 mm protruding into the lumen with smooth margins and broad bases, located on the posterior bladder wall (Figure 1). Both exhibited homogeneous contrast enhancement, with no obvious extravesical invasion, no significantly enlarged lymph nodes, and no mass lesions outside the bladder within the scanning area. On MRI, the tumors showed mild hyperintensity on T2-weighted imaging and marked diffusion restriction on diffusion-weighted imaging, with one tumor considered to have a stalk (Figure 2). Cystoscopy showed that the tumors had smooth borders and intact overlying mucosa, findings suggestive of submucosal tumors (Figure 3).

Figure 1.

Contrast-enhanced CT demonstrated two bladder tumors with smooth margins and broad bases (white arrows) in sagittal (a) and axial (b, c) sections.

Figure 2.

MRI demonstrated two bladder tumors (white arrows) with intermediate hyperintense signals on axial T2-weighted imaging (a, b) and significant diffusion restriction on axial diffusion-weighted imaging and the ADC map (c, d). The caudal tumor had a stalk based on sagittal T2-weighted imaging (e).

Figure 3.

Cystoscopy revealed that the tumors (white arrows) had smooth margins and displayed no abnormalities on the mucosal surface.

Based on cystoscopic findings suggestive of submucosal tumors, mesenchymal tumors, including leiomyoma, were considered. However, MRI showed marked diffusion restriction, suggesting malignancy and one tumor was considered to have a stalk. Given that 90% of bladder tumors are bladder cancer, a preoperative diagnosis of bladder cancer was made, and transurethral resection of the bladder tumor (TUR-BT) was performed.⁶

The pathological specimen of TUR-BT showed a cluster of atypical lymphocyte-like cells with plasma cells in the stroma on hematoxylin and eosin staining with lymphoepithelial lesions (Figure 4). The specimen contained muscle layers, with no tumor invasion observed. Immunohistochemistry revealed that the neoplastic lymphoid cells expressed CD20, a pan-B-cell marker, and BCL2, an anti-apoptotic protein (Figure 4). The Ki-67 proliferation marker level was low. Immunohistochemistry for CD3, CD5, and CD10 was negative. A diagnosis of low-grade MALT lymphoma was made based on the results of immunohistochemistry and presence of lymphoepithelial lesions. The resection margins could not be evaluated in the collected pathological specimens.

Figure 4.

Histopathological sections showed a cluster of atypical lymphocyte-like cells with plasma cells in the stroma along with lymphoepithelial lesions within the uroepithelium (black circles) on HE staining (a). Immunohistochemistry revealed that these cells expressed CD20 (b) and BCL2 (c).

FDG-PET/CT was performed for staging of MALT lymphoma, which showed intense uptake in the bladder tumor bed (SUVmax of 20.4 and 24.4), being higher than in urine, but no corresponding mass was found (Figure 5). No significant FDG uptake in other organs suggestive of systemic lymphoma involvement was observed (Figure 5). Thus, a final diagnosis of primary bladder MALT lymphoma was made. Since pathological analysis confirmed the diagnosis of low-grade MALT lymphoma, the patient did not receive chemotherapy or radiation therapy but was closely monitored instead. FDG-PET/CT performed 5 months later showed the absence of accumulation in the bladder tumor bed after TUR-BT. She has been followed regularly every three months since that time and remained free of recurrence for 11 months following TUR-BT.

Figure 5.

A maximum intensity projection (MIP) image of FDG-PET showed FDG uptake in the lesion area (white arrows) and diffuse FDG accumulation in the thyroid gland (black arrows) corresponding to pre-existing chronic thyroiditis, and mild accumulation in cervical lymph nodes (black arrowheads), considered to represent reactive change (a). FDG accumulation was observed (white arrows) in the lesion area on axial images, with SUVmax of 20.4 and 24.4 (b, c), but no corresponding mass was found on fused PET/CT or non-contrast CT images (d, e, f, g).

DISCUSSION

Primary bladder malignant lymphoma is a rare condition, accounting for less than 1% of all primary bladder tumors, due to the absence of lymphoid tissue in the bladder.^{1, 2} The most common histological type is MALT lymphoma, with a male-to-female ratio of 1:5.6.^{4, 7} According to our research, only 24 cases of primary MALT lymphoma of the bladder have been reported in Japan, with a total of 64 cases worldwide.^8-14 There are no clearly defined risk factors, and symptoms are nonspecific, including hematuria, recurrent urinary tract infections, and dysuria.^{3, 4} One hypothesis proposes that chronic inflammation, such as cystitis, may lead to the growth of extranodal lymphoid tissue, increasing the risk of progression to lymphoma.^{1, 15}

Only a few case reports describe radiological findings of primary bladder MALT lymphoma, and characteristic imaging features are not clearly defined. Previous reports indicated that CT shows the tumor with a smooth margin and uniform contrast enhancement, and MRI reveals a similar morphology, with a low signal on T1-weighted, intermediate signal on T2-weighted, and high signal on diffusion-weighted imaging.^{2, 8, 16} On cystoscopy, these tumors present as well-defined submucosal masses with intact overlying mucosa, occasionally accompanied by edema or ulceration.^{8, 17} Additionally, one report of primary bladder MALT lymphoma included FDG-PET/CT after TUR-BT, which showed marked FDG uptake with SUVmax of 17.3.⁹ Our imaging findings were largely consistent with these previous reports. However, we found no previous reports of multiple lesions, as observed in our patient.

Several factors may account for the strong FDG uptake observed in the tumor bed in this case. First, a residual tumor is one possibility, as reported in a previous case.⁹ However, no mass corresponding to the area of FDG uptake was identified. Additionally, although a correlation between the Ki-67 index and SUVmax has been documented in malignant lymphoma,^{18, 19} a discrepancy was noted in this case. Second, postoperative inflammation might have been contributory, although SUVmax was unusually high for typical postoperative changes.^20-23 Lastly, physiological FDG accumulation in the posterior bladder, known as “posterior bladder layering”, occurs in approximately 4% of cases and could potentially explain uptake in the postoperative area.²⁴ However, this finding was not reproduced on follow-up FDG-PET/CT. Therefore, the significance of FDG uptake in the tumor bed remains unclear. Differential diagnoses of bladder tumors include: bladder cancer as an epithelial tumor; and leiomyoma, paraganglioma, hemangioma, leiomyosarcoma, rhabdomyosarcoma, and malignant lymphoma as submucosal tumors.⁶ Differentiating primary bladder MALT lymphoma from these tumors is challenging, but several imaging findings may support the diagnosis. MRI findings can be useful to distinguish submucosal tumors from benign tumors with a more characteristic appearance, such as leiomyoma and paraganglioma.⁶ Cystoscopy helps determine whether the tumor is mucosal or submucosal. If FDG uptake in the tumor bed after TUR-BT is due to a residual tumor, FDG-PET may be useful for diagnosing primary bladder MALT lymphoma. Submucosal bladder tumors with strong FDG uptake include sarcomas and malignant lymphomas. Sarcomas tend to be invasive and are unlikely to present as multiple lesions. MALT lymphoma is the most common histological type of primary bladder malignant lymphoma and also known to be FDG-avid when occurring in other organs.^{19, 25} Thus, the cystoscopic findings of a submucosal tumor, along with high SUVmax on FDG-PET and tumor multiplicity may facilitate a diagnosis of primary bladder MALT lymphoma.

CONCLUSION

We report a rare case of primary bladder MALT lymphoma. Differentiating it from bladder cancer based solely on CT and MRI findings is challenging. Cystoscopic findings of a submucosal tumor, combined with high SUVmax on FDG-PET and tumor multiplicity may contribute to the diagnosis of primary bladder MALT lymphoma. If FDG uptake in the tumor bed after TUR-BT is due to a residual tumor, FDG-PET could serve as a useful diagnostic adjunct for an accurate diagnosis. Further studies are warranted to evaluate the utility of FDG-PET for patients with primary bladder MALT lymphoma.

COI

The authors report no conflicts of interest directly relevant to the content of this article.

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