原発性脾臓リンパ腫における臨床的および診断的課題：SMZLとDLBCLを鑑別するために脾臓摘出が必要なのはいつか？

Primary splenic lymphoma (PSL) is a rare form of lymphoproliferative neoplasm, accounting for less than 2% of all lymphomas and approximately 1% of non-Hodgkin lymphomas. While the spleen is often involved in advanced lymphoproliferative disorders, its isolated primary involvement presents significant diagnostic challenges. PSL includes both indolent subtypes, such as splenic marginal zone lymphoma (SMZL), and aggressive forms such as diffuse large B-cell lymphoma (DLBCL).

We report two cases of PSL with distinct clinical and histological features. The first case involves a 65-year-old male with prior radioiodine treatment for thyroid carcinoma and occupational radiation exposure, who presented with progressive splenomegaly over four years. Splenectomy revealed SMZL characterised by infiltration of small, mature CD20-positive B-cells and a low proliferative index. The patient remains under observation without systemic treatment. The second case concerns a 78-year-old female with a history of breast cancer and hysterectomy, presenting with constitutional symptoms, splenomegaly and diffuse abdominal pain. Imaging demonstrated hypermetabolic splenic lesions and cervical lymphadenopathy. Splenectomy confirmed DLBCL with high Ki-67 index and CD20, BCL6 expression. Systemic chemotherapy with modified rituximab, cyclophosphamide, hydroxydaunorubicin (doxorubicin), vincristine and prednisone (R-CHOP) was initiated.

These cases underscore the diagnostic complexity of PSL, highlighting the importance of histopathological confirmation to distinguish between indolent and aggressive subtypes. In patients with isolated splenomegaly and non-specific systemic symptoms, timely evaluation - including imaging and splenectomy - is critical for accurate diagnosis and appropriate management.

Unexplained splenomegaly warrants consideration of primary splenic lymphoma in the differential diagnosis, even without lymphadenopathy or classic B symptoms, particularly when detected during abdominal imaging for hepatosplenic evaluation.Histopathologic and molecular confirmation is essential, as indolent subtypes (e.g. SMZL) and aggressive subtypes (e.g. DLBCL) may present identically but require fundamentally different treatment approaches.PET/CT-guided diagnostic strategy optimises outcomes by enabling accurate staging, identifying the most metabolically active site for biopsy, and informing the need for splenectomy versus direct systemic therapy.