A Rare Case of Primary Follicular Lymphoma in the Retroperitoneum.

Primary retroperitoneal non-Hodgkin lymphoma (NHL) is considered sporadic, difficult to diagnose, and has an atypical presentation. NHL typically presents with non-tender peripheral lymphadenopathy in two-thirds of patients. Very rarely, it may present with abdominal swelling and obstructive symptoms of the respiratory and GI tracts. The primary gastrointestinal NHL accounts for 5-20% of all extra-nodal lymphomas. However, there are only a few reported cases of primary retroperitoneal lymphomas. Among them, most were diffuse large B cell lymphomas. This case is of a 54-year-old male patient who presented to the emergency department with abdominal swelling for the past four months. The CT abdomen and pelvis were indicative of massive retroperitoneal and mesenteric adenopathy with concerns for lymphoma with some volume ascites. He underwent an interventional radiology (IR)-guided retroperitoneal lymph node biopsy. Given his pleural effusion on the CT abdomen and pelvis, a CT chest was ordered, which showed loculated left-sided pleural effusion and trace pleural effusion on the right side. His lymph node biopsy results revealed grade II follicular lymphoma. This case highlights the atypical presentation of follicular lymphoma and that it should be considered in the differential diagnosis when a patient presents with pleural effusion or ascites, even though there are no B symptoms. Because of its indolence, follicular lymphoma is often diagnosed when a patient presents with obstructive symptoms and other complications like peritoneal carcinomatosis. Early diagnosis and treatment can lead to good clinical outcomes.