The incidence of extranodal presentation of the disease is less than 10% in follicular lymphomas. Follicular lymphoma with plasmacytic differentiation arising in an extranodal site like subcutaneous tissue and bone is uncommon and its natural history and treatment are poorly described in the literature.

Sharing an unusual case report of a resistant extranodal follicular lymphoma with plasmacytic differentiation transformed in Diffuse Large B Cell lymphoma ABC subtype undergoing successful treatment with bone marrow transplantation.

In November 2012 a 48-year-old woman was complaining about knee pain during movements. A CT done at that time demonstrated an osteolytic lesion in her right knee in the lateral condyle. The biopsy of the lesion was consistent with the diagnosis of follicular lymphoma with plasmocytic differentiation. Bone marrow aspiration and total body CT were normal without evidence of other tumor masses. The patient underwent radiation therapy and was in perfect condition until late 2017 when she was presented to the hematology consultation because of some subcutaneous masses on her body. PET CT scan revealed several subcutaneous masses with high FDG uptake, one in her right shoulder (3.5 × 1.8 cm), two on her right breast (6.0 × 3.4 cm and 2.1 × 1.3 cm), one on the left side of her neck (1.5 × 0.6 cm), and one on her left inguinal region (4.0 × 2.3 cm). A biopsy of the mass in her inguinal region revealed the diagnosis of follicular lymphoma with plasmacytic differentiation (CD10, CD20, CD138, and MUM1 positive). She was referred to the hematology department for further evaluation and treatment. On admission, the bone marrow aspiration and biopsy showed no malignant diseases. Due to the perfect clinical condition of the patient, we decided to go with Rituximab monotherapy. But after 4 courses no improvement was seen. So, we decided to go with RCVP therapy but still, the disease was refractory, and the PET CT showed other than the subcutaneous masses, a lytic bone lesion in her left talus. We went with 2 RCHOP therapies and 4 RCHOEP plus Bortezomib and only after that, the patient went into total remission. One year later, the masses started to grow in the same location. A second biopsy revealed high-grade follicular lymphoma. We continued with R-lenalidomide but the disease was still refractory. A third biopsy performed showed a high-grade DLBCL ABC subtype. In this condition, we started salvage therapy with 2 cycles of R-BEGEV protocol and referred the patient to a clinic abroad for autologous bone marrow transplantation. The patient underwent total remission after the protocol and autologous bone marrow transplant. She has been in remission since July 2022.

The transformation of follicular lymphoma with plasmacytic differentiation, positive for MUM1 has a high probability according to literature to be resistant to standard therapy and to progress to diffuse large B cell lymphoma ABC subtype. Therefore, the need for aggressive treatment combined with bone marrow transplantation is important.