Multiple myeloma (MM) is a malignant plasma cell disorder that typically occurs sporadically. Familial clustering is rare, with only a limited number of cases reported worldwide. Such familial presentations suggest a possible hereditary predisposition or shared environmental risk factors contributing to disease development [1,2]. Here, we present two siblings with distinct plasma cell neoplasms: one with recurrent extramedullary plasmacytoma and the other with multiple myeloma.

The first case was a 69-year-old woman who underwent surgery in 2017 for a proximal femoral mass, diagnosed as plasmacytoma. In 2024, she presented with a cervical swelling; excisional biopsy of a right level-5 lymph node again revealed plasmacytoma. Bone marrow biopsies performed at that time did not show features of multiple myeloma.

Her brother, one year older, was diagnosed with multiple myeloma in June 2025. PET-CT revealed lytic lesions in the axial skeleton, and systemic therapy was initiated.

Familial occurrence of plasma cell neoplasms is exceedingly uncommon. Reported cases often involve either multiple relatives with MM or, less frequently, different manifestations of plasma cell disorders within the same family [3,4]. The present siblings illustrate divergent clinical phenotypes: persistent extramedullary plasmacytoma without myeloma progression in the sister, versus classical MM with lytic bone disease in the brother. This highlights the potential role of shared genetic background with variable penetrance and expression.

Genetic susceptibility loci, immune dysregulation, and epigenetic mechanisms have all been proposed as contributors to familial myeloma [5]. Recognizing such familial patterns may have implications for surveillance strategies in high-risk relatives.

We report a rare familial clustering of plasma cell neoplasms in siblings, underlining the importance of considering hereditary predisposition in plasma cell disorders. Further genetic and epidemiological studies are warranted to elucidate the underlying mechanisms.