Low-grade B-cell non-Hodgkin lymphoma in the sternocleidomastoid muscle: a case report and review of literature.

Primary non-Hodgkin lymphoma (NHL) arising in skeletal muscle is exceedingly rare. We present a case of low-grade B-cell NHL involving the sternocleidomastoid muscle. A 67-year-old male presented with an asymptomatic, slowly growing left neck mass. Imaging revealed a 17 mm × 11 mm intramuscular nodule, initially suggestive of a benign lesion. Excisional biopsy demonstrated lymphoid proliferation with muscle infiltration. Immunohistochemistry revealed CD20(+), PAX5(+), BCL2(+), and monoclonal IgH rearrangement, consistent with low-grade B-cell NHL, most likely extranodal marginal zone lymphoma (ENMZL), non-gastric/non-cutaneous type. Staging confirmed Lugano stage I disease. Given the indolent nature, the patient was managed with active surveillance. This case highlights an unusual intramuscular presentation of NHL, emphasizing the importance of histopathological and molecular evaluation for accurate diagnosis and tailored management.