A hidden site: positron emission tomography-computed tomography (PET-CT) unveiling subclinical masseter involvement...

Extranodal lymphoma involvement refers to lymphoma arising outside primary lymphatic organs. Muscular involvement is rare. This case report highlights the critical role of positron emission tomography-computed tomography (PET-CT) in detecting underestimated extranodal lesions, especially in muscles. We report the case of a 66-year-old patient with no prior significant medical history who presented with 3 months of cervical lymphadenopathy, asthenia, and night sweats. Examination revealed supracentimetric elastic cervical and inguinal lymphadenopathy and splenomegaly. Computed tomography suggested nodal disease above and below the diaphragm. Biopsy confirmed grade 3B follicular lymphoma and bone marrow infiltration. Positron emission tomography-computed tomography revealed intense fluorodeoxyglucose (FDG) uptake in nodal regions, spleen, bone marrow, and left masseter muscle, which was not visible on CT. The disease was staged as Ann Arbor stage IV, and the patient was planned for 8 cycles of R-CHOP and intrathecal methotrexate. As a final point, ¹⁸F-FDG PET/CT is indispensable for accurate staging and assessment of treatment response in follicular lymphoma, permitting detection of extranodal disease not visible on CT.