Lymphoma or Hematoma: Emergency Department Recognition of a Diagnostic Pitfall.

Neck swelling with upper-extremity edema in a cancer patient presenting to the emergency department (ED) is a concerning clinical picture that may indicate underlying venous obstruction or malignancy. A 74-year-old woman developed a rapidly enlarging left supraclavicular mass after core needle biopsy of a lesion later diagnosed as high-grade follicular large B-cell lymphoma. Follow-up imaging initially suggested a postprocedural hematoma; however, bedside Doppler ultrasound in the emergency department demonstrated internal vascular flow, prompting repeat contrast-enhanced computed tomography (CT), which confirmed aggressive tumor progression with venous compression rather than bleeding. The patient was admitted for emergent inpatient Rituximab, Cyclophosphamide, Doxorubicin, Vincristine, and Prednisone (R-CHOP) chemotherapy, which resulted in rapid symptomatic improvement and complete metabolic remission. This case highlights the diagnostic ambiguity of rapid postbiopsy enlargement of malignant neck masses and underscores the importance of bedside ultrasound, multidisciplinary evaluation, and reconsideration of initial imaging interpretations to avoid delays in definitive therapy.