Background: Anaplastic large cell lymphoma (ALCL), ALK-positive, is a rare subtype of T-cell non-Hodgkin lymphoma in childhood and adolescence. Primary presentation in soft tissues with neuro-meningeal involvement is uncommon, often mimicking benign musculoskeletal conditions and leading to diagnostic delays.Case Presentation Summary: A previously healthy 16-year-old female presented to the emergency department (ED) with a one-week history of pain, warmth, and swelling of the left leg, without trauma or fever. No improvement was observed with non-steroidal anti-inflammatory drugs. She denied hormonal contraceptive use, smoking, or recent travel. Physical examination revealed calf firmness, cold foot, and pain on dorsiflexion. Doppler ultrasound excluded deep vein thrombosis and suggested intramuscular hematoma. The patient was discharged with symptomatic treatment.Six weeks later, she returned to the ED due to persistent pain and a new nodular swelling in the proximal left leg. Ultrasound revealed two nodular lesions with atypical vascularization, and magnetic resonance imaging suggested rhabdomyosarcoma. Biopsy confirmed ALK-positive ALCL (CD30+, ALK+). Positron emission tomography revealed regional lymph node involvement. Cerebrospinal fluid cytology showed 38.4% CD30+ T lymphocytes, confirming neuro-meningeal involvement. Chemotherapy was initiated with a favorable clinical response.Discussion: Persistent musculoskeletal pain in adolescents, even without systemic symptoms, can be the first sign of rare malignancies such as ALK-positive Anaplastic Large Cell Lymphoma. Clinical reassessment and targeted imaging are crucial for early diagnosis when initial findings are inconclusive. Multidisciplinary collaboration among pediatrics, radiology, pathology, and oncology ensures prompt diagnosis and initiation of therapy. This case serves as a teaching point for clinicians to avoid delayed diagnosis in atypical presentations, optimizing outcomes.
