Radiotherapy for Iliopsoas Muscle Metastasis from Undifferentiated Nasopharyngeal Carcinoma in a Young Patient: A Rare Case Report

Nasopharyngeal carcinoma (NPC) typically metastasizes to bone, lung, liver, and distant lymph nodes, whereas skeletal muscle involvement is exceptionally rare and often diagnostically challenging. We report a 15-year-old patient with locally advanced undifferentiated nasopharyngeal carcinoma (UCNT) who achieved locoregional control after induction chemotherapy followed by definitive concurrent chemoradiotherapy, but developed early distant relapse manifesting as a symptomatic iliopsoas muscle metastasis with associated iliac bone destruction. Imaging findings were suggestive but not specific, and histopathologic confirmation was required to establish the diagnosis. Palliative radiotherapy delivered to the iliopsoas lesion (30 Gy in 10 fractions) resulted in marked pain relief, functional improvement, and sustained local stabilization, despite subsequent systemic progression. This case underscores the aggressive potential of UCNT, highlights skeletal muscle as a rare site of dissemination, and reinforces the pivotal role of radiotherapy in achieving durable local control and meaningful symptomatic benefit in this uncommon metastatic presentation.