A 37-year-old man was admitted with an extrathoracic desmoid tumor invading the brachial plexus. Magnetic resonance imaging (MRI) of the thorax demonstrated a 20 x 9 x 14 cm mass in the supraclavicular fossa, axillary fossa and the right side of the chest. It invaded the brachial plexus and circumscribed the subclavian and axillary arteries concentrically. Biopsy revealed desmoid tumor which was resected subtotally with the brachial plexus. The gross residual mass was treated postoperatively with radiotherapy (60 Gy) which resulted in major regression of the mass. Following radiotherapy, ethodolac with ascorbic acid were administered. The tumor was clinically indiscernible 35 months post-radiotherapy. This case shows the high effectiveness of radiotherapy along with less toxic medical treatment modalities in instances where local control is hard to achieve with surgery and highlights the importance of using multidisciplinary treatments to maintain good functional results.