An unusual case of a lytic, expanding lesion of the manubrium with histological diagnosis of aneurysmal bone cyst in a 13-year-old girl is presented. After a recurrence following primary surgery, the patient was treated successfully by external beam radiotherapy. A total dose of 25.2 Gy was delivered using conventional fractionation (1.8 Gy day(-1)) to the whole sternum. She remains recurrence- and symptom-free 46 months after the end of the treatment. This is the sixth patient with primary aneurysmal bone cyst in the sternal region, the first paediatric patient for this location, and the first case of its kind treated exclusively by radiotherapy ever reported in the literature. The histopathological, radiological and clinical findings of the patient are presented, relevant literature is reviewed, and radio therapeutic management of such lesions is discussed.