An 83-year-old female was treated with mastectomy and postoperative radiotherapy for breast cancer 30 years prior to developing a new small patch of thickening and scaliness on her left upper back, within the previous radiotherapy field. Serial excision biopsies revealed this to be a lymphangioendothelioma with no suggestion of malignancy. In early 2006 the lesion began to enlarge and take on a more erythematous plaque-like appearance. She was reviewed by a specialist dermatologist and an excision biopsy revealed a low-grade cutaneous angiosarcoma; this was approximately 38 years after radiotherapy to this region, the longest reported period between irradiation and in-field angiosarcoma development (the previous being 16 years). To our knowledge, this is the first case of post-radiotherapy angiosarcoma with a diagnosed precursor lesion. The lesion was treated with surgical excision and adjuvant radiotherapy. After further in- and out of- field recurrences, low dose radiotherapy elicited a surprisingly rapid and complete response within the treated areas; this was unusual in that these tumours are characteristically radiation-resistant. The radiosensitive case we report here raises the possibility that radiation should be more widely considered in the therapy of this disease. Methods of treatment of this rare malignancy are discussed.