92-year-old man presents with 15 year history of basal cell carcinoma on left thigh. The cancer is now a 9.3 cm x 8.6 cm superficial plaque with a few ulcerations. Patient is being evaluated for treatment with superficial radiation therapy.
Large superficial basal cell carcinomas of the skin in cosmetically non-critical areas such as the thigh can be treated with successfully with radiation therapy. Superficial radiotherapy (SRT) with office based X-ray machines can have significant limitations with such large lesions. The size of the lesion requires a large treatment applicator. In case of Xstrahl 100 SRT machine, a 10 cm diameter applicator is available. However, treating to the edges of this large field is not recommended. Given the 15 cm SSD (source to surface distance), the divergence angles of the 10 cm treatment area are large, especially at the edges. This creates a penumbra effect with variable energy delivery at the edges of the field. (see diagram) In the case of the large thigh lesion, electron based radiotherapy with a linear accelerator allows about 100 cm SSD. Such distance to the lesion results in smaller beam angles at the field edge, limiting the penumbra effect. Thus, in the case of large lesions, electron beam therapy delivers a more uniform treatment with radiation that office-based SRT cannot.