Comparison of skin doses to large fields using tangential beams from cobalt-60 gamma rays and 4-MV x rays

Excess radiation to the skin during external beam megavoltage radiation therapy has reportedly caused excessive erythema in patients treated with the Clinac 4 linear accelerator on sloping surfaces, but not for similar treatments with cobalt 60. Doses at the epidermal level were measured under geometries simulating sloping surfaces for a Clinac 4 and an Eldorado 8 cobalt-60 teletherapy machine. For equal doses to the axilla, doses to the epidermal layer were similar. When the tumor dose was calculated for the mediastinum, the dose to the skin in the axillary region was 12% higher for the Clinac 4.