Physical aspects of a rotational total skin electron irradiation

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Abstract
A technique for rotational total skin electron irradiation [for malignancy] is presented in which the patient stands on a slowly rotating platform (SSD [source-skin distance] = 285 cm) in a large uniform linear accelerator electron field (E0 = 3.5 MeV). The beam is scattered by the transmission ionization chamber and by a special Pb/Al scattering filter, and then degraded by a sheet of lucite. A Farmer chamber is used as a patient dose monitor and a method for absolute dose calibration is presented. The field is uniform to within .+-. 5% for dimensions of 180 .times. 40 cm2. The surface dose for rotational therapy is equal to 45% of the maximum dose in a stationary beam. The rotating beam exhibits a dose maximum on the surface, falls to 80% at 0.5 cm and has an X-ray contamination of .apprx. 4%. The surface dose rate is about 25 cGy[gray]/min for the rotating beam. The rotational beam percentage depth dose distributions, calculated using stationary beam information, agree well with measured data. The stationary beam exhibits a dose maximum at 4 mm in tisuse, a surface dose of 93%, 80% dose at a depth of 1 cm, a practical range of 1.75 cm, and an X-ray contamination of 2.5%. The rotational total skin electron irradiation significantly reduces the patient treatment and setup time and solves the problem of beam matching, when compared to standard multiple-beam techniques.

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