Implementation of total skin electron therapy using an optional high dose rate mode on a conventional linear accelerator.

A technique for total skin electron therapy (TSET) has been implemented using a standard accelerator that has been equipped with an optional special procedures mode to permit high dose-rate therapy with a 6-MeV electron beam. Patients are treated in a standing position using dual angled fields at a source to skin distance of 3.6 m. Dosimetric characteristics of the dual field technique were investigated for the 6-MeV beam as well as for a lower energy beam produced by the introduction of an acrylic beam degrader. A treatment stand, which incorporates the degrader in addition to devices used for patient support and shielding, is described. Acceptable beam uniformity and depth dose have been achieved while maintaining a low level of x-ray contamination. Treatment times are reasonably short since the output of the machine in the high-dose-rate mode is 25 Gy/min at the isocenter. Beam uniformity, dose rate, and x-ray contamination are relatively unaffected by the presence of the beam degrader if it is positioned near the treatment plane. The high dose-rate electron option is a useful treatment mode that provides the advantage of reduced treatment times while retaining proper functioning of all accelerator dosimetry systems and interlocks. Use of a dual field technique permits TSET in a treatment room of standard dimensions. The machine is easily set up for treatment, and patient setup is simplified through use of a customized support system.