SU‐FF‐T‐387: Rotational Total Skin Electron Irradiation Using a Commercially Available Linear Accelerator with a High Dose Rate Total Body Electron Mode

Abstract

Purpose: During the past 25 years, 185 patients have been treated with a rotational total skin electron irradiation (RTSEI) technique in our center. To modernize the technique we recently transferred it from a linac (Varian Clinac‐18) with a custom modified beam line to a linac (Varian 21EX) with a commercially available electron mode intended for total skin electron irradiation. Methods and Materials: The new technique uses a “high dose rate” mode and a “high dose per Monitor Unit” mode in conjunction with a custom‐made flattening filter to produce a uniform beam at an extended SSD of 378 cm. The accessory tray holds the custom‐made flattening filter and automatically selects the beam energy (6 MeV) and high dose rate (888 MU/min) while moving the collimators to the maximum 40 × 40 cm2 field size. Beam parameters are monitored using the record‐and‐verify (VARIS) system. Results: Reference dosimetry for the stationary and rotational electron fields was performed to allow delivery of the prescription dose using the linac's transmission ionization chamber. Patients are treated on a rotating platform with a high dose rate rotational electron beam having a zmax at the skin surface, an R50 at 15 mm and a bremsstrahlung contamination of the order of 3%. The nominal dose rate to water at zmax (surface) for the rotational technique was determined to be 24.1 cGy/1000 MU, and beam delivery is monitored with a secondary Farmer‐type ionization chamber located near the patient in the treatment field. Conclusions: Treatment times with the rotational total skin electron irradiation technique at an SSD of 378 cm for a daily dose of 2.0 Gy are of the order of 9.5 minutes and to date we have treated 15 patients with this technique.