Purpose: Total skin electron therapy (TSET) generally requires equipment modifications to meet its specific needs. Since the electron beams are used at an extended distance, the dose rate decreases significantly on the patient skin. Previously, to increase dose rate for treatments at 700 cm, a customized 6 MeV TSET electron beam from a Varian Clinac-6 was created by removing the X-ray target, flattening filter, etc. from the electron beam path while operating in photon mode (Chen, et. al., IJROBP 59 p.872, 2004). This study, however, investigates an electron beam without electron scattering foils and explores the feasibility of using it to treat TSET patient at a distance less than 700 cm.
Methods: TSET requires the field size of the composite electron beams at the patient treatment plane to be 200 cm x 80 cm, which promotes dosimetrically required dose uniformity at the specified patient treatment distances. The EGSnrc system was used to generate 6 MeV electron beams with and without scattering foils. The simulated beams were analyzed to obtain the change of beam characteristics as a function of extended distances.
Results: The simulated electron beams are able to reproduce the measured data and show that the beam dose rate can be increased by up to 8 times by removing the electron scattering-foils. The electron fluence as a function of distances off beam central axis drop faster when scattering foils are removed. The electron fluence at 50 cm off axis relative to central axis is 75% (39%), 90% (79%), and 95% ((91%) for a 6 MeV beam with (without) electron scattering foils at a distance of 300 cm, 500 cm and 700 cm, respectively.
Conclusion: A scatter-foil free electron beam is feasible to provide required dose uniformity at SSD ≥500 cm by using a dual-field technique.