Purpose: Electron beam from a Linear accelerator is commonly used in total skin electron Therapy (TSET) for advance cutaneous T-cell lymphoma. The high percentage bremsstrahlung dose up to 5% was reported for a 6 MeV electron beam at extended distance of 500 cm from source by Das et al. (Med Phys 21, p.1733, 1994). The significant increase in bremsstrahlung dose is a clinical concern as the electron beam for TSET is always used at extended distances. This study re-evaluates the reported high bremsstrahlung dose by both measurements and Monte Carlo simulations.
Methods: We measured bremsstrahlung doses of 6 MeV HDTSe beams from Varian accelerators by using different detectors at extended distances. The Monte Carlo simulations were done by using the EGSnrc Monte Carlo codes, BEAMnrc/DOSXYZnrc, to simulate incident beams and to calculate dose distribution in the measurement phantoms. The simulated beams with jaws opening to 40 x 40 cm2 were benchmarked with experiments in water phantom.
Results: The result of previous reported high bremsstrahlung dose up to 5% at extended distances can only be reproduced by measurement using a parallel plate ion chamber. The results from Monte Carlo simulations show an insignificant (1%) of percentage bremsstrahlung even at SSD=500 cm. The measurements by using cylindrical chambers and other detectors confirmed the results form Monte Carlo simulations.
Conclusion: Previously reported significant high percentage bremsstrahlung doses for 6 MeV electron beam at extended distances were inaccurate because a parallel plate ion chamber was used in the measurements. The reason that an incorrect high bremsstrahlung dose is measured by a parallel plate chamber needs further investigation. Although the bremsstrahlung tail increases with the increasing of extended distances, the percentage bremsstrahlung doses of a 6 MeV electron beam are 0.5% to 1% for SSD from 100 to 500 cm respectively.
Not Applicable / None Entered.