Purpose: The electron beams for total skin electron therapy (TSET) are always used at extended distances and degraded by a scatter plate. Dose measurements according to AAPM TG-51 dosimetry protocol require the beam quality conversion factor, kQ, which is derived from a standard beam (SSD=100 cm with 10×10 cm²). Since kQ is directly related to the water/air stopping-power ratios (SPRs) at electron reference depth, dref, it is important to kwon if the SPRs of a clinical degraded beam differ from that of a standard beam. This study calculates SPRs for these beams in order to determine if kQ is still applicable.
Methods: The calculation of SPRs in this study is carried out in two steps, viz. the generation of the clinical incident beams followed by their use as incident beams to compute the SPRs. The Monte Carlo code BEAMnrc is used to simulate clinical 6 MeV HDTEe beams form Varian accelerators.
Results: R₅₀ in water decreases approximately 1 mm for each additional 100 cm extended distance ranging from 2.24 cm at SSD=100 to 1.72 cm at SSD=700 cm. For beams passing through a scatter plate, R50 is 1.76 cm (1.14) at SSD=300 and 1.48 cm (0.85 cm) at SSD=600 cm with an Acrylic plate thickness of 3 mm (9 mm) respectively. Although significant differences in calculated SPRs as a function of depth occur at depth beyond dmax between a clinical beam and a standard beam, less than 1% variation is found at dref for all beams.
Conclusion: The SPRs as a function of depth for a standard electron beam provided by AAPM TG-51 protocol may not be accurate to convert a measured depth-ionization curve into a depth-dose curve accurately. However, kQ in the protocol, is still applicable in the dose determination at dref for a beam used in TSET.
Not Applicable / None Entered.