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[Dose Distribution Combinations of Different Electron Beam Energy for Treatment Region Expansion in High-energy Electron Beam Radiation Therapy: A Feasibility Study].

INTRODUCTION
External electron beams have excellent distributions in treatment for superficial tumors while suppressing influence deeper normal tissue. However, the skin surface cannot be given a sufficient dose due to the build-up effect. In this study, we have investigated the combination of electron beams to expand the treatment region by keeping the dose gradient beyond dmax.


MATERIALS AND METHODS
The percentage depth doses of different electron beams were superimposed on a spreadsheet to determine the combinations of electron beams so that the treatment range was maximized. Based on the obtained weight for electron beams, dose distributions were calculated using a treatment planning system and examined for potential clinical application.


RESULTS
With the combination of 4 MeV and 9 MeV electron beams, the 90% treatment range in the depth direction increased by 8.0 mm, and with 4 MeV and 12 MeV beams, it increased by 4.0 mm, with the same maximum dose depth and halfdose depth of the absorbed dose. The dose calculations were performed using the treatment planning system yielded similar results with a matching degree of ±1.5%.


CONCLUSIONS
Although the influences of low monitor unit values and daily output differences remain to be considered, the results suggest that the proposed approach can be clinically applied to expand treatment regions easily.