188 Orthovoltage rotational breast radiotherapy: A Monte Carlo and experimental study

Purpose To investigate depth-dose characteristics of a new technique for orthovoltage rotational breast radiotherapy (RT) via Monte Carlo (MC) simulations and experimental phantom study, following the initial work by Boone [1] . Methods Geant4 MC simulations were developed to calculate dose distributions generated by an orthovoltage (320 kVp, 4 mm Cu filtration) or kilovoltage (150 kV, 1 mm Cu) X-ray beams rotating 360° around a cylindrical phantom (polyethylene or PMMA) placed at isocentre to simulate prone patient’s breast. Horizontal beam collimation was 1–14 cm. Radial and longitudinal depth-dose distributions were performed for homogeneous phantoms (with or without lesions simulated for different depths in phantom). Experimental validation was performed on the same geometry with the 150 kV beam. Farmer-type air ionization chamber and radiochromic films are used to measure radial dose profiles. Results MC simulations at 320 kVp show that rotational kilovoltage radiotherapy assures sufficient skin sparing during breast RT. Centre to periphery dose ratio is less than 20:1 (1 cm collimation in the horizontal plane at 320 kVp). The vertical dose spread profile shows limited influence of the scatter dose at distance of 10 mm from the irradiated slice. MC simulations will be compared to laboratory measurements at 150 kV. Conclusions We showed the potential of orthovoltage external beam rotational RT for breast cancer with satisfactory skin sparing, though using a kilovoltage beam.