Sharpening the penumbra of high energy electron beams with low weight narrow photon beams

Abstract

Background and purpose: High energy (20–50 MeV) electron beams, available from the MM50 Racetrack Microtron, can be used for the treatment of deep-seated tumors. A disadvantage is the increasing penumbra width as a function of depth. By the addition of a narrow (typically 1 cm wide) photon beam near the field edge, the 50–90% penumbra width, of the electron beam is reduced, yielding a significantly increased effective field size. Materials and methods: For rectangular electron beams in a water phantom (energies 25 and 40 MeV, field sizes 5×5–15×15 cm 2) a computer program was used to optimize the photon beam parameters (position, weight and width) to obtain a combined beam with the sharpest penumbra at the optimization depth and a beam flatness within certain constraints. The study furthermore included penumbra sharpening of an irregular multileaf collimator-shaped field. Results and conclusion: At optimization depths near R 90, photon beam addition reduces the penumbra width by 40–50% (from 15–20 mm to 8–10 mm). Beam flatness at the optimization depth is within ±5% and hot-spots are ≤120% for all depths. By the addition of narrow photon beams around the rectangular or irregular field, the electron field width can be reduced by 1–3 cm, while the effective field size is maintained.