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

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.

For rectangular electron beams in a water phantom (energies 25 and 40 MeV, field sizes 5 x 5-15 x 15 cm2) 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.

At optimization depths near R90, 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 < or =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.