A NEW PENUMBRA GENERATOR FOR ELECTRON FIELDS MATCHING

Abutment of two or more electron fields to irradiate extended areas ma y lead to significant dose inhomogeneities in the junction region. Thi s paper describes the geometric and dosimetric characteristics of a de vice developed to modify the penumbra of an electron beam and thereby improve the dose uniformity in the overlap region when fields are abut ted. The device is a Lipowitz metal block placed on top of the electro n applicator's insertion plate and positioned to stop part of the elec tron beam on the side of field abutment. The air-scattered electrons b eyond the block increase the penumbra width from about 1.4 to 2.7-3.4 cm with an SSD of 100 cm. The modified penumbra is broad and almost li near at all depths for the 9 and 12 MeV electron beams used in this st udy. Film dosimetry was used to obtain beam profiles and isodose distr ibutions of single modified beams and matched fields of 9 and 12 MeV a s well as matched fields of both energies. Computer simulation was use d to optimize the skin gap to be used and to quantify the dose uniform ity as a function of the field separation for both modified and nonmod ified beams. Results are presented for various field configurations. W ithout the penumbra generator, lateral setup errors of 2-3 mm may intr oduce dose variations of 20% or more in the junction region. Similar s etup errors cause less than 5% dose variations when the penumbra gener ator is used to match the fields. The potential of the technique for t he irradiation of curved surfaces is presented. A possible method for implementing the modified penumbra into a conventional treatment plann ing system is evaluated. (C) 1997 American Association of Physicists i n Medicine.