title: A new penumbra generator for electron fields matching.
abstract: Abutment of two or more electron fields to irradiate extended areas may lead to significant dose inhomogeneities in the junction region. This paper describes the geometric and dosimetric characteristics of a device developed to modify the penumbra of an electron beam and thereby improve the dose uniformity in the overlap region when fields are abutted. The device is a Lipowitz metal block placed on top of the electron applicator's insertion plate and positioned to stop part of the electron beam on the side of field abutment. The air-scattered electrons beyond 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 linear at all depths for the 9 and 12 MeV electron beams used in this study. Film dosimetry was used to obtain beam profiles and isodose distributions of single modified beams and matched fields of 9 and 12 MeV as well as matched fields of both energies. Computer simulation was used to optimize the skin gap to be used and to quantify the dose uniformity as a function of the field separation for both modified and nonmodified beams. Results are presented for various field configurations. Without the penumbra generator, lateral setup errors of 2-3 mm may introduce dose variations of 20% or more in the junction region. Similar setup errors cause less than 5% dose variations when the penumbra generator is used to match the fields. The potential of the technique for the irradiation of curved surfaces is presented. A possible method for implementing the modified penumbra into a conventional treatment planning system is evaluated.
authors.affiliation: Centre Hospitalier Universitaire de Québec, Canada.
authors.forename: B
authors.initials: B
authors.lastname: Lachance
authors.forename: D
authors.initials: D
authors.lastname: Tremblay
authors.forename: J
authors.initials: J
authors.lastname: Pouliot
mesh_terms: D004583:Electrons; D006801:Humans; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011878:Radiotherapy / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D013909:Thorax