THE USE OF TERTIARY COLLIMATION FOR SPINAL IRRADIATION WITH EXTENDED SSD ELECTRON FIELDS

Purpose: The spine can be treated with an electron beam when its maxim um posterior depth is within the therapeutic range of electrons, Elect ron fields treated at extended source-to-surface distances (SSDs), how ever, have larger penumbras and narrower therapeutic isodose widths re lative to those at the standard SSD of 100 cm, We investigated the use of tertiary collimation close to the patient surface for these fields to sharpen the penumbra, minimizing dose to normal tissue and maximiz ing target coverage. Methods and Materials: Using film dosimetry in a polystyrene phantom, we measured the dose distribution for electron fi elds at extended SSD under varying collimation conditions, Beam penumb ra and therapeutic width as a function of depth, SSD, applicator inser t size, and tertiary collimator opening were determined, We also measu red the dose distributions in the junction region for various gaps bet ween x-ray fields and an electron field as used for craniospinal irrad iation. Results: Measurements show that tertiary collimation close to the skin surface reduces penumbra width (lateral distance between the 90 and 20% isodose lines) by 56% and increases therapeutic isodose wid th (lateral width of the 90% isodose curve) by 25% at a depth of d(max ) relative to standard collimation, These numbers change to 23 and 13% , respectively, at an average depth of the spine. When lateral brain a nd posterior spine fields are used to irradiate the entire craniospina l axis, tertiary collimation aids in reducing the volume of the hot sp ot in the junction region by as much as 10% without compromising targe t coverage. Conclusions: Tertiary collimation for extended SSD electro n fields is preferable to standard collimation in order to minimize do se to normal tissue and increase target coverage, This technique can b e applied to both spinal and craniospinal irradiation, Support structu res for the tertiary blocking are needed because the weight of the lea d is usually too great for placement on the skin. (C) 1997 Elsevier Sc ience Inc.