ABUTTING ORTHOGONAL ELECTRON AND PHOTON BEAMS IN THE HEAD AND NECK REGION USING ASYMMETRICAL PHOTON-BEAM EDGES

Irradiation of target volumes within the ethmoid sinus and nasal cavit y region has, at our institution, been performed using one anterior el ectron beam, abutted to two opposing lateral photon beams, in order to exclude the orbit from the irradiated volume. However, this technique give rise to larger dose hetereogeneities within the target volume, a s well as undesired hot spots in the chiasma region. To reduce the inh omogenious distribution, we have introduced asymmetrical photon beam e dges to broaden the penumbra of the two opposing photon beams resembli ng the dose gradient of the perpendicular electron beam. The achieved reduction in dose heterogeneity, measured from the differential dose-v olume-histogram, is in the order of 30-40% when applying the asymmetri cal abutting technique compared with the conventional two opposing pho ton beam technique. As demonstrated by calculating the tumour control probability (TCP), the increase in dose homogeneity within the target volume, may be of clinical significance.