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 cavity region has, at our institution, been performed using one anterior electron 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, as well as undesired hot spots in the chiasma region. To reduce the inhomogenious distribution, we have introduced asymmetrical photon beam edges to broaden the penumbra of the two opposing photon beams resembling the dose gradient of the perpendicular electron beam. The achieved reduction in dose heterogeneity, measured from the differential dose-volume-histogram, is in the order of 30-40% when applying the asymmetrical abutting technique compared with the conventional two opposing photon 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.