Dosimetric effects of abutting extended source to surface distance electron fields with photon fields in the treatment of head and neck cancers

Purpose: In the management of head and neck cancer, it is often necessary to junction photon and electron fields. When patients are treated supine, electron cones or applicators may have to be positioned at source to surface distance (SSD) greater than the standard 100 cm because of the patient's shoulders. We have studied the dosimetric effects of abutting 6 MV photon fields with 9 MeV electron fields at extended SSDs to assess changes in the 90% isodose width, dose uniformity in the target, and the extent of hot and cold spots in the junction region. Methods and Materials: Four independent film studies were conducted, in a polystyrene phantom, for evaluation of the dose distribution in the junction region. Measured distributions were also compared with computer generated distributions using a treatment planning computer system. Results: At the junction line between photon and extended SSD electron beams, hot and cold spots were observed. A 20% hot spot occurred on the photon side because of the electron scatter from the adjoining field. The width of this hot spot increased in dimension but not in magnitude as electron SSDs were increased. The cold spot occurred on the electron side due to the decrease in 90% isodose width at extended electron SSDs. This cold spot was minimal (less than 10%) at shorter electron SSDs, but increased as electron SSDs approached 120 cm. Computer dosimetry underestimated these results because of limitations of the electron beam treatment planning algorithm. Conclusion: These hot and cold spots may be clinically acceptable compared to the loss of treatment accuracy and the added possibility of more extensive hot or cold spots if the patient were to be removed from the original supine position and repositioned on his/her side.