Dosimetric effects of matching electron fields with cobalt 60 fields in the management of head and neck cancer.
The dose distributions in a blocked Co 60 beam with the area under the block irradiated with a 9-MeV electron beam at a nominal source to surface distance (SSD) of 100 cm and at an extended SSD of 115 cm were determined. The photon block was simulating the spinal cord shielding used when treating the posterior neck. A hot spot of 10% appeared close to the junction on the side of the photon field when electron treatment was given at a nominal SSD of 100 cm. At 115 cm SSD, the magnitude of the hot spot on the side of photon field increased to 20% and a cold spot on the order of 10% appeared in the distal area of the irradiated volume treated with electrons. A gap of 0.5 cm between the light field edges of the photon field and the electron field reduced the hot spot from 20% to 10% on the side of photon field. However, the magnitude of the cold spot on the side of electron field increased to 20%, and it appeared across the irradiated volume. Considering the limitation of the design of the electron applicators, which precludes the use of a nominal SSD of 100 cm for head and neck treatments, it is recommended that no gap should be allowed between the photon fields and the extended SSD electron fields. Clinically a limited hot spot of 20% is considered more acceptable than a cold spot of 20% extending from proximal to distal areas of the treatment volume.
PubMed Abstracts
PubMed Abstracts