An intra-oral cone for an 18 MeV linear accelerator

A set of intra-oral cones has been constructed for electron beam therapy from a linear accelerator. We have modified the 4 cm X 4 cm treatment accessory provided with this machine by simply removing the lower collimator and replacing it with a brass plate into which all the cones can fit, so that they are easily interchangeable. Six circular cones, with diameters ranging from 27 mm and to 45 mm, plus two elliptical cones are currently in use. A light field system has been built that fits into the acrylic compartment directly above the base, and provides a clear, well-illuminated view of the treatment field. The dosimetry for these cones shows that the percent depth dose curve for a 41 mm diameter field is the same as that obtained with the 4 cm X 4 cm treatment accessory, and that the isodose curves are very similar. This intra-oral cone system has been in operation for over two years and we have found it very useful in boosting the dose to specific primary lesions in the oral cavity, for example, oral tongue, floor of mouth, retromolar trigone, and soft palate.