AN ANTERIOR APPOSITIONAL ELECTRON FIELD TECHNIQUE WITH A HANGING LENS BLOCK IN ORBITAL RADIOTHERAPY - A DOSIMETRIC STUDY

A technique for orbital radiotherapy is presented consisting of an anterior, appositional electron beam with a hanging lens block. The beam was modified by introducing two 1.6 mm thick plastic spoilers, at about 3 cm and 15 cm from the lens, to boost in-scattering of electrons under the block. The 9 mm diameter, 2 cm long stainless steel cylindrical block was suspended 0.5-1.0 cm above the eye. We performed film, TLD (Thermo Luminescent Dosimetry), and diode dosimetry to determine the dose fill-in behind the lens. The introduction of the spoilers dramatically changed the dose distribution. The maximum dose under the block increased from 66% to 85% of the open field dose. Moreover, the dose to the posterior surface of the globe directly underneath the block, at a depth of 3 cm, increased from 48% to 76% of maximum dose, while the dose to the lens was still below 20%. This is a simple and easily reproducible treatment and is an improvement on a previously described technique. The dose distribution is adequate for cases where the target volume surrounds and is posterior to the globe.