New dosimetric approach for multidimensional dose evaluation in gamma knife radiosurgery - Technical note

During the past two decades, the progress in computerized treatment plannin g systems has led to more accurate imaging and therapy by using the gamma k nife, especially with the smallest collimators (4 mm). However, the ionizat ion chambers that have been used to calibrate the gamma knife are not usefu l with the smallest collimators because the chambers are too big compared w ith the irradiated volume. Therefore, it is important to develop more suita ble dosimeters. This study proposes a new dosimeter method. The FriXyGel method proposed here is based on a phantom dosimeter, an acqui sition chain, and dedicated software. This dosimeter uses an agarose gel in to which a ferrous sulphate solution (Fricke solution) and a metal ion indi cator (xylenol orange) are incorporated. The absorbed dose is detected thro ugh measurements of visible light transmission, imaged by means of a charge -coupled device camera provided with a suitable optical filter. Gel layers are imaged before and after irradiation, and the differences in light absor ption are related to the absorbed dose. By choosing convenient thickness of gel layers and by building up a phantom with different gel slices, it is p ossible to obtain a three-dimensional (3D) representation of the absorbed d ose. The final 3D representation is reached after several mathematical processes have been applied to the images. The first step identifies and reduces all factors that could alter the original data, such as nonuniformity in illum ination. Then, after calibration procedures, it is possible to obtain absor bed dose values and to discover their 3D representation. This goal has been reached by developing appropriate software that performs all the calculati ons necessary for spatial representation routines and prompt comparison wit h theoretical calculations.