Build-up Region of Narrow Photon Beams in Linear Accelerator Radiosurgery : Tissue Maximum Ratio Measurements Using Diamond Detector and Microchamber

Abstract

Linear accelerator radiosurgery has been used to treat small intracranial lesions. However, when the lesion is superficial, the dose assessment point comes closer to the build-up peak. In this case, the tissue maximum ratio(TMR)is critical in dose assessment, and therefore TMR and build-up peak should be measured accurately for each field. The authors conducted comparative TMR dosimetry of 10 MV x-rays in small circular fields ranging from 6.7 mm to 31.4 mm in diameter at the isocenter using a microchamber and a diamond detector. Both measurements were within 3% of each other at depths ranging from 10 mm to 120 mm. The TMR shift was clearly observed. The position of the build-up peak shifted from 15.0 mm to 27.0 mm with the diamond detector as the field size changed from 6.7 mm to 31.4 mm in diameter.