The effects of focus-skin distance and shaping block tray on the calibration factor of in-vivo dosimetry diodes.

Abstract

In-vivo dose measurements during conformal treatment require correction factor evaluations for differences in block shapes, field size and source-skin distance (SSD). The aim of this paper is to evaluate a single correction factor, CFentranceSSD, depending only on source-skin distance, which takes into account both shape and size of blocked fields, in pelvic treatments. A set of measurements was performed to investigate the effects of different block shapes and sizes, source-skin distances and collimator settings on the entrance dose values. For this reason EDP 20 Scanditronix diodes, with 20 mm water-equivalent build-up cap, were irradiated by 2 Varian linear accelerators (Clinac 1800 and Clinac 2100) 18 MV photon beams with 3 different collimator settings. Diodes were calibrated by comparison with a Farmer 2571 ionization chamber at reference conditions (10 cm x 10 cm open field, source-skin distance 100 cm, build-up depth 3.3 cm) in order to convert the semiconductor signal into water absorbed dose. Three sets of different measurements (regarding open beams, tray fields and shaped fields) were performed. Diode responses were compared with the ionization chamber ones, in the same irradiation conditions. The CFentranceSSD trends can be described by linear fits in dependence upon source-skin distance, and the effect with tray alone or with tray and blocks is stronger than in the open field. The observed effects can be interpreted by the influences of a) the different source distances of diode and reference point in the phantom and b) the secondary electron contamination of the photon beam upon the calibration factor of the diode. The effects of source-skin distance and of the shaping blocks and tray upon the diode calibration factor can be effectively accounted for by a single CFentranceSSD correction factor.