LIQUID IONIZATION-CHAMBER MEASUREMENTS OF DOSE DISTRIBUTIONS IN SMALL6 MV PHOTON BEAMS

A new liquid ionization chamber (LIC) design optimized for high spatia l resolution was used for measurements of dose distributions in radiat ion fields intended for stereotactic radiosurgery (SRS). This work was mainly focused on the properties of this detector in radiation fields from linear accelerators for clinical radiotherapy (pulsed radiation with dose rates from approximately 0.5 to 5 Gy min(-1) and beam diamet ers down to 8 mm). The narrow beams used in stereotactic radiosurgery require detectors with small sizes in order to provide a good spatial resolution. The LIC is investigated to see whether it can be used as a detector for dose measurements in beams currently used for stereotact ic radiosurgery. Its properties are compared with those of silicon dio des. The comparisons include output factor (OF), depth dose and profil e measurements in 6 MV photon fields of different sizes. For OF measur ements, an NACP air ionization chamber was also used in the comparison . The dependence of the response on the detector orientation in the ph oton beam is also investigated for the diodes and the LIC. The results suggest that LICs can provide better properties than diodes for measu ring dose distributions in narrow photon beams.