The effect of the horizontal metallic drive on reference dosimetry in the SNC 3D scanner water tank

Abstract

Accurate quantification of absorbed radiation dose is important for safe and effective delivery of radiation therapy. An important aspect to this is reference dosimetry, which is performed under reference conditions specified by international codes of practice. Such measurements are usually performed in a water phantom. In the Sun Nuclear Corporation (SNC) three-dimensional (3D) scanner water tank system the detector holder is fixed to a horizontal metallic drive rail (MDR) which is in close proximity to the active volume of the detector. In this project, the dosimetric effects of the MDR on reference dosimetry were investigated for MV photons, MeV electrons, and kV photons by comparing reference dosimetry measurements in the SNC 3D scanner against similar measurements in a Standard Imaging (SI) one-dimensional (1D) tank and against measurements in the SNC 3D scanner with an additional, custom-made spacer placed beneath the chamber holder to increase the chamber -MDR separation. A second experiment investigated the difference in chamber reading dependent on chamber to MDR separation by fixing the chamber in the tank independently of the MDR and successively moving the MDR vertically to alter the separation. The results showed that measurements in the SNC 3D scanner agree with both SI 1D tank and SNC 3D scanner with spacer to within ±0.3% for MV photons, ±0.1% for electrons and ±1.2% for kV photons within the calculated setup uncertainty. The second experiment showed that the contribution of backscatter from the MDR was significant if the distance between MDR and chamber was reduced below the chamber's designed position in the SNC 3D scanner. The exception was for kV photons where the contribution of backscatter from the MDR was measured to be 0.5% at the designed distance. Further investigation would be useful for kV photons, where the experiment showed relatively large measurement uncertainties. K E Y W O R D S3D scanner, backscatter, reference dosimetry, water tank ---

Mapping 3D doses in water with a cable robot equipped with plastic scintillator

“…Comparison of different commercial motorized water phantom have shown that they can all achieve equivalent level of accuracy: dose difference within 1% and distance to agreement within 1 mm of each other [3]. Nevertheless, these phantoms do suffer from drawbacks; their large metallic arms can interfere with the radiation beam [4] and can cause waves that lead to measurement innacuracies [5] of up to 5% [6]. Risks of interference with the metallic arm usually restrict irradiations at normal incidence (i.e.…”