Commissioning and cross-comparison of four scanning water tanks

Abstract

Purpose: Water scanning systems are commonly used for data collection to characterize dosimetric properties of photon and electron beams, and the commissioning of such systems has been previously described. The aim in this study, however, was to investigate tank-specific dependencies as well as conduct a dosimetric comparison between four distinct water scanning systems.

Methods: Four water scanning systems were studied including the PTW MP3-M Phantom Tank, the Standard Imaging DoseView 3D, the IBA Blue Phantom, and the Sun Nuclear 3D Scanner. Mechanical accuracy and reproducibility was investigated by driving the chamber holder to nominal positions relative to a zero point and using a leveled caliper with 30 cm range to measure the actual position. Dosimetric measurements were also performed not only to compare percent-depth-dose (PDD) curves and profiles between tanks but also to assess dependencies such as directionality, scanning speed, and reproducibility for each tank individually. A PTW Semiflex 31010 ionization chamber with a sensitive volume of 0.125 cc was used at a Varian Clinac 2300 linear accelerator.

Results: Mechanical precision was ensured to within 0.1 mm with the standard deviation (SD) of reproducibility <0.1 mm for measurements made with calipers. Dependencies on scanning direction and speed are presented. 6 MV PDDs between tanks agreed to within 0.6% relative to an averaged PDD beyond d_max and within 2.5% in the build-up region. Specifically, the maximum difference was 1.0% between MP3-M and Blue Phantom at 6.1 cm depth. Lateral profiles agreed between tanks within 0.5% in the central 80% of the field. 6 MeV PDD maximum difference was 1.3% occurring at the steepest portion, where the R₅₀ was nevertheless within 0.6 mm across tanks. Setup uncertainties estimated at ≤1 mm are presumed to have contributed some of the difference between water tank data.

Conclusion: Modern water scanning systems have achieved high accuracy across vendors, but commissioning tests nevertheless reveal tank-specific dependencies. This study not only ensures confidence in the individual systems but also provides the medical physicist with an understanding of variation in water tank properties between vendors.