Session Item

For commissioning of source position for ring applicators of Flexitron an autoradiography method was used. The purpose was to develop a “digital method” for determining center of film blackening on Gafchromic EBT film sheets.

A set of Interstitial Ring CT/MR Applicators (Elekta) of 26, 30 and 34 mm radius were investigated. Gafchromic EBT film sheets were placed on the acrylic glass support underneath of ring applicators. Five film sheets were used for every applicator. Four dwell positions from proximal to distal direction and step size of 2cm were selected. After exposing films were scanned and data analysis was performed by image processing software ImageJ, Microsoft Exel and Oncentra 4.5 measuring tools. Coordinates of plot profile of blackening area was extracted by ImageJ (Figure1, a) and inserted into Exel (Figure1, b) for post processing. The ''U'' shaped graph is well described by parabola. Solving quadratic equation gives coordinates of center film blackening which are setting into the film (Figure1, c). The position of the source was compared to the Measured Source Path provided by manufacturer in Oncentra 4.5 (Figure1, d). Collected data was compared with deviation from “manual method” where dot was putting into center of film blackening manually (based on estimation by eye). The mean calculated center and the standard deviation (SD/mean) were calculated.

“Digital method” allows to reconstruct source dwell position with accuracy of 0.1mm, “manual method” – 0.5mm. An example data for ring (60˚ Ø26mm) is presented  in Table 1.

 Table 1. Mean absolute differences from the measured position of the source determined autoradiographically to the Measured Source Path provided by manufacturer for ring (60˚ Ø26mm).

Because “digital method” is independent from human sensitivity, it gives an appropriate reproducible result for determination a center of film blackening comparing to “manual method”.