ESTRO 2022 - Abstract Book

S1610

Abstract book

ESTRO 2022

cavity/path is one approach to get information about source position traveling through. For this purpose the dummy source cable was installed into extended/modified Source Position Simulator Set instead of dummy/check cable. Step size of the dummy moving was 1 cm. The fluoroscopic images of the source inside applicator channel in different dwell positons was analysed using standard measuring tools. For the point of origin/beginning of “z-coordinate” was taken the centre of source travel channel Ø 3 mm (Fig., Z=0, 3) and the positive direction of “z-coordinate” it was taken direction of the applicator tandem. Special care was undertaken (using angle indicators) to adjust angle of the fluoroscopic image direction view to be parallel to plate of ring applicator fixation.

Figure. Flexitron Ring Applicator Ø 26 mm, Side View, „Z-coordinate“of dummy source dwell positions. 1,2 – “300 mm” dwell position of the dummy source for Measurement type 1 and type 2, respectively. 3 - Source travel path cavity, diameter 3 mm.

Results On the figure the curves 1 and 2 correspond to two types/groups of obtained measurements: 1- moving of dummy source into last distal position “300 mm” with Z= ~+0,5 mm; 2 - dummy source, spontaneously, moving into last distal position “300 mm” with Z= ~-0,4 mm; For start dwell positions inside ring (“240-260” positions) these curves almost coincide between each other, but for distal dwell positions (“270-300”) they, unexpectedly, differs by ~1mm. Z-coordinate for library applicator in dwell position “300” close to Measurement 1 result and in dwell position “270” close to Measurement 2. For dwell positions “240-260” library Z-coordinate differ from our measurement by ~0,5 mm. Conclusion For all dwell positions inside Ring Applicator Ø 26 mm library “z-coordinates” don’t differ from our measurement more than 1 mm (i.e. gives contribution ≤ 5% difference in dose at 4 cm distance), which can be evaluated/valued as small rate. So, for creating adapted to clinical user Ring Aapplicator library the “z-coordinates” of source dwell positions, at the first stage, can be copied from library data.

PO-1806 Autoradiography method for determination of actual source dwell positions in Elekta ring applicators

A. T š i ž ik 1 , N. Saveljev 1

1 The North Estonia Medical Centre Foundation, Radiotherapy, Tallinn, Estonia

Purpose or Objective 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. Materials and Methods 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.