ESTRO 36 Abstract Book

S399 ESTRO 36 2017 _______________________________________________________________________________________________

PO-0764 A study of Tandem systems incorporating three thermoluminescent dosimetry materials. V. Nelson 1 1 Macarthur Cancer Therapy Centre, Medical Physics, Campbelltown, Australia Purpose or Objective Tandem systems, incorporating a pair of TL materials, have been shown to be very useful for the determination of effective energy in radiation beams with unknown radiation energy spectrum. Tandem curves in all these tandem systems exhibits maximum TL response ratio between 40-50 keV and similar TL response ratios on either side of this energy (E max ), making it difficult to determine if the effective energy is less than or greater than the E max (Fig.1(a)). However, if a third TL material, with energy dependent TL response different to the other two materials, is included in the tandem, two tandem curves can be obtained. If these two tandem curves significantly differ from each other, the effective energy of the radiation beam can be identified as either less than or greater than E max , Fig. 1(b), hence improving the dosimetry in this energy range. The aim of this work was to test a number of different TL materials and find a TL material which fulfils this criteria.

the B-field. Two B-field strengths of B = 0 T and B = 1.5 T were used. All films were exposed to the B-field for 7 min. A subsequent set of films was irradiated to 4 Gy using the same setup and B = 1.5 T field strength. The films were exposed to the B-field for 6, 10, or 30 minutes. All films were scanned with an Epson 10000 XL flatbed scanner prior to and 24 hours after irradiation, first with the reference edge parallel (PA) to the scan direction and then perpendicular (PA90). Red channel pre- and post-scans were analyzed with ImageJ software. Percent differences (%diff) with respect to B = 0 T were calculated for PA and PA90 films. %diff between PA and PA90 were also determined. Results All films exhibited an under-response at each dose level when compared to irradiation at B = 0 T. Less than -2.0 % difference was determined in the PA scan direction at all dose levels for both orientations in the B-field (solid and dashed black in Figure 1). The under-response increased with increasing dose for RE90 films scanned in PA90 direction (dashed grey in Figure 1). For RE0 the maximum %diff was -1.1 % (solid grey in Figure 1). The %diff in scan direction increased with increasing dose from 11.0 % to 12.4 % in RE0 orientation and from 9.6 % to 11.1 % in RE90 orientation. Table 1 shows that increasing the time the films remained in the B-field resulted in less than 1.0 % over-response for 6 and 30 min and -0.9 % under-response for 7 and 10 min. The %diff in scan direction was about 12 % for all times.

Material and Methods Five different TL materials TLD100, TLD100H, TLD200, TLD400 and TLD500, were investigated. Each type of TL dosimeter was irradiated to the eight different qualities of x-radiation. Mean of the response of the 5 dosimeters for a certain x-radiation with effective energy Eeff was taken as the energy dependent TL response of that type of TL dosimeter. For each type of TL detector, energy dependence curves were determined by fitting the experimental results with a polynomial function. Tandem curve pairs for six different combinations were generated; ;1.TLD100H, TLD200, TLD400, 2.TLD100H, TLD200, TLD500, 3.TLD100H, TLD400, TLD500, 4.TLD100H, TLD200, TLD400,5.TLD100, TLD200, TLD500 and 6.TLD100, TLD400, TLD500.TL response ratios at different energies was calculated and compared with two TL material tandem systems. Results All Tandem curves exhibited maximum TL response ratio, E max , at approximately 45 keV, with reduction in TL response ratios at energies above and below this energy level. All tandem combinations, except the combinations (1) and (4) showed that at energies in the 30 to 80 keV range, where the TL response ratio of tandem pair (i) is same, TL response ratio tandem pair (ii) differs by 20-30%,

Conclusion Radiochromic films can measure doses delivered by a magnetic resonance-image guided radiotherapy treatment unit and can be considered for quality assurance of MR- guided treatment units. The duration of exposure to the B-field did not affect the response of the film and neither did the orientation of the reference edge as all determined %diff were less than the uncertainty of film measurements. However, the orientation of the reference edge with respect to scan orientation did have a significant effect on the response of the film. Maintaining consistent orientation of films both during the irradiation in a B-field and also on the flatbed scanner still remains essential to acquire results with the lowest %diff.