ESTRO 36 Abstract Book

S403 ESTRO 36 _______________________________________________________________________________________________

Conclusion Rare-earth doped scintillating silica, thanks to their high light yield and favorable spectral properties, offer a true alternative to perform optical fiber dosimetry, in different clinical and preclinical conditions, eliminating in a reliable and robust way the influence of the stem effect, without the need of complex and time-consuming calibrations. PO-0763 Characterizing the response of Gafchromic EBT3 film in a 1.5 T magnetic field Y. Roed 1,2 , H. Lee 2 , L. Pinsky 1 , G. Ibbott 2 1 University of Houston, Physics, Houston, USA 2 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA Purpose or Objective To assess the influence of a magnetic field (B-field) on the response of radiochromic film. Irradiation at different orientations of the film with respect to the B-field was assessed as well as different durations of exposure of the films to the B-field. Material and Methods EBT3 films were placed at 5 cm depth in an acrylic phantom and irradiated to 2, 4, and 8 Gy using a cobalt source while exposed to the B-field from an electromagnet. The film surfaces were perpendicular to the incident beam while a reference film edge was oriented either parallel (RE0) or perpendicular (RE90) to 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.

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. 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.