ESTRO 36 Abstract Book

S761 ESTRO 36 2017 _______________________________________________________________________________________________

to optimize the dose sensitivity and the Signal-to-Noise Ratio. In particular, geometrical distortion, image homogeneity, artifacts, image texture, dose accuracy and resolution, limit of detectability (LOD) and quantification (LOQ), Fe 3+ spatial diffusion (Fricke-gels) and dose rate dependence were evaluated. Finally, a pre-treatment dosimetry of a SBRT plan was acquired and a relative planar profiles comparison with a standard dosimeter (Gafchromic EBT2) was performed. Ad hoc Matlab codes were developed for images analysis. Results The chemical composition, MRI acquisition and reconstruction parameters were optimized for each gel system. No image correction maps were needed, since geometrical distortion, artifacts and inhomogeneity were always negligible, and no dependence on photon beam dose rate was observed. 3D spatial resolution (voxel dimension) was 1x1x3mm 3 . Dose accuracy was under 4% in the range 18-25Gy, but worst for lower doses. Dose resolution was about 1Gy, while LOD was less than 0.5Gy. Differences between gel systems and Gafchromic profiles’ FWHMs were in the range 0,5mm – 5,5mm, mean dose deviations in flat region were always around 2%, while penumbra differences were about 2mm. Negligible diffusion and time effects were observed up to 3 hours from irradiation for all gel systems. Conclusion This study showed that both Fricke/Polymer- gel dosimeter could be a suitable tool to perform pre- treatment QA, with particular focus on SBRT and SRS treatments, thanks to their optimal spatial resolution, their practicability and their capability to perform 3D dosimetry. Further studies are ongoing to standardize a protocol to perform 3D pre-treatment dosimetry. EP-1443 Measurement of 3D dose distributions from an MR Linac with gel dosimetry Y. Roed 1,2 , 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 demonstrate the potential value of polymer gels to measure 3D dose distributions delivered with an MR-image guided radiotherapy delivery machine. Material and Methods Polymer gels were obtained from MGS Research Inc (Madison, CT) in custom-designed glass cylinders of 4 cm height and 5 cm diameter. Irradiations were delivered with a non-clinical MR-linac pilot system (MR-Linac, Elekta AB, Stockholm) that combined a 1.5 T MR scanner with a 7 MV linac. Two dosimeters were positioned separately in a phantom with their midplanes at isocenter distance. A total of 750 MU (~5 Gy) was delivered with 3x3 cm² fields at three gantry positions. The gantry was positioned at 0 ⁰ , 90 ⁰ , and 180 ⁰ for the first irradiation and at 0 ⁰ , 270 ⁰ , and 180 ⁰ for the second irradiation. All four cardinal angles weren’t feasible due an asymmetric phantom design. MR images across the entire volume of the dosimeter were acquired with a 3T GE scanner using a 2D spin echo sequence (TR = 1000 ms, TE = 10, 20, 60, 100 ms) 24h after irradiation. Spin-spin relaxation rate (R2) maps were generated. Both field size and penumbra widths were calculated on the central slice. R2 maps were concatenated into a 3D matrix. The experiment was performed while the magnet of the MR component (B-field) was turned off and will be repeated once the B- field is turned back on. Results The small fields were captured and resolved within each dosimeter. The field width measured along the central cross-plane R2 profile from each dosimeter was 28 mm and 29 mm, respectively. The penumbra widths were 5 mm at

both field edges in each dosimeter. The 3D R2 matrix visualized the irradiated volume of the dosimeter well. In order to study the influence of the B-field on the dose distribution in 3D, the results in the presence and absence of the MR component (B-field) will be compared and presented.

Conclusion Polymer gels offer an excellent means to measure 3D relative dose distributions delivered with an MR-Linac in a clinically relevant fashion. Previous experiments with polymer gels have already shown that steep dose gradients could be measured when irradiated with an MR-Linac. The current study encourages further study of polymer gels for measuring 3D dose distributions in the presence of B-fields. EP-1444 Reliable error detection in radiochromic film dosimetry with optimal density curves and corrections H. Park 1 , Y. Bae 2 , J. Park 3 , M. Kim 1 , Y. Oh 1 , M. Chun 1 , O. Noh 1 , O. Cho 1 , J. Lee 2 1 Ajou University Medical Center, Dept. of Radiation Oncology, Suwon, Korea Republic of 2 Konkuk University Medical Center, Dept.of Convergent Medical Physics and Dept. of Radiation Oncology, Seoul, Korea Republic of 3 University of Florida, Dept. of Radiation Oncology, Gainesville, USA Purpose or Objective To minimize variation of dosimetric errors caused by correction methods and to suggest optimal conditions in gafchromic film dosimetry using a flatbed scanner, feasible scanning and post analysis procedures were investigated with impacts on error detection in gamma analysis. Material and Methods When a rectangular piece (5 × 4 cm) of EBT3 film was placed at a 5 cm depth of the water-equivalent solid water phantom, doses were delivered to film pieces from 0 cGy to 20 Gy with every 50 cGy under 500 cGy and 100 cGy over 500 cGy. To find an optimal sensitometric curve having a large range of optical density (OD) and linearity in doses of interest, a set of exposed films was scanned in a flatbed scanner with different conditions by adjusting brightness, contrast, and highlight from -50 to 50. Sensitometric curves of a red and a green channel were obtained with each scanning condition and used to compare gamma distributions. In addition, to clarify the