ESTRO 36 Abstract Book

S770 ESTRO 36 _______________________________________________________________________________________________

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.

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 effects of applicable corrections, particularly for light scattering and non-uniform responses between a film layer and a panel, dose errors before and after correction were visualized and quantified. Each effect by sensitometric curves and correction methods on detection of dose errors in gamma analysis was evaluated in a square field of 10 cm, a 45° wedged field, and an intensity-modulated field for prostate cancer. Results Both sensitometry curves of the red and the green channel could reach two times higher OD of 2.2 at 20 Gy and the gap of OD was gradully more distinguishable over 4 Gy in scanning with high contrast. The sensitometric curve of green channel showed differentiated linearity in the dose range under 2 Gy. The difference in the gradient of OD brought out maximum 10% difference of gamma passing rate in both wedged and intensity-modulated fields. The primary positions of failed points in gamma analysis were different according to sensitometric curves. When the optimal sensitometric curve was applied, uniformity correction caused maximum 8% difference in gamma passing rate.

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.