Abstract Book

S1184

ESTRO 37

(Z eff ), the same HU can represent different RED. Saito [1] has proposed a method for the diagnostic CT to generate a linear relationship between the RED and the weighted subtraction of a high and low energy scan of a phantom. The purpose of this study was to investigate the applicability and robustness of this method to linac-based cone beam CT (CBCT) imaging with the aim of facilitating dose of the day calculations in future. Material and Methods Dual energy (DE) scans of a Catphan 503 and a CIRS 062MQA phantom with the electron density inserts positioned at the isocenter plane were acquired (two consecutive scans; 70 kVp (LE) and 130 kVp (HE); Elekta XVI 5.0 equipped with an anti-scatter grid [2]). The CIRS was scanned in standard (CIRS M) and small configuration (CIRS S) to examine the effect of the different scatter conditions. The HE and LE scans were combined according to ΔHU=(1+α) HU HE – α HU LE [1]. α was determined by maximizing the value of the R 2 -parameter of the ΔHU-RED linear fit. To characterize the robustness of this DE method, α and the fit parameters from one phantom were applied in turn to the images of the other two phantoms and the absolute difference in RED with the fitted curve was calculated. For comparison, a bilinear interpolation was performed for a 120 kVp single- energy (SE) reference scan. Results Excellent goodness of fits (R 2 >0.99) were observed for both SE and DE and all phantoms (Fig. 1). Consequently, small differences between estimated and specified RED were found when appliying the calibration curves to the same phantom for both SE and DE (Fig 2). Considerable deviations were observed when applying the SE CIRS M or S phantom HU-RED calibration to the Catphan and vice versa due to the differences in Z eff between the phantoms. These differences were greatly reduced using the DE scan (Fig. 2). On the other hand, applying the calibration of the CIRS M/S to the CIRS S/M yielded larger deviations for DE compared to SE.

Conclusion The ΔHU-RED calibration method has been successfully implemented on a linac integrated CBCT scanner. It allows the discrimination of materials that have the same HU but different electron densities. The robustness with respect to the object size due to changes in scatter conditions is reduced compared to the SE method. Further evaluation of the off-axis accuracy and robustness is needed and the trade-off between dose, image quality and HU-RED calibration has to be explored.

[1] Med.Phys ; 2012 ;Apr; 39 ( 4 ):2021-30 [2] Med.Phys;2016;Mar;43(3):1057-64

EP-2147 Comparison of Spatial-Distortion Maps for MR- Sim Versus MR-Linac in the Brain and Pelvis at 1.5T R.J. Goodburn 1,2 , R.H.N. Tijssen 2 , M.E.P. Philippens 2 1 Cambridge University Hospital NHS Foundation Trust,

Medical Physics, Cambridge, United Kingdom 2 University Medical Center Utrecht, Dept. of Radiotherapy, Utrecht, The Netherlands

Purpose or Objective The MR-Linac is a new technology that uses onboard MRI to directly guide radiotherapy. Geometric fidelity is very important for both the MR-Linac and the MR-Sim used in the treatment planning phase. Here, we evaluate whether there are significant differences between the 1.5T MR-Sim and MR-Linac systems in our radiotherapy department. Material and Methods Geometric fidelity in MRI is primarily determined by two factors: System-specific gradient non-linearities and patient-induced B0 inhomogenity. We expect patient- induced distortions to be similar in the MR-Sim and MR- Linac since both have 1.5T B0 fields. Distortions caused by gradient non-linearities are system specific, and may differ substantially due to the split gradient design of the MR-Linac. A Philips Geometric-QA phantom was imaged on a Philips Ingenia 1.5T MR-Sim and an Elekta Unity 7MV, 1.5T MR- Linac. The main body coil was used for RF transmission and signal detection. Two T1-weighted 3D gradient-echo sequences were acquired with readout along the AP and PA directions respectively. The Philips QA software was then used with the acquired images to calculate distortion in each orthogonal direction. In order to remove distortion due to B0 inhomogenity induced by the phantom, the arithmetic mean of the marker locations in the AP and PA images was calculated.