ESTRO 2020 Abstract book

S1032 ESTRO 2020

PO-1761 A cross-platform daily QA of MRI simulator using an ACR MRI phantom O.L. Wong 1 , Y.W. Ho 1 , J. Yuan 1 , M.W.K. Law 1 , C.C. Ho 1 , C. Kin Yin 1 , Y. Siu Ki 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong, Hong Kong SAR China Purpose or Objective This study establishes a cross-platform quality assurance (QA) procedure for the use of MR-simulator (MR-sim) in radiotherapy. In contrast to vendor-provided procedures which are commonly platform-specific and customized from diagnostic counterparts, the proposed one uses widely available ACR MRI phantom and is based on ACR MRI phantom test protocol and acceptance test. Material and Methods The reference point of an ACR MRI large phantom was first aligned with the external 3-point laser (EPL) equipped with an MR-sim, and scanned using a representative 3D-T1W- TSE sequence for brain MR-sim scan (FOV=256´256´176mm 3 , isotropic 1mm voxel size, other parameters as close as possible for different vendors) with a typical and constant RF coil setup. A protocol was developed for the following tests. EPL shift relative to MR- sim isocenter was evaluated by the image location of the reference point, with a tolerance limit of ±2mm according to acceptance test (ATP). Geometric distortion was assessed by measuring the inner diameter ( D ) and inner length ( L ) with a tolerance limit of ±2mm compared to the ground truth ( D =190mm; L =148mm) according to ATP. Spatial resolution of 1mm was assessed following the ACR MRI protocol. RF coil integrity was checked by assessing the mean image intensity with ATP baseline values, and percent integral uniformity (PIU) (pass criterion >85% for 1.5T) using the ACR MRI protocol. Image artifact was visually checked. Functionality of patient safety accessories was tested. This protocol was applied by medical physicists indaily QA of two 1.5T MR-sims (vendor V A and V B ). The inter-observer agreement was assessed using intra-class correlation (ICC). Results Results of 69 longitudinal daily QA datasets (V A : n =36 and V B : n=33) were reported in Table 1.The whole daily QA procedure including setup, scan and analysis could be completed within 15 minutes in both MR-sims. All test results passed the set criteria. Small EPL shifts were obtained in SI (V A = 0.31±0.46 mm, V B = -0.11±0.47 mm), AP(V A = 0.02±0.23 mm, V B = -0.35±0.49 mm) and RL (V A = 0.58±0.71 mm, V B = -0.13±0.77 mm) directions. All measurements showed geometric distortion of <1mm (D: V A = +0.50±0.53 mm, V B = -0.33±0.76 mm; L: V A = - 0.02±0.62 mm, V B = -0.18±0.55 mm). 1mm resolution insert was always resolved. Consistent mean intensity (V A = 180.72±8.29, V B = 1797.78±102.00) and PIU were obtain (V A = 96.31%±0.34%, V B = 93.71%±0.61%). No obvious artifacts were observed. Excellent inter-observer agreement was achieved between 2 physicists for EPL shift (ICC>0.85), geometric distortion (D: ICC >0.95, L: ICC>0.95), resolution (ICC=1), mean signal intensity (ICC >0.95) and PIU (ICC >0.95). Customized phantom cradle and automated image analysis are to be implemented in the future.

Figure 1: Dose distributions from a single beamlet created from a 10 mm diameter proton beam focused with a 250 T/m magnet. A) 2D transverse dose at 10 mm WED. B) 2D dose distribution taken in the vertical midplane. C) 2D transverse dose at 97 mm WED. D) and E) 1D transverse dose profiles taken at various WED. A modulated beam was generated using a 12 mm incident proton beam and a 250 T/m focusing magnet as this is more indicative of potential clinical usage case. Three beamlets were utilized with a beam separation of 9.25 mm to optimize the 90% isodose for coverage of a 15 mm diameter target (see Figure 2). FWHM at 10 mm water equivalent depth was 1.7 mm and 17.7 mm in the horizontal and vertical directions respectively for the modulated beamlet, while for an unmodulated beamlet the same parameters were 1.8 mm and 17.8 mm.

Figure 2: Dose distributions for the modulated beam described above. 2D distributions taken in, A) the horizontal midplane, and B) the vertical midplane. C) 2D transverse distributions taken at 85, 89, 93 and 97 mm WED along the SOBP, and D) 1D transverse distributions taken along the horizontal (blue trace) and vertical (red trace) axes of the profiles in panel C). The dashed white lines in panel A represent the depth of cross-sectional profiles in panels C and D. Conclusion The data presented suggests that magnetic focusing can be used to produce minibeams in proton therapy with similar dosimetric properties yet greater efficiency and fewer secondary particles than collimation. Magnetic focusing technology for pMBRT can potentially be applied both in passive and active proton delivery and is the subject of ongoing physics and radiobiological research at our institution.