Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_3_3146 - Measurement Validation of Beam Model for High-Field MR-Linac Treatment Planning

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Measurement Validation of Beam Model for High-Field MR-Linac Treatment Planning
X. Chen1, E. S. Paulson1, E. E. Ahunbay2, S. Klawikowski1, A. Sanli1, and A. Li1; 1Medical College of Wisconsin, Milwaukee, WI, 2Medical College of Wisconsin, Department of Radiation Oncology, Milwaukee, WI

Purpose/Objective(s): MR-Linacs integrating an MRI scanner and a linear accelerator are being introduced into radiation therapy (RT) clinics. The presence of the magnetic field has non-negligible dosimetry effects due to tilting of the dose kernel and electron returning effect (ERE). These effects should be accurately accounted for by the beam models in the treatment planning system (TPS) for MR-Linac. The purpose of this study is to perform a measurement verification of the TPS beam model for a high-field MR-Linac.

Materials/Methods: Relative and absolute dosimetry and geometry accuracy measurements were performed on a non-clinical MR-Linac prototype consisting of 7MV photon beam and a 1.5T MRI scanner and were compared to the corresponding beam model calculations from the TPS for the MR-Linac. Measurement devices included ion chambers (IC), diamond detector, radiochromic film, an MR-compatible ion chamber array and diode array. The dosimetry measurements data with the ICs and diamond detector were taken within a water phantom. Relative measurements included percentage depth dose (PDD) and beam profiles (both in-plane and cross-plane). The PDD curves were measured for field sizes 1x1, 2x2, 3x3, 5x5, 10x10, 15x15, 20x20 and 30x22 cm2. The in-plane and cross-plane beam profiles at various depths for the same field sizes are measured with different devices. The absolute dosimetry measurements were performed with an IC in liquid water based on a modified TG-51 protocol which takes into account the magnetic field effects.

Results: It was observed in both measurements and TPS calculations that, in the presence of the 1.5T transverse magnetic field, the depth of maximum dose is reduced and the maximum intensity of cross-plane profile shifts to the patient left, especially for small fields, as compared to the similar situations without the magnetic field. For PDD comparison, the differences between measurements and the TPS calculation for most data points was <1% (range +/-2%) beyond the buildup region. The beam profiles measured with different detectors agree well with those from the beam models, with no more than 1 % differences in the regions outside the penumbra and no more than 2% for most regions within the primary beam, except within the penumbra regions. In the penumbra regions, the agreement is within 1.0 mm for most data points of all field sizes studied. The absolute dose at the measurement point was determined to be 87.66 cGy per 100 MU. A dose calibration of 111.8 cGy/100 MU was obtained at the depth of maximum dose with SAD setup (dmax, SAD) when a TMR of 0.784 was used to convert the dose at the measurement point to the calibration point. The absolute dose calibration was verified by IC measurement in a cylindrical solid water phantom.

Conclusion: Both relative and absolute dosimetry measurements agreed well with the TPS calculations, indicating that the beam model for the MR-Linac properly accounts for the magnetic field effects.

Author Disclosure: X. Chen: None. E.S. Paulson: Research Grant; Elekta AB, Siemens Healthcare. E.E. Ahunbay: None. S. Klawikowski: Employee; Green Bay Oncology. A. Sanli: None. A. Li: None.

Xinfeng Chen, PhD

Department of Radiation Oncology, Medical College of Wisconsin

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