Characterizing Magnetically Focused Contamination Electrons by Off-axis Irradiation on an Inline MRI-Linac

Overview

Journal J Appl Clin Med Phys

Specialties Biophysics
General Medicine

Date 2022 Mar 25

PMID 35333000

Authors

Elizabeth Patterson

Bradley M Oborn

Dean Cutajar

Urszula Jelen

Gary Liney

Anatoly B Rosenfeld

Peter E Metcalfe

Affiliations

Soon will be listed here.

Abstract

Purpose: The aim of this study is to investigate off-axis irradiation on the Australian MRI-Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field.

Methods: Dosimetric measurements were performed using a microDiamond detector, Gafchromic EBT3 film, and MOSkin . Three field sizes were investigated including 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm . Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source-to-surface distance (SSD) of 1.8 m for fields collimated centrally and off-axis. PDD measurements were also acquired at isocenter for each off-axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination.

Results: Off-axis irradiation separates the majority of electron contamination from an x-ray beam and was found to significantly reduce in-field surface dose. For the 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm field, surface dose was reduced from 120.9% to 24.9%, 229.7% to 39.2%, and 355.3% to 47.3%, respectively. Monte Carlo simulations generally were within experimental error to MOSkin and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero.

Conclusions: Experimental and simulation data were acquired for a range of field sizes to investigate off-axis irradiation on an inline MRI-Linac. The skin sparing effect was observed with off-axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.

Citing Articles

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PMID: 39475282 PMC: 11788252. DOI: 10.1002/mp.17495.

Electron streaming dose measurements and calculations on a 1.5 T MR-Linac.

Patterson E, Powers M, Metcalfe P, Cutajar D, Oborn B, Baines J J Appl Clin Med Phys. 2024; 25(7):e14370.

PMID: 38661097 PMC: 11244671. DOI: 10.1002/acm2.14370.

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac.

Patterson E, Stokes P, Cutajar D, Rosenfeld A, Baines J, Metcalfe P Phys Eng Sci Med. 2023; 46(2):787-800.

PMID: 36988905 PMC: 10209272. DOI: 10.1007/s13246-023-01251-6.

Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac.

Patterson E, Oborn B, Cutajar D, Jelen U, Liney G, Rosenfeld A J Appl Clin Med Phys. 2022; 23(6):e13591.

PMID: 35333000 PMC: 9195023. DOI: 10.1002/acm2.13591.

References

. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP publication 103. Ann ICRP. 2007; 37(2-4):1-332. DOI: 10.1016/j.icrp.2007.10.003. View

Xhaferllari I, Kim J, Liyanage R, Liu C, Du D, Doemer A . Clinical utility of Gafchromic film in an MRI-guided linear accelerator. Radiat Oncol. 2021; 16(1):117. PMC: 8236160. DOI: 10.1186/s13014-021-01844-z. View

Buckley J, Smith A, Sidhom M, Rai R, Liney G, Dowling J . Measurements of human tolerance to horizontal rotation within an MRI scanner: Towards gantry-free radiation therapy. J Med Imaging Radiat Oncol. 2020; 65(1):112-119. DOI: 10.1111/1754-9485.13130. View

Marroquin E, Herrera Gonzalez J, Camacho Lopez M, Villarreal Barajas J, Garcia-Garduno O . Evaluation of the uncertainty in an EBT3 film dosimetry system utilizing net optical density. J Appl Clin Med Phys. 2016; 17(5):466-481. PMC: 5874103. DOI: 10.1120/jacmp.v17i5.6262. View

Bielajew A . The effect of strong longitudinal magnetic fields on dose deposition from electron and photon beams. Med Phys. 1993; 20(4):1171-9. DOI: 10.1118/1.597149. View

De Deene Y, Wheatley M, Dong B, Roberts N, Jelen U, Waddington D . Towards real-time 4D radiation dosimetry on an MRI-Linac. Phys Med Biol. 2020; 65(22):225031. DOI: 10.1088/1361-6560/abb9f7. View

Stecker M, Balter S, Towbin R, Miller D, Vano E, Bartal G . Guidelines for patient radiation dose management. J Vasc Interv Radiol. 2009; 20(7 Suppl):S263-73. DOI: 10.1016/j.jvir.2009.04.037. View

. 1990 Recommendations of the International Commission on Radiological Protection. Ann ICRP. 1991; 21(1-3):1-201. View

Oborn B, Metcalfe P, Butson M, Rosenfeld A . Monte Carlo characterization of skin doses in 6 MV transverse field MRI-linac systems: effect of field size, surface orientation, magnetic field strength, and exit bolus. Med Phys. 2010; 37(10):5208-17. DOI: 10.1118/1.3488980. View

10.

Park J, Shin K, Kim J, Park S, Jeon S, Choi N . Air-electron stream interactions during magnetic resonance IGRT : Skin irradiation outside the treatment field during accelerated partial breast irradiation. Strahlenther Onkol. 2017; 194(1):50-59. DOI: 10.1007/s00066-017-1212-z. View

11.

Barten D, Hoffmans D, Palacios M, Heukelom S, Van Battum L . Suitability of EBT3 GafChromic film for quality assurance in MR-guided radiotherapy at 0.35 T with and without real-time MR imaging. Phys Med Biol. 2018; 63(16):165014. DOI: 10.1088/1361-6560/aad58d. View

12.

Raaijmakers A, Raaymakers B, Lagendijk J . Magnetic-field-induced dose effects in MR-guided radiotherapy systems: dependence on the magnetic field strength. Phys Med Biol. 2008; 53(4):909-23. DOI: 10.1088/0031-9155/53/4/006. View

13.

Bol G, Lagendijk J, Raaymakers B . Compensating for the impact of non-stationary spherical air cavities on IMRT dose delivery in transverse magnetic fields. Phys Med Biol. 2015; 60(2):755-68. DOI: 10.1088/0031-9155/60/2/755. View

14.

Roberts N, Patterson E, Jelen U, Causer T, Holloway L, Liney G . Experimental characterization of magnetically focused electron contamination at the surface of a high-field inline MRI-linac. Med Phys. 2019; 46(12):5780-5789. DOI: 10.1002/mp.13847. View

15.

Reynolds M, Fallone B, Rathee S . Dose response of selected ion chambers in applied homogeneous transverse and longitudinal magnetic fields. Med Phys. 2013; 40(4):042102. DOI: 10.1118/1.4794496. View

16.

Liney G, Jelen U, Byrne H, Dong B, Roberts T, Kuncic Z . Technical Note: The first live treatment on a 1.0 Tesla inline MRI-linac. Med Phys. 2019; 46(7):3254-3258. DOI: 10.1002/mp.13556. View

17.

Keyvanloo A, Burke B, St Aubin J, Baillie D, Wachowicz K, Warkentin B . Minimal skin dose increase in longitudinal rotating biplanar linac-MR systems: examination of radiation energy and flattening filter design. Phys Med Biol. 2016; 61(9):3527-39. DOI: 10.1088/0031-9155/61/9/3527. View

18.

Patterson E, Oborn B, Cutajar D, Jelen U, Liney G, Rosenfeld A . Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac. J Appl Clin Med Phys. 2022; 23(6):e13591. PMC: 9195023. DOI: 10.1002/acm2.13591. View

19.

Steinmann A, OBrien D, Stafford R, Sawakuchi G, Wen Z, Court L . Investigation of TLD and EBT3 performance under the presence of 1.5T, 0.35T, and 0T magnetic field strengths in MR/CT visible materials. Med Phys. 2019; 46(7):3217-3226. PMC: 9673483. DOI: 10.1002/mp.13527. View

20.

De-Colle C, Nachbar M, Mnnich D, Boeke S, Gani C, Weidner N . Analysis of the electron-stream effect in patients treated with partial breast irradiation using the 1.5 T MR-linear accelerator. Clin Transl Radiat Oncol. 2021; 27:103-108. PMC: 7856390. DOI: 10.1016/j.ctro.2020.12.005. View