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Magnetic Field Dose Effects for Various Radiation Beam Geometries for Patients Treated with Hypofractionated Partial Breast Irradiation

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S Lim-Reinders

S Lim-Reinders1,2*, B Keller1,3 , C McCann1,3 , A Sahgal1,3 , J Lee1,3 , A Kim1,3 , (1) Sunnybrook Odette Cancer Centre, Toronto, Canada (2) University of Toronto, Department of Physics (3) University of Toronto, Department of Radiation Oncology


SU-F-T-432 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Hypofractionated partial breast irradiation (HPBI) is being used at our clinic to treat inoperable breast cancer patients who have advanced disease. We are investigating how these patients could benefit from being treated in an MRI-linac, where real-time daily MRI tumor imaging and plan adaptation would be possible. As a first step, this study evaluates the dosimetric impact of the magnetic field for different radiation beam geometries on relevant OARs.

Methods: Five patients previously treated using HPBI were selected. Six treatment plans were generated for each patient, evaluating three beam geometries (VMAT, IMRT, 3DCRT) with and without B₀=1.5 T. The Monaco TPS was used with the Elekta MRI-Linac beam model, where the magnetic field is orthogonal to the radiation beam. All plans were re-scaled to the same isocoverage with a prescription of 40Gy/5 to the PTV. Plans were evaluated for the effect of the magnetic field and beam modality on skin V₃₀, lung V₂₀ and mean heart dose.

Results: Averaged over all patients, skin V₃₀ for 3DCRT was higher than VMAT and IMRT (by +22% and +21%, with B₀-ON). The magnetic field caused larger increases in skin V₃₀ for 3DCRT (+8%) than VMAT (+3%) and IMRT (+4%) compared with B₀-OFF. With B₀-ON, 3DCRT had a markedly lower mean heart dose than VMAT (by 538cGy) and IMRT (by 562cGy); for lung V₂₀, 3DCRT had a marginally lower dose than VMAT (by -2.2%) and IMRT (also -2.2%). The magnetic field had minimal effect on the mean heart dose and lung V₂₀ for all geometries.

Conclusion: The decreased skin dose in VMAT and IMRT can potentially mitigate the effects of skin reactions for HPBI in an MRI-linac. This study illustrated that more beam angles may result in lower skin toxicity and better tumor conformality, with the trade-off of elevated heart and lung doses.

Funding Support, Disclosures, and Conflict of Interest: We are receiving funding support from Elekta

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