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Dosimetric Effects of Magnetic Field in MRI-Guided Radiation Therapy Delivery for Breast Cancer

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G Chen

G Chen*, A Currey , X Li , Medical College of Wisconsin, Milwaukee, WI


SU-E-P-14 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:MRI-guided radiation therapy (RT) delivery would be beneficial for breast irradiation. The electron return effect due to the presence of a transverse magnetic field (TMF) may cause dosimetric issues on dose on skin and at the lung-tissue interface. The purpose of this study is to investigate these issues.

Methods:IMRT plans with tangential beams and VMAT plans with 200 degree arcs to cover ipsilateral breast were generated for 10 randomly selected breast cancer cases using a research planning system (Monaco, Elekta) utilizing Monte Carlo dose calculation with or without a TMF of 1.5 T. Plans were optimized to deliver uniform dose to the whole breast with an exclusion of 5 mm tissue under the skin (PTV-EVAL). All four plans for each patient were re-scaled to have the same PTV-EVAL volume to receive the same prescription dose. The skin is defined as the first 5 mm of ipsilateral-breast tissue, plus extensions in the surrounding region.

Results:The presence of 1.5 T TMF resulted in (1)increased skin dose, with the mean and maximum skin dose increase of 5% and 9%, respectively; (2) similar dose homogeneity within the PTV-EVAL; (3) the slightly improved (3%) dose homogeneity in the whole breast; (4) Averages of 9 and 16% increases in V5 and V20, respectively, for ipsilateral lung; and (5) increased the mean heart dose by 34%. VMAT plans don’t improve whole breast dose uniformity as compared that to the tangential plans.

Conclusion: The presence of transverse magnetic field in MRI-guided RT delivery for whole breast irradiation can result in slightly improved dose homogeneity in the whole breast, increased dose to the ipsilateral lung, heart, and skin. Plan optimization with additional specific dose volume constraints may eliminate/reduce these dose increases.

Funding Support, Disclosures, and Conflict of Interest: This work is partially supported by Elekta Inc.

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