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Field-In-Field Breast Planning for a Jawless, Double-Stack MLC Linac Using Flattening-Filter-Free Beams

R Morris1*, B Cai2 , J Hilliard3 , G Hugo4 , S Mutic5 , (1) ,,,(2) Washington University School of Medicine, St. Louis, MO, (3) Washington University School of Medicine, St Louis, MO, (4) Washington University School of Medicine, St. Louis, MO, (5) Washington University in St Louis, St Louis, MO


(Sunday, 7/29/2018) 4:00 PM - 4:55 PM

Room: Karl Dean Ballroom A2

Purpose: To develop a planning strategy to achieve clinically acceptable 3D conformal breast plans with the Varian Halcyon linear accelerator using a field-in-field (FiF) technique. Unlike conventional accelerators, the Halcyon employs a jawless, double-stack MLC for beam modulation and utilizes a 6 MV flattening-filter-free (FFF) beam that introduces new challenges in forward planning.

Methods: Ten clinical breast FiF plans were anonymized and imported into the Halycon-capable Eclipse treatment planning system (v15.5). Clinically, seven plans utilized 6MV tangents and three used 10MV tangents due to a large bridge separation. Breast PTV volumes ranged from 518.6–1210.7cc. For the Halcyon plans, each plan used the clinical tangent gantry angles, but energy was changed to 6FFF. The unconventional planning workflow involved normalization to the depth of maximum dose on central axis as opposed to a dosimetrist-defined weight point location. In addition, collimator rotation allowed non-standard FiF segmentation to account for the off-axis softening of the FFF beam. Total number of segments per field, including those utilizing the Halcyon flattening sequence, was limited to five with a minimum of 5 monitor units (MU) per segment. Plans were normalized for comparison by matching DVH coverage of the PTV at the prescription dose. Plan quality was evaluated by analysis of hot spot, mean target coverage, and ipsilateral lung mean dose.

Results: Mean coverage of the PTV remained within ±2% of the clinically delivered plan. Global plan hot spots were -2/+6%, and ipsilateral lung doses were -2/+4% with the Halcyon plans. Isodoses were qualitatively evaluated by a radiation oncologist and deemed clinically acceptable.

Conclusion: A clinically acceptable and standardized planning workflow has been developed that allows dosimetrists to efficiently plan 3D breast FiF on the Halcyon platform.


Not Applicable / None Entered.


TH- External beam- photons: General (most aspects)

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