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Dose to Mass in Lung Cancer IMRT Optimization

W Watkins

W.T. Watkins1*, J.A. Moore2, C Dial1, G.D. Hugo1, J.V. Siebers1, (1) Virginia Commonwealth University, Richmond, Virginia, (2) Johns Hopkins University, Baltimore, Maryland

TU-G-BRB-5 Tuesday 4:30:00 PM - 6:00:00 PM Room: Ballroom B

Purpose: To assess the efficacy of intensity modulated radiotherapy (IMRT) optimization for lung cancer based on dose-to-structure mass objectives.

Methods: Three patient cases are planned for two radiotherapy deliveries; each optimized using either dose-volume histogram (DVH) constraints or dose-mass histogram (DMH) constraints. An inhale-phase optimization simulates breath hold (BH) treatment; an average CT (aCT) optimization simulates treatment under free breathing. DVH plan objectives include 70 Gy to 98% of the PTV while minimizing lung V20, esophagus V25, heart V30. DMH optimization utilizes the same beam angles with corresponding objectives; 70 Gy to 98% of the PTV mass and minimum dose to relative mass of risk structures. Relative mass at objective dose levels are compared for DVH and DMH optimization.

Results: For BH plans, DMH optimization maintains or improves target-mass coverage (up to 2% increase in mass at 70 Gy) and decreases lung mass by up to 2%, heart mass by up to 4%, and esophagus mass by up to 2% at the objective dose levels. The DMH plans optimized on aCT increase target mass coverage by <1% in all cases and do not improve risk structure mass sparing compared to DVH based plans. The DVH to DMH optimization differences are patient and radiation path dependent. For a complete set of voxels composing a given volume, irradiated voxels are a subset which can have a different mean density than the entire set. This leads to variations in DVH and DMH solutions for the PTV, lungs, esophagus, and heart. Preferred (reduced) radiation path-lengths through regions of lower density lung are observed in BH-DMH optimization.

Conclusions: DMH optimization has the potential to improve the therapeutic ratio on well-defined geometry (for example during BH-RT). This may have advantages in the heart, however, functional lung sub-units must be further investigated.

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