Dose to mass for evaluation and optimization of lung cancer radiation therapy

Purpose To evaluate potential organ at risk dose-sparing by using dose-mass-histogram (DMH) objective functions compared with dose-volume-histogram (DVH) objective functions. Methods Treatment plans were retrospectively optimized for 10 locally advanced non-small cell lung cancer patients based on DVH and DMH objectives. DMH-objectives were the same as DVH objectives, but with mass replacing volume. Plans were normalized to dose to 95% of the PTV volume (PTV-D95v) or mass (PTV-D95m). For a given optimized dose, DVH and DMH were intercompared to ascertain dose-to-volume vs. dose-to-mass differences. Additionally, the optimized doses were intercompared using DVH and DMH metrics to ascertain differences in optimized plans. Mean dose to volume, D_v‾, mean dose to mass, D_M‾, and fluence maps were intercompared. Results For a given dose distribution, DVH and DMH differ by >5% in heterogeneous structures. In homogeneous structures including heart and spinal cord, DVH and DMH are nearly equivalent. At fixed PTV-D95v, DMH-optimization did not significantly reduce dose to OARs but reduced PTV-D_v‾ by 0.20 ± 0.2 Gy (p = 0.02) and PTV-D_M‾ by 0.23 ± 0.3 Gy (p = 0.02). Plans normalized to PTV-D95m also result in minor PTV dose reductions and esophageal dose sparing (D_v‾ reduced 0.45 ± 0.5 Gy, p = 0.02 and D_M‾ reduced 0.44 ± 0.5 Gy, p = 0.02) compared to DVH-optimized plans. Optimized fluence map comparisons indicate that DMH optimization reduces dose in the periphery of lung PTVs. Conclusions DVH- and DMH-dose indices differ by >5% in lung and lung target volumes for fixed dose distributions, but optimizing DMH did not reduce dose to OARs. The primary difference observed in DVH- and DMH-optimized plans were variations in fluence to the periphery of lung target PTVs, where low density lung surrounds tumor.