Integral dose based inverse optimization objective function promises lower toxicity in head-and-neck

Abstract

Purpose: The voxels in a CT data sets contain density information. Besides its use in dose calculation density has no other application in modern radiotherapy treatment planning. This work introduces the use of density information by integral dose minimization in radiotherapy treatment planning for head-and-neck squamous cell carcinoma (HNSCC).

Materials and methods: Eighteen HNSCC cases were studied. For each case two intensity modulated radiotherapy (IMRT) plans were created: one based on dose-volume (DV) optimization, and one based on integral dose minimization (Energy hereafter) inverse optimization. The target objective functions in both optimization schemes were specified in terms of minimum, maximum, and uniform doses, while the organs at risk (OAR) objectives were specified in terms of DV- and Energy-objectives respectively. Commonly used dosimetric measures were applied to assess the performance of Energy-based optimization. In addition, generalized equivalent uniform doses (gEUDs) were evaluated. Statistical analyses were performed to estimate the performance of this novel inverse optimization paradigm.

Results: Energy-based inverse optimization resulted in lower OAR doses for equivalent target doses and isodose coverage. The statistical tests showed dose reduction to the OARs with Energy-based optimization ranging from ∼2% to ∼15%.

Conclusions: Integral dose minimization based inverse optimization for HNSCC promises lower doses to nearby OARs. For comparable therapeutic effect the incorporation of density information into the optimization cost function allows reduction in the normal tissue doses and possibly in the risk and the severity of treatment related toxicities.

Keywords: Dose; IMRT; Imaging; Integral dose; Mass; Optimization.

Figure 1

Flow chart of the optimization scheme used to perform the inverse planning with both – DV-and Energy-based optimization schemes. Both schemes utilized multi-stage dose reduction to the OARs used as dose-limiting structures.

Figure 2

Comparison of dose-volume-histograms (DVHs) and isodoses for a single case. The solid DVHs are derived from DV optimization, while the dashed lines are derived from Energy optimization. The DVHs for the different OARs are color-coded in the figure legend. On the right side of the figure there are comparisons of isodose plots in two orthogonal planes, where the far right column is with isodose plots from the Energy-based plan, while the column next to the DVHs plot represents the DV plan isodoses.