Purpose: QUANTEC recommends whole-mean radiotherapy dose constraints for various organs-at-risk, but mean dose constraints allow dose to concentrate in arbitrary regions of the constrained volume. Here we demonstrate a radiotherapy treatment planning method that uses sub-segmentation to improve spatial control of modulated dose distributions.
Methods: An open-source binary plug-in script for Varian Eclipse™, “Plan n Check,” was developed for structure sub-segmentation, applying constraints, and automating Volume Modulated Arc Therapy (VMAT) plan optimization. Plan n Check was used on 6 head-and-neck VMAT patients (70Gy, 35 fractions) to divide laryngopharynx structures into 8 sub-structures of equal volume using nested segmentation. An institutional whole-mean dose constraint of 45Gy (adapted from QUANTEC) was imposed on each sub-structure in an effort to mitigate the concentration of dose to sub-regions of the laryngopharynx. Plans were optimized with Plan n Check using the Analytical Anisotropic Algorithm (AAA). In addition, a standard plan which did not apply sub-segment dose constraints was created for each patient as a control. The number of sub-structures violating the adopted dose constraint was compared for plans with and without sub-structure constraints. The standard deviation of mean dose within laryngopharynx sub-structures was also calculated for all plans to determine whether sub-structure optimization can reduce intra-structure dose variability.
Results: The number of laryngopharynx sub-structures with dose violations was significantly reduced (mean: 2.2 to 0.33) when plans imposed sub-structure whole-mean constraints (Student’s t(5) = 3.38, p < 0.02). Imposing sub-structure constraints significantly reduced the standard deviation (8.2Gy to 4.1Gy) of mean laryngopharynx sub-structure doses (t(5) = 2.58, p < 0.025). This technique did not significantly impact other organ or target volume dose distributions.
Conclusion: Mean dose constraints may in some cases insufficiently constrain organ doses. Imposing additional sub-segment constraints is a practical method for exerting greater spatial control and reducing intra-organ dose variability.