(Tuesday, 7/31/2018) 11:00 AM - 12:15 PM
Room: Karl Dean Ballroom A1
Purpose: Simultaneous optimization for dual-energy (6MV, 18MV) IMRT may produce complex fluence maps, which could impact deliverability resulting in a poor dosimetric outcome. This study focuses on reducing the complexity of simultaneously optimized dual-energy fluence maps (DEFMs) by controlling sum of positive gradients (SPG) weighting factors based on modulation index (MI).
Methods: This study includes eight prostate cancer plans that had the dose prescription of 7920 cGy/44 fractions. For each plan, DEFMs were simultaneously optimized using a linear programming model including smoothening constraints based on SPG with four different weighting factors (W = 0 (unsmooth), 1, 10 and 100), respectively. For each weighting factor, the summation of SPG(m)�ₓ from every beam in a plan ,SPG(T)ₒ(t)�(l) per plan, was calculated for both energies. The degree of fluence modulation resulting from each SPG weighting factor was studied by computing MI for 63×2 DEFMs from all 8 plans. The plan quality was measured based on homogeneity index (HI) to the PTV and Dmean, Dmax to the prostate, bladder, rectum and femoral heads. The SPG(T)ₒ(t)�(l), MI, and plan quality resulting from unsmooth fluences were statistically compared with the ones resulting from smoothed fluences (W=1, W=10, W=100), respectively.
Results: For both energies, there was no significant (P≥0.05) difference in SPG(T)ₒ(t)�(l) and MI resulting from unsmooth fluences vs. W=1. The SPG(T)ₒ(t)�(l) reduced significantly at W=10 and W=100 compared to unsmooth fluences. However, mean MI value for both W=1 and W=10 remained comparable (P≥0.05) to unsmooth fluences. In comparison to unsmooth fluences, the degree of fluence modulation reduced significantly (P≤0.05) at W=100 (0.66±0.08 vs. 0.58±0.06 for 6MV and 0.66±0.08 vs. 0.59±0.06 for 18MV) without significant difference in HI and dosimetric parameters (P≥0.05).
Conclusion: By controlling the SPG weighting factor based on MI, the degree of modulations in DEFMs were significantly reduced without compromising the plan quality.
Keywords
Numerical Analysis, Optimization, Inverse Planning
Taxonomy
TH- External beam- photons: General (most aspects)
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