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)â‚?â‚“ from every beam in a plan ,SPG(T)â‚’(t)â‚?(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 63Ã—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)â‚’(t)â‚?(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 (Pâ‰¥0.05) difference in SPG(T)â‚’(t)â‚?(l) and MI resulting from unsmooth fluences vs. W=1. The SPG(T)â‚’(t)â‚?(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 (Pâ‰¥0.05) to unsmooth fluences. In comparison to unsmooth fluences, the degree of fluence modulation reduced significantly (Pâ‰¤0.05) at W=100 (0.66Â±0.08 vs. 0.58Â±0.06 for 6MV and 0.66Â±0.08 vs. 0.59Â±0.06 for 18MV) without significant difference in HI and dosimetric parameters (Pâ‰¥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.