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(Saturday, 3/30/2019)
Room: Exhibit Hall
Purpose: To perform risk-based plan evaluation within the Eclipse treatment planning system using normal tissue complication probability (NTCP) models and derive planning guidance based on intermediate sequential boost levels.
Methods: Utilizing the Varian Eclipse Scripting API we created a module implementing dose volume histogram manipulation including normalization to standard fractionation and reduction to organ-equivalent doses (OED) and generalized-equivalent-uniform-doses (gEUD). Lyman-Kutcher-Burman NTCP models (QUANTEC recommended) were used for rectum (D₅₀=76.9Gy) and small bowel (D₅₀=59Gy) organs at risk (OARs). We randomly selected 27 patients treated for prostate cancer with radiation therapy to 77.4Gy in 1.8Gy/fraction with 45Gy to the pelvis followed by sequential boosts to the prostate and seminal vesicles. Volumetric modulated arc therapy was used for all patients. We calculated OEDs, gEUDs, and NTCP values in all plans and total dose plan sums. Spearman’s rank correlation coefficients (ρ) and polynomial fits were calculated for the relationship between NTCP estimated from the total plan dose (NTCPₓ) and dose metrics from the sequential plans.
Results: Small bowel NTCP correlated most strongly (p<0.01) with gEUD₄₅ and Dmean₄₅, with ρ=0.91 and 0.82, respectively. For the sequential boosts (54Gy/77.4Gy) significant albeit weaker correlation was found with ρ=0.56/0.57 and 0.72/0.74, respectively. Values of NTCP₄₅ were within 8.3%±2.0% of NTCPₓ on average. Correlation between NTCPₓ and gEUD₄₅ was significant (p=0.03) with ρ=0.41, while Dmean₄₅ was not significantly associated. Stronger correlation (p<0.01) was found for gEUD and Dmean on sequential boost plans with ρ=0.76/0.72 and 0.64/0.66, respectively. Values of gEUD₄₅ leading to 5% NTCPₓ were 29.5Gy and 36.7Gy for small bowel and rectum, respectively.
Conclusion: Our scripting module can be used for NTCP-based plan evaluation. Using the correlation between gEUD₄₅ and NTCPₓ we can derive gEUD targets that can be used in optimizing 45Gy primary prostate plans to achieve a certain acceptable NTCP after sequential boosts to 77.4Gy.
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