Program Information
Efficient Scatter Correction for Direct Ray-Tracing Based Dose Calculation
M Chen*, S Jiang , W Lu , UT Southwestern Medical Center, Dallas, TX
Presentations
SU-E-T-355 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To propose a scatter correction method with linear computational complexity for direct-ray-tracing (DRT) based dose calculation. Due to its speed and simplicity, DRT is widely used as a dose engine in the treatment planning system (TPS) and monitor unit (MU) verification software, where heterogeneity correction is applied by radiological distance scaling. However, such correction only accounts for attenuation but not scatter difference, causing the DRT algorithm less accurate than the model-based algorithms for small field size in heterogeneous media.
Methods: Inspired by the convolution formula derived from an exponential kernel as is typically done in the collapsed-cone-convolution-superposition (CCCS) method, we redesigned the ray tracing component as the sum of TERMA scaled by a local deposition factor, which is linear with respect to density, and dose of the previous voxel scaled by a remote deposition factor,
D(i)=aρ(i)T(i)+(b+c(ρ(i)-1))D(i-1),
where T(i)=e^(-αr(i)+β(r(i))^2) and r(i)=Σ_(j=1,...,i)ρ(j) .
The two factors together with TERMA can be expressed in terms of 5 parameters, which are subsequently optimized by curve fitting using digital phantoms for each field size and each beam energy.
Results: The proposed algorithm was implemented for the Fluence-Convolution-Broad-Beam (FCBB) dose engine and evaluated using digital slab phantoms and clinical CT data. Compared with the gold standard calculation, dose deviations were improved from 20% to 2% in the low density regions of the slab phantoms for the 1-cm field size, and within 2% for over 95% of the volume with the largest discrepancy at the interface for the clinical lung case.
Conclusion: We developed a simple recursive formula for scatter correction for the DRT-based dose calculation with much improved accuracy, especially for small field size, while still keeping calculation to linear complexity. The proposed calculator is fast, yet accurate, which is crucial for dose updating in IMRT optimization.
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