Room: AAPM ePoster Library
Purpose: CyberKnife provides the option of using either Monte Carlo(MC) or ray-tracing dose calculation algorithm in treatment planning; MC are known as most accurate in predicting dose, especially for low density medium. This study focuses on the clinical implications of quantitively comparing MC and ray-tracing.
Methods: An anthropomorphic digital phantom(XCAT) was used to generate a CT set. Each tumor(GTV) was modeled as a cylinder of 1, 2, or 3-cm diameter/length, and at three different tumor locations (close to spine(SP), chest wall(CW), and left wall(LW)), with center at the same CT slice. Several OARs were contoured, including chest walls, spine, and heart. Clinically acceptable plans(60 Gy to 95% PTV coverage) were first developed using the ray-tracing using VOLO optimizer. The same beam sets were then recalculated with MC; each dose calculation utilized the same beam orientations. The MC prescription was then normalized to achieve the same PTV coverage as the ray-tracing plan.
Results: In the comparison of ray-tracing and MC for lung plans, monitor units in MC plans were 7.5-14.5% higher than ray-tracing plans to maintain same PTV coverage. Under this circumstance, ray-tracing algorithm consistently underestimated the target and the organ dose. For the 1-cm diameter tumor, the maximum target dose dropped 6.76±1.7% from MC to ray-tracing. For the organ maximum dose comparison, from MC to ray-tracing, an 8 to 18% drop for chestwall, and 4 to 11 % drop for heart were observed. For the large tumor, organ doses were similar. The mean dose difference in organs such as the chestwall and heart did not vary greatly with tumor location, while max dose has more location-dependency.
Conclusions: Significant differences between MC calculations versus ray-tracing can influence dose accuracy, due to the improved ability of Monte Carlo algorithms for photon and electron transport at interfaces between lung/soft tissues or lung/tumors.