Room: Exhibit Hall | Forum 3
Purpose: To determine the impact on dose calculation accuracy of approximations frequently used in VMAT plan optimization. To implement an accurate fluence calculation method for VMAT which accounts for the continuous motion of the MLC leaves.
Methods: The impact of the discrete dose calculation angle approximation and the discrete aperture approximation (which assumes the collimator leaves shift instantaneously from one aperture to the next) was determined by comparing the dose for a VMAT plan recalculated at successively smaller dose calculation angle spacing using one of two schemes. The first tested both approximations, while the second used our accurate fluence calculation method (continuous aperture), and therefore only tested the discrete dose angle approximation. VMAT plans were optimized using the matRad treatment planning system on three cases, using both fluence calculation
methods: discrete and continuous aperture. The benefit of the continuous aperture method over the discrete approximation was determined by comparing the plan quality of both plans, where dose was recalculated using the continuous aperture method.
Results: Dose calculated using the discrete aperture approximation at 4Â° spacing resulted in significant dose errors (10%-35%, depending on the anatomical site) as compared to the reference dose (continuous aperture method at 0.5Â° spacing). These errors were greatly reduced (to 0.8%-2%) with the continuous aperture method even at 4Â° spacing, implying that the discrete aperture approximation contributes the largest error. Whereas all dose volume histogram (DVH) objectives appeared to be met by the discrete aperture VMAT optimized plan, many of them failed when the dose was recalculated with the continuous aperture method. All DVH objectives were met when the plan was re-optimized with the continuous aperture method.
Conclusion: The proposed continuous aperture fluence calculation method was successfully used to optimize VMAT plans, with the final calculated dose correctly accounting for the continuous nature of VMAT delivery.