SU‐E‐T‐409: A Probability Density Function Based Dose Calculation for Gated Volumetric Modulated Arc Therapy in Lung Stereotactic Body Radiotherapy

Abstract

Purpose: To evaluate the accuracy of probability density function based dose calculation for phase gated volumetric modulated arc therapy (VMAT) in lung stereotactic body radiotherapy, and to quantify the dose uncertainty induced by tumor motion. Methods: During VMAT gating delivery the tumor position, gantry, MLC leaves and dose rate change independently and continuously. To compute the accumulated dose, we convert the target motion into MLC motion for each control point by convolution of MLC leaf positions with target motion probability density function. The new VMAT plan file was imported back to treatment planning system (Eclipse) to reconstruct the dose distribution. The planar dose distribution was compared to a mobile phantom (QUASAR) measurement with the gating technique with an amplitude gating window of 5 mm. This method mentioned above was used to compute the gated VMAT dose distribution and evaluate the motion effect to the PTV and GTV coverage and normal tissues. Results: The probability density function based VMAT gated dose calculation can greatly simplify the dose calculation procedure since only the reference/planning image is used for the accumulated dose calculation. Our results show good agreement between the modified plan and measured dose distribution. Calculated results show that motion can greatly reduce the PTV coverage but its effect on GTV coverage is insignificant for the gated VMAT for the treatment of lung cancer. The gated dose distribution is comparable to the static plan for spinal cord and lung dose. Conclusion: The dose distribution comparison shows good agreement between the modified plan and the measured dose distribution. The VMAT gated dose distribution can be calculated using the probability density function so that the accumulated dose calculation procedure can be greatly simplified.