Assessing the dosimetric impact of real-time prostate motion during volumetric modulated arc therapy

Abstract

Purpose: To develop a method for dose reconstruction by incorporating the interplay effect between aperture modulation and target motion, and to assess the dosimetric impact of real-time prostate motion during volumetric modulated arc therapy (VMAT).

Methods and materials: Clinical VMAT plans were delivered with the TrueBeam linac for 8 patients with prostate cancer. The real-time target motion during dose delivery was determined based on the 2-dimensional fiducial localization using an onboard electronic portal imaging device. The target shift in each image was correlated with the control point with the same gantry angle in the VMAT plan. An in-house-developed Monte Carlo simulation tool was used to calculate the 3-dimensional dose distribution for each control point individually, taking into account the corresponding real-time target motion (assuming a nondeformable target with no rotation). The delivered target dose was then estimated by accumulating the dose from all control points in the plan. On the basis of this information, dose-volume histograms and 3-dimensional dose distributions were calculated to assess their degradation from the planned dose caused by target motion. Thirty-two prostate motion trajectories were analyzed.

Results: The minimum dose to 0.03 cm(3) of the gross tumor volume (D0.03cc) was only slightly degraded after taking motion into account, with a minimum value of 94.1% of the planned dose among all patients and fractions. However, the gross tumor volume receiving prescription dose (V100%) could be largely affected by motion, dropping below 60% in 1 trajectory. We did not observe a correlation between motion magnitude and dose degradation.

Conclusions: Prostate motion degrades the delivered dose to the target in an unpredictable way, although its effect is reduced over multiple fractions, and for most patients the degradation is small. Patients with greater prostate motion or those treated with stereotactic body radiation therapy would benefit from real-time prostate tracking to reduce the margin.

Figure 1

Trajectory determination from 2-dimensional (2D) fiducial positions measured with electronic portal imaging device cine MV imaging. The blue, green, and red crosses represent the positions of the 3 fiducials as derived from their 2D localization. The black solid line corresponds to the determined prostate displacement. AP = anteroposterior; LAT = lateral.

Figure 2

Gross tumor volume dose-volume histogram (DVH) degradation for 5 different prostate trajectories measured in 3 different patient treatments. The green and black lines represent the trajectories for the 2 volumetric modulated arc therapy arcs. If the beam targeting is good (row 1), no significant degradation is seen in the DVH (planned DVH depicted in blue and delivery DVH depicted in red in the right column). In all the other trajectories, motion has an impact on the absorbed dose in the GTV (prostate).

Figure 3

Volume covered by the prescription dose (V_100%, first row), minimum dose that irradiates 0.03 cm³ of the volume (D_0.03cc, second row) normalized to the planned D_0.03cc as 100% reference, and mean displacement along the treatment fraction for each fraction (third row) with its standard deviation. The circles in the 2 upper rows represent the planned values, and the crosses the actual values for each of the fractions, for the 8 patients.

Figure 4

Dose distribution reconstructed in the target by accumulating the dose due to each control point in each voxel. The axial, coronal, and sagittal slices are taken through the isocenter, depicted by the orange cross. The interplay effect can be seen, affecting the absorbed dose received in the inner part of the tumor. This dose reconstruction corresponds to the treatment fraction with the trajectory seen in Figure 2, row 4. The underdosage of the tumor in this fraction is significant, with some points receiving close to 10% less of the prescribed dose in the target.