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 nondeform-able 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 cm3 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.

Multileaf Collimator Tracking Improves Dose Delivery for Prostate Cancer Radiation Therapy: Results of the First Clinical Trial

“…A simulation study performed using dose reconstruction for conventional and subvolume boost treatments for prostate showed that MLC tracking is effective at mitigating the dosimetric effects of prostate motion and allows for successful GTV and PTV dose delivery (14), as demonstrated in the present study. The dosimetric results for the without-MLC tracking delivery also corresponds closely with other observational prostate dose reconstruction studies (15,16) that have modeled the effect of intrafraction prostate motion on the dose delivery of standard non-MLC tracking treatments. The outlier fractions for which the dose distribution was improved by MLC tracking tended to be lower PTV and CTV doses, lower rectum doses, and higher bladder doses.…”

The Practicality of ICRU and Considerations for Future ICRU Definitions

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Characterization of a novel liquid fiducial marker for multimodal image guidance in stereotactic body radiotherapy of prostate cancer

“…Intrafraction prostate motion is a predominant cause for treatment delivery inaccuracies as multiple studies have shown that the prostate tends to move in an unpredictable way in approximately 50% of all patients with observed motion amplitudes up to 2 cm for certain extreme cases . The dosimetric impact of this residual intrafraction motion is nonnegligible and should be mitigated by accurate monitoring of the prostate position during treatment delivery . To this end, a wide variety of motion monitoring techniques are currently utilized in clinical practice or are under development …”