Abstract
In radiotherapy, abdominal and thoracic sites are candidates for performing motion tracking. With real-time control it is possible to adjust the multileaf collimator (MLC) position to the target position. However, positions are not perfectly matched and position errors arise from system delays and complicated response of the electromechanic MLC system. Although, it is possible to compensate parts of these errors by using predictors, residual errors remain and need to be compensated to retain target coverage. This work presents a method to statistically describe tracking errors and to automatically derive a patient-specific, per-segment margin to compensate the arising underdosage on-line, i.e. during plan delivery.The statistics of the geometric error between intended and actual machine position are derived using kernel density estimators. Subsequently a margin is calculated on-line according to a selected coverage parameter, which determines the amount of accepted underdosage. The margin is then applied onto the actual segment to accommodate the positioning errors in the enlarged segment.The proof-of-concept was tested in an on-line tracking experiment and showed the ability to recover underdosages for two test cases, increasing in the underdosed area about and , respectively. The used dose model was able to predict the loss of dose due to tracking errors and could be used to infer the necessary margins.The implementation had a running time of 23 ms which is compatible with real-time requirements of MLC tracking systems. The auto-adaptivity to machine and patient characteristics makes the technique a generic yet intuitive candidate to avoid underdosages due to MLC tracking errors.
Highlights
In radiotherapy, dose conformity, the ratio of actual to intended dose deposition, is impaired by the change of patient anatomy during treatment and between treatment fractions
On-line multileaf collimator (MLC) control has become available on the treatment machines of major radiotherapy vendors and first tracked deliveries were performed in vivo (Colvill et al 2015)
The quality of conformity using MLC tracking is to a high extent influenced by the underlying system delay, which can amount up to several hundreds of milliseconds (Hoogeman et al 2009, Tacke et al 2010, Depuydt et al 2011, Fast et al 2014, Bedford et al 2015, Glitzner et al 2015)
Summary
Dose conformity, the ratio of actual to intended dose deposition, is impaired by the change of patient anatomy during (intra) treatment and between (inter) treatment fractions. A planned reference segment, shifted to the new target position in beam’s eye view (BEV), is sent to the MLC controller. All of these comp onents exhibit an inherent time delay which cause lag and misalignment between the target and the treatment beam. As a simplification, these time delays are usually quantified using sinusoidal motion patterns, assuming a linear phase behavior of the entire MLC system (Glitzner et al 2015). The machine response will show complex over- and undershoots, which cannot be explained by a constant lag alone
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