Being certain about uncertainties: A robust evaluation method for high-dose-rate prostate brachytherapy treatment plans including the combination of uncertainties

Abstract

In high-dose-rate (HDR) prostate brachytherapy the combined effect of uncertainties cause a range of possible dose distributions deviating from the nominal plan, and which are not considered during treatment plan evaluation. This could lead to dosimetric misses for critical structures and overdosing of organs at risk. A robust evaluation method to assess the combination of uncertainties during plan evaluation is presented and demonstrated on one HDR prostate ultrasound treatment plan retrospectively. A range of uncertainty scenarios are simulated by changing six parameters in the nominal plan and calculating the corresponding dose distribution. Two methods are employed to change the parameters, a probabilistic approach using random number sampling to evaluate the likelihood of variation in dose distributions, and a combination of the most extreme possible values to access the worst-case dosimetric outcomes. One thousand probabilistic scenarios were run on the single treatment plan with 19.0% of scenarios passing all eight clinical objectives. The prostate D90 had a standard deviation of 4.7%, with the worst case decreasing the dose by up to 27.6%. The urethra D10 was up to 38% higher than planned in the worst case. All DVH metrics in the probabilistic scenarios were found to be within acceptable clinical constraints for the plan under statistical tests for significance. The clinical significance of the results from the robust evaluation method presented on any individual treatment plan needs to be compared in the context of a historical data set that contains patient outcomes with robustness analysis data to ascertain a baseline acceptance.

A robust evaluation of 49 high‐dose‐rate prostate brachytherapy treatment plans including all major uncertainties

“…Uncertainty magnitudes were obtained from literature as described in previous work 15 and are technique‐specific but not clinic specific. Applying uncertainties for a specific clinic is critical for the accuracy in simulating uncertainty scenarios and, therefore, the resulting robustness measurement.…”

“…The robust evaluation method described in previous work 15 has been developed further to run on a graphics‐processing unit (GPU). In this method, 14 different uncertainties in the HDR prostate BT technique were found to cause five common changes to the structures (prostate, urethra, rectum, dwell points) in the nominal plan and cause variation to dwell times.…”

“…Table 1 describes the parameter variables' action on the nominal plan and corresponding magnitudes. Where possible, these magnitudes were obtained from literature 15 …”

“…The worst‐case scenarios were conceptualized from expert knowledge of the movements likely to cause extreme DVH metric values, 15 and where these differed from the probabilistic method, are described in Table 1. The worst‐case values used for each parameter variable were defined to be 90% CI limits from the six probabilistic normal distributions (and zero when appropriate), and all values are shown in Table 1.…”

“…Our previous work 15 illustrated a framework for robust evaluation in HDR prostate BT on one patient plan. It included the most substantial uncertainties, using probabilistic and worst‐case approaches.…”