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

Uncertainties in radiotherapy cause deviation from the planned dose distribution and may result in delivering a treatment that fails to meet clinical objectives. The impact of uncertainties is unique to the patient anatomy and the needle locations in HDR prostate brachytherapy. Evaluating this impact during treatment planning is not common practice, relying on margins around the target or organs-at-risk to account for uncertainties.

A robust evaluation framework for HDR prostate brachytherapy treatment plans was evaluated on 49 patient plans, measuring the range of possible dosimetric outcomes to the patient due to 14 major uncertainties.

Patient plans were evaluated for their robustness to uncertainties by simulating probable uncertainty scenarios. Five-thousand probabilistic and 1943 worst-case scenarios per patient were simulated by changing the position and size of structures and length of dwell times from their nominal values. For each uncertainty scenario, the prostate D₉₀ and maximum doses to the urethra, D_0.01cc , and rectum, D_0.1cc , were calculated.

The D₉₀ was an average 1.16 ± 0.51% (mean ± SD) below nominal values for the probabilistic scenarios; the D_0.01cc metric was 2.24 ± 0.90% higher; and D_0.1cc was greater by 0.48 ± 0.30%. The D_0.01cc and D₉₀ metrics were more sensitive to uncertainties than D_0.1cc , with a median of 79.0% and 84.9% of probabilistic scenarios passing the constraints, compared to 96.5%. The median pass-rate for scenarios that passed all three metrics simultaneously was 63.4%.

Assessing treatment plan robustness improves plan quality assurance, is achievable in less than 1-min, and identifies treatment plans with poor robustness, allowing re-optimization before delivery.