Robust optimization of dose-volume metrics for prostate HDR-brachytherapy incorporating target volume delineation uncertainties

In radiation therapy planning, uncertainties in target volume definition yield a risk of underdosing the tumor. The classical way to prevent this in the context of external beam radiotherapy (EBRT) has been to expand the clinical target volume (CTV) with an isotropic margin to obtain the planning target volume (PTV). However, the EBRT-based PTV concept is not directly applicable to brachytherapy (BT) since it can lead to dose escalation (Tanderup et al., 2010). This work presents a treatment plan optimization model that uses worst-case robust optimization to account for delineation uncertainties in interstitial high-dose-rate BT. A scenario-based method was developed that handles uncertainties in index sets. Heuristics were included to reduce the calculation times to acceptable proportions. The method was applied on data from prostate cancer patients, and evaluated in terms of commonly used dosimetric performance criteria for the CTV and relevant organs at risk. The robust optimization approach was compared against the classical PTV margin concept and against a scenario-based CTV margin approach. The results show that both the scenario-based margin and the robust optimization method are well capable of reducing the risk of underdosage to the tumor. As expected, the scenario-based CTV margin approach leads to dose escalation within the target, whereas this could be prevented with the robust model.