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 undesirable dose escalation \citep{Tanderup: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 of the prostate. A scenario-based method was developed that handles uncertainties in index sets. Heuristics were included to reduce the calculation times to acceptable proportions. The approach was extended to account for delineation uncertainties of an OAR as well. 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. For cases where rectum sparing was a binding restriction, including uncertainties in rectum delineation in the planning model led to a reduced risk of a rectum overdose, and in some cases to reduced target coverage.
Citation
Unpublished, Tilburg University Warandelaan 2 5037 AB Tilburg, The Netherlands December 2015