Dwell time modulation restrictions do not necessarily improve treatment plan quality for prostate HDR brachytherapy

Inverse planning algorithms for dwell time optimisation in interstitial high-dose-rate (HDR) brachytherapy may produce solutions with large dwell time variations within catheters, which may result in undesirable selective high-dose subvolumes. Extending the optimisation model with a dwell time modulation restriction (DTMR) that limits dwell time differences between neighboring dwell positions has been suggested to eliminate this issue. DTMRs may additionally reduce the sensitivity for uncertainties in dwell positions that result from catheter reconstruction errors and source positioning inaccuracies. This study quantifies the reduction of high-dose subvolumes and the robustness against these uncertainties by applying a DTMR to template-based prostate HDR brachytherapy implants. Three different DTMRs were consecutively applied to a linear dose-based penalty model (LD) and a dose-volume based model (LDV), both obtained from literature. The models were solved with DTMR levels ranging from no restriction to uniform dwell times within catheters. Uncertainties were simulated on clinical cases using in-house developed software, and dose-volume metrics were calculated in each simulation. For the assessment of high-dose subvolumes, the dose homogeneity index (DHI) and the contiguous dose volume histogram were analysed. Robustness was measured by the improvement of the lowest $D_{90\%}$ of the planning target volume (PTV) observed in the simulations. For (LD), a DTMR yields an increase in DHI of 30% and reduces the size of the largest high-dose volume by 2 to 5 cc. This came at a cost of a reduction in $D_{90\%}$ of the PTV of 10%, which often implies that it drops below the desired minimum of 100%. For (LDV), none of the DTMRs improved high-dose volume measures. DTMRs were not capable of improving robustness of PTV $D_{90\%}$ against uncertainty in dwell positions for both models.