  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.
