Dosimetric impact of positional uncertainties and a robust optimization approach for rectal intensity-modulated...

Intensity-modulated brachytherapy (IMBT) employs rotating high-Z shields during treatment to decrease radiation in certain directions and conform the dose distribution to the target volume. Prototypes for dynamic IMBT have been proposed for prostate, cervical, and rectal cancer.

We considered two shielded applicators for IMBT rectal cancer treatment and investigated how rotational uncertainties in the shield angle and translational uncertainties in the source position affect plan evaluation criteria.

The effect of rotational errors of $3^{\circ }$ , $5^{\circ }$ and ${10}^{\circ }$ , and translational errors of 1, 2 and 3 mm on evaluation criteria were investigated for shields with ${180}^{\circ }$ and ${90}^{\circ }$ emission windows. Further, a robust optimization approach based on quadratic penalties that includes scenarios with errors was proposed. The extent to which dosimetric effects of positional errors can be mitigated with this model was evaluated compared to a quadratic penalty model without scenarios with errors. A retrospective rectal cancer data set of ten patients was included in this study. Treatment planning was performed using the Monte Carlo-based treatment planning system, RapidBrachyMCTPS.

For the largest investigated rotational error of $\pm {10}^{\circ }$ , the clinical target volume  $D_{90}$ remained, on average, within $5\%$ of the result without error, while the contralateral healthy rectal wall experienced an increase in the mean $D_{0.1cc}$ , $D_{2cc}$ , and $D_{50}$ of $26\%$ , $9\%$ , and $1\%$ for the ${180}^{\circ }$ shield and of 32%, 9%, and 2% for the ${90}^{\circ }$ shield. For translational errors of $\pm 2$  mm, there were increases in dosimetric indices for both the superior (sup) and inferior (inf) dose spill regions. Specifically, for the ${180}^{\circ }$ shield, the $D_{0.1cc}$ , $D_{2cc}$ , and $D_{50}$ increased by $13\%$ , $11\%$ , and $10\%$ , respectively, for the sup region, and by $26\%$ , $15\%$ , and $11\%$ , respectively, for the inf region. Similar results were obtained with the ${90}^{\circ }$ shield. Overall, the robust and traditional models had similar results. However, the number of active dwell positions obtained with the robust model was larger, and the longest dwell time was shorter.

We have quantified the effect of rotational shield and translational source errors of various magnitudes on evaluation criteria for rectal IMBT. The robust optimization approach was generally not able to mitigate positional errors. However, it resulted in more homogeneous dwell times, which can be beneficial in conventional high-dose-rate brachytherapy to avoid hot spots around specific dwell positions.