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

BackgroundIntensity-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.PurposeWe 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.MethodsThe effect of rotational errors of 3 degrees$3<^>\circ$, 5 degrees$5<^>\circ$ and 10 degrees$10<^>\circ$, and translational errors of 1, 2 and 3 mm on evaluation criteria were investigated for shields with 180 degrees${\rm 180}<^>\circ$ and 90 degrees${\rm 90}<^>\circ$ 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.ResultsFor the largest investigated rotational error of +/- 10 degrees$\pm 10<^>\circ$, the clinical target volume D90${\rm D}_{90}$ remained, on average, within 5%$5\%$ of the result without error, while the contralateral healthy rectal wall experienced an increase in the mean D0.1cc${\rm D}_{0.1cc}$, D2cc${\rm D}_{2cc}$, and D50${\rm D}_{50}$ of 26%$26\%$, 9%$9\%$, and 1%$1\%$ for the 180 degrees${\rm 180}<^>\circ$ shield and of 32%, 9%, and 2% for the 90 degrees${\rm 90}<^>\circ$ shield. For translational errors of +/- 2$\pm 2$ mm, there were increases in dosimetric indices for both the superior (sup) and inferior (inf) dose spill regions. Specifically, for the 180 degrees${\rm 180}<^>\circ$ shield, the D0.1cc${\rm D}_{0.1cc}$, D2cc${\rm D}_{2cc}$, and D50${\rm D}_{50}$ increased by 13%$13\%$, 11%$11\%$, and 10%$10\%$, respectively, for the sup region, and by 26%$26\%$, 15%$15\%$, and 11%$11\%$, respectively, for the inf region. Similar results were obtained with the 90 degrees${\rm 90}<^>\circ$ 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.ConclusionsWe 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.