Room: AAPM ePoster Library
Purpose: To evaluate the suitability of ¹6?Yb-based dynamic-shield intensity modulated brachytherapy (IMBT) for head & neck cancers. IMBT delivery is proposed using an ¹6?Yb source combined with platinum-shielded interstitial needles and a shield rotation device (AIM-Brachy).
Methods: Treatment planning was performed using RapidBrachyMCTPS, a Geant4-based treatment planning system for brachytherapy applications. The IMBT plans were generated for various head & neck tumor sites including oral tongue, lip, and base of tongue. The activated dwell positions were extracted from the clinical plan. Tissue heterogeneities were taken into account by assigning voxel-by-voxel tissue composition (soft tissue, air or bone) and physical density (CT-to-density curve). The dose distributions and dose-volume metrics were compared to conventional ¹?²Ir-based high dose rate brachytherapy (HDR BT) for the planning target volume (PTV) and organs at risk (mandible, parotids, spinal cord). All plans were normalized such that at least 95% of the PTV receives the prescription dose (PD).
Results: Given equal PTV V100 coverage, the mandible D1cc was elevated with ¹6?Yb-based IMBT compared to conventional HDR BT for the oral tongue (66.7% PD vs. 36.7% PD), lip (58.7% PD vs. 37.1% PD), and base of tongue (120% PD vs. 63.0% PD). Conformity of the 100% isodose (excluding regions in mandible where dose may exceed 100% PD) to the PTV improved marginally for the oral tongue and base of tongue cases, and worsened for the lip case. Dose reduction to parotids and spinal cord was negligible or not clinically relevant (<2% PD).
Conclusion: ¹6?Yb-based IMBT delivers a higher dose to bone than conventional HDR BT. The angular modulation of the dose distribution is not sufficient to overcompensate for the increased absorption of lower-energy photons in dense bony structures. This study suggests that ¹6?Yb-based IMBT is not an appropriate technique to improve plan quality for head & neck cancers.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Collaborative Health Research Projects (grant number 523394-18) and Natural Sciences and Engineering Research Council of Canada (grant number 241018). G.F. acknowledges support by the Natural Sciences and Engineering Research Council of Canada.