3117 - Low-Dose-Rate Brachytherapy for Peripheral Lung Tumor using Robotic-Assisted Bronchoscopy

Purpose/Objective(s): Robotic-assisted navigation bronchoscopy with cone-beam CT (RB-CBCT) enables minimally invasive access to peripheral lung lesions while avoiding transpleural needle traversal and thus reduced risk of pneumothorax compared to a percutaneous approach (approximately 1-4% vs 12-45%). Low-dose-rate brachytherapy (LDR-BT) provides highly conformal dose with rapid falloff and has demonstrated feasibility in CT-guided and emerging bronchoscopic workflows. Patients with interstitial lung disease are at increased risk for radiation induced pneumonitis following stereotactic body radiation therapy (SBRT), where large volumes of healthy lung receive low-to-moderate dose. Techniques to reduce integral lung dose may mitigate pneumonitis risk. We evaluated the technical feasibility, workflow, and dosimetry of RB-CBCT-guided Cs-131 seed implantation for a peripheral lung tumor.

Materials/Methods: A 54-year-old female with interstitial lung disease and medically inoperable stage IA3 lung adenocarcinoma underwent RB-CBCT-guided LDR-BT. Preplanning was performed using commercial software utilizing TG-43 formalism to generate a peripheral loading strategy and seed coordinates to optimize target coverage while minimizing dose to organs at risk (OAR). All source strengths were assay-confirmed and decay-corrected to the implant date. Four Cs-131 seeds (mean air-kerma strength (AKS) 15.08 U/seed, 60.32 U total; apparent activity 23.67 mCi/seed, 94.68 mCi total) were bronchoscopically deployed via an 18-gauge needle under fluoroscopic guidance. CBCT was used to verify positioning. Total procedure time was approximately 1 hour. Post-implant CT-based dosimetry was performed.

Results: With a prescription of 100 Gy, post-implant dosimetry demonstrated target coverage of V100 = 89.23%, V150 = 64.06%, and D90 = 98.36%. OAR doses were acceptable: Chest Wall D2cc = 78.02 Gy, Rib D2cc = 42.64 Gy, and Lung-CTV D1500cc = 0.22 Gy (D1000cc = 0.53 Gy). One-month follow-up CT demonstrated no seed migration and early radiographic response with tumor reduction from 9.28 cc to 3.59 cc (threshold -150 HU). No procedure-related complications were observed. Six-month imaging follow-up is planned.

Conclusion: RB-CBCT-guided delivery of Cs-131 LDR brachytherapy is technically feasible with favorable dosimetry. Preplanning with 3D seed coordinates and iterative CBCT verification enabled controlled seed placement and clinically acceptable target coverage with limited normal tissue dose. This case demonstrates a novel minimally invasive seed delivery technique for peripheral lung tumors and highlights the role of medical physics in treatment planning, image-guided implantation, and post-implant dosimetric evaluation. Further investigation is warranted to define clinical efficacy, potential dosimetric advantages relative to external beam approaches, and outcomes/adverse events such as radiation induced pneumonitis.