225 - Early Clinical Insights into FDG-Guided Radiotherapy Planning and Delivery on a Novel PET-Linac Platform in Patients with Lung and Bone Lesions: Pioneering Real-Time Biology-Guided Radiotherapy

B. Cai¹, T. Banks¹, C. Shen¹, R. Prasad², Y. K. Park¹, M. H. Lin¹, A. R. Godley¹, A. Pompos¹, T. Dan³, S. N. Badiyan⁴, A. Garant¹, K. D. Westover¹, D. J. Sher¹, O. K. Öz⁵, R. D. Timmerman¹, and S. B. Jiang¹; ¹Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, ²University of Texas Southwestern Medical Center, Dallas, TX, ³Department of Radiation Oncology, The University of Texas Southwestern Medical Center, Dallas, TX, ⁴Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, TX, ⁵University of Texas Southwestern Department of Radiation Oncology, Dallas, TX

Purpose/Objective(s): Present center’s initial experience of the treatment of lesions in lungs and bones with recently FDA-approved novel real-time biology-guided radiotherapy (BgRT) linear accelerator incorporating FDG emission guidance. Materials/

Methods: 2 patients, 1 with spine metastasis at L4 and another with primary lung cancer in left upper lobe, underwent CT simulation for BgRT treatment. Each patient received 15mCi FDG injection each time for both PET modeling session required for BgRT planning and the subsequent treatment fractions. For spine target, a BgRT planning target volume (PTV) was defined with a 5 mm expansion from the PET avid gross tumor volume (GTV). PTV received a dose of 12Gy in one fraction with a simultaneous integrated boost dose of 16Gy to the GTV. For lung target, 4D CT was utilized to define the GTV on a breathing phase close to the average PET tumor position. BgRT PTV was then defined with a 5mm expansion from the phase GTV and received a dose of 55Gy in 5 fractions. Biological tracking zone (BTZ) was defined for both cases, encompassing the targets motion envelope and BgRTs setup/tracking uncertainties. Planning required 95% coverage of Rx while meeting institutional constraints for key organs-at-risk (OARs). BgRT delivery feasibility was assessed using Activity Concentration (AC > 5 kBq/ml), Normalized Target Signal (NTS > 2), and Gamma Passing Rate (Gamma > 95%) of DVH points, evaluated in pre-scan session before each BgRT fraction. Patient-specific QA (PSQA) conducted using ionization chamber (IC) for point dose and patient-specific quality assurance (PSQA) detector array for 3D dose validation. Treatment efficiency evaluated by recording the treatment time of each fraction. Post-delivery dose calculation based on real-time PET images and machine log files performed to verify actual dose received by OARs.
Results: Patients with lesions in the spine and lung completed all planned fractions successfully with 8 FDG injections averaging 15.4 mCi. Initial plans for both cases achieved RX 95% coverage while adhering to key organ-at-risk constraints. Lung patient BgRT PTV volume exhibited 30% reduction compared to the ITV-based approach. PSQA yielded <2% IC point dose variation and >97% Gamma passing rate (2%/2mm) for both plans. Average treatment time was 79 mins for 6 fractions. Although AC, NTS, and Gamma for the lung patient showed a downward trend, they remained above required threshold for a robust BgRT delivery. Post-delivery dose calculation confirmed key OARs constraints were still met despite the stochastic delivery nature.
Conclusion: BgRT with FDG signal guidance is feasible. With appropriate inclusion criteria and real time tracking, BgRT treatment showed potential benefit for fiducial-less tracking during treatment and margin reductions especially for moving target.

Abstract 225 – Table 1

	Spine		Lung
	Planning	FX1	Planning	FX1	FX2	FX3	FX4	FX5
AC (kBq/ml)	29.9	29.8	30.7	29.4	29.3	23.0	21.9	17.9
NTS	17.7	13.1	39.1	27.5	26.8	20.4	20.4	14.6
Gamma(%)	NA	100	NA	100	100	100	99	98