2227 - Single Fraction SBRT Using a SIB Approach Delivered by BgRT to a Lumbar Spine Metastasis: A Novel Multimodality Planning Strategy Combining BgRT and IGRT

Purpose/Objective(s): The PET/CT linac is a dual-modality system that enables both Biology-guided Radiotherapy (BgRT) and Image-guided radiotherapy (IGRT). We hypothesize that these two modalities could be combined to generate a hybrid single target (HST) treatment approach where IGRT is applied to a larger treatment volume and BgRT to a smaller volume—similar to the concept of a Simultaneous Integrated Boost (SIB). This work aims to investigate the different SIB ratios that would enable the joint optimization of both modalities.

Materials/Methods: A TCIA database provided images of a patient with a lumbar spine bone metastasis. Several boost doses —20 Gy, 18 Gy, 16 Gy —were planned to the PET-avid GTV using BgRT and 14 Gy, 12 Gy, 10 Gy, 8 Gy were planned to the IGRT CTV. The GTV volume was 2.7cc and the CTV volume was 41.7cc. The Dmax, CI, CI80, CI50, percent GTV and CTV coverage and estimated treatment times for each treatment area were recorded.

Results: The average GTV and CTV coverage for the 20 Gy, 18 Gy and 16 Gy variants of 14 Gy, 12 Gy, 10 Gy, 8 Gy were 99.84/99.76/99.81% and 95.28/95.32/95.43%, respectively. The average treatment times for the BgRT and IGRT treatment areas for the 20 Gy, 18 Gy, and 16 Gy variants were 13.50/12.80/12.80 min and 12.96/11.75/11.04 min, respectively. The CTV Dmax for the 20 Gy, 18 Gy and 16 Gy variants were 27.90, 25.06 and 22.15 Gy, respectively. The 20 Gy, 18 Gy, and 16 Gy variants average CI, CI80 and CI50 were 1.28/1.23/1.21, 2.11/2/04/1.97, and 5.33/5.11/4.91.

Conclusion: This preliminary work demonstrates that as the escalated GTV boosted dose increases in reference to the CTV base dose, there is an average increase in max dose, CI, CI80, CI50, and IGRT treatment times, respectively. Hence, there may be a range of preferred ratios for HST SIB using the PET/CT linac for jointly optimized plans. Further studies are required to confirm and refine these findings, along with deliverability studies for similar SIB HST approaches.