2272 - Initial Experience with a CBCT-Based Adaptive Radiotherapy System

A. Eldib Jr¹, J. Panetta¹, L. Chen¹, D. M. Yang¹, L. Wang¹, A. T. Clark², R. A. Price Jr³, and C. M. C. Ma¹; ¹Fox Chase Cancer Center, Philadelphia, PA, ²Fox Chase Cancer Center, philadelphia, PA, ³Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA

Purpose/Objective(s): Adaptive radiotherapy was initiated in our department in February 2023. Physics parameters explored during our initial experience included mechanical/dosimetric accuracy and machine stability for SBRT, imaging artifacts caused by motion or due to the presence of metal implants, and the efficiency of aligning moving targets. In this work, we present our approach to those physics concerns faced with our adaptive radiotherapy system. Materials/

Methods: We used a CBCT-based adaptive radiotherapy system, to treat more than 180 patients comprising more than 850 adaptive sessions. Machine dosimetric and mechanical accuracy were recorded daily prior to initiating treatment. Records were analyzed to evaluate machine stability over the past year. Our machine was upgraded with a new imaging panel in September 2023 for the purpose of advancing our imaging quality. A phantom investigation was done to quantify improvement after the upgrade. During the adaptive contouring step, we developed the propagation of a bony landmark structure combined with fiducials and clinical target volume, especially for lesions with low soft-tissue contrast. Dose calculation during adaptive sessions utilizes synthetic CTs generated by deforming planning CTs onto daily cone beam CTs. Those synthetic CTs were exported to our treatment planning system for careful examination of the dose differences when compared to original treatment plans for some patient cases.
Results: The maximum deviation from the ideal isocenter was 0.62mm with a 0.009mm standard deviation. The maximum deviation in daily output was on the order of 1.8%. The propagation approach proved to be efficient and was used in almost all adaptive cases. It also aids in addressing positional uncertainties caused by internal movement due to patient respiration. The new imaging panel showed shorter imaging time (< 7 seconds) and better image quality with thoracic and abdominal cases imaged with breathhold; however, distortion was still seen with free breathing images. Our phantom study confirmed that this distortion was caused by motion. The shorter imaging time facilitated the adoption of breathhold treatments and we concluded that the optimal use of the CBCT-based adaptive radiotherapy system would require a method of breathhold monitoring. Synthetic CT was shown to resolve some of the artifacts caused by metal implants, and at this stage, direct use of CBCT for dose calculation is not recommended for such cases.
Conclusion: Adaptive radiotherapy can be helpful for many treatment sites; however, it requires the focus and attention of team members to gain the full benefit of this new and exciting advanced technique. Abstract 2272 - Table 1: Treatment time of adaptive therapy for various treatment sites analyzed for this work.

Procedure	Average Tx time (min)
	Before the upgrade		After
	Prostate	Abdomen	Prostate	Abdomen
CBCT + Contouring + Evaluation	12	29	13	20
Mobius QA	4	3	3	4
2nd CBCT+ Delivery	9	15	10	11
Total	25	47	26	35