2159 - Real Time Target Tracking and Radiation Therapy Adaptation with SCINTIX
Presenter(s)
Y. Liu1, S. Dumont2, C. Han1, A. Liu1, and W. T. Watkins1; 1Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, 2Reflexion Medical, Hayward, CA
Purpose/Objective(s): The SCINTIX algorithm performs real-time tracking and adaptation of radiation therapy (RT) based on PET active targets. However, RT does not only treat PET-active regions but also includes surrounding tissue to account for subclinical disease (CTV) and spatial uncertainty (PTV). Furthermore, tumors have differential PET-activity and only certain regions of target volumes may meet the SCINTIX signal-to-noise criteria. This study evaluated planning, delivery, tracking, and plan adaptation for large PTVs with small PET-active GTVs.
Materials/Methods: A point source embedded in the patient-specific quality assurance (PSQA) phantom on a motion platform was used to evaluate plan quality, delivered plan dosimetry, real-time PET tracking, and real-time plan adaptation for various PTV expansions of the PET-active region. PTVs ranging from 45-864 cc corresponding to PET-active GTV expansions up to 70 mm were evaluated. Planning indices including 100% dose to PTV (V100) and PTV Heterogeneity Index (HI) were constrained to =95% and =125%. Conformity Indices CI100 and CI50 were evaluated. Plan delivery and tracking were evaluated for all plans which met the V100 and HI constraints, and plan adaptation was evaluated for a subset of plans based on the real-time tracking results. The biological tracking zone (BTZ) was defined as PTV expansions of 3 and 10 mm (PTV+3mm and PTV+10mm) to evaluate real time tracking and plan adaptation in different intra-fraction motion scenarios.
Results: All plans with superior-inferior (SI) dimension <5 cm met V100>95% and HI<125% with BTZ of PTV+3mm. CI ranged from 1.04-1.33 for PTV volumes ranging up to 462 cc planned in this plan set. Increasing BTZ to PTV+10mm and SI dimensions to >5cm led to worse plan quality. Tracking and plan adaptation revealed two ways to utilize the current SCINTIX BgRT technology: (1) real-time tracking without adaptation or (2) real-time tracking w/ adaptation. Real-time tracking without adaptation utilizes a small BTZ expansion of the PTV (or BTZ=PTV) and can detect motion <=3mm. In this case, by using tight margins, the RefleXion system becomes more sensitive to intra-fraction motion and produces improved plan quality and delivery metrics. In testing this scenario with motion =3 mm, the system reliably interrupted treatment, required re-imaging, and continued with successful delivery. By expanding BTZ, the system will adapt treatment in real time if intra-fraction motion is detected. Delivered dose was accurately verified with up to 8 mm shifts mid treatment.
Conclusion: This study validated BgRT treatment planning, tracking, and delivery for varying PTV sizes and intra-fraction motion. Our results indicate real-time tracking and real-time adaptation can be controlled via BTZ design. Large BTZ and SI dimensions >5cm may not be desirable in high precision delivery.