3138 - A Multi-Institutional Evaluation of a Novel PET and Dose Congruence (PDC) Metric for Biology-Guided Radiotherapy
Presenter(s)
M. Surucu1, D. Pham1, L. Vitzthum1, N. Kovalchuk1, B. Han1, J. Chang2, R. J. Lalonde2, M. S. S. Huq2, N. S. McCall2, W. T. Watkins3, A. Liu3, D. J. Carlson4, H. S. M. Park4, and H. Chen4; 1Department of Radiation Oncology, Stanford University, Stanford, CA, 2Department of Radiation Oncology, UPMC Hillman Cancer Center, Pittsburgh, PA, 3Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, 4Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT
Purpose/Objective(s):
To introduce a novel PET and Dose Congruence (PDC) metric for evaluating the alignment between acquired PET signals and administered doses in Biology-guided Radiotherapy (BgRT), and to assess the effectiveness of the PDC metric across multiple institutions for patients treated for lung and bone indications.
Materials/Methods:
BgRT utilizes PET emissions from tumors to convert the PET data into beamlets, directing the radiotherapy beam toward the target. During the BgRT planning, a functional modeling session captures the PET emissions from the patient and a BgRT plan is optimized. The GTV is delineated using the end-expiration phase of a 4DCT simulation for lung tumors and a 5 mm margin added to create the PTV. For bone tumors, a 3D CT simulation was conducted, and in certain cases, a CTV was defined, followed by the addition of a 5 mm margin to establish the PTV. After treatment, a PET image is reconstructed using the emissions captured during BgRT, and the delivered dose is calculated based on the beamlets used during treatment. For each treatment, the mean PET intensity within the GTV is calculated to threshold the signal, enabling the identification of the centroid of the PET biodistribution. The delivered prescription isodose line is then converted into a volume to determine the centroid of the delivered doses. The distance between the centroids of the PET signal and the prescription isodose volume is calculated to assess the congruence between the PET data and the delivered BgRT doses for each fraction (PDC). A total of 7 patients with lung tumors and 8 patients with bone tumors were treated across four institutions, with 29 fractions evaluated for lung tumors and 12 for bone tumors.
Results:
The median GTV volumes were 14.4 cc (range: 3.2 to 30.9) for lung tumors and 20.8 cc (9.7 to 39.6) for bone tumors. The median PTV volumes were 35.9 cc (13.6 to 67.3) for lung tumors and 62.6 cc (13.3 to 87.9) for bone tumors. The change in mean PET activity within the GTV from simulation to each treatment was 3.3% (-34.0% to 36.8%) for lung tumors and 4.4% (-27.1% to 37.3%) for bone tumors. For lung tumors, the median PDC was 2.0 mm (0.4 to 7.1), with 2 out of 29 delivered fractions exceeding 5 mm. For bone tumors, the median PDC was 4.1 mm (1.0 to 8.1), and 3 out of 12 treatment fractions had PDC greater than 5 mm. The median PDC values in the left/right, anterior/posterior, and superior/inferior directions were 1.1 mm, 1.0 mm, and 0.7 mm for lung tumors, and 1.7 mm, 1.7 mm, and 1.5 mm for bone tumors, respectively.Conclusion:
This study is the first to assess the accuracy of BgRT delivery across multiple institutions. For 36 out of 41 treatment fractions, the BgRT doses closely aligned with the centroid of the PET scans within the commonly used 5 mm GTV to PTV margin. Proper alignment of the PET signal with the target during BgRT planning may improve the PDC agreement. Considering the 2.1 mm pixel size for dose calculations and the 4 mm PET pixel size, the observed agreement in PDC offers a novel way to validate the geometric accuracy of BgRT delivery.