2084 - Variations of PET Metrics for Target and Surrounding Tissue Metabolic Activity in the Course of Biology-Guided Radiotherapy for Bone and Lung Lesions
Presenter(s)
C. Han1, G. Bal2, W. T. Watkins1, T. M. Williams1, and A. Liu1; 1Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, 2RefleXion Medical, Inc., Hayward, CA
Purpose/Objective(s): In biology-guided radiotherapy (BgRT), real-time positron emission tomography (PET) signals are used to guide radiation delivery. Two PET evaluation metrics, the normalized target signal (NTS) and activity concentration (AC), determine whether PET uptake in the tumor is sufficient for BgRT. For NTS and AC to work, several additional PET metrics are evaluated during BgRT planning and delivery. We report PET metric variations in clinical BgRT courses at a single institution.
Materials/Methods: Data from ten patients with bone lesions and two patients with lung lesions who were treated at our institution and was reviewed. Of the ten bone cases, five were treated for one fraction, two treated for 2 fractions, and three treated for 3 fractions. The two patients with lung tumors were treated with 3 and 5 fractions, respectively. Each patient received a functional modelling (FM) PET scan prior to treatment planning and a pre-treatment PET scan (prescan PET) prior to daily treatment. The AC and NTS were calculated using PET data inside the planning target volume (PTV) and a shell outside the biology target zone (BTZ). Three additional PET metrics were evaluated: the net PTV fraction (nPTVf) is calculated as the difference between mean PET activity in the PTV and in the BTZ minus the PTV, divided by the mean PTV PET activity; the net PTV fraction ratio (nPTVfr) is the ratio of nPTVf in the prescan PET and the FM PET; and the mean activity concentration in the biology-tracking zone (BTZmAC) is the mean activity concentration in the BTZ in prescan PET. Operating limits for all PET metrics were applied during treatment planning and each treatment fraction to determine BgRT eligibility.
Results: Based on FM PET images, the BgRT plans showed an average AC of 16.00±9.38 kBq/ml, NTS of 7.13±1.73, nPTVf of 0.69±0.19, and BTZmAC of 13.89±10.12 kBq/ml. The average time gap between FM PET and the first BgRT treatment fraction was 8±3 days (range: 5 – 15 days). In the first treatment fraction, the prescan PET showed an average AC of 14.57±8.13 kBq/ml, NTS of 5.45±1.89, BTZmAC of 13.51±9.44 kBq/ml, and nPTVfr of 0.98±0.33. For patients treated with multiple BgRT fractions, the length of the treatment course ranged from 3 to 7 days. Relative to the values in the first fractions, the entire treatment course showed average AC variations of 1.0±23.7% (range: -45.1 – 49.0%), NTS variations of 5.2±22.0% (range: -49.3 – 51.9%), BTZmAC variations of 5.7±24.8% (range: -56.8 – 44.9%), and nPTVfr variations of -0.9±8.5% (range: -13.0 – 19.4%). All PET metrics passed machine operating limits for BgRT delivery throughout the treatment course for all patients, although the AC and nPVTfr fell within 10% of the operating limits in one treatment fraction.
Conclusion: Variations of key PET metrics were within machine operating limits in this patient cohort despite daily variations of PET uptake in the tumor.