Author: Girish Bal, Ke Colin Huang, Christopher F. Njeh, Oluwaseyi Oderinde, Yong Yue 👨🔬
Affiliation: Purdue University, RefleXion Medical, Indiana University School of Medicine, Department of Radiation Oncology 🌍
Purpose: Biology-guided radiotherapy (BgRT) uses real-time positron emission tomography (PET) signals to precisely direct radiotherapy beamlets. The success of BgRT planning and delivery relies on PET avidity in tumors meeting specific thresholds, aiding in the identification of suitable candidates. This study examines tumor-to-background ratios in diagnostic PET and their correlation with synthetic PET metrics to identify optimal BgRT candidates before treatment.
Methods: Twenty patients with liver metastases treated with stereotactic body radiotherapy (SBRT) were retrospectively selected from a database. Diagnostic clinical PET/CT images were co-registered with planning CT scans using MIM software. PET-defined gross tumor volumes (GTVs) were contoured, expanded by 5 mm to create BgRT planning target volumes (PTVs), and further expanded by 5 mm to define biology-tracking zones (BTZs). Target regions were defined as areas with >80% of the maximum activity concentration in the GTV, while background regions were created by expanding the BTZ by 5 mm. The mean activity-concentration (mAC) and normalized-target-signal (mNTS) were computed for target and background regions. For diagnostic PET, tumor-to-background (TBR) was calculated using SUVmax and SUVmean, resulting in TBRmax and TBRmean, respectively.
Results: The mean mAC and mNTS for the twenty patients were 13.2 kBq/ml and 6.2, respectively. Five patients failed the PET evaluation thresholds (mAC≥5kBq/ml and mNTS≥2.7); four had mAC<5kBq/ml, three had mNTS<2.7, and two failed both metrics. For diagnostic PET, TBRmean and TBRmax were significantly correlated with mAC and mNTS (p<<0.001). Notably, TBRmean (≥1.42) yielded identical results to those obtained using the mAC and mNTS thresholds.
Conclusion: This study is an initial step that demonstrates the utility of PET metrics in selecting patients with liver metastases for BgRT planning and treatment. The findings suggest that TBR values derived from diagnostic PET could provide early insights for patient selection, with significant implications for future BgRT treatments.