3036 - Synthetic PET Evaluation as a Pre-Treatment Patient Eligibility Screening Tool for Biology-Guided Radiation Therapy
Presenter(s)
A. Kessler1, J. J. Park1, Z. Xu1, H. Matani2, E. R. Zhang-Velten1, J. C. Ye2, K. Woods1, and J. P. Schiff1; 1Department of Radiation Oncology, Keck School of Medicine, University of Southern California, Los Angeles, CA, 2Department of Radiation Oncology, University of Southern California Keck School of Medicine, Los Angeles, CA
Synthetic PET Evaluation as a Pre-Treatment Patient Eligibility Screening Tool for Biology-guided Radiation Therapy
Purpose/Objective(s): Biology-guided radiation therapy (BgRT) requires a functional modeling (FM) imaging session to confirm the target positron emission tomography (PET) signal is sufficient for near real-time tracked delivery. This study evaluates the accuracy of synthetic PET plan (synth plan) metrics in predicting FM success and signal stability during treatment.
Materials/Methods: Seven patients consented for BgRT. Prior to or after CT simulation and FM session, diagnostic PET-CT images with physician-contoured gross tumor volumes were imported into the treatment planning system (TPS) to generate a synth plan. Clinical eligibility was defined as a normalized Activity Concentration (AC) > 5 kBq/mL and a Normalized Target Signal (NTS) > 2.7. AC was normalized to 555 MBq. Predictive accuracy was assessed by calculating ratios between synth, FM, and the mean of all delivered fractions (avg).
Results: Five patients met synth eligibility, completed one FM session a mean of 43 days (9–88 days) after diagnostic PET-CT, and completed BgRT. Two patients had acceptable AC and NTS values during FM obtained a mean of 86 days (84-87 days) after diagnostic PET-CT but ultimately were not BgRT candidates due to proximity to an FDG-avid OAR. One of these patients had a synth plan calculated post-FM that failed due to low NTS (2.66). The other patient passed the synth plan evaluation. Using BgRT, four lung lesions were treated with 50–54 Gy in 3-5 fractions, while one bone lesion received 30 Gy in 3 fractions. The mean administered FDG activity was 392 MBq (305-476 MBq) during the diagnostic PET-CT, 538 MBq (511–570 MBq) during the FM session and 558 MBq (470–592 MBq) across treatment fractions. The AC means for synth, FM, and avg were 23.78 (11.38–60.28), 24.28 (9.97–50.81), and 22.52 (8.22–50.89), respectively. The NTS means for synth, FM, and avg were 15.27 (4.38–40.08), 13.40 (11.00–18.04), and 10.51 (6.38–16.95), respectively. The mean synth/FM ratio was 0.98 (0.45–1.19) for AC and 1.04 (0.40–2.22) for NTS. The mean synth/avg ratio for AC was 1.07 (0.55–1.38) and 1.36 (0.69-2.36) for NTS.
Conclusion: Synth plan evaluation showed high concordance with FM and treatment metrics in this cohort. Despite inter-fraction variability, all metrics remained above clinical thresholds during BgRT. This suggests synth plan evaluation may be a reliable predictor of BgRT feasibility and signal stability.
Table 1: Raw Synth, FM, and avg values for NTS and AC
a: TPS aborted AC calculation after NTS fell below cutoff
| Metric | Case | Synth | FM | avg |
| NTS | 1 | 40.08 | 18.04 | 16.95 |
|
| 2 | 12.35 | 11.11 | 8.98 |
|
| 3 | 4.38 | 11.00 | 6.38 |
|
| 4 | 9.34 | 12.67 | 6.76 |
|
| 5 | 10.21 | 14.17 | 10.51 |
|
| 6 | 2.66 | 3.44 | N/A |
|
| 7 | 13.45 | 7.15 | N/A |
| AC (kBq/mL) | 1 | 60.28 | 50.81 | 50.89 |
|
| 2 | 18.67 | 17.85 | 16.38 |
|
| 3 | 12.20 | 27.32 | 22.19 |
|
| 4 | 11.38 | 9.97 | 8.22 |
|
| 5 | 16.37 | 15.44 | 14.89 |
|
| 6 | N/A? | 6.85 | N/A |
|
| 7 | 41.45 | 33.31 | N/A |