1160 - ¹⁸F-FDG PET Image Metric Analysis of Clinical Patients Treated Using Biological-Guided Radiotherapy (BgRT)

Purpose/Objective(s): This study investigates the PET evaluation metric values from initial clinical BgRT cases to assess the feasibility, safety, and accuracy of BgRT. The utility of the standard and novel PET metrics is evaluated to quantify the tumor PET uptake over the BgRT treatment course.

Materials/Methods: Eleven patients received ~15mCi FDG injections for PET imaging during both the functional modeling session (FM) and the BgRT treatment sessions (TX). Clinically used standard PET metrics in terms Activity Concentration (AC), Normalized Target Signal (NTS) were analyzed for screening at FM and TX(AC > 5kBq and NTS > 2.7 for FM and AC > 5kBq and NTS > 2 for TX). A total of 37 sessions (12 FM and 25 TX) were analyzed. Additionally, the Dose-Volume histogram (DVH) was reviewed to compare delivered vs. planned dose per each TX. 6 patients (total 25 sessions) were further analyzed using two novel metrics: net PTV AC per fraction (nPTVf) and nPTVf ratio (nPTVfR). nPTVf was defined as: (mean PET AC within the PTV - mean PET AC within Biological Tracking Zone (BTZ) excluding PTV) / mean PET AC within the PTV. nPTVfR was defined as: nPTVf pre-TX / nPTVf at FM.

Results: Nine patients (7 lung and 2 bone cases) met the PET metric screening criteria for both FM and TX (AC = 14.69±7.48, NTS = 7.36±2.95). Two patients did not pass the FM PET evaluation and subsequently received conventional radiotherapy (AC <5). One lung patient aborted BgRT due to logistical reasons and one bone case was discontinued from BgRT after the first fraction due to a 2cm non-PET avid area within the PTV. Eight patients completed BgRT treatment in 25 sessions. Post-treatment DVH reviews confirmed physician-approved dose agreement. Further analysis of 6 patients showed nPTVf = 0.93±0.08, nPTVfR = 0.97±0.05. Additionally, one lung session did not meet the pre-TX PET screening despite prior FM approval. In two other TX, the BgRT system detected patient movement, while tumor moved outside the BTZ and subsequent treatment interruption, which resumed after repeated localization CT and pre-TX PET evaluation.

Conclusion: Early implementation of BgRT demonstrated the feasibility, safety, and accuracy of utilizing PET signals. The PET evaluation metrics nPTVf and nPTVfR, in addition to AC and NTS, are valuable for patient eligibility for BgRT and may improve the ability to detect discrepancies between planned and TX PET uptake, enhancing the BgRT effectiveness through the treatment course.