3213 - Evaluating the Use of <sup>68</sup>Ga-PSMA for Biology-guided radiotherapy (BgRT) Treatment on PET-Linac Platform
Presenter(s)
B. Cai1, R. Prasad2, C. Shen1, T. Banks1, E. Phillips2, G. Bal3, A. Garant1, A. Bhatnagar4, R. D. Timmerman1, O. K. Öz5, and N. B. Desai1; 1Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, 2University of Texas Southwestern Medical Center, Dallas, TX, 3RefleXion Medical, Inc., Hayward, CA, 4Department of Radiation Medicine, MedStar Georgetown University Hospital, Washington, DC, 5University of Texas Southwestern Department of Radiation Oncology, Dallas, TX
Purpose/Objective(s): BgRT is an innovative PET-guided radiotherapy system that uses the real-time PET signals emitted from the tumor to direct the external beam radiation to the target. 68Ga-PSMA-11 is a radioactive PET tracer that specifically binds to the prostrate-specific membrane antigen (PSMA) expressed by the cancer cells. The detection sensitivity and specificity of 68Ga-PSMA tracer is very high and hence ideal to detect small tumors. In this work, we aim to investigate the imaging capabilities of the BgRT device for 68Ga-PSMA-11 and the potential suitability of this tracer for BgRT
Materials/Methods: A total of 25 patients have been approved for the non-significant risk (NSR) study using 68Ga-PSMA. So far, five patients have been imaged on the BgRT system. The tumor locations of these five patients were in the subcarinal node, pelvic lymph node, and for the last three patients in the prostate. Typically, 15 mCi of FDG is used for BgRT. In this NSR study, we enrolled patients who have undergone a routine 68Ga PET-CT scans, with a mean injected dose of 5.52±0.61 mCi. The patient was first scanned on a commercial PET-CT system. The gross tumor volume (GTV), planning target volume (PTV) = GTV + 5mm expansion, biology-tracking zone (BTZ) = motion extend+5 mm and RTSS were drawn using the SimCT and clinical PET data. The patient, in the meantime, was transported to the BgRT PET system and the functional modeling PET data was acquired. The mean time duration between 68Ga-PSMA injection and BgRT PET scan was 125±18 minutes. The FM PET data and SimCT were used to create the BgRT treatment plan for 68Ga-PSMA with the objective of delivering either 35 Gy or 40 Gy in 5 fractions to the tumor.
Results: On the BgRT system, the activity-concentration (AC) for the five patients were 3.1, 1.52, 2.68, 26.09 and 2.78 while the normalized-target-signal (NTS) were 9.51, 4.62, 5.25, 3.24 and 2.48. The corresponding SUV max values on the commercial PET-CT system were 18.3, 7.66, 27 and 9.04. The lower AC observed on the BgRT system was likely due to the short half-life of Ga68 (68 minutes) and the extended wait time of 125±18 minutes between 68Ga-PSMA injection and imaging on the BgRT system. Clinically acceptable treatment plans with a coverage of 95% were successfully generated for four cases, with a conformity-index (CI) = 1.29±0.16, homogeneity-index (HI) = 1.2±0.8 and treatment time of 17±5 minutes. For the fifth patient, the lesion in the prostate was located adjacent to the hot bladder and did not meet the BgRT criteria of no PET avid OAR within 1.5 cm from the BTZ.
Conclusion: This is the first time a non-18F based PET tracer has been evaluated for BgRT. In the imaging only study, we aim to investigate the use of 68Ga-PSMA for tumors located throughout the body, expanding beyond the currently BgRT approval for tumors in lung and bone. The findings from the NSR study will be presented at the conference.