City of Hope National Medical Center Duarte, California, United States
C. Han1, G. Bal2, J. Y. C. Wong1, 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): The medical technology company X1 system is a biology-guided radiotherapy (BgRT) machine with SCINTIX® treatment capability, which uses real-time positron emission signals from the tumor to guide radiation delivery. This study aims to evaluate the feasibility of using the Gallium-68 (Ga-68) prostate-specific membrane antigen (PSMA)-11 radiotracer as a bio-guide for SCINTIX® treatment on the X1 machine in the future for bony metastases in prostate cancer patients. Materials/
Methods: A retrospective review was performed on patients who underwent recent diagnostic Ga-68 PSMA-11 PET/CT scans at our institution. Five patients with PET-avid bone metastases identified in the scans were included. A gross tumor volume (GTV) was drawn for a PET-avid bone lesion for each patient. The PTV was created by adding a 5 mm margin to the GTV, and a biology tracking zone (BTZ) was created to encompass the potentially full range of motion of the tumor plus margin. To create SCINTIX treatment plans on the X1 machine, the diagnostic PET images were first converted to simulated treatment planning PET images on the X1 machine to account for differences in PET detector size and reconstruction algorithms. Then the activity concentration (AC) and normalized target signal (NTS) were calculated. The SCINTIX plans used a jaw width of 2 cm and a prescription dose of 27 Gy in 3 fractions given to the PTV using four couch passes. A 64-leaf binary MLC was used to modulate a 6-MV flattening filter-free photon beam on a ring gantry that rotates around the patient at 60 RPM. Results: Patients received an average Ga-68 injection dose of 4.86±0.73 mCi (range: 4.1 – 5.6 mCi) prior to PET/CT scans. In the diagnostic Ga-68 PSMA-11 PET images, the average maximum and mean PET activities in the GTVs were 25.2±12.0 kBq/ml (range: 13.3 – 43.5 kBq/ml) and 16.44±5.6 kBq/ml (range: 14.3 – 24.2 kBq/ml), respectively. The average AC and NTS values for targets in the simulated treatment planning PET images were 11.5± 5.2 kBq/ml (range: 6.2 - 18.9 kBq/ml) and 12.6±5.5 (range: 8.7 - 22.2), respectively. With an average PTV volume of 15.7±9.6 cc (range: 7.0 – 30.9 cc), the average treatment beam-on time was 1,678±290 seconds (range: 1,416 – 2,120 sec). Bounded dose-volume histograms (bDVH) were calculated to estimate the range of delivered dose due to PET signal and patient position uncertainties. The average PTV coverage, conformity index, and homogeneity index were 93.6±1.9%, 1.14±0.1, and 1.63±0.1, respectively. Conclusion: This is one of the first studies on Ga-68 based tracers for SCINTIX planning as the current plan creation software was developed for F-18 based tracers. Metastatic bony lesions from prostate cancer could have sufficient positron emission strength for SCINTIX treatment when Ga-68 PSMA-11 radiotracer is used at diagnostic dose levels. This study demonstrated the feasibility of using Ga-68 PSA-11 radiotracer for future PET-guided SCINTIX treatments on the X1 machine.
