Ga-68 PSMA-11 tracer could guide SCINTIX radiation therapy for metastasized prostate cancer

Chunhui Han, PhD

Credit: City of Hope

Gallium-68 (Ga-68) prostate-specific membrane antigen (PSMA)-11 radiotracer may feasibly guide positron emission tomography (PET)-based proprietary radiation therapy SCINTIX in the treatment of bony metastases in patients with prostate cancer, according to new data.

Findings published at the American Society for Radiation Oncology (ASTRO) 2024 Annual Meeting in Washington, DC, this weekend show the Ga-68 PSMA-11 tracer can help guide SCINTIX therapy to treat metastasized prostate cancer—despite the software of the novel radiation procedure being originally developed to work with F-18 tracers. The California-based investigation team, supported by RefleXion Medical, believe the data reasonably support the use of Ga-68 PSA-11 radiotracer in PET-guided SCINTIX treatments conducted through the X1 machine.

The X1 system is a biology-guided radiotherapy machine designed to deliver targeted radiation based on real-time PET feedback of tumor positioning in patients with cancer; according to the company, it is delivered in ≤5 fractionated doses and touts capability to reduce the risk of radiation-based adverse events. Investigators led by Chunhui Han, PhD, of the City of Hope National Medical Center, sought to evaluate Ga-68 PSMA-11 tracer’s feasibility to guide SCINTIX treatment through the X1 machine for patients with bone-metastasized prostate cancer.

The team conducted a retrospective analysis of patients who underwent diagnostic scans at their institution via Ga-68 PSMA-11 PET or computed tomography scans. Their review ultimately included 5 patients with PET-avid bone metastases.

Investigators then drew gross tumor volumes (GTVs) for a PET-avid bone lesion per patient. Planning target volumes (PTVs) were defined by adding 5 mm margins to GTVs, while biology tracking zone (BTZ) was created to define the potentially full range tumor motion 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,” investigators explained. “Then the activity concentration (AC) and normalized target signal (NTS) were calculated.”

Mean Ga-68 injection dose was 4.86 mCi (range, 4.1 – 5.6) per patient prior to PET or CT scans. Average maximum PET activity in the GTVs was 25.2 kBg/ml (range, 13.3 – 43.5), and mean PET activity was 16.44 kBg/ml (range, 14.3 – 24.2). Targets in the simulated treatment planning PET images had a mean AC value of 11.5 kBg/ml (range, 6.2 – 18.9), and a mean NTS value of 12.6 kBg/ml (range, 8.7 – 22.2).

Average treatment beam-on time was 1678 seconds (range, 1416 – 2120), and mean PTV volume was 15.7 cc (range, 7.0 – 30.9). Mean PTV coverage was 93.6%; conformity index was 1.14; and homogeneity index was 1.63.

Han and colleagues expressed high interest in the findings, which they note may be among the first studies to observe the feasibility of Ga-68 based tracers for planning SCINTIX treatment instead of F-18 radiotracers.

“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,” they concluded. “This study demonstrated the feasibility of using Ga-68 PSA-11 radiotracer for future PET-guided SCINTIX treatments on the X1 machine.”

References

1. ^{Han C, Wong JYC, Williams TM, Liu A. Feasibility of Biology-Guided Radiotherapy with Gallium-68 PSMA-11 Radiotracer for Bony Metastases in Prostate Cancer Patients. Poster presented at: American Society for Radiation Oncology (ASTRO) 2024 Annual Meeting. Washington, DC. September 29 – October 2, 2024.}
2. ^{SCINTIX Therapy. Accessed September 28, 2024. https://scintixtherapy.com.}