New PET tracer emerges in the staging of primary prostate cancer

Australian researchers have studied the performance of an F-18-based prostate specific membrane antigen (PSMA) tracer in patients with biopsy-proven prostate cancer. They found that it was significantly correlated with the clinical criteria for assessing their risk of metastasis during primary staging.

“These results are an important step in validating the role of F-18 PSMA-1007 PET / CT in the primary staging of prostate cancer,” wrote lead author Venkata Chikatamarla of the University of Queensland in Brisbane, in Australia.

PSMA is a protein found on the surface of prostate cancer cells. PSMA molecules have been developed that bind to the protein, and when combined with isotopes such as gallium-68 (Ga-68) or F-18, radiotracers allow researchers to locate these tumors on PET imaging.

Tests for the circulation of prostate specific antigen (PSA) in the blood and scores based on the aggressiveness of cancer cells appearing in biopsies are used to determine the risk of metastases in patients who have undergone treatment. The International Society of Urological Pathology (ISUP) system, for example, which is based on a modified Gleason score, ranks cancer on a scale of 1 to 5, with 1 being the least aggressive and grade 5 the most aggressive.

During the primary phase, determining the risk patients face is essential for physicians to develop therapeutic approaches.

In this study, the Australian group investigated whether F-18 PSMA-1007 CT scans during primary staging could provide similar diagnostic information about patient risk as PSA levels and ISUP grades.

Researchers performed a retrospective review of 194 consecutive patients who underwent F-18 PSMA-1007 CT scans (Biograph Vision, Biograph mCT or Biograph mCT Flow, Siemens Healthineers) for the initial staging of prostate cancer proven by biopsy at Royal Brisbane. and Women’s Hospital between May 2019 and May 2020. The median age was 70 years, with 36.6% of patients considered to be at intermediate risk and 60.8% of patients at high risk.

A 70-year-old man with unfavorable intermediate risk prostate cancer (ISUP grade 3; PSA 5.2 ng / mL) who underwent F-18 PSMA-1007 PET / CT scan which showed a solitary greedy focus of F-PSMA in the left medioposterolateral part of the prostate gland (SUVmax 14.9). (a) Axial CT without contrast, (b) PET image corrected with axial attenuation, and (c) Axial fusion PET / CT image at prostate level. (d) Maximum Intensity Projection Image (MIP). Image courtesy of EJNMMI research.

Associations between imaging variables (primary tumor SUVmax) and clinical variables (PSA level and ISUP grades) were assessed using descriptive statistics, a logistic regression model, and receptor functioning characteristics (ROC) analysis.

Analysis showed that the SUVmax (maximum tracer absorption) in primary prostate tumors was higher in patients with ISUP grades 4 and 5, compared to ISUP grades 2 and 3 (median SUVmax 19.6 vs. 14.9). Likewise, the primary tumor SUVmax was higher in the group of patients with PSA> 20 ng / mL than in the group with PSA levels between 10 ng / mL and 20 ng / mL.

Researchers found a statistically significant association between ISUP grade (p = 0.004) and PSA level (p max primary prostate tumor seen on F-18 PSMA-1007 CT scans. Higher primary tumor SUVmax was also detected in the high risk group compared to the intermediate risk group (median SUVmax 20.2 against 11.9; p

“This is the first study demonstrating a positive correlation between ISUP grade and PSA level with increased likelihood of metastatic disease using F-18 PSMA-1007 PET / CT for primary prostate cancer staging “, wrote the authors.

PSMA can be coupled to F-18 or Ga-68 for diagnostic imaging, and PSMA-1007 is one of several PET tracers in clinical development for the diagnosis of prostate cancer.

The United States Food and Drug Administration approved Ga-PSMA-11 in December 2020 for imaging in patients with prostate cancer with suspected metastases, as well as in patients with suspected recurrence due to elevated rates by PSA.

However, only small amounts of Ga-68 labeled tracer can be produced at a time as this requires a specific generator, rather than on-site cyclotrons used for the production of F-18 tracers. Additionally, F-18 has a longer half-life than Ga-68 and lower positron energy, resulting in better spatial resolution, according to the authors.

Previous preliminary studies have shown that PET / CT F-18 PSMA-1007 has excellent disease detection efficiency comparable to and potentially superior to PET / CT Ga-68 PSMA-11, the researchers noted.

“The validation of these results further supports the use of F-18 PSMA-1007 PET / CT for the primary staging of prostate carcinoma,” they concluded.

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