18F rhPSMA-7.3 PET/MRI demonstrates high detection in recurrent prostate cancer

Simultaneous ¹⁸F rhPSMA-7.3 PET/MRI imaging demonstrated a high detection rate of true positive lesions in patients with biochemically recurrent prostate cancer, even at low prostate-specific antigen (PSA) levels, according to data from a phase 2 trial (NCT04978675) presented at the 2024 American Society for Radiation Oncology (ASTRO) Annual Meeting in Washington, DC.¹

A change in the radiation therapy treatment plan following 18F rhPSMA-7.3 PET/MRI imaging occurred in 79% of patients.

“In the setting of biochemical recurrence of prostate cancer, detection of small volume disease is usually a challenge with conventional imaging,” explained lead author Devaki Shilpa Surasi, MD, of MD Anderson Cancer Center in Houston, Texas, during the presentation at ASTRO. “Simultaneous PET/MRI has the advantage of combining both the metabolic information from PET and the spatial resolution of MRI to detect disease with greater accuracy than conventional imaging.”

In total, the prospective pilot study enrolled 29 patients between August 2021 and January 2023 who had biochemical recurrence of prostate cancer following prostatectomy. Of those, 28 patients underwent initial ¹⁸F rhPSMA-7.3 PET/MRI imaging on a simultaneous 3T PET/MRI scanner. The median age of patients was 66 years (IQR, 47-76 years) at the time of imaging. After initial imaging, patients received standard fractionated radiotherapy with at least 6 months of hormonal therapy.

Those with a positive scan at the initial timepoint returned for a second scan following treatment. Of the 20 patients with a positive initial scan, 16 returned for the second timepoint imaging and 4 chose to undergo treatment elsewhere.

The primary aim for the study was to evaluate the positive predictive value (PPV) of ¹⁸F rhPSMA-7.3 PET/MRI in detecting recurrent disease. The secondary outcome measures were to evaluate changes in the radiation therapy treatment plan following imaging as well as treatment response.

Data showed a median PSA of 0.3 ng/mL (IQR, 0.2-1.5 ng/mL) prior to the first scan, with 86% (26) of patients having a PSA less than 0.5 ng/mL. The median PSA following treatment, but prior to the second scan, was less than 0.1 ng/mL (IQR, <0.1-0.1 ng/mL).

Overall, 15/16 patients had a true positive scan based on confirmatory imaging and/or biopsy, equating to a PPV of 93.75%. One patient had a false positive scan. Among patients with a PSA value of 0.2 ng/mL to less than 0.5 ng/mL, the PPV was 92.31% (12/13 patients).

A change in the radiation therapy treatment plan following ¹⁸F rhPSMA-7.3 PET/MRI imaging occurred in 79% (22/28) of patients.

Major changes to the treatment plan occurred in 36% (8/22) of patients, which included extension of the clinical target volumes to cover PSMA positive lesions within the pelvis and the cancellation of radiotherapy due to the presence of polymetastatic or visceral metastatic disease. Minor changes to the treatment plan occurred in 41% (9/22) of patients and included dose escalation to gross disease or dose de-escalation to the rest of the prostate fossa.

Both major and minor changes to the treatment plan were noted in 23% (5/22) of patients.

All patients who underwent a combination of standard fractionated radiotherapy and hormonal therapy (n = 14) demonstrated a complete response on the second ¹⁸F rhPSMA-7.3 PET/MRI scan.

Based on these data, the authors concluded, “Simultaneous 18F rhPSMA-7.3 PET/MRI imaging can potentially serve as a ‘one stop shop’ to stratify patient treatment and tailor the radiation treatment fields.”

References

1. Surasi DS, Bathala T, Choi S, et al. A prospective pilot study investigating 18F rhPSMA-7.3 PET/MRI to detect recurrent disease and guide radiotherapy planning in patients with biochemically recurrent prostate cancer post-prostatectomy. Presented at: 2024 American Society for Radiation Oncology Annual Meeting. September 29-October 2, 2024. Washington, DC. Abstract 1005. https://www.redjournal.org/article/S0360-3016(24)02982-1/fulltext