Simultaneous 18F-rhPSMA-7.3 PET/MRI imaging demonstrates 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.
A change in the radiation therapy treatment plan following 18F-rhPSMA-7.3 PET/MRI imaging occurred in 79% of patients, highlighting the potential of this imaging modality to refine treatment strategies.
Improved Detection with PET/MRI
"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. "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."
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.
Patients 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, while 4 chose to undergo treatment elsewhere.
The primary aim of the study was to evaluate the positive predictive value (PPV) of 18F-rhPSMA-7.3 PET/MRI in detecting recurrent disease. Secondary outcome measures included evaluating changes in the radiation therapy treatment plan following imaging, as well as treatment response.
High Positive Predictive Value
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 out of 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).
Impact on Treatment Planning
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."
