Jeremie Calais, MD, highlights study of PSMA-PET vs bone scan in metastatic prostate cancer

In this video, Jeremie Calais, MD, PhD, highlights the background and key findings from the study, “Phase 2 trial of PSMA PET CT versus planar bone scan and CT in prostate cancer patients progressing while on androgen deprivation therapy,” (NCT04928820), for which he served as the senior author. Calais is a nuclear medicine physician at the University of California, Los Angeles.

Video Transcript:

Bone scan or bone scintigraphy is the main standard of care imaging method for decades to stage metastatic prostate cancer patients. The bone tropism of metastasis from prostate cancer made that bone scan was the method of choice. Bone scan is an indirect imaging method to detect prostate cancer metastasis because it detects the surrounding reaction of the bone, around the bone metastasis, but still using indirect methods. It was used for again, decades, with some strong correlation to outcomes with the Prostate Cancer Working Group Criteria 2+2 rules.

PSMA-PET has been in the landscape of prostate cancer staging now for about 5 to 10 years. So, [there is] a lot about how do we convert and translate from bone scan to PSMA-PET staging? Depending on the patient population, you will have different answers. In the primary staging setting, untreated disease, biochemical recurrence setting, for sure PSMA-PET scan has a lot of superiority of a bone scan. Much more specific, much more accurate. There is a lot of false positive findings on the bone scan. When the prevalence of the metastases is low, such as in the untreated disease stage, early disease, biochemical recurrence, hormone sensitive, you have a lot of false positive findings.

Now when you move to the more advanced patient population, metastatic castration-resistant, for example, then of course the prevalence of bone metastasis is much higher. So, the specificity of the bone scan increases, specifically on a per patient basis. So, that study was an attempt to answer to that question. How do these 2 imaging modality compare to stage metastatic prostate cancer patients who are progressing under hormonal treatment, getting castration-resistant. So, it's an early mCRPC disease stage that we choose here. These are not the super late stage advanced, and we wanted to see how these 2 scans compare.

At the time of the study design, PSMA-PET was not yet FDA approved, so it was an incentive for patients to get a PSMA-PET scan. They could get into the study if they would do also a pair bone scan within 2 weeks of the PSMA-PET. So, it enabled to recruit patients. But during the trial, PSMA-PET got FDA approved, so it got widely available to patients that could get it covered by insurances. At some point there was much more difficulty to recruit patient to the trial because they don't need to do a bone scan, they don't want to do specifically a research trial, they can just get the scan as first-line of care. So, we had to terminate the trial early, instead of having 100 patients—that was the initial study design. The initial study design was based on the hypothesis that there would be a 35% detection rate with PSMA-PET/CT vs bone scan/CT 23% detection rate.

In fact, the 2 imaging modalities were deemed quite equivalent in the study. I'm going to go back on that, but we had to end the study at 20 patients instead of 100. So, the study was underpowered because of difficulty to recruit patients. Also, we were able to show that in the first 20 patient sample we had, the statistical signal that we got would have probably made the study impossible to succeed in terms of detection rate difference because bones scans and PSMA-PET in that specific patient subset performed quite equivalently on a per patient basis. There was no patient with bone scan positive and PSMA-PET negative detection rates, and there was no patient with PSMA-PET positive and bone scan negative detection rates.

Both scans were read by 3 imaging blinded readers. We used 3 blinded readers for the bone scan reads, different than the 3 imaging readers for the PSMA-PET/CT reads. We used a majority score for each imaging modality, 2 vs 1 in case of disagreement. That's the result; at the end there was no major difference observed on a patient level. On a lesion level, there were still more lesions detected with PSMA-PET, but not on a patient level. So, because of that absence of sign of [a statistical difference,] even in the first 20 patient data sets, [and] because of the difficulty to recruit patients after FDA approval, we terminated the study early at 20 patients instead of 100, and that's what we reported here.

This transcript was AI generated and edited by human editors for clarity.