Abstract
P589
Introduction: Prostate-specific membrane antigen (PSMA) positron emission tomography (PET) has become standard of care for prostate cancer imaging. Radiohybrid (rh) 18F-rhPSMA-7.3 is a novel high affinity PSMA-targeting PET radiopharmaceutical. The LIGHTHOUSE study (NCT04186819) evaluated the diagnostic performance of 18F-rhPSMA-7.3 in patients with newly diagnosed prostate cancer. Here, we report the inter- and intra-reader reproducibility of 18F-rhPSMA-7.3 image interpretation using data from LIGHTHOUSE.
Methods: Treatment-naïve patients with unfavorable intermediate to very high-risk prostate cancer who had elected to undergo radical prostatectomy with regional pelvic lymph node dissection underwent PET/Computed Tomography (CT) 50-70 minutes after IV administration of 8 mCi (296 MBq) ± 20% 18F-rhPSMA-7.3. Scans were evaluated by 3 independent central PET readers who had completed 18F-rhPSMA-7.3 scan interpretation training and were blinded to all clinical information including the results of any baseline or additional imaging.
The inter-reader agreement was assessed using the Cohen’s kappa (κ) statistic for pairwise agreement between any 2 readers. Overall agreement among all 3 readers was assessed using Fleiss’ κ coefficient. Cohen’s κ was also used to assess the intra-reader agreement between the first read and a repeat read (≥4 weeks later) of 20% of randomly selected images for each of the 3 readers.
Both analyses were performed at a patient-level and for the following regions: prostate, pelvic lymph nodes and other (extra-pelvic) regions (i.e., extra-pelvic lymph nodes, soft tissue/parenchyma, bones).
Results: The inter-reader agreement for image interpretation in 352 men with evaluable 18F-rhPSMA-7.3 PET/CT is presented in Table 1. Overall agreement was ≥95% across the pair‑wise inter‑reader comparisons with a Fleiss’ κ=0.26 across all 3 readers. On a regional basis, the agreement was ≥81% across the pair‑wise inter‑reader comparisons for all regions, with a Cohen’s κ ≥0.45 for all comparisons in pelvic lymph nodes and other (extra-pelvic) regions. For the prostate, the Cohen’s kappa was not estimable for Reader 1 vs 2 and 1 vs 3 but was 0.50 for Reader 2 vs 3. A Fleiss’ κ of 0.24 was achieved for the overall comparison across all 3 readers in the prostate with values of 0.74 and 0.55 achieved for pelvic lymph nodes and other (extra-pelvic) regions, respectively.
The overall intra-reader agreement (Table 2) was ≥96% for each reader. The Cohen’s κ was not estimable for Reader 1 or 2 and was 0.39 for Reader 3. On a regional basis, intra-reader agreement was high (≥87%) for each reader in each region, with Cohen’s κ shown to be higher for pelvic lymph nodes and other (extra-pelvic) regions than for the prostate where it was only estimable for 1 of the 3 readers.
Conclusions: The data here show 18F-rhPSMA-7.3 scans can be reliably interpreted, with a high degree of inter- and intra-reader agreement observed across 3 blinded readers given the same set of scans and having completed identical training. Inter- and intra-reader agreement were ≥95% overall and while highest in the prostate region, they were also high in extra-prostatic regions. Being able to reliably determine the presence of nodal and metastatic disease is particularly important in this newly diagnosed population given that such findings would be more likely to influence treatment decisions.
Low, or non-estimable Cohen’s κ values were observed for corresponding very high agreement data in some analyses. This is consistent with the well-reported high agreement, low κ paradox that is a consequence of the κ value’s propensity to be affected by the prevalence of rare findings, such as negative 18F-rhPSMA-7.3 scans.
The high reproducibility of results across all regions observed here offers further support to the clinical utility of 18F-rhPSMA-7.3 PET for patients with prostate cancer.
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