Tumor Sink Effect in ⁶⁸Ga-PSMA-11 PET: Myth or Reality?

Abstract

We aimed to systematically determine the impact of tumor burden on ⁶⁸Ga-prostate-specific membrane antigen-11 (⁶⁸Ga-PSMA) PET biodistribution by the use of quantitative measurements. <b>Methods:</b> This international multicenter, retrospective analysis included 406 men with prostate cancer who underwent ⁶⁸Ga-PSMA PET/CT. Of these, 356 had positive findings and were stratified by quintiles into a very low (quintile 1, ≤25 cm³), low (quintile 2, 25-189 cm³), moderate (quintile 3, 189-532 cm³), high (quintile 4, 532-1,355 cm³), or very high (quintile 5, ≥1,355 cm³) total PSMA-positive tumor volume (PSMA-VOL). PSMA-VOL was obtained by semiautomatic segmentation of total tumor lesions using qPSMA software. Fifty prostate cancer patients with no PSMA-positive lesions (negative scan) served as a control group. Normal organs, which included salivary glands, liver, spleen, and kidneys, were semiautomatically segmented using ⁶⁸Ga-PSMA PET images, and SUV_mean was obtained. Correlations between the SUV_mean of normal organs and PSMA-VOL as continuous and categoric variables by quintiles were evaluated. <b>Results:</b> The median PSMA-VOL was 302 cm³ (interquartile range [IQR], 47-1,076 cm³). The median SUV_mean of salivary glands, kidneys, liver, and spleen was 10.0 (IQR, 7.7-11.8), 26.0 (IQR, 20.0-33.4), 3.7 (IQR, 3.0-4.7), and 5.3 (IQR, 4.0-7.2), respectively. PSMA-VOL showed a moderate negative correlation with the SUV_mean of the salivary glands (<i>r</i> = -0.44, <i>P</i> < 0.001), kidneys (<i>r</i> = -0.34, <i>P</i> < 0.001), and liver (<i>r</i> = -0.30, <i>P</i> < 0.001) and a weak negative correlation with the spleen SUV_mean (<i>r</i> = -0.16, <i>P</i> = 0.002). Patients with a very high PSMA-VOL (quintile 5, ≥1,355 cm³) had a significantly lower PSMA uptake in the salivary glands, kidneys, liver, and spleen than did the control group, with an average difference of -38.1%, -40.0%, -43.2%, and -34.9%, respectively (<i>P</i> < 0.001). <b>Conclusion:</b> Tumor sequestration affects ⁶⁸Ga-PSMA biodistribution in normal organs. Patients with a very high tumor load showed a significantly lower uptake of ⁶⁸Ga-PSMA in normal organs, confirming a tumor sink effect. As similar effects might occur with PSMA-targeted radioligand therapy, these patients might benefit from increased therapeutic activity without exceeding the radiation dose limit for organs at risk.