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Background
The use of prostate-specific membrane antigen (PSMA) targeted PET imaging has revolutionized the primary staging, therapy monitoring and assessment of recurrent disease in men with prostate cancer, with a significant impact on management decisions and often on clinical outcomes. Given its high sensitivity and specificity for the detection of recurrent and metastatic disease, its use indications are becoming broader. It is estimated that PSMA-ligand PET can detect more than 90% of nodal metastases with a short axis of at least 4.0 mm. As for primary staging, its diagnostic accuracy is superior to the combination of bone scintigraphy and CT, in patients with unfavorable intermediate or high risk disease, leading to a significant change in management in up to 22-28% of cases. In therapy response assessment, it is considered a useful biomarker for classifying patients as responders (stable disease, partial or complete response) or nonresponders (disease progression, defined as an increase in tumor burden greater than 30%), leading to essential changes in management. Finally, in the biochemical recurrence scenario, which is one of the most common indications, PSMA-PET has been shown to have detection rates of up to 38% in patients with PSA under 0.5 ng/ml, as well as in up to 62% of cases with negative conventional imaging, with such a significant impact that it can change the management intent, from a palliative to a potentially curative approach in up to 20% of cases.

Educational Goals / Teaching Points
To review, with demonstrative cases, the role and clinical impact of PSMA-ligand PET in primary staging of prostate cancer, restaging at time of biochemical failure and therapy monitoring.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Introduction, PSMA-ligand PET detection rates and diagnostic performance, PSMA-ligand PET in prostate cancer staging, PSMA-ligand PET in therapy response assessment, and PSMA-ligand PET in patients with biochemical failure.

Conclusion
PSMA-targeted PET imaging has revolutionized the staging and restaging of prostate cancer and has a significant impact on treatment decisions. For certain indications, its benefit and superiority over other imaging modalities have been widely demonstrated. It should be considered the modality of choice for staging patients with unfavorable intermediate or high risk disease, restaging men with biochemical recurrence even at very low serum PSA and negative conventional imaging, and can be a useful tool for monitoring treatment response.