The aim of this study is to provide robust data on the head-to-head comparison of the two ligands of the prostate specific membrane antigen (PSMA) available in Switzerland for positron emission tomography (PET)-imaging, i.e. 68Ga-PSMA-11 und 18F-PSMA-1007.

Prostate Cancer (PC) is the most common malignancy in men and the second leading cause of cancer-related death in men. Despite initial therapy at early stage disease, biochemical recurrence remains a commonly encountered entity and presents a challenge for conventional imaging modalities given their limited abilities to detect disease at early stages of recurrence.

PET/CT with ligands of the prostate specific membrane antigen has been shown to have a significant impact on treatment and is now the sine qua non for staging of recurrent PC. For example, accurate identification of PC lesions allows for more accurate radiotherapy planning, allowing for an individualised treatment strategy. There is therefore a substantial clinical requirement for the accurate identification and stratification of individuals in whom prostate cancer is diagnosed and at earlier stages of recurrent disease when the chance of a curative treatment is at its highest.

It is in this context that PSMA has become the focus of much attention owing to its high levels of expression on PC cells and has rapidly established itself as the investigation of choice in recurrent PC. Furthermore, PSMA-directed radioligand therapy is a rapidly evolving treatment modality for metastatic disease, creating an additional therapeutic role for PSMA-ligand molecular imaging, for which the term "theragnostics" has been coined. The challenge for nuclear medicine is therefore to develop tracers and examination protocols that provide optimal detection and characterisation of disease, thus improving upon this promising technique.

There are currently no published prospective head-to-head studies comparing these two tracers in recurrent PC. Because of this lack of data, there are no clear recommendations about which tracer to use and in which situation.

This study aims to fill this gap and provide comprehensive data with the potential to improve the diagnosis of PC. By providing robust data comparing the two tracers, such data will also provide guidance to clinicians faced with the scenario of an initially negative 68Ga-PSMA-11 PET as to the diagnostic utility of an additional 18F-PSMA-1007 PET, or vice-versa, and in which scenarios repeated scanning may be justified.

Finally, the application of the radiotracer into the same patient allows for a comparison of tracer kinetics. Although radiotracer kinetics are well known from the original pioneering dosimetric publications, they have never been compared in a head to head fashion and not in biochemical recurrence. Obtaining dynamic scans over the first hour post injection will allow intra-individual dosimetry and a head-to-head comparison of parametric imaging parameters, allowing a direct comparison of the radiotracer's affinity using standard parametric imaging techniques.

Inclusion Criteria:

• Patients with known biochemical recurrence of a histologically confirmed prostate cancer post radical prostatectomy, defined as two consecutive prostate specific antigen (PSA) values > 0.2 ng/ml:
• Post prostatectomy: Patients > 18 y/o
• PSA measured within ± 4 weeks of the first PSMA-PET/CT
• Patients providing written informed consent
• No change in PC treatment in the period between the first and second scans

Exclusion Criteria:

• Patients receiving androgen deprivation therapy (ADT) within 6 months prior to the PSMA-PET/CT
• Patients with contraindication to diuresis with 20mg Furosemide
• Patients with renal dialysis or relevant renal impairment (eGFR < 35 ml/min)
• Inability to provide written informed consent
• Inability to schedule and attend two consecutive PET examinations
• Patients undergoing active treatment for a second non-prostatic malignancy at the time of the first scan.
• Known sensitivity or allergy to PSMA-ligands or one of the components of the radiotracer solutions used.

Data available upon reasonable request

Independent read by masked investigators for assessment of the primary outcome.