In order to accurately interpret nuclear medicine studies, a knowledge of the normal physiological distribution of the radiotracer is required. PSMA expression is present in normal prostatic tissue, though at a low level, and can also be found in the salivary and lacrimal glands, nasal cavity, larynx, liver, spleen, intestines, kidneys, and sympathetic ganglia 2 . Below is an image of the normal radiotracer distribution for [18F]-PSMA PET.
In our institution, we starting using [18F]-PSMA-1007 PET in April 2022. The image below is from the first patient to be imaged. He was a gentleman with a diagnosis of prostate cancer since 2008. He had multiple nodal, lung and bone metastases. Despite ongoing treatment with Gonadotrophin Releasing Hormone Agonists, his prostate specific antigen (PSA) was continually rising.
The physiological uptake of PSMA ligands in various normal tissues is an important pitfall in the PSMA-targeted imaging. In particular, uptake in sympathetic trunk ganglia can pose challenges to nuclear medicine readers and may mimic lymph nodes 3. Cervical chain and coeliac ganglia may demonstrate higher uptake with [18F]-PSMA 4.
The images demonstrate a challenging case of uptake in the cervical chain sympathetic ganglia. This was initially described as lymph nodes and would have up-staged this patient’s disease and altered their treatment. On review and multi-disciplinary team discussion, the uptake was in-fitting with ganglion uptake.
These images demonstrate uptake In coeliac trunk ganglia.
An interesting finding noted in PSMA PET/CT has been the emergence of benign, focal bone uptake. It has been suggested that this uptake in benign lesions occurs more frequently with [18F]F-PSMA compared with [68Ga]Ga-PSMA 5. This is important to be aware of while interpreting [18F]F-PSMA PET/CT, as this can be easily mistaken for metastases, which may subsquently result in erroneously up-staging a patient. Below are two such cases of benign rib uptake that we have encountered:
The phenomenon of a "superscan" was first described in 1975 for bone scintigraph 6 and is considered to be widespread, intense, and largely symmetrical uptake of the osseous radiotracer, typically accompanied by minimal or absent tracer accumulation in the urinary system and soft tissues. This is generally a finding that occurs in technetium-99 HMDP bone scans, however, there have been some reports of the "superscan" appearance with PET/CT, with some reports of it occurring with [68Ga]Ga-PSMA PET 7. Below is an example of a "superscan" appearance with [18F]F-PSMA-1007:
Uptake in renal lesions, particularly primary renal malignancies, have been described 8. Below is a case of a renal lesion that was initially identified on [18F]F-PSMA-1007. This avid lesion was worked up with dedicated triple phase renal CT and ultrasound-guided biospy. It was histologically proven to be an oncocytoma.
PSMA PET/CT has proven itself to be very useful in the evaluation of disease disconcordance. This is a case of a gentleman who had two negative transperineal prostate biopsies with MRIs demonstrating a possible PIRADS 3 lesion in the left gland.
Despite two negative systematic prostate biospsies, this gentleman's prostate-specific antigen (PSA) was continually rising and the primary referring team had a high index of suspicion for occult disease. He was referred for [18F]F-PSMA PET/CT, which demonstrated a focus of increased radiotracer uptake in the anterior left gland.
Using the PSMA PET/CT findings, a third, more focused, transperineal biopsy of the prostate was taken. Histology was consistent with Gleason 7 disease in the PSMA-avid region. This demonstrates the significant clinical utility of PSMA PET/CT in challenging clinical cases and for clinical problem-solving.