Teaching Case

A 75-year-old male patient with symptoms of obstructive uropathy and elevated PSA levels (19 ng/ml) was sent for 68Ga-PSMA PET/CT to rule out prostate carcinoma. The rectal examination of prostate and transrectal ultrasound (TRUS) were unremarkable. There was no history of hepatitis or diffuse liver disease.

Ga-PSMA PET/CT images (Fig.1 a and b) showed a large, nearly round, focal intense PSMA expressing lesion in the right lobe of the liver. Corresponding non-contrast CT image revealed the well-defined hypodense mass in the liver. Fig. 1 c and d show normal prostate gland with normal mild diffuse physiological uptake and no evidence of intense focal PSMA avid area, ruling out primary prostate carcinoma. Fig. e shows PET whole body Maximum Intensity Projection (MIP) image clearly depicting the large focal intense uptake by the mass in the liver. No other foci of PSMA avid lesions are made out, thus ruling out the possibility of distant lymph nodal or skeletal metastatic focus.

Background

Molecular imaging with 68Ga-PSMA hybrid PET/CT in prostate cancer (PCa) is well documented, gaining highest clinical impact during the last few years as it is reported to be superior to other imaging modalities in detecting primary PCa and recurrence at low PSA levels due to its high sensitivity and specificity, and presenting metastatic lymph node and bone lesions with excellent contrast. However, it is not yet considered an evidence-based investigation for patients with elevated PSA levels, and in particular to detect, map and rule out intraprostatic lesions. Prostate-specific membrane antigen (PSMA) is a glycoprotein strongly expressed in the human prostate, but also found in kidney, liver, intestine, and brain. PSMA has been suggested to be involved in tumour angiogenesis. There have been a few reports of PSMA expression in non-prostatic cancers like metastatic clear cell renal carcinoma [1], metastatic breast cancer [2] and in primary gliomas [3]. Though 95% of hepatocellular carcinoma (HCC) stain positive for PSMA [4], the application of 68Ga-PSMA PET/CT for imaging of HCC for primary and metastatic workup is not yet documented.

Clinical Perspective

Rectal examination, TRUS and biopsy are the components used in prostate cancer screening with rising PSA. This patient was referred from outside for PSMA workup to rule out PCa.

No fasting/special patient preparation is required prior to the study. Dose of 3-4mCi 68Ga PSMA is injected intravenously. The tracer is well tolerated. Whole body PET scan and non-contrast CT scan are done on 6 slice PET/CT machine 1 hour after injection.

Imaging Perspective

With short half life of 68Ga, excellent radiolabelling chemistry, fast background clearance, early imaging is possible and high resolution PET images are produced. Focal avidity seen in prostate indicate PCa and focal overexpression in lymph nodes/skeleton suggest metastases. In this case, no abnormal uptake is seen in prostate gland, instead a focal large avid area is seen in the liver.

Outcome

With serum AFP level also elevated, the patient underwent MDCT, which showed typical HCC features, and biopsy proved primary HCC. As it was a large HCC and due to graft unavailability, he was admitted for right hepatectomy.

Take Home Message

18F-FDG PET/CT is not the best imaging option for primary and metastatic workup of HCC, as all HCCs are not FDG-avid. Hybrid 68Ga-PSMA PET/CT might be useful for imaging in HCCs, in addition to its high sensitivity in detecting PCa lesions, and therefore its role needs further investigation.