Visualize hidden tumor cells

In cases of prostate carcinoma, depending on the specific risk profile, imaging techniques may be used to check for the spread of cancer to lymph nodes, bones and other organs. The risk profile of prostate cancer is determined as part of the initial diagnosis (primary staging) by means of palpation, PSA level measurements and tumour differentiation following a biopsy (Gleason score or ISUP classification).

• Low risk: PSA <10 ng/ml, a Gleason score of 6 (ISUP 1) and a tumour in category cT1c or cT2a
• Intermediate risk: PSA >10 to 20 ng/ml, Gleason score of 7 (ISUP 2 and 3) and a tumour in category cT2b
• High risk: PSA >20 ng/ml, Gleason score of 8 to 10 (ISUP 4 and 5) and a tumour in category >cT2c 

^{S3 Guidelines on Prostate Carcinoma, Version 6.2 – October 2021}

Diagnostic imaging procedures are not recommended for low-risk prostate cancer due to the low probability of cancer spread. Imaging procedures should be performed on patients with a high risk profile, where the cancer is more likely to have spread. As for patients in the intermediate risk category, the benefits of diagnostic procedures to determine spread remain unclear.

These procedures can involve a number of imaging techniques. Bone scintigraphy examines whether any conspicuous lesions are present in the patient’s skeletal system. Cross-sectional imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) can help to identify abnormalities in soft tissue, such as in the pelvic lymph nodes and other organs.

Positron emission tomography (PET)

Positron emission tomography (PET) has taken on an increasingly significant role in recent years, usually in combination with computed tomography (PET/CT), with a growing influence on the treatment of prostate carcinoma. A PET/CT scan uses weakly radioactive molecules, which are injected intravenously and then attach to and subsequently are taken up by prostate cancer cells. The selection of the molecular target structure is of crucial importance. The prostate-specific membrane antigen (PSMA) is an ideal target, as this protein appears on the surface of prostate cancer cells in considerably higher quantities – sometimes 100 to 1,000 times higher – than on normal prostate tissue. The development of molecules (called ‘ligands’ or ‘tracers’) capable of marking PSMA has created a unique boom in the use of PET scans for prostate carcinoma diagnostics in recent years (for an overview, see: Maurer et al. Nat. Rev. Urol 2016).

PSMA PET scans can therefore identify and visualise even small foci in lymph nodes and bones – which is beyond the abilities of the conventional techniques mentioned above. For this reason,

PSMA PET scans are increasingly replacing conventional imaging techniques as a means of determining the spread of cancer.
At present, the primary area of application for PSMA PET is biochemical recurrence, i.e. when a patient’s PSA level rises again following surgery or radiotherapy. In such cases, successful treatment is dependent on being able to localise the prostate cancer cells as precisely as possible. Indeed, PSMA PET has also proven itself superior to classic imaging techniques for determining the spread of cancer in primary staging for patients with a high risk.

What do we see in the picture above?

A PSMA-PET in black and white, the images from a computed tomography in gray, and the combined view of both in color.

In a patient with PSA levels rising again after radical prostatectomy, two foci of bone can be clearly seen on the PSMA-PET (red arrows), while the two bone metastases are not (yet) visible on the computed tomography (circle).

In another patient with high-risk prostate carcinoma, the PSMA PET shows spread into a pelvic lymph node (arrow) that does not appear abnormal on computed tomography.