Image of the month

During the last decade, PSMA-PET/CT became a mainstay in the imaging of prostate cancer. However, PSMA is also expressed in the neo-vasculature of various non-prostatic tumors [1]. Amongst other somatostatin receptor-targeted tracers, DOTATOC-PET/CT is the current gold standard for imaging of well-differentiated neuroendocrine tumors arising in the gastrointestinal tract [2].

This image presents a patient who was originally diagnosed with a high-grade prostate carcinoma (Gleason score 9, initial PSA 48 ng/ml). Consequently, an atypically located liver lesion was histologically proven to be a well-differentiated neuroendocrine tumor (G2). Due to his transplant kidney, the patient was not allowed to receive contrast media; hence, he received staging with [18F]F-PSMA-1007 and [68 Ga]Ga-DOTATOC PET/CT.

In the PSMA-PET, the uptake of the prostate carcinoma was SUVmax 55.5 and the suspected liver metastasis was SUVmax 106.2; no uptake was seen in the neuroendocrine primary tumor and only faint uptake (SUVmax 8.5) in a peritoneal lymph node. In the DOTATOC-PET, the uptake of the duodenal neuroendocrine primary tumor was SUVmax 67.9, the positive lymph node in the related drainage was SUVmax 91.2, and the liver metastasis was SUVmax 221.3; the adenocarcinoma of the prostate was somatostatin-receptor negative.

The presented image contains two interesting points. First, it demonstrates the high variability of PSMA expression in non-prostatic tumor lesions and its dependency of the local tumor microenvironment (e.g., site of metastasis, neoangiogenetic activity, organ-specific perfusion effects). Second, it serves as a reminder that an uncritical belief in strong biological signals obtained with highly specific tracers can lead to an overestimate of the “histo-radiological” performance of molecular imaging. A clinically uncommon situation—such as suspicion of liver metastatic prostate cancer without additional lymph node or bone metastases—still needs histopathological validation!

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