PSMA-PET/CT Detection of Prostate Cancer Metastasis to Thyroid and Cricoid Cartilages

Introduction: Worldwide, prostate cancer is the sixth leading cause of death in men and is projected to have one of the largest increases in cancer incidence among men following the year 2020. Most prostate cancer patients present with high serum prostate-specific antigen (PSA) levels with metastatic disease occurring commonly in the pelvic lymph nodes, bones, lungs, and liver. In literature, very few cases have been described of metastatic lesions from prostate carcinoma to the thyroid cartilage and cricoid cartilage. This may be due to their relatively small size and heterogeneity on conventional computed tomography (CT) and Tc-99m MDP bone scan.

Prostate Specific Membrane Antigen (PSMA) is a Type II integral membrane glycoprotein with a large extracellular domain, with increased expression in metastatic disease and disease recurrence. There have been several radiotracers developed for PET/CT which target PSMA with the two most widely used being F-18 piflufolasta (Pylarify) and Ga-68 PSMA-11 (Illuccix). These agents have revolutionized imaging of prostate cancer, both in initial staging, as well as in the evaluation of biochemical recurrence. We present 3 cases of prostate cancer metastasis to the thyroid cartilage and cricoid cartilage detected on PSMA PET/CT imaging which suggests this may not be so rare of an entity.

Methods: We conducted an internal institutional review of the F-18 Pylarify and Ga-68 Illuccix PSMA examinations that were performed at our single center Academic Teaching Hospital since we started offering PSMA PET/CT from April 2022 to December 2022. Out of a total of 75 examinations, we found 3 patients diagnosed with prostate adenocarcinoma demonstrating PSMA-avid metastatic disease to the thyroid cartilage or cricoid cartilage, amounting to 4% cases in this limited dataset.

Results: Two cases demonstrated PSMA-avid metastatic disease to the thyroid cartilage and the third showed an intensely PSMA avid-lesion in the cricoid cartilage with concurrent CT scans showing concordant sclerotic lesions. All three cases demonstrated diffuse osseous metastatic disease burden. Two cases demonstrated no abnormal activity in the prostate gland and third case with history of prostatectomy, showed no activity in the surgical bed to suggest local recurrence. One of the three cases demonstrated mildly PSMA-avid in the retroperitoneal lymph nodes. None of the three patients had PSMA-avid metastatic disease to the liver or lungs.

Conclusions: Accurate staging and restaging of prostate cancer is of high importance for treatment decisions and patient management. As mentioned previously, prostate cancer most often spreads to lymph nodes, bones, liver, and lungs. High PSMA expression has been demonstrated in adenocarcinoma of the prostate and its metastatic sites. Although prostate cancer can metastasize to any part of the body, laryngeal cartilage metastasis including thyroid and cricoid cartilage are rare with few cases being reported in literature with approximately 16 cases have been reported. Of these, most were detected using the Ga-68 PSMA PET/CT, however some reports have also used conventional CT imaging, MRI, and F-18 Fluorocholine PET/CT modalities. Here, we present 3 cases of metastatic lesions to thyroid and cricoid cartilage detected on PSMA PET/CT in the first 75 patients scanned at our institution. This limited data set suggests that prostate cancer metastasis to the thyroid and cricoid cartilages may be far more common than previously presumed.

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