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Abstract
Abstract We present the first described case of retroperitoneal metastasis from follicular thyroid carcinoma (FTC). This was incidentally discovered as a PSMA (prostate-specific membrane antigen)-positive lesion on PSMA-positron emission tomography (PET)-computed tomography (CT) in a patient with synchronous prostate cancer (PCa).The expanding utilization of PSMA-PET-CT has revealed tracer uptake in several nonprostatic conditions. A 68-year-old man investigated for PCa, underwent magnetic resonance imaging, which revealed an 18-mm retroperitoneal soft tissue nodule lateral to the left psoas and a left pelvic node. PSMA-PET-CT showed tracer uptake in the primary PCa, retroperitoneal lesion, and pelvic node with an incidental high-grade focus in the thyroid. A CT following a period of androgen deprivation demonstrated no response in the retroperitoneal lesion, while the pelvic node became smaller. Fine-needle aspiration (FNA) of the thyroid was performed, although an ultrasound was initially reported as benign. FNA cytology (FNAC) was interpreted as a benign nodule. However, CT-guided biopsy of the retroperitoneal lesion revealed follicular thyroid tissue. The differential diagnoses were ectopic thyroid tissue and FTC. FNAC and ultrasound were reviewed at the thyroid multidisciplinary meeting (MDM) and upgraded to follicular atypia and suspicious for malignancy, respectively. Left hemithyroidectomy confirmed an angioinvasive follicular carcinoma. Completion thyroidectomy revealed a small incidental micropapillary carcinoma. Single photon emission computed tomography (SPECT)-CT post-131I treatment showed intensely iodine-avid tissue within the thyroid bed and retroperitoneal deposit. On follow-up 123I-SPECT-CTs, there was no abnormal iodine uptake and the retroperitoneal deposit decreased from 18 to 5 mm, presumed as scar tissue. Thyroglobulin reduced from 7.7 to < 0.1 ug/L. MDM recommended 6 monthly surveillance. PSMA-positive lesion evaluation can be challenging due to PSMA expression in nonprostatic conditions. As illustrated by this case, unusual distribution of tracer uptake requires further investigations and a multidisciplinary approach to guide management. High PSMA expression in differentiated thyroid cancer was associated with shorter progression-free survival and may be considered a marker of aggressiveness. Such tumors could be candidates for targeted PSMA-radioligand therapy (e.g., 177lutetium), particularly in radioiodine-negative/refractory cases, which are difficult to treat.
Highlights
We present the first described case of retroperitoneal metastasis from follicular thyroid carcinoma (FTC)
We present a unique case of a solitary retroperitoneal metastasis from follicular thyroid carcinoma (FTC), incidentally detected by a staging [18F]-prostate-specific membrane antigen (PSMA)-1007-positron emission tomography (PET)computed tomography (CT) for prostate cancer (PCa) and confirmed by subsequent investigations
Practitioners should be aware of the spectrum of PSMA expression
Summary
We present a unique case of a solitary retroperitoneal metastasis from follicular thyroid carcinoma (FTC), incidentally detected by a staging [18F]-prostate-specific membrane antigen (PSMA)-1007-positron emission tomography (PET)computed tomography (CT) for prostate cancer (PCa) and confirmed by subsequent investigations. The expanding utilization of PSMA-PET-CT has revealed PSMA-ligand uptake in other tissues/conditions, including normal nonprostatic epithelial cells, inflammation/infection, nonprostatic neoplastic cells, and tumor-associated neovasculature.[1] As exemplified by this case, it is important to investigate unusual sites of uptake on PSMA-PET-CT to exclude synchronous tumors. Initially thought to represent metastasis from PCa, the atypical retroperitoneal location of the lesion, lack of response to treatment, and incidental PSMA-tracer uptake in the thyroid prompted further investigations to rule out a synchronous primary. On repeat 123I-SPECT-CTs at 4 and 10 months, the retroperitoneal lesion remained nonavid and became miniscule (5 mm), presumed as fully ablated scar tissue This indicated an excellent, likely complete response to treatment. MDM recommended 6 monthly surveillance with 123ISPECT-CT and thyroglobulin antibody levels for hidden micrometastases and early recurrence
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