甲狀腺癌腎臟轉移偽裝為腎細胞癌-病例報告

陳冠甫¹、黃子豪^1,2、黃逸修^1,2

¹臺北榮民總醫院泌尿部；2國立陽明交通大學醫學院泌尿學科及書田泌尿科學研究中心

Occult Follicular Thyroid Carcinoma Presenting as a Solitary Renal Mass Mimicking Renal Cell Carcinoma - A case report
Kuan-Fu Chen¹, Tzu-Hao Huang^1,2, Eric Yi-Hsiu Huang^1,2

1 Department of Urology, Taipei Veterans General Hospital, Taipei, Taiwan

2 Department of Urology, College of Medicine and Shu-Tien Urological Science

Research Center, National Yang Ming Chiao Tung University, Taipei, Taiwan

Introduction: Renal metastases from other primary malignancy are uncommon and often clinically silent. Among them, metastasis from thyroid carcinoma is particularly rare and may radiologically mimic primary renal cell carcinoma (RCC), posing a diagnostic challenge. Here, we present a case of a metastatic thyroid carcinoma presenting as an incidental renal tumor and suspected to be renal cell carcinoma.

Case Presentation: A 71-year-old woman with a history of follicular thyroid adenoma status post bilateral subtotal thyroidectomy in 2011 was found to have an incidental renal mass during routine health screening in November 2025. She was asymptomatic. Contrast-enhanced computed tomography demonstrated a 2.1 cm enhancing lesion in the left kidney with suspected renal sinus fat invasion, suggestive of renal cell carcinoma without evidence of metastasis. The patient underwent robotic-assisted left partial nephrectomy on 2025/12/18 after shared decision making. A 2.5 cm endophytic tumor with renal sinus and collecting system involvement was identified and resected during the operation. The warm ischemia time was 39 minutes, while estimated blood loss was 200ml. Post-operative course was smooth and the patient was discharged on post-operative day (POD) 5.

Pathological examination revealed metastatic well-differentiated thyroid carcinoma, with positive immunohistochemical staining for PAX8, TTF-1, and thyroglobulin. BRAF VE1 and NRAS Q61R was negative. The patient was then referred to thyroid specialists for further management.

Discussion: Renal metastases are uncommon and frequently originate from lung, breast, or gastrointestinal malignancies. Metastasis from thyroid carcinoma is rare, and presentation as a solitary renal mass is particularly unusual, often leading to misdiagnosis as primary renal cell carcinoma (RCC).

Follicular thyroid carcinoma typically spreads hematogenously, most commonly to the lungs and bones. Renal involvement is rare and usually occurs in the context of disseminated disease. In this case, the lesion mimicked RCC both clinically and radiologically, including features suggestive of renal sinus invasion, highlighting the limitation of imaging in distinguishing primary from metastatic renal tumors.

In the present case, an important diagnostic challenge arises from the patient’s prior thyroid pathology, which was reported as follicular adenoma. The subsequent detection of metastatic disease raises the possibility of previously unrecognized minimally invasive follicular carcinoma, as the distinction between adenoma and carcinoma depends on identification of capsular or vascular invasion, which may be focal and easily missed. Efforts were made to retrieve the original slides from the original hospital for review; however, they were unavailable as they had been discarded after reaching the mandatory archival period limit.

Of note, “thyroid-like follicular renal cell carcinoma” (TLFRCC), a rare subtype of non-clear cell RCC, represented a critical differential diagnosis in this case. With only fewer than 20 cases reported, this rare diagnosis is made upon combining clinical, radiological, and immunohistochemical confirmation. We had a comprehensive discussion with the pathologist before achieving the final diagnosis. Primary thyroid origin was confirmed by positive immunoreactivity for TTF-1 and thyroglobulin, both of which are characteristically negative in TLFRCC.

The long interval between initial thyroid surgery and detection of renal metastasis further reflects the indolent yet persistent nature of well-differentiated thyroid carcinoma. This case underscores the importance of considering metastatic disease in the differential diagnosis of renal masses, particularly in patients with a history of thyroid lesions, and highlights the essential role of histopathological confirmation in guiding management.

Conclusions: This case underscores a rare but clinically significant diagnostic pitfall in which metastatic thyroid carcinoma presents as a solitary small renal mass with radiologic features highly suggestive of primary renal cell carcinoma, including apparent renal sinus invasion. Notably, the absence of overt malignant findings in prior thyroid pathology further obscured the diagnosis, highlighting the potential for occult or previously misclassified follicular thyroid carcinoma to manifest as late distant metastasis. Multidisciplinary evaluation and histopathological confirmation are essential for accurate diagnosis and appropriate management.