This is a report of an ileocecal adenocarcinoma metastasizing to a follicular papillary thyroid carcinoma (PTC). Primary colorectal metastases to thyroid carcinomas are extremely rare. This work aims to describe the 7th ever reported case and to highlight the inhomogeneous reporting on this topic.

An ileocecal adenocarcinoma was endoscopically diagnosed in a 64-year-old woman. Preoperative computed tomography (CT) and positron emission tomography (PET) two months postoperatively were negative for distant lesions and right hemicolectomy allowed staging of this malignancy as a T4bN2bM0 ACP stage C high-grade invasive ileocecal adenocarcinoma. A year later a metastasis was found in segment 8 of the liver and treated with neoadjuvant chemotherapy and partial hepatectomy. Subsequently, a 6 and a 7 mm lesion were found in the lungs bilaterally on PET-CT with SUV 0.94 and 0.97. These were initially managed with surveillance but then required bilateral resection 4 years after initial diagnosis when surveillance PET-CT established SUV of 2.0 and 1.7. Another hepatic lesion was simultaneously found and resected from segment 6, and a suspect metastasis was found on surveillance PET-CT and biopsied in the left thyroid. A malignant lesion in the thyroid gland was characterized by CDX2 and TTF1 positive immunostaining and a left hemithyroidectomy was performed. Histopathology showed a partly necrotic metastatic colorectal adenocarcinoma measuring 35×20×10 mm adjacent to a separate 3.5 mm lesion identified as PTC, follicular variant. While the metastatic adenocarcinoma was focally extending into the extra thyroidal soft tissue and blending with diathermy artifact at the peripheral surgical margin, the PTC appeared to be histologically confined to the thyroid. Immunohistochemistry staining further demonstrated CDX2 positive and KRAS gene mutation for the metastatic colorectal adenocarcinoma segment and TTF-1 positive and BRAF gene mutation for the PTC (Figure 1).

Six cases of primary colorectal carcinoma metastasizing to thyroid carcinomas were identified in the scientific literature (Table 1). Metastatic colorectal cancer is common, primarily to regional lymph nodes, liver and lungs [1]. However, metastases to the thyroid gland are uncommon. Autopsy series have shown a prevalence of 1.9–24% [2]. The most common primary sites are renal cell, colorectal, lung, and breast. However, colorectal adenocarcinoma metastasizing to the thyroid gland with an adjacent primary thyroid neoplasm is extremely rare. Some authors have postulated that cancer changes the cellular structures in the thyroid gland, predisposing it as a site of metastasis [1],[2],[3],[4],[5]. The most commonly involved thyroid carcinoma is PTC, with 1 follicular variant PTC, and 1 medullary carcinoma (Table 1). Colorectal adenocarcinoma is the most common metastasis to be found within thyroid cancers, followed by renal cell carcinoma and lung adenocarcinoma.

The cases reported in Table 1 were described as tumor-to-tumor metastases, collision tumors, or tumors found on the same anatomical site not further categorized by the authors. While most of these cases’ histopathology was described as having an abrupt transition between the two groups of cancer cells—including those classified as collision tumors and tumor-to-tumor metastasis—some cases reported inter-tumor distribution.

Our patient had metastatic deposits in liver, lung, thyroid, and lymph nodes, but the only site with two distinct groups of cancerous cells was the thyroid gland.

Primary colorectal metastases to thyroid carcinomas are rare, and involve invasive oncological processes with patients having multiple sites of metastases. Adopting a standardized approach to describing the microscopic findings would enhance reporting of these occurrences and improve management and follow-up for patients.