Metachronous Collision Tumors of the Thyroid Gland Unveiled in Real Time by a Change in the Thyroid Nodule Phenotype

Introduction: The term collision tumor refers to the coexistence of two histologically distinct neoplastic tumors within the same mass. Collision tumor of the thyroid (also referred to as tumor-to-tumor metastasis) with metastatic colorectal carcinoma to a preexisting thyroid adenomatoid nodule is rare. Preoperative diagnosis of this disorder is exceedingly difficult. This report highlights the rare occurrence of a tumor-to-tumor metastasis of colorectal cancer to a preexisting thyroid adenomatoid nodule unveiled in real time by serial positron emission tomography CT scans (PET/CT). Since cytological diagnosis of these lesions is often insufficient, we present a stepwise diagnostic approach with a combination of investigations in the preoperative setting. The patient management is reviewed together with a rationalized use of targeted therapy based on the genetic status of the tumor. Methods: Ultrasound (US)-guided fine needle aspiration (FNA) biopsies of the collision tumor of thyroid were performed and analyzed for cytologic, immunologic, and molecular tumor phenotyping. KRAS gene analysis of the metastatic lesion was also performed. Results: The samples revealed adenocarcinoma of the colon (Tg-negative, TTF-1-negative, HBME-1-negative, galectin-3 negative, CEA-positive, CK-20 positive) intense glucose transporter-1(GLUT-1) expression and KRAS positivity. The patient was then brought to surgery for a left hemithyroidectomy and ipsilateral central compartment node dissection. Final histology revealed metastatic colonic carcinoma, invading the benign adenomatoid nodule. His recovery was uneventful and the post-op chemotherapeutic regimen was guided by the KRAS analysis of the