What are the differentials for the incidental finding in the left neck?
The findings most likely represent a synchronous primary thyroid neoplasm with locoregional nodal involvement, favoured a papillary carcinoma given the early metastasis to the local lymph nodes and patient age. The main differential would be a colorectal metastatic disease to the thyroid and neck lymph nodes which, although previously described, is much rarer than having a asynchronous primary tumour.
What is the next step?
Ultrasound of the neck and FNA of the lesions.
Selected images of an FDG-PET CT study demonstrates a region of increased uptake in the left thyroid lobe and also some uptake within some ipsilateral lymph nodes. The remainder of the study (not shown) showed uptake in the rectum/sigmoid (the primary tumour), in perirectal lymph nodes, and some peritoneal implants.