Ultrasound is the preferred screening modality for thyroid nodules and is required for almost all patients. However, ultrasound is not a panacea, and its results are highly subjective and closely related to the surgeon’s experience, approach, and instrumentation. If a doctor is considering removing a nodule, a more “objective” assessment is often needed to evaluate the condition in its entirety during the preoperative evaluation. This is where CT comes in.
What are the advantages of CT?
Thyroid CT is the most preferred “objective” method of evaluation. It has the following advantages:
Clear
CT clearly shows the thyroid gland and the location, size, and relationship of the nodule to the surrounding tissue.
The anatomy of the neck is very complex, with a variety of important tissues and structures surrounding the thyroid, and some thyroid cancers may invade surrounding organs, and CT is the best way to evaluate precisely these risks.
Complete
Ultrasound can only “see” the part of the nodule in the neck because the sternum and clavicle are obscured, and some thyroid tumors may “fall” into the chest cavity because of gravity, and ultrasound alone may underestimate the extent of the lesion and misjudge the surgical approach. In this case, CT is also needed to give a full picture of the thyroid gland.
Accurate assessment of cervical lymph node metastases
Another important component of CT evaluation is the presence of metastases in the cervical lymph nodes.
Lymph nodes that metastasize from thyroid cancer may be located both around the gland and along the internal jugular vein. On ultrasound, some of the lymph nodes may not be explored due to the obscuration of layers of tissue, resulting in a missed diagnosis and possibly missing this treatment in patients who should have had a lymph node dissection in the neck.
Unlike CT, however, it can fully visualize all tissue structures in a profile without fear of obscuring other tissues. For example, a CT showing liquefied, necrotic, calcified lymph nodes is often a sign of metastatic thyroid cancer lymph nodes, suggesting the need for lymph node dissection.
In view of these advantages, the American Thyroid Association (ATA) guidelines state that CT is strongly recommended for patients with clinically advanced thyroid cancer with extensive lymph node metastases. That is, patients with earlier-stage thyroid cancer do not always need CT, but those who may have lymph node metastases should have CT.
In addition to CT of the thyroid, some patients also need CT of other sites, mainly to look for distant metastases.
Do you want to do an “enhanced” or “plain” scan?
Do you want to do a CT scan?
When you get a CT, you may hear “plain” or “enhanced,” depending on whether contrast is used. So which one should you choose?
The doctor’s recommendation is: an enhanced CT (CT with contrast). It shows the tissue structure of the neck more clearly, helping doctors determine the benignity of the tumor and more accurately assess the condition.
The contrast agent currently used is an iodine-containing reagent that, when injected into the body, clears completely within 4 to 8 weeks without concern for interfering with subsequent isotope therapy.
The majority of patients can safely undergo enhanced CT, but there are some patients who cannot undergo this test. For example, if you have “hyperthyroidism” (“hyperthyroidism”) that is not properly treated, or if you are allergic to contrast, you may need to consider CT scan or magnetic resonance imaging (MRI). In particular, if you have not had an enhanced CT before and do not know if you are allergic to contrast, you should first have a skin test. If the skin test is positive (allergic to the contrast agent), you will not be able to do this test. Do not underestimate contrast allergy; it can have serious consequences.
Summary:
After the ultrasound, your doctor may also recommend a thyroid CT during the preoperative evaluation to help develop an accurate treatment plan. Your doctor will also determine if CT of other areas is needed based on your actual condition.
Co-written by Dr. Naisi Huang, Fudan University Cancer Hospital