Overview of Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation
Thyroid cancer is the most common type of cancer that affects the endocrine system, which is the group of glands that produce hormones in the body. This type of cancer makes up about 2% of all cancers in the United States. There are different types of thyroid cancer such as papillary and follicular types which are known as differentiated carcinomas. These types respond well to treatment and have a survival rate of up to 93% within the first ten years.
However, there are types of thyroid cancer that are not as easy to treat. These include medullary and anaplastic subtypes, which are known as poorly differentiated carcinomas. These types of thyroid cancer are more aggressive and only have a survival rate of 10% to 60%.
There are several tools that doctors have to diagnose thyroid cancer. The first step usually involves using an ultrasound and a fine-needle aspiration, which is a type of biopsy where a small needle is used to take a sample of cells from the thyroid. This is especially used for patients who are at high risk, for example, those with a noticeable lump in their neck or those with a family history of cancer.
In certain cases, more detailed imaging methods, such as a PET/CT scan, may be needed. This is particularly the case for more aggressive types of thyroid cancer or when trying to identify if the cancer has spread to other parts of the body.
Anatomy and Physiology of Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation
When it comes to identifying different types of cancer, a technique often used involves the use of radiotracers. These are special substances that highlight particular biological aspects of diseases and are also used to check how the disease is progressing or spreading (called metastasis).
In the case of the thyroid gland, there are cells (called follicular epithelial cells) that take up iodide very effectively. This process is done through a transporter called sodium-iodide symporter. Even in cancerous cells, this transporter can still somewhat work and allows for the successful use of a radioactive form of iodide in treating and diagnosing the disease. But, this isn’t the case for all types of thyroid cancer. Some types, called poorly differentiated carcinomas, don’t act in the same way, which makes their treatment more difficult.
However, we can use another type of radiotracer called F18 fluorodeoxyglucose (18 F FDG) to tackle this issue. This radiotracer works by highlighting areas of high sugar (glucose) use, which is characteristic of fast-growing cancer cells. Interestingly, sometimes certain cancer cells start acting differently, behaving more like poorly differentiated or anaplastic cells (called the “flip-flop phenomenon”), and this also increases their uptake of FDG.
Lastly, there are certain types of thyroid cancers (like medullary carcinoma) that show unique features. They increase production of specific proteins (somatostatin receptors) and a chemical process called decarboxylation of dopamine. Due to these characteristics, radiotracers like F-DOPA and 68-gallium DOTA peptides can be used specifically for their diagnostic imaging.
Why do People Need Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation
If you have already been treated for a specific type of thyroid cancer called differentiated thyroid cancer, the American Thyroid Association (this is a leading group of experts on thyroid health) has some specific guidelines about when to use a particular type of imaging test known as FDG PET/CT. This test is often used when the doctors believe there is a high chance that the cancer may have spread (metastasis) or come back (recurrence). The test could be especially useful if your levels of a protein called thyroglobulin (this protein can increase when thyroid cancer is present) go above 10 ng/ml and another type of scan (radioiodide scan) doesn’t show anything unusual.
The Association also recommends that if you have another type of thyroid cancer known as medullary carcinoma and your calcitonin levels (calcitonin is a hormone produced by the thyroid gland) reach more than 150 pg/ml, you should have further evaluation. Calcitonin levels can increase in some types of thyroid cancer.
Other reasons doctors may advise this type of imaging test are to monitor a tumor that was previously identified by FDG PET/CT. This helps to understand if your treatment is effective or not. The test might also be used if doctors suspect the cancer has spread after you’ve had your thyroid gland removed (total or near-total thyroidectomy) due to an aggressive tumor. Lastly, this test can be used as a second evaluation before surgery if the radioiodide scan doesn’t show any signs of remaining disease after a thyroidectomy.
When a Person Should Avoid Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation
There are no clear reasons why someone shouldn’t have a PET/CT scan. Nevertheless, poor preparation before the scan can lead to it being postponed. For instance, high sugar levels in the blood can reduce how well FDG, a chemical used in the scan, works in the areas that are being studied. Therefore, if a patient’s blood sugar level is over 200 m/dl, it can lessen the accuracy of the test. In this case, the scan will be delayed until the patient can control their blood sugar levels.
Equipment used for Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation
The most frequently used substance in medical imaging is called 18F FDG. This is made in a machine called a cyclotron. The significant aspect of 18F FDG is that it has a relatively long half-life – around 110 minutes. This means it lasts long enough for it to be produced and sent to the place where the study will be carried out on the same day.
Other imaging substances are available for use in clinical studies and research, but their formal use is still a disputed matter.
The imaging procedure requires a device known as a PET/CT scanner. This is a combined system that includes a single table where back-to-back images of the body’s inner structures are taken using PET and CT scanners. This is different from the gamma camera usually used in scans that use radioiodide, as it provides a clearer picture by using high-energy gamma rays emitted from positrons, without needing additional devices, called collimators, to improve the image’s quality.
Furthermore, this process of creating the image, coupled with the cross-sectional CT scan, offers a better understanding of increased activity in specific body structures.
Who is needed to perform Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation?
The images taken by a PET/CT scan machine need to be interpreted by a highly specialized doctor called a radiologist or nuclear medicine physician who has substantial ongoing experience and education in this task. These doctors should have examined at least 150 of these types of scans over a period of 3 years.
These images can be taken by a professionally registered radiographer (a specialist in creating medical images). Also, a medically-trained expert known as a nuclear medicine technologist can take them, or a radiation therapist who is qualified to operate CT and radiopharmaceuticals, which are medicines that contain small amounts of radioactive material.
Lastly, every organization approved to use radiopharmaceuticals for health purposes needs to have a safety radiation officer (SRO). The SRO is in charge of making sure that radiopharmaceuticals are handled properly – from receiving and storing them to safely disposing of them. They also make sure that all the tools used for measuring radiation are working properly and that the use of radiation in the organization is safe.
Preparing for Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation
Before undergoing a medical scan that involves the use of a radiotracer, patients are typically asked to avoid eating or drinking anything for about 4 to 6 hours. This ensures that the radiotracer can work effectively. If you have diabetes, you need to make sure that your blood sugar levels are below 200 mg/dl on the day of your test. Make sure to avoid any strenuous physical activity before the scan to prevent the tracer from going into the muscles instead of where it’s needed.
Patients are also advised to use the bathroom right before the scan. This helps to clear out any background signals in the urinary system that could interfere with the test results. Women who are able to have children should have a pregnancy test before the scan. This is because the radiotracer can potentially harm an unborn baby.
If you are breastfeeding, you do not need to stop before the scan. However, it’s advisable to limit direct contact with your baby for the first 12 hours after the radiotracer has been administered.
How is Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation performed
The recommended dosage of the injected solution for adults is between 370-740 mBq/kg (m stands for milli, Bq stands for Becquerel – a unit to measure radioactivity, and kg stands for kilogram – a unit to measure weight), and for children, it ranges from 5.18-7.4 MBq/kg (M stands for Mega which is a million times, Bq stands for Becquerel and kg stands for kilogram). Doctors advise that this solution should be injected on the side of the body opposite to the site of the disease if possible. Images from inside the body are captured around an hour after the injection.
The images captured cover the whole body, from the base of the skull down to the feet which is especially useful in cases where the site of the original tumor is unknown or when signs of the disease spreading to the other parts of the body are found.
There are two ways the images can be captured: enhanced and non-enhanced. Regardless of the method, every study must include a specific type of image called a CT tomogram or scout views. Non-enhanced studies use a low-dose CT scan followed by a Positron Emission Tomography (PET) scan. On the other hand, the enhanced method includes a whole-body scan which takes 45 to 60 seconds to accommodate for the blood flow in the belly area, followed by a PET scan.
Both PET and CT scans are important to accurately interpret the results, especially when the images are unclear due to bodily movement. Also, normal uptake (process by which substances are absorbed into the body) and infection or inflammation can lower the detection of the solution. Normal uptake can be noticed in the brain, heart muscles, lymphatic system, gut, kidneys, muscles, uterus, ovaries, testes, and a specialized type of fat called brown adipose tissue.
Possible Complications of Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation
A PET/CT scan is a safe medical procedure. It involves using a tiny amount of radioactive drugs, called radiopharmaceuticals, to help capture clear images of the body. These drugs are generally safe and don’t usually have side-effects. However, some patients might have an allergic reaction to the iodine solution used to enhance the visibility of the organs and tissues in the scan. Symptoms can include a skin rash or more serious allergic reactions.
Furthermore, getting an intravenous (IV) line for the scan might result in mild bruising and leakage of the IV fluid into the surrounding tissue, which is referred to as extravasation. These are minor complications that can occur with any procedure involving a needle to draw blood or insert an IV line.
What Else Should I Know About Nuclear Medicine PET/CT Thyroid Cancer Assessment, Protocols, and Interpretation?
PET/CT scans are very helpful in managing thyroid cancer, especially when it comes back or spreads to other parts of your body. This is particularly useful for aggressive types of thyroid cancer that have higher chances of spreading. Essentially, PET/CT scans are great at picking out dangerous tumor cells which may not absorb iodine very well.
Though PET/CT scans don’t play a huge role in monitoring specific types of thyroid cancer, they are quite useful in patients whose scans don’t show any iodine uptake. Research has shown that these scans have more than 80% accuracy in detecting widespread disease. Certain factors like the degree of differentiation (how much cancer cells resemble normal cells), tumor size, and levels of Thyroid Stimulating Hormone (TSH) in the blood can improve the accuracy of these scans. Higher uptakes in the scan generally mean aggressive tumors and bad prognosis.
There are some other types of traces used in PET/CT scans like 18 F DOPA that can detect thyroid cancer. They seem promising, especially in cases of medullary carcinoma (a type of thyroid cancer) that have differentiated well. Although these scans have shown improved results in detecting far-reaching disease, whether or not to use them routinely is still under debate.
Lastly, PET/CT scans have a significant advantage in that they can detect “incidentalomas,” or tumors found by accident while doing scans for other cancers. These accidental findings have a 24% to 36% chance of being cancerous, necessitating further diagnosis with tissue sampling. However, if the scan shows a widespread thyroid uptake, it could just be thyroiditis, or inflammation of your thyroid, and does not require more testing according to Level 3, ATA guidelines.
