Chest CT can be used both for screening and diagnostic purposes in the context of lung cancer, but the findings will be approached differently depending on why you're ordering the scan. In this video, you'll find out what to look for, why a chest CT can completely miss cancer, and the 4 clues to look out for when evaluating non-specific lesions.
Join our Chest CT Essentials course now!
Master this step-by-step process for diagnosing the most common cardiothoracic diseases using chest CT. The complexity of the pathophysiology revealed by chest CT can be overwhelming. This course will teach you the step-by-step process to reading a chest CT, how to categorize and understand your findings, and the patterns of findings seen in the most common cardiothoracic diseases. By the end of this course, you will know the 20% of radiology needed to understand 80% of your findings on chest CT. Start the course now!
All imaging falls into one of two categories, screening or diagnostic. Screening tests look for disease in a healthy person who has no history or symptoms of that disease. They're useful in finding diseases early when treatment may work better. Diagnostic imaging evaluates symptoms that may indicate an underlying disease, or further defines a known disease.
Knowing the difference between these two types of exams can help you determine what scan to get and how to interpret the findings. Let's discuss these two types of imaging tests in the context of lung cancer evaluation. We screen for lung cancer in patients who are asymptomatic, but at high risk for the disease.
The most common example of this is cigarette smokers, who are at high risk for developing small cancerous lung nodules. The US Preventive Services Task Force recommends an annual screening CT to look for small lung nodules in anyone aged 50 to 80 years, who has a 20 pack year smoking history and currently smokes or who has quit within the past 15 years.
A screening chest CT is done with a lower radiation dose than a standard CT, a technique known as low-dose CT. The screening exam is also done without intravenous contrast. Due to the decreased radiation, a low dose CT image is noisier and the image quality isn't as good. As such, low-dose CT should not be used to evaluate fine structures such as the interstitium.
But, they are good enough when our only question is whether or not a nodule is present. Once a nodule is found on the screening scan, it can be followed with the same low radiation non-contrast technique to see whether it changes in size. When people have signs or symptoms that suggest cancer, such as unexplained weight loss, new ptosis, or chronic cough, but they do not have a known cancer, we want to look for a source of those symptoms.
We look for cancer, but we keep in mind other potential causes of these symptoms. Since for the symptomatic patients, we're not only looking for small lung nodules, we do a diagnostic CT, also known as a Standard CT with full radiation and contrast doses. The diagnostic CT will give us the best view of all the anatomy, and allow for a thorough evaluation.
If a chest CT is performed on a patient with signs or symptoms suggestive of cancer, but it doesn't reveal a suspicious lesion, it can mean that either the cause of the lesion is not in the chest, or chest CT is not the appropriate diagnostic test. An example of the lesion not being in the chest is when an esophageal mass is located in the pharynx.
In this case, a chest CT will show a normal appearing esophagus in a patient with esophageal cancer. The CT is simply too low to see the tumor. A neck CT would be more appropriate. An example of CT not being an appropriate test, is when diagnosing esophageal or thyroid cancers for which CT is relatively insensitive.
This CT image shows a normal appearing thyroid with a cyst, yet the patient actually has thyroid cancer. The American College of Radiology appropriateness guidelines can be very helpful in choosing the correct imaging exam for your specific patient. If a chest CT performed on a patient with signs or symptoms suggestive of cancer and a lesion is found, the first question to ask is whether it's benign or nonspecific.
Recall from our previous discussion of pulmonary nodules that CT cannot diagnose whether nodule is malignant or not. If characteristics specific to benign nodules are present, such as stability over time, we can call it benign. Otherwise, we call it nonspecific. That is, it may be benign or it may not be.
The majority of neoplastic lesions seen on a single CT scan, without prior CT's to indicate stability or growth, are nonspecific in appearance. For example, although most spiculated soft tissue nodules are cancer, 15 percent or more can be benign post-inflammatory scarring, particularly in smokers. Likewise, although the majority of non-calcified soft tissue nodules that are less than 10 millimeters are benign, many turn out to be neoplastic.
Nonspecific lesions often require further evaluation, especially if they have worrisome features. So what makes a nonspecific lesion worrisome? There are four characteristics to look out for when evaluating nonspecific lesions. The first one is the fact that the finding is new. Although many ideologies can lead to new findings, including infection, inflammation, and simple aging, new is the opposite of the most benign characteristic stability. So if a finding is new, it needs further evaluation.
Whether this is by simple surveillance imaging or by other means will be defined by the specific clinical scenario. The second concerning characteristic is if the lesion is growing. Growth itself doesn't always mean cancer. A very rapidly enlarged nodule, or mass that has a doubling time of less than 30 days, is most likely inflammatory and benign.
A slow growing nodule or mass that doubling time of greater than 450 days, is also most likely benign. But, if a lesion shows growth between these rates, particularly if their rate of growth increases over time, further evaluation is warranted. The third concerning characteristic is invasion into adjacent structures. If a lesion is displacing the structure but not invading it, it may be benign, such as a reactive lymph node, or malignant, such as lymphoma.
The only time a benign lesion will invade an adjacent structure is in the case of an infection. So, if there is destruction of any part of a normal structure, and no evidence of infection, be very concerned that this is a neoplasm. The fourth concerning characteristic is the unexplained enlargement of a normal structure. On CT, some cancers can look identical to the tissues that surround them. In these cases, the unexplained increase in size signals that something may be afoot.
Examples of this include asymmetric thyroid enlargement, vocal bulging in the muscle, and asymmetric thickening of the gastroesophageal junction. The sequential CT images show an enlarging bronchus, that actually represents an esthetic chondrosarcoma. Although any of these four worrisome and characteristics may be benign, they may also be malignant. The lesion must be considered nonspecific and further workup must be pursued.