How is the cause of pulmonary nodules diagnosed?
Though most lung nodules are not malignant, it is very important that those representing cancer are identified early in their course, when they are curable.
Chest X-rays and CT scans: Usually, the first sign that a pulmonary nodule is present is a spot on the lung that shows up on a chest X-ray or a CT scan. These tests are usually done when a person sees the doctor for a respiratory illness.
If the X-ray film or CT scan indicates there is a pulmonary nodule, your doctor will ask you about your medical history, including whether you have had cancer in the past. He or she will want to know whether you are a smoker or former smoker and about any exposure to environmental chemicals that may be toxic.
The doctor will look at the X-ray to evaluate the size and shape of the nodule, its location, and its general appearance. Single pulmonary nodules seen on chest x-rays are generally at least 8 to 10 millimeters in diameter. If they are smaller than that, they are unlikely to be visible on a chest X-ray. The larger the nodule is, and the more irregularly shaped it is, the more likely it is to be cancerous. Those located in the upper portions of the lung are also more likely to be cancerous.
If you have any older chest X-rays, you should let your doctor look at them to determine the growth rate of the nodule. In general, malignant nodules double in size every one to six months. Nodules with a slower or faster growth rate are less likely to be cancerous.
Your doctor may recommend that you undergo a CT scan to obtain a more detailed image of the nodule, or your nodule may have first been identified by a CT scan. CT scans can give information about the specific features of the nodule, including its shape, size, location and internal density. CT scans are more accurate than chest x-rays in determining the nature of the nodule. A CT scan can find very small nodules, as small as 1-2 mm in diameter.
If the nodule is small enough or if its features suggest a very low likelihood that it represents a cancer, your doctor is likely to follow the nodule over time with repeated chest imaging. If the nodule does not grow over time, it is confirmed to be benign. If a concerning pace of growth is noted, then additional evaluation would be suggested. The interval between scans and the length of follow-up depends on the size of the nodule and the risk of malignancy.
Positron emission tomography (PET): A PET scan can also help to find out if a nodule is malignant or benign. A PET scan uses a radiolabeled substance such as glucose that is absorbed by the nodule, and provides a picture of the nodule’s metabolic activity level. Malignant cells have faster metabolic rates than normal cells, so they require more energy and thus absorb more of the radiolabeled substance. Nodules can light up on PET imaging if they are malignant or if there is active inflammation. We have to be careful with the PET scan interpretation when someone has nodules smaller than 8-10 mm because they are not seen well by PET imaging.
Biopsy: A biopsy is a procedure in which a small tissue sample is removed from the nodule so it can be examined under a microscope. It may be performed when other tests are inconclusive to rule out the chance that a growth is malignant.
There are two ways, short of going through surgery, to collect samples from lung tissue. The method used depends on the size and location of the nodule, as well as the comfort of the medical team with these procedures.
- Bronchoscopy: This procedure is used if it appears the nodule can be reached through the breathing tubes. It uses a bronchoscope, which is a thin, lighted flexible tube that can be inserted into the mouth or nose and through the windpipe (trachea) into the bronchus (airway) of the lung. The bronchoscope has a very small camera at its end. Biopsy tools can be passed through the camera to reach the nodule.
- Needle biopsy (also known as transthoracic needle aspiration): This test is most successful when the nodule is towards the edge of the lung, near the chest wall. A needle is inserted through the chest wall and into the nodule, usually under the guidance of a CT scan.
If the nodule has a very concerning appearance or growth pattern, or it is somewhat concerning and its nature is not able to be clarified by the above tests, the best step may be to remove the nodule. This will clarify its nature while treating it. This requires the patient be fit enough to undergo the surgery.