Solitary Pulmonary Nodule
SPN Exams and Tests continued...
A tuberculin skin test is used to help determine whether the SPN has been caused by the bacteria Mycobacterium tuberculosis. The test involves injecting the tuberculin antigen (a substance that triggers the immune system to produce cells that attack and try to destroy the antigen) into the skin and observing the body’s response. If the SPN has been caused by tuberculosis, the injection site swells and reddens.
- Because SPNs are first detected on chest X-rays, ascertaining whether the nodule is in the lung or outside it is important. A chest X-ray taken from a side position, fluoroscopy, or a CT scan may help confirm the location of the nodule.
- Although nodules of 5 mm diameter are occasionally found on chest X-rays, SPNs are often 8-10 mm in diameter.
- Patients who have an older chest X-ray should show it to their health care provider for comparison. This is important because the growth rate of a nodule can be determined. The doubling time of most malignant SPNs is one to six months, and any nodule that grows more slowly or more rapidly is likely to be benign.
- Chest X-rays can provide information regarding size, shape, cavitation, growth rate, and calcification pattern. All of these features can help determine whether the lesion is benign or malignant. However, none of these features is entirely specific for lung cancer.
- Characteristics that may help establish the diagnosis with reasonable certainty include (1) a benign pattern of calcification, (2) a growth rate that is either too slow or too fast to be lung cancer, (3) a specific shape or appearance of the nodule consistent with that of a benign lesion, and (4) unequivocal evidence of another benign disease process.
- The CT scan is an invaluable aid in identifying features of the nodule and determining the likelihood of cancer. In addition to the features seen on a chest X-ray, a CT scan of the chest allows better assessment of the nodule. The advantages of a CT scan over chest X-ray include the following:
- Better resolution: Nodules as small as 3-4 mm can be detected. Features of the SPN are better visualized on CT scan, thereby aiding the diagnosis.
- Better localization: A nodule's location can be more accurately determined.
- Areas that are difficult to assess on X-rays are visualized better on a CT scan.
- CT scanning provides more details of the internal structures and more readily shows calcifications.
- If the CT scan demonstrates fat within the nodule, the lesion is benign. This is specific for a benign lesion.
- CT scanning helps distinguish between a neoplastic abnormality and an infective abnormality.