While there are multiple etiologic causes of infection, TB and fungal infections (Cryptococcosis, Histoplasmosis, Coccidioidomycosis, Blastomycosis, and Aspergillosis) are the most widely cited as origins of false positive PET/CT findings (Bunyaviroch and Coleman, 2005). A study conducted by Stellwagen et al. (2005) found a correlation between increased FDG activity and the presence of infectious organisms in patients with lymphoma or leukemia. They reported that 22 of 26 patients with a positive scan had at least one organism detected on culture or histopathology. None of the four patients without an identified cause for their increase in FDG activity had positive cultures or results on histopathology.
Other causes of false positive results include inflammation due to other disease processes such as sarcoidosis or radiation therapy, however, these cases are usually recognized by other means before being subjected to a PET scan.
It is important to note that a negative PET scan does not rule out the possibility of active infection. A study conducted by Stellwagen et al. (2005) showed that only about half of all patients who were determined to have an active infection based on other methods had positive scans. This suggests that a negative PET scan may be useful in ruling out active infection if other methods are used to make this determination first. However, a negative PET scan cannot definitively rule out active infection if no other tests are done to confirm the absence of disease.
A CT scan generates twice as many false alarms as an X-ray. They were then tracked for another year. The findings were presented at the American Society of Clinical Oncology's annual meeting. In 33% of patients, the second CT scan yielded false-positive cancer results. In 2% of patients, the scans caused doctors to recommend surgery when there was no cancer.
False positives are readings on testing equipment or images that appear to indicate a problem but don't require further action. They can occur for several reasons, including:
• Random noise in the test equipment can cause random spikes in results. This is called "random error."
• Sometimes there are true changes in health status that the test cannot detect. For example, if a patient has cancer but its cells are not active enough for it to be visible on imaging tests, such as a CT scan. These areas of inactive cancer are called "fibers" or "nodes." A node may or may not contain cancer cells, so this information should be taken into account by any doctor who interprets follow-up CT scans.
• Sometimes there are actual changes in health status that the test does pick up. For example, if a patient has cancer and starts taking chemotherapy treatments, the tumors will begin to shrink, which will make them appear less intense on CT scans.
All medical tests have the potential to produce false positive and false negative results. A false positive can result in wasteful therapy, whereas a false negative can result in a mistaken diagnostic, which is extremely dangerous since a condition has been overlooked. Medical professionals must be aware of these risks and make efforts to minimize them.
FDG is not just a cancer-specific imaging agent; it can also provide false positive findings in cases of benign illnesses. In regions of active inflammation or infection, false positive findings are prevalent (Gupta et al., 2000), with a reported false positive rate of 13% and a false negative rate of 9%. (Alavi et al., 2002). The cause for these false positives is thought to be due to increased glucose metabolism by inflammatory cells. False negatives may occur if the tumor is small enough to be missed on FDG-PET or if it does not produce enough glucose to be detected.
Benign tumors and lesions can also appear on PET scans as FDG uptake depends on the presence of metabolically active tissue. Some of the more common causes of false positives include:
Inflammatory processes such as sarcoidosis, tuberculosis, and histoplasmosis can all show up on PET scans as FDG-avid lesions even though they are noncancerous. Other disorders that can cause false positives include autoimmune diseases such as lupus, scleroderma, and polymyositis, and metabolic conditions such as diabetes, hyperthyroidism, and vitamin D intoxication. False positives can also be seen in patients receiving steroid therapy for inflammatory diseases or post-transplantation.
It is important to distinguish between true positives and false positives because the treatment for these two types of lesions is different.