Can a PET Scan Be Wrong? Understanding False Results

PET scans are among the most powerful imaging tools in medicine, but they are far from infallible. False positives, where the scan flags something that turns out not to be cancer, and false negatives, where it misses disease that is actually present, both occur regularly in clinical practice. The reasons range from ordinary biology to technical quirks of the scanner itself, and understanding them matters if you or someone you know is making treatment decisions based on a PET result.

Why PET Scans Light Up When There Is No Cancer

The most common type of PET scan uses a radioactive sugar called FDG. Cancer cells tend to burn through sugar faster than healthy cells, so the scan highlights areas of high sugar uptake. The catch is that cancer cells are not the only things with a big appetite for glucose. Infections caused by bacteria, fungi, or tuberculosis can look strikingly similar to tumors on a PET scan, as can inflammatory conditions like sarcoidosis, radiation-related lung inflammation after treatment, and even healing surgical sites.¹ Any process that activates your immune system and draws energy-hungry white blood cells to a location can mimic the metabolic signature of a tumor.²

This is not a rare edge case. If you have a recent infection, granulomatous disease, or have had surgery in the weeks before your scan, the radiologist is already watching for these kinds of misleading hot spots. But even with that awareness, distinguishing an inflamed lymph node from a cancerous one based on FDG uptake alone can be genuinely difficult.

When PET Scans Miss Real Cancers

A false negative on a PET scan is arguably more dangerous than a false positive, because it can create false reassurance. Certain cancers are simply less metabolically active and do not consume enough sugar to register clearly on the scan. In breast cancer, for instance, small tumors and low-grade tumors are significantly more likely to be missed. Research has found that tumors measuring 10 mm or smaller and those with lower histologic grades are independently associated with false negative PET results.³

The same principle applies to other cancer types. Some slow-growing tumors, including certain well-differentiated neuroendocrine tumors, low-grade lymphomas, and some forms of prostate and thyroid cancer, may show little FDG uptake. If the cancer cells are not burning through glucose rapidly, the scan has less signal to detect. This is one reason PET is often used alongside other imaging methods rather than as a standalone diagnostic tool, and why a “clean” PET does not always mean you are cancer-free.

PET’s accuracy also varies by the clinical question being asked. For axillary lymph node staging in breast cancer, a systematic review of nine studies found sensitivity of only about 52%, meaning roughly half of truly positive lymph nodes were missed. Specificity was much better at around 92%, and overall accuracy sat near 77%.⁴ Those numbers are a reminder that PET performs differently depending on what you are looking for and where you are looking.

Blood Sugar and Its Surprising Influence

Your blood glucose level at the time of the scan can change how the images look. Since FDG is essentially a sugar molecule, it competes with the glucose already circulating in your blood. High blood sugar means more competition, which can reduce FDG uptake in certain tissues. A large meta-analysis of over 20,000 individual measurements found that elevated blood glucose was significantly correlated with decreased tracer uptake in the brain and muscles, while liver and blood pool uptake went up. In tumors, the effect was less consistent, though patients with blood glucose above 200 mg/dL showed meaningfully lower tumor uptake.⁵

A separate systematic review confirmed this pattern, showing that brain uptake had a strong inverse relationship with blood glucose, while effects on liver and blood pool were milder.⁶ This is why you are told to fast before a PET scan and why diabetic patients sometimes need careful glucose management beforehand. A poorly controlled blood sugar level can make a real lesion appear dimmer than it actually is, increasing the risk of a false negative, or shift background activity in the liver, complicating the reading.

Brown Fat and Other Body Quirks

One of the more surprising sources of false positives is brown fat. Unlike regular white fat, brown fat is metabolically active tissue that your body uses to generate heat. When activated, particularly in cold environments or in younger, leaner patients, brown fat can avidly take up FDG, producing hot spots that look worryingly like cancer. The neck and upper chest are classic locations. One study found that neck fat appeared as bright spots on about 2.3% of scans, with average peak uptake values of 7.7, a level intense enough to be easily confused with a malignant lymph node.⁷ Brown fat activity has also been documented in the mediastinum, along the spine, and around the kidneys.⁸

Beyond brown fat, the body has plenty of normal structures that light up on PET. The brain, heart muscle, and urinary tract routinely show high FDG activity. The digestive tract, thyroid, skeletal muscle, bone marrow, and reproductive organs all show variable uptake that can look abnormal to an untrained eye.⁹ The stomach, liver, spleen, bowel, kidneys, ureters, and bladder all display some degree of FDG uptake as part of normal metabolism and excretion.¹⁰ An experienced reader knows to expect these patterns, but when uptake is unexpectedly intense or asymmetrical, it can trigger further workup.

Metal Implants and Technical Glitches

Modern PET scans are almost always combined with a CT scan (PET/CT). The CT portion helps the system correct for how much radiation the body’s tissues absorb, a process called attenuation correction. Metallic implants like hip replacements, dental hardware, or surgical clips throw a wrench into this process. The metal creates bright streaking artifacts on the CT images, and those artifacts carry over into the PET images, making it look like there is abnormally high tracer uptake near the implant.¹¹ Hip prostheses are a well-known culprit, generating artifacts that can mimic increased FDG uptake in the pelvis.¹²

Breathing during the scan is another technical pitfall. PET and CT images are acquired at different speeds: the CT is a quick snapshot, while the PET data is collected over minutes. If you breathe normally during PET acquisition and then take a breath-hold during CT, structures near the diaphragm may end up in slightly different positions on the two image sets. Research has documented cases where liver dome lesions were mislocalized to the right lung base on PET/CT because of exactly this kind of respiratory mismatch.¹³ A lesion that appears to be in the lung rather than the liver could lead to a very different clinical path.

Medications That Alter What the Scan Shows

Several commonly used medications can change PET scan appearances in ways that create false readings. Among the most significant are growth factor injections like pegfilgrastim and filgrastim (G-CSF), which are routinely given to cancer patients after chemotherapy to boost white blood cell production. These drugs rev up the bone marrow, and all that metabolic activity lights up on PET. Pegfilgrastim in particular has been shown to increase FDG uptake in both the bone marrow and the spleen.¹⁴ In one case report, the patchy bone marrow activity caused by G-CSF was initially mistaken for widespread bone metastases.¹⁵

The timing between these injections and your PET scan matters a great deal. If the scan is performed too soon after a growth factor dose, the bone marrow signal may be so bright that it obscures or mimics real disease. This is why oncologists and nuclear medicine physicians coordinate scan scheduling with chemotherapy cycles.

Exercise Before Your Scan Can Muddy the Picture

Physical activity before a PET scan is another factor that most patients do not think about. When you exercise a muscle, it burns glucose at a much higher rate. That elevated metabolism shows up on the scan as increased tracer uptake in the muscles you used. In one study, strenuous exercise with a single arm produced a roughly 200% increase in tracer uptake in the activated biceps compared to the resting arm, and even muscles used incidentally to maintain posture showed uptake increases of more than 100%.¹⁶ This asymmetry can make a scan harder to interpret and may create false hot spots in areas where the radiologist would normally not expect to see activity. You are typically advised to rest quietly and avoid exercise for at least 24 hours before the scan for this reason.

Scans After Radiation Therapy

One of the trickiest clinical scenarios for PET is distinguishing cancer recurrence from radiation necrosis, the tissue death and inflammation that radiation treatment itself causes. Both conditions involve metabolic activity, and on a standard FDG PET scan, they can look remarkably similar. This is a particular concern in brain imaging after radiosurgery, where a new or growing area of enhancement on MRI could be either recurrent tumor or treatment-related damage. Specialized tracers like FET (fluoroethyl-tyrosine) are sometimes used in these cases to help differentiate the two when standard MRI findings are inconclusive.¹⁷

The general rule is that PET performed too soon after treatment is more likely to give a misleading result. Residual inflammation from surgery, chemotherapy, or radiation can persist for weeks to months, and all of it can produce FDG-avid signals that look suspicious. Timing protocols vary by cancer type and treatment, but most guidelines recommend waiting a defined interval after therapy before scanning.

Newer Tracers Have Their Own Blind Spots

FDG is not the only tracer available. In prostate cancer, PSMA-targeted tracers have become increasingly important. These tracers bind to prostate-specific membrane antigen, a protein overexpressed on prostate cancer cells, offering much better sensitivity for prostate cancer than FDG does. But PSMA tracers have their own false positive problem. PSMA is not expressed exclusively by prostate cancer cells; it appears in some benign bone lesions, healing fractures, certain non-cancerous tumors, and inflammatory conditions.

In one study of patients who had already had their prostates removed and had undetectable PSA levels, meaning they should have been cancer-free, over 80% showed at least one focus of “non-physiological” PSMA uptake on their scans, including spots in bones and soft tissue.¹⁸ A prospective multi-center trial found false positive PSMA results in about 8% of patients who had their findings validated. Most of those false positives occurred in patients who had received radiation therapy, where residual treated adenocarcinoma with treatment effects was the main source of confusion. False negatives occurred at a similar rate, primarily because of small metastases or uptake obscured by nearby bladder activity.¹⁹ Familiarity with the normal distribution of PSMA tracer and its known pitfalls is essential for accurate interpretation.²⁰

PET in Dementia and Brain Imaging

PET scans are also used to evaluate neurodegenerative diseases, particularly Alzheimer’s disease, using tracers that bind to amyloid plaques in the brain. These amyloid PET scans can detect the protein deposits that characterize Alzheimer’s with high sensitivity. However, they come with a notable limitation: roughly 20% to 30% of cognitively normal older adults show positive amyloid scans, especially those carrying the ApoE4 genetic variant. A positive amyloid PET does not necessarily mean Alzheimer’s disease is the cause of a patient’s symptoms, because amyloid positivity can be age-related or incidental alongside a different type of dementia.²¹

When amyloid PET and FDG PET are used together, the combination performs better. In an autopsy-confirmed study of dementia patients, amyloid PET alone had 96% sensitivity for detecting Alzheimer’s pathology but 86% specificity. When both scans agreed, the combined sensitivity reached 97% and specificity climbed to 98%. About a quarter of cases showed disagreement between the two scans, and in many of those cases the patients had mixed pathologies, with Alzheimer’s coexisting alongside other neurodegenerative conditions.²² The takeaway is that even in brain imaging, a single PET scan rarely tells the whole story.

Children and Young Adults Face Different Pitfalls

PET scans in younger patients have their own set of interpretive challenges. The thymus, a gland in the upper chest that is large and active in children, naturally takes up FDG. Thymic uptake was found in 80% of patients younger than 10 years, compared to only 8% of those in the 31-to-40 age group. After chemotherapy, the thymus can enlarge further (a phenomenon called thymic rebound or hyperplasia), creating even brighter signals that can be confused with residual or recurrent disease in the chest.²³

Routine PET surveillance after treatment in pediatric cancers has also come under scrutiny because of high false positive rates. In a review of pediatric Hodgkin disease patients, PET scans obtained during routine follow-up had a positive predictive value of only 11%, with identifiable causes of false positives including fibrosis, thymus activity, and unrelated conditions like appendicitis or abdominal hernias.²⁴ An 11% positive predictive value means that for every scan that flagged something suspicious during routine surveillance, it was right only about one in nine times. That kind of false alarm rate raises serious questions about whether routine surveillance PET is worthwhile in this population, given the anxiety and follow-up procedures it generates.

Human Readers Disagree More Than You Might Expect

Even when the scan itself produces a clear image, two radiologists looking at the same PET can arrive at different conclusions. In a study evaluating PET scans for recurrent laryngeal cancer after radiation, interobserver agreement was only moderate.²⁵ Post-treatment scans are especially vulnerable to reader disagreement, because the visual landscape is complicated by treatment effects and residual changes in tissue. When there is ambiguity in the image, individual radiologists bring different thresholds for calling something suspicious versus benign, and those differences affect patient management.

This is one reason second opinions on PET scans are sometimes recommended, particularly when a result would change your treatment plan. It is also why clinical context matters so much: a radiologist who knows your full medical history, recent treatments, and symptoms is better positioned to distinguish artifact from disease than one reading the scan in isolation.

Can PET/MRI Reduce False Results?

One technological evolution aimed at reducing ambiguous PET findings is PET/MRI, which pairs the metabolic information of PET with the soft-tissue detail of MRI instead of CT. A systematic review and meta-analysis comparing PET/MRI to PET/CT found broadly comparable or superior performance across multiple cancer types. For detecting liver metastases, PET/MRI showed dramatically higher sensitivity, and in breast and colorectal cancer staging, accuracy differences favored PET/MRI by wide margins.²⁶

A separate study found that PET/MRI reduced the rate of indeterminate findings — cases where the radiologist could not say definitively whether something was benign or malignant — compared to PET/CT. Fewer indeterminate findings meant fewer additional diagnostic tests, biopsies, or anxious waiting periods for patients.²⁷ PET/MRI is not yet widely available and comes with its own challenges, including longer scan times and higher cost, but it represents a meaningful step toward reducing the gray areas that lead to false results.

The Downstream Costs of Getting It Wrong

False PET results are not just academic curiosities. A false positive in melanoma staging, for example, can lead to unnecessary biopsies, additional imaging, treatment delays, and significant patient distress.²⁸ A false negative can do worse: patients may be incorrectly reassured that their cancer has not spread, potentially missing the window for more aggressive treatment.

The anxiety dimension is often underappreciated. Patients undergoing PET scans for cancer already experience moderate to high levels of anxiety, driven by concern about results, the clinical situation itself, and the uncertainty of the waiting period. When scans produce ambiguous or false findings, that anxiety is extended or amplified through additional testing cycles. Clinicians increasingly recognize that the psychological toll of imaging needs to be weighed against its diagnostic value, particularly when scans are ordered as part of routine surveillance rather than in response to specific clinical concerns.

When PET Scans Perform at Their Best

Understanding PET’s limitations is useful, but it is equally important to recognize that the technology remains extraordinarily valuable when used in the right context. PET scans are most reliable when evaluating metabolically active cancers of reasonable size, when the patient has been properly prepared with fasting and blood sugar management, when adequate time has passed since treatment, and when the radiologist has full access to the patient’s clinical history. The accuracy improves further when PET is combined with other imaging modalities, either through hybrid PET/CT or PET/MRI, or through correlation with separate MRI, ultrasound, or biopsy results. No single imaging test provides a perfect answer on its own, and the most accurate diagnoses come from integrating PET findings into the broader clinical picture rather than treating any single scan as the final word.