PET scan results
What Causes a — False Positive on a PET Scan?
A bright spot on a PET scan can come from many sources besides cancer. Before your team can tell you what a finding means, they need to work out which source it is — and that investigation follows a clear, ordered path.
Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed September 2026
- Glucose uptake, not cancer — The scan measures how fast a tissue uses glucose. Cancer uses it intensely, but so does infection, inflammation, and several normal body processes.
- Many causes mimic cancer on PET — Tuberculosis, sarcoidosis, healing wounds, and brown fat can all produce bright spots that your radiologist must interpret carefully.
- India's TB burden is relevant — In a country with high tuberculosis rates, TB-related lymph node uptake is one of the most common reasons a scan needs further investigation.
- Only tissue confirms the diagnosis — A suspicious finding on a scan is the start of an investigation, not a diagnosis. Tissue biopsy is the only way to confirm or rule out cancer.
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A PET scan lights up anywhere glucose is used intensely — not only in cancer. Infection, inflammation, healing tissue, brown fat, and normal organs such as the brain and bladder all produce uptake. A finding that is not clearly cancer is resolved through clinical review and, where needed, tissue diagnosis.
CION's PET-CT scans in Hyderabad start at Rs 10,499 — among the lowest published prices in the city, with no hidden charges. Indicative price, as of September 2026.
Why does a PET scan light up for things that are not cancer?
A PET scan uses a glucose-based tracer called FDG. Any tissue that is actively using glucose will take it up — cancer cells do this because they grow rapidly, but so does every other tissue with high metabolic activity.
This is the core limitation of the technology. It is excellent at detecting areas of intense activity, but it cannot tell you on its own why that area is active.
Your radiologist interprets the scan alongside your CT images, your clinical history, and your blood results. A suspicious finding is the beginning of an investigation, not a conclusion.
What else can light up on a PET scan?
- Active infectionTuberculosis, pneumonia, fungal infections, and abscesses all cause intense FDG uptake. In India, TB is one of the most frequent reasons a lymph node lights up on a cancer staging scan.
- Inflammation without infectionSarcoidosis, autoimmune conditions, and inflammatory bowel disease can produce bright areas that closely resemble metastatic spread.
- Healing tissue after surgery or radiationAn operated site or a previously radiated area causes local inflammation for months. It often shows uptake on the first post-treatment scan, even when there is no remaining cancer.
- Brown fat activityBrown adipose tissue, concentrated in the neck, shoulders, and upper chest, generates heat by burning glucose. It is more active in cold or anxious patients and has a characteristic symmetrical appearance.
- Normal physiological uptakeThe brain, heart, kidneys, ureters, and bladder always show uptake. These are expected findings — but they can sometimes make it harder to assess a nearby area.
- Muscle tension during the scanMuscles that were active or tense in the period before the scan — from talking, chewing, or anxiety — can take up tracer and appear on the images.
- Certain benign tumoursSome non-cancerous growths, including certain thyroid nodules and adrenal adenomas, are FDG-avid and may be flagged for further investigation.
How often do false positives happen on PET scans?
There is no single figure that applies across all cancers and all body sites. False positive rates vary considerably by cancer type, the region being scanned, and the local burden of infection and inflammation.
ESMO and NCCN both note that the specificity of FDG-PET — its ability to correctly identify non-cancerous findings — is reduced in settings where granulomatous infections such as tuberculosis are common. India sits in this category, and your oncologist will factor this into how they interpret your scan.
What matters more than a population figure is your specific situation. Ask your oncologist to explain the likelihood that this particular finding, in this location, in your clinical context, is something other than cancer.
PET-CT Partner Centres
PET-CT Scan Centres in Hyderabad
CION offers PET-CT scans through 4 trusted partner PET-CT centres across Hyderabad, so you can choose the one closest to you. Call 18002028726 and we’ll guide you to the earliest available appointment.
PET-CT Centre — Punjagutta
PET-CT Centre — Himayatnagar
PET-CT Centre — Narayanaguda
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How does your team work out what a suspicious finding is?
Radiologist review
Your radiologist examines the pattern, intensity, and location of uptake against the known signatures of physiological and benign causes. Many are identified at this stage without further testing.
Clinical correlation
Your oncologist places the finding in context — your cancer type, your symptoms, any recent procedures, and whether there is a history of infection or inflammatory disease.
Additional imaging
A dedicated CT, MRI, or ultrasound can show whether the finding has structural features that suggest cancer or point towards an alternative explanation.
Repeat PET after an interval
Some findings are rescanned after a short period. A persistent or growing area carries more weight than one that has reduced or resolved.
Tissue biopsy
When doubt remains, a sample from the suspicious area is the definitive step. A scan cannot confirm cancer; tissue diagnosis can.
What should you do while your team investigates?
Waiting is genuinely hard. A finding under investigation is not a confirmed diagnosis — it is an unanswered question your team is working to answer.
Ask your oncologist two direct questions: what do you think this finding is, and what is the next step to confirm it? A clear answer to both is reasonable to expect at any appointment.
If the plan involves further imaging or a biopsy, ask how long each step will take. You should not be waiting without knowing what you are waiting for.
Did you know?
Tuberculosis-infected lymph nodes take up FDG intensely and can be indistinguishable from metastatic cancer on the scan image alone.
In a country with high TB prevalence, this is not a rare edge case — it is a routine part of how PET scan findings are interpreted in Indian oncology practice.
Source: ESMO Clinical Practice Guidelines: FDG-PET in oncology; WHO Global Tuberculosis Report
CION does PET-CT in Hyderabad from Rs 10,499 — among the lowest published prices in the city — including an oncologist-reviewed report and a free Rs 950 consultation. Indicative price, as of September 2026.
Explore 94 more Understanding Your PET-CT Report topics
False Positives, False Negatives & Pitfalls
- Bladder, Kidney and Ureter Activity on PET
- Bone Marrow Uptake After Growth Factor Injections
- Bowel Uptake: Normal, Inflammation or Something Else?
- Brown Fat Uptake: A Common Cause of Confusing Findings
- Does a Suspicious PET Finding Always Need a Biopsy?
- Getting a Second Read of Your PET-CT Images
- How Often Is a PET-CT Scan Wrong?
- Muscle Uptake After Exercise or Injections
- Thymic and Rebound Uptake in Younger Patients
- Uptake After Radiotherapy: Inflammation or Disease?
- Uptake at a Biopsy or Surgery Site
- What Causes a False Negative on a PET Scan?
- What Causes a False Positive on a PET Scan?
- What Happens If Two Scans Disagree?
- Which Cancers Do Not Show Well on PET-CT?
- Why the Brain Always Lights Up on PET
Incidental Findings
- A Second Unrelated Cancer Found on PET-CT
- An Incidental Bone Finding on PET-CT
- An Incidental Breast Finding on PET-CT
- An Incidental Kidney or Adrenal Finding
- An Incidental Liver Lesion on PET-CT
- An Incidental Lung Nodule Found on PET-CT
- An Incidental Prostate Finding on PET-CT
- An Incidental Thyroid Nodule on Your PET Scan
- Do All Incidental Findings Need Investigation?
- Focal Bowel Uptake: Could It Be a Polyp?
- Incidental Findings on a PET-CT Scan: What They Are
- Incidental Heart Findings on a Cancer PET Scan
- The Cost of Chasing an Incidental Finding
- What to Ask Your Doctor About an Incidental Finding
Infection, TB & Inflammation on PET
- Can Tuberculosis Look Like Cancer on a PET Scan?
- Dental Infection and Jaw Uptake on PET
- Fungal Infection and Abscess on PET-CT
- How Long Should You Wait After an Infection Before a PET Scan?
- If It Might Be Infection, What Test Comes Next?
- Inflammation After Injections, Vaccines and Cannulas
- Old Healed TB and Calcified Nodes on PET-CT
- PET-CT for Fever of Unknown Origin
- PET-CT for Suspected Infection in Implants and Prostheses
- PET-CT for Vasculitis and Inflammatory Conditions
- Pneumonia and Recent Chest Infection on PET
- Sarcoidosis on PET-CT: The Great Mimic
- TB or Cancer? How Doctors Tell Them Apart
- Thyroid Uptake on PET: Diffuse vs Focal
Reading Your Report: Core Terms
- 'Cannot Be Excluded' and Other Cautious Phrases
- 'Deauville Score' on a Lymphoma PET Report
- 'FDG Avid': What It Means on Your Report
- 'Hypermetabolic': What the Word Actually Means
- 'Hypometabolic' and 'Photopenic' Findings
- 'Impression' vs 'Findings': Which Section Should You Read?
- 'Interval Change' and 'Stable Since Previous Study'
- 'Krenning Score' on a DOTANOC or DOTATATE Report
- 'Lymphadenopathy' on Your PET Report
- 'Metabolically Active Disease': What It Tells You
- 'Metastasis' and 'Distant Spread' on Your Report
- 'Mildly FDG Avid' vs 'Intensely FDG Avid'
- 'No Abnormal FDG Uptake': Is That a Clear Scan?
- 'Physiological Uptake': Normal Areas That Always Light Up
- 'Residual Disease' vs 'Complete Metabolic Response'
- 'Sub-Centimetre Node' and Why Size Matters on PET
- 'Suspicious for Malignancy': How Certain Is That?
- How Long Does a PET-CT Report Take?
- How to Read Your PET-CT Report: A Section-by-Section Guide
- Should You Read Your Own PET-CT Report Before the Consult?
- TNM Staging on a PET-CT Report
- Why Your Report Mentions the Liver and Blood Pool
Response Assessment Scores & Criteria
- Complete Metabolic Response: What It Means
- Deauville Score 1 to 5: The Lymphoma Scale Explained
- End-of-Treatment PET Scan: What It Decides
- How Doctors Judge Whether Treatment Is Working on PET-CT
- Interim PET Scan During Chemotherapy: Why It Is Done
- Metabolic Response vs Size Response: Why They Differ
- PERCIST Criteria: How Metabolic Response Is Measured
- Partial Metabolic Response: Is That Good Enough?
- Progressive Metabolic Disease: What Happens Next
- RECIST vs PERCIST: Size vs Activity
- Stable Metabolic Disease: Good News or Bad?
- Tumour Flare and Pseudoprogression on PET
- What Is a Deauville 3 and Why Is It Ambiguous?
- Why the Scan Says One Thing and You Feel Another
SUV Values Explained
- Comparing SUV Between Two Scans: Is It Meaningful?
- Does a Higher SUV Mean a More Aggressive Cancer?
- Factors That Change Your SUV Reading
- High SUV but Not Cancer: When the Number Misleads
- Is My SUVmax of 4 High? Reading Your Specific Number
- Low SUV but Still Cancer: The PET-Negative Cancers
- SUV in Non-Cancer Conditions
- SUVmax, SUVmean and SUVpeak: What Is the Difference?
- What Is SUV on a PET Scan?
- What Is a Normal SUV Value?
- What SUV Value Indicates Cancer?
- Why Blood Sugar Changes Your SUV Result
- Why Did My SUV Go Down but Not to Zero?
- Why Did My SUV Go Up After Treatment?
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Common questions
Frequently asked questions
What is the most common cause of a false positive PET scan?
In India, active tuberculosis and TB-related lymph node changes are among the most frequently encountered causes. Brown fat is a common cause everywhere and is usually the first thing a radiologist rules out when uptake appears symmetrically in the neck or shoulders. Sarcoidosis and post-procedural inflammation are also frequent. The answer depends on your clinical context — which is why your radiologist and oncologist interpret the finding together rather than relying on the scan image alone.
Can tuberculosis cause a false positive PET scan?
Yes, and this is well recognised in guidance from ESMO. TB-infected lymph nodes take up FDG intensely and can be indistinguishable from cancer spread on the scan image. If TB is suspected — even in someone already being treated for cancer — your team may need to test specifically for it before concluding that a new lymph node finding represents disease progression. This is a clinical judgement that requires your full history, not a limitation of the scan.
Does brown fat always mean the result is a false positive?
Brown fat uptake has a characteristic appearance — typically symmetrical, in the neck, shoulders, supraclavicular regions, and upper chest — that an experienced radiologist usually recognises. It is not a sign of cancer. If brown fat uptake obscured an area your team needed to assess, a repeat scan with steps taken to suppress it — including keeping you warm during the uptake period — can usually be arranged. Your radiologist will advise whether that is needed in your case.
How long does it take to resolve a suspicious PET finding?
It depends on which step is needed. A radiologist's review of the images happens quickly. Additional CT or MRI results may be available within a few days. A biopsy, including laboratory analysis of the sample, typically takes one to two weeks from the procedure date. If the plan your team has given you does not include a timeline, ask for one. You should not be waiting without knowing what each step is and roughly how long it will take.
Should I get a second opinion on a suspicious PET result?
A second opinion is a reasonable step, particularly if the finding is in an unusual location, if there is genuine uncertainty between teams, or if the recommended next step is a significant intervention such as stopping current treatment or proceeding to major surgery. Ask for the images to be reviewed by a nuclear medicine physician or a radiologist with specific PET expertise. Your oncologist should support this request — it is a standard part of managing uncertain findings in oncology.
What happens if the biopsy shows it is not cancer?
If the biopsy confirms a benign cause — such as infection, sarcoidosis, or a non-cancerous growth — your team will treat the underlying condition where needed and update your management plan accordingly. This is a genuinely good outcome, even when it does not feel like one after weeks of uncertainty. A biopsy result is a definitive answer where the scan result was not. The weeks of investigation were the correct and necessary path to get there.