Quintessential PET scan does not detect cancer cells. - Macs Blogs (2023)

I very often have patients who ask me “is there a test that can detect cancer cells in the body?” or “shall we do PET CT and confirm the presence of cancer and then give next treatment?” Well, the answer is NO! The PET scan can only detect cancer when it is at least larger than 5mm in size in a given place. This is about 10 million cells sitting in one place. Its accuracy is better when the size is 10mm which is about 100 million cells. So, PET detects cancer nodules and not cells.

Over the recent years, PET/CT imaging has become a valuable tool in oncology or cancer treatment. Based on the higher sensitivity and specificity of staging compared to conventional imaging modalities such as CT or MRI alone, PET/CT is preferred in staging. In addition, PET/CT may also be used for identifying patients with

cancer who are at high risk of relapse and predicting benefit from treatment.

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PET scan detects cancer by its high level of activity. The cancer cells multiply in number very rapidly and so consume a lot of glucose. PET CT detects this and shows cancer as an orange activity as shown in the image.

The role of PET CT is well defined in cancer care:

1. Diagnosis: Has very limited role in the diagnosis of cancer as many other diseases show activity and are mistaken for cancer. However, when we find cancer in that is already spread and are unable to locate the origin, we use PET CT to locate primary cancer.
2. Staging : After the biopsy confirms cancer, to assess the extent of disease before the start of treatment.
3. ResponseEvaluation: Assessment of response to treatment during or after therapy. This allows us to change treatment if it is not working.
4. Restaging: Assessment of the extent of the disease after treatment or after confirmed recurrence
5. Suspected Recurrence: When blood tests suspect recurrence in an old case of cancer, it can help us to locate it.

6. Follow-up or Surveillance: Often we request for yearly scans. Depending on the type of cancer we may ask for PET CT.
7. Radiotherapy Planning (RT): To plan the area that has to receive radiation. This allows for the normal area to be spared. Fewer side effects.

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Is PET useful in all cancers?

Though most cancers are picked up on PET CT, there are a few which do not. The most important of these would be cancer of stomach (signet cell type). In such cases performing this test would be waste. However, there are cancers which are very sensitively detected which include lymphoma, GIST, etc. Your doctor will tell you when it is useful.

Quintessential PET scan does not detect cancer cells. - Macs Blogs (2)

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Author:Dr. Sandeep Nayak


Author is a leading surgical oncologist of Bangalore presently heading MACS Clinic as well as Fortis Hospital, Bangalore. He is a well acclaimed specialist in robotic and laparoscopic cancer surgeries. He has been a teaching faculty at Kidwai Cancer Institute, Bangalore, which is one of the leading teaching institutes in Oncology in India. He has keen interest in public education and awareness. He shares his thoughts and facts on this blog.

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Why would cancer not show up on a PET scan? ›

The PET scan can only detect cancer when it is at least larger than 5mm in size in a given place. This is about 10 million cells sitting in one place. Its accuracy is better when the size is 10mm which is about 100 million cells. So, PET detects cancer nodules and not cells.

Is PET scan enough to detect cancer? ›

A PET/CT scan can be more sensitive than other imaging tests and may find cancer sooner than other tests do. Not all tumors take up the radiotracer, but PET/CT is highly accurate in differentiating from the benign and malignant tumors it finds, particularly in some cancers such as lung and musculoskeletal tumors.

Are there cancers that don't show up on a PET scan? ›

PET scans must be interpreted carefully because noncancerous conditions can look like cancer, and some cancers do not appear on PET scans.

How accurate is PSMA PET scan? ›

The PSMA-PET scan was 92% accurate in detecting metastatic tumours (or the lack thereof), compared to only 65% accuracy for CT and bone scans. Accuracy relies on two measures, specificity and sensitivity: Specificity is the ability to correctly identify those without metastatic tumours (true negative rate).

Can you have cancer that doesn't show up on a CT scan? ›

Some cancers, such as prostate cancer, uterine cancer, and certain liver cancers, are pretty much invisible or very hard to detect on a CT scan. Metastases to the bone and brain also show up better on an MRI.

What is the best scan to detect cancer? ›

A CT scan (also known as a computed tomography scan, CAT scan, and spiral or helical CT) can help doctors find cancer and show things like a tumor's shape and size. CT scans are most often an outpatient procedure. The scan is painless and takes about 10 to 30 minutes.

How many cancer cells does it take to show up on a PET scan? ›

Discussion: The detection limit of PET is in the magnitude of 10(5) to 10(6) malignant cells.

What are the limitations of PET scan? ›

PET scanning can give false results if chemical balances within the body are not normal. Specifically, test results of diabetic patients or patients who have eaten within a few hours prior to the examination can be adversely affected because of altered blood sugar or blood insulin levels.

Is PET scan always accurate? ›

PET scan images can detect cellular changes in organs and tissues earlier than CT and MRI scans. Your healthcare provider may perform a PET scan and CT scan at the same time (PET-CT). This combination test produces 3D images that allow for a more accurate diagnosis.

Can a PET scan miss metastasis? ›

Based on the imaging, PSMA PET-CT was 27% more accurate than the standard approach at detecting any metastases (92% versus 65%). Accuracy was determined by combining the scans' sensitivity and specificity, measures that show a test's ability to correctly identify when disease is present and not present.

How often should a PET scan be repeated? ›

After completion of treatment, fluorodeoxyglucose positron emission tomography with diagnostic computed tomography (PET/CT) scan 3–6 months later is often performed to assess disease response due to its high sensitivity and negative predictive value [1,2,3,4,5,6,7,8].

How small a tumor can a PET scan detect? ›

PET scans may play a role in determining whether a mass is cancerous. However, PET scans are more accurate in detecting larger and more aggressive tumors than they are in locating tumors that are smaller than 8 mm a pinky nail (or half of a thumb nail) and/or less aggressive cancers.

How sensitive is PSMA PET to PSA? ›

The sensitivity rates of PSMA PET/CT according to PSA levels are 55-60% (0.2-0.5 ng/mL), 72-75% (0.5-1.0 ng/mL), 93% (1.0-2.0 ng/mL) and 97% (≥2.0 ng/mL).

Do all prostate cancers express PSMA? ›

Because prostate specific membrane antigen (PSMA), a transmembrane protein, is expressed by virtually all prostate cancers, and its expression is further increased in poorly differentiated, metastatic, and hormone-refractory carcinomas, it is a very attractive target.

What is the PSA cut off for PSMA? ›

Generally speaking, PSMA PET Scans using PSMA (18F-DCFPyL) have been shown to be effective at PSA levels above 0.2ng/mL. Also generally speaking, the higher the PSA level (when restaging patients), the more likely a PSMA PET Scan is to detect and identify recurrent prostate cancer.

Can CT scan miss metastasis? ›

CT is a useful modality for detecting metastases, with a sensitivity of 83% and a specificity of 85%, while PET has sensitivity and specificity of 87% and 83% respectively. Although the diagnostic accuracy of the two modalities is similar, they are complementary.

Does a CT scan of the abdomen and pelvis show colon cancer? ›

A CT scan can help with staging and follow-up.

The best test for staging and follow-up of colorectal cancer is a CT scan of the chest, abdomen, and pelvis. A CT scan is an X-ray that uses a special kind of dye. It highlights parts of your body that the doctor needs to see.

Can a radiologist tell if you have cancer? ›

While even the most advanced imaging technology doesn't allow radiologists to identify cancer with certainty, it does give them some strong clues about what deserves a closer look.

What scan is better than a PET scan? ›

CT scans are more accurate than PET scans, but PET scans can be used to diagnose cancer at an early stage. Both scans are painless and take about 30 minutes to complete. If you're unsure which scan is right for you, be sure to talk to your doctor about PET and CT protocols.

What cancer is the hardest to detect? ›

Pancreatic cancer is hard to find early. The pancreas is deep inside the body, so early tumors can't be seen or felt by health care providers during routine physical exams. People usually have no symptoms until the cancer has become very large or has already spread to other organs.

Which is better to detect cancer PET scan or MRI? ›

PET scans show metabolic changes that occur at a cellular level in your organ or tissue, which is where diseases can first be detected. MRIs cannot view the tissue at this scale, but rather are useful for detecting larger changes in the organ or tissues.

Do you need a biopsy after a PET scan? ›

For nodules that are not metabolically active during the PET/CT scan, it's not recommended to follow up with a biopsy. So patients avoid the risks of an unnecessary procedure. However, if the nodule positively reacts to the radiotracer, further investigation and a biopsy are strongly recommended.

What percentage of PET scans are positive for cancer? ›

The diagnostic accuracy of PET-CT was 93.5%, and the false positive rate was 6.50%.

Is a PET scan better than a biopsy? ›

Conclusion. PET/CT-guided percutaneous bone biopsy, compared with CT-guided bone biopsy, is an effective and safe alternative that yields a high diagnostic performance in the evaluation of hypermetabolic bone lesions to diagnose bone tumors and tumor-like lesions.

Can a PET scan be inaccurate? ›

In cancer cells, there is an overproduction of glucose transporters and, as a result, increased FDG uptake. However, not all PET-positive lesions are cancer, and in many instances, PET findings can be false positive."

How conclusive is a PET scan? ›

PET/CT is the most useful test for determining the stage of cancer. It is more accurate than any other test in finding local or metastatic tumors. Although PET can't detect microscopic cells, it can detect clusters of tumor cells that metastasized, or spread, to other tissues or organs.

Can inflammation be seen on a PET scan? ›

FDG-PET/CT can provide information on active inflammatory lesions. It is a very sensitive but non-specific imaging modality. The distribution patterns of inflammatory foci sometimes suggest specific disease.

What is the success rate of PET scans? ›

Whole-body PET can detect unsuspected extrathoracic metastases in up to 10% of patients when CT scanning fails to detect them, and may also alter management in up to 40% of cases. PET may be used to evaluate adrenal masses, with sensitivities and specificities of PET reported to be 100% and 80–100% respectively[7].

Should I get second opinion on PET scan? ›

For the best diagnostic results, you need a PET/CT second opinion. Studies have found that not every radiologist will interpret the same PET/CT picture in exactly the same way. Your course of treatment depends on the exam results.

How common is false negative in PET scan? ›

The retrospective study by Leblanc et al. including 125 patients with cervical cancer in stage IB2 –IIA with negative pre-operative CT scans or MRI, FDG PET or PET/CT showed a sensitivity of 33% in PA lymph nodes detection with false negative rate of 67% [10].

Do cancerous lymph nodes show up on PET scan? ›

PET scan: A PET scan, which uses a small amount of radioactive material, can help show if an enlarged lymph node is cancerous and detect cancer cells throughout the body that may not be seen on a CT scan.

What does no evidence of metastatic disease mean? ›

Achieving NED means your doctor can't find any signs of cancer using tests such as imaging, blood tests, or a biopsy. Obtaining NED is a promising sign that your cancer treatment was effective, but it's still possible for your cancer to return.

What cancers show up on a PET scan? ›

PET scans can show solid tumors in the brain, prostate, thyroid, lungs, and cervix. The scans can also evaluate the occurrence of colorectal, lymphoma, melanoma, and pancreatic tumors. They are also used to assess if treatment is being effective in eliminating cancer.

How many PET scans does Medicare allow per year? ›

If ordered by a doctor, Medicare Part B will typically cover 80% of the cost of a PET scan, up to three times per year. You'll be responsible for the remaining 20% after you meet your annual deductible for Medicare Part B, which is $233 per year in 2022.

What causes false PET scan results? ›

Infection is one of the most common causes of false-positive 18F-FDG PET-CT findings post-chemotherapy. Chemotherapy patients are susceptible to a wide variety of infections, including upper respiratory chest infections, pneumonia, colitis and cholecystitis.

What causes hot spots on PET scans? ›

Areas of the body that use a lot of glucose, such as the brain and heart, will pick up this radioactive material and appear hot. Abnormal cells in the body that use a lot of glucose will also appear as "hot spots." Cancer cells are highly metabolic and use a lot of sugar.

Can a CT scan tell if a lymph node is cancerous? ›

A CT scan of the chest or abdomen can help detect an enlarged lymph node or cancers in the liver, pancreas, lungs, bones and spleen. The noninvasive test is also used to monitor a tumor's response to therapy or detect a return of cancer after treatment.

How big does a tumor have to be to show up on a CT scan? ›

A CT scan can find lesions as small as 2-3 mm. However, the location of the tumor may play a role in how big it must grow before it's visible. Compared to traditional X-rays, CT scans can provide more information about the size of suspicious nodules and how harmful they may be.

Can PET scan differentiate benign and malignant tumors? ›

The PET imaging with FDG has proven useful in differentiating malignant tumors from benign lesions based on differences in their metabolic activity for tumors of the central nervous system4,5 and various body tumors.

What cancers do not show up on a PET scan? ›

Though most cancers are picked up on PET CT, there are a few which do not. The most important of these would be cancer of stomach (signet cell type). In such cases performing this test would be waste. However, there are cancers which are very sensitively detected which include lymphoma, GIST, etc.

Is PSMA PET covered by Medicare? ›

As of January 1, 2022, CMS provided PSMA PET Scan (PSMA) with a dedicated billing code, which in turn facilitated Medicare creating a consistent reimbursement schedule, making PSMA PET Scan reimbursable.

How long does a PSMA PET scan last? ›

The scan can also be used in conjunction with care at any other hospital where the patient is being treated. The total time it takes for a PSMA PET scan is about two hours.

What percentage of prostate cancers express PSMA? ›

Prostate-specific membrane antigen (PSMA) is highly expressed in prostate cancer. PSMA is expressed in >80% of men with prostate cancer.

What PSA indicates metastatic prostate? ›

Based on results from some small, single centre studies published in the beginning of the 1990s, PSA levels above 100 ng/mL have been used as a proxy for metastatic prostate cancer [3–5].

What should a 70 year old PSA? ›

For men aged 70 to 79, they suggested a normal serum PSA reference range of 0.0–6.5 ng/mL (0.0–6.5 μg/L).

What are the side effects of PSMA PET scan? ›

PSMA PET scans are safe for most patients. Some may experience temporary symptoms, such as headache, taste changes, or fatigue, that will go away on their own. Rarely, there may be an allergic reaction, particularly in patients with a history of allergy to other medications and foods.

When is a PSMA PET scan indicated? ›

Currently PSMA PET/CT is not intended to be a screening tool. It is for those who have been diagnosed with prostate cancer and who either have a higher chance of it spreading or who have a recurrence and a rising PSA level after initial treatment.

Can a PET scan light up and not be cancer? ›

Not all that lights up on a PET scan can or should be considered malignancy, and clinical context can become even more pertinent. Inflammatory cells, sarcoidosis, and other thoracic etiologies such as silicone-induced granuloma, can cause PET scans to light up, giving false positives [3,4,5].

What interferes with PET scan? ›

Certain factors or conditions may interfere with the accuracy of a PET scan, including:
  • High blood glucose levels in diabetics.
  • Caffeine, alcohol, or tobacco consumed within 24 hours of the procedure.
  • Excessive anxiety (may affect brain function)
  • Medicines such as insulin, tranquilizers, and sedatives.

Can a positive PET scan be anything but cancer lung? ›

Many researchers have used the PET-CT to diagnose lung cancer, but false positive can appear.

Will a PET scan show cancer in lymph nodes? ›

PET scans are sometimes used to look for cancer in the lymph nodes in the centre of the chest.

What is the biggest disadvantage in using a PET scan? ›

The major drawback to standard PET is that the images are of substantially lower resolution than, for example, those of CT and MRI, and PET is generally poor at delineating anatomic detail.

How can a PET scan be wrong? ›

FDG is not only a cancer specific imaging agent, false positive results may be observed with benign diseases. False positive results are commonly observed in areas of active inflammation or infection (Gupta et al., 20000), with a reported false positive rate of 13% and false negative rate of 9% (Alavi et al., 2002).

Can a PET scan give a false reading? ›

In cancer cells, there is an overproduction of glucose transporters and, as a result, increased FDG uptake. However, not all PET-positive lesions are cancer, and in many instances, PET findings can be false positive."

What is the success rate of PET scan? ›

[7] In assessing FDG-PET in staging patients with non-small cell carcinoma, FDG-PET had a higher sensitivity (71% vs 43%), positive predictive value (44% vs 31%), negative predictive value (91% vs 84%) & accuracy (76% vs 68%) than computed tomography (CT) scan for N2 lymph nodes.

What is the success rate of PET-CT scan? ›

In this study, we found that PET/CT showed specificity, sensitivity, and accuracy PET/CT in detection of primary tumor as 95%, 79%, and 87%, respectively.

What does it mean when lymph nodes light up on a PET scan? ›

PET scans detect the rate at which cells are using sugar. When the scan lights up brightly, it means there is metabolic activity. Most aggressive cancers light up brightly, but the caveat is inflammation in the body also lights up because inflammatory cells are also metabolically active.

Can lung cancer be missed on a PET scan? ›

Positron emission tomography (PET) with 18-fluorodeoxyglucose (FDG) is recommended for the non-invasive diagnosis of pulmonary nodules suspicious for lung cancer. In populations with endemic infectious lung disease, FDG-PET may not accurately identify malignant lesions.

Is a PET scan more accurate than a biopsy? ›

A total of 33 patients exhibited bone marrow involvement; of these, eight were identified by biopsy and 32 were identified by 18F-FDG PET/CT. PET/ CT was more sensitive (94 vs 24%; p < 0.001), demonstrated a higher negative predictive value (98 vs 80%) and was more accurate (98 vs 81%) than the biopsy procedure.

What are the symptoms of Stage 1 lung cancer? ›

When stage 1 lung cancer does cause symptoms, they may include:
  • New cough that persists.
  • Chronic cough that gets worse.
  • Coughing up bloody mucus.
  • Shortness of breath.
  • Ongoing chest pain.
  • Frequent lung infections.


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