FDG PET Scan — how it works, what it shows.

What is an FDG PET scan?

One whole-body FDG PET/CT can answer several questions at once: where a cancer has spread (staging), whether it has come back (recurrence, often prompted by a rising tumour marker), and whether treatment is working (response assessment). It is the workhorse scan for most common solid cancers and lymphoma.

At Theranostic Physicians, FDG PET/CT is performed in the Department of Nuclear Medicine at Fortis Memorial Research Institute (FMRI), Sector 44, Gurugram, with image interpretation led by Dr. Ishita B. Sen and Dr. Dharmender Malik.

The scan is only as good as who reads it.

An FDG PET result depends on two things working together: a good machine to acquire a clean, high-resolution study, and a good reader — an experienced nuclear medicine physician — to interpret it correctly. FDG makes this especially true, because the tracer is not cancer-specific.

• The institution. Scans are performed at Fortis Memorial Research Institute, Sector 44, Gurugram — a leading tertiary hospital with a dedicated Department of Nuclear Medicine and modern PET/CT imaging.
• The reader. Every study is interpreted by Dr. Ishita B. Sen and Dr. Dharmender Malik. With FDG, infection, inflammation, tuberculosis and healing tissue can all light up and mimic cancer — and some low-grade cancers barely light up at all. Telling the difference, by reading the images against your history, is exactly what years of focused experience are for.

A scan is only ever as reliable as the equipment that acquires it and the physician who reads it. We hold both to the same standard.

How an FDG PET works.

FDG is a molecule built to behave like glucose — the body's basic fuel — with a positron-emitting fluorine-18 atom attached. The scan works because active cells, especially cancer cells, are hungry for sugar:

• The tracer. ¹⁸F-FDG is injected into a vein. It is carried into cells by the same transporters that handle glucose.
• Metabolic trapping. Once inside, FDG is partly processed and then “stuck” — it cannot be broken down further the way glucose is, so it accumulates inside metabolically active cells. The more active the cell, the more FDG it holds.
• The Warburg effect. Most cancers consume glucose far faster than normal tissue, so they concentrate FDG and stand out clearly against the background.
• Imaging. The PET scanner detects the signal and builds a three-dimensional map; the CT component pins down the exact anatomy. Areas of high uptake are reported, often using a measure called the SUV (standardised uptake value).

Some FDG uptake is entirely normal: the brain (which runs on glucose), the heart, the liver, the kidneys and urinary tract (which clear the tracer), and active muscle and bowel. Recognising this normal pattern is part of reading the scan correctly.

Who needs an FDG PET scan?

FDG PET/CT is requested by your oncologist, surgeon or physician in a range of situations across most common cancers:

• Staging a new cancer. To map how far disease has spread — lymph nodes, distant organs — before deciding on surgery, radiotherapy or systemic treatment. Common in lung cancer, lymphoma, head and neck, oesophageal, colorectal, cervical and melanoma, among others.
• Detecting recurrence. When a tumour marker rises or symptoms suggest the cancer has returned, FDG PET can locate it.
• Assessing treatment response. To see whether chemotherapy, immunotherapy or radiotherapy is working — falling FDG activity usually signals a response. In lymphoma this is a routine, well-validated use.
• Characterising a lesion. Helping tell whether a mass, such as a solitary lung nodule, is likely benign or malignant.
• Finding an unknown primary. When cancer is found in a lymph node or organ but the original site is unknown.

FDG also has uses beyond cancer — for example in investigating infection or inflammation of unknown cause — but oncology is where it is used most.

What to expect on the day.

An FDG PET scan is an outpatient test, but the preparation matters more than for some other scans — chiefly the fasting requirement. Plan for about two to three hours at the centre.

Out-of-town and international patients are helped with scheduling so the scan, the review and any onward plan can be arranged in a single short trip.

What an FDG PET shows — and what it can't.

FDG PET/CT is powerful precisely because it images cancer biology, not just anatomy — it finds active disease that a CT alone can miss, and tells active tumour from inactive scar tissue. But its strength and its limitation are the same fact: FDG marks activity, and activity is not unique to cancer.

What it shows well

• Active, glucose-avid cancer throughout the body in a single scan.
• Spread to lymph nodes and distant organs that may be normal-sized on CT.
• Whether disease is responding to treatment, as uptake falls.
• The difference between active tumour and treated, inactive tissue.

Its limitations

• False positives. Infection, inflammation, tuberculosis and other granulomatous disease, and healing after surgery or radiotherapy all take up FDG and can look like cancer. This matters particularly in regions where TB is common.
• False negatives. Some low-grade or slow-growing cancers — and certain types such as many prostate cancers and well-differentiated neuroendocrine tumours — take up little FDG and can be underestimated.
• Small lesions. Disease below the scanner's resolution may not be seen; the brain's high background also makes FDG poor for brain metastases, where MRI is preferred.

This is why FDG findings are always read in context — against your history, blood results and other imaging — and why, for some cancers, a targeted tracer such as PSMA or DOTATATE is chosen instead. For how molecular PET differs from an ordinary CT, see PET scan vs CT scan.

Cost of an FDG PET scan in India.

The FDG PET/CT scan is billed by Fortis Memorial Research Institute. The price you pay is the same hospital counter price for everyone — with no commission added for booking and coordinating through our team. We do not discount; the value is in the accuracy of the scan and the physician who reads it.

Is an FDG PET scan safe?

Yes — an FDG PET scan is a very safe, well-tolerated test. It is non-invasive apart from a single small injection.

Tracer & side effects

• The FDG tracer is given in a microdose; allergic or adverse reactions are rare and usually mild.
• There is no sedation and none of the contrast-dye reactions associated with iodinated CT contrast.
• Drink fluids afterwards to help flush the tracer, which is cleared mainly through the kidneys.

Radiation

• The radiation dose is low to moderate — in the range of other routine diagnostic PET/CT studies, and a small fraction of any therapeutic dose.
• The radioactivity decays quickly. As a routine precaution we may advise limiting prolonged close contact with pregnant women and young children for a few hours after the scan.

Diabetes, pregnancy & breastfeeding

• Diabetic patients need their blood sugar managed for an accurate scan — tell the team in advance so timing and medication can be planned.
• Tell the team beforehand if there is any possibility of pregnancy or if you are breastfeeding, so appropriate precautions can be taken.

Book an FDG PET scan.

FDG PET/CT is performed at the Department of Nuclear Medicine, Fortis Memorial Research Institute, Sector 44, Gurugram — the single centre where we deliver scans — with interpretation led by Dr. Ishita B. Sen and Dr. Dharmender Malik. Our team coordinates directly with the department so you skip the phone queues, and can usually secure a priority slot on a best-effort basis.

Already have PET images from another centre? You can request an independent expert opinion through our Second Read service without travelling.