PET Outputs
A PET scan works by injecting a small amount of radioactive tracer — typically a form of glucose — into the bloodstream. Active cells, including cancer cells, consume more glucose than normal cells, so the tracer accumulates in those areas. The scanner detects the radiation the tracer emits, producing images that highlight regions of high metabolic activity. PET is often combined with CT, known as PET/CT, pairing that functional data with detailed anatomy so metabolic activity can be precisely located within the body's structures.
Reading the image
- Hot spots — brighter areas indicating higher metabolic activity; these can point to cancerous tumors, infection, or inflammation.
- Cold spots — darker areas indicating reduced metabolic activity, which can be associated with certain tumor types or tissue damage.
Tracer uptake is also measured quantitatively using the Standard Uptake Value (SUV). Higher SUV values tend to indicate more aggressive or active disease, while lower values are more often associated with benign or less aggressive conditions. Radiologists combine this metabolic data with the anatomical picture to identify abnormalities and assess how far a disease has progressed.
What PET helps evaluate
| Area | What it's used for |
|---|---|
| Cancer detection and staging | Identifies malignant tumors, determines their size, and detects metastasis, which supports accurate staging and treatment planning tailored to the cancer's location and activity level |
| Heart disease | Evaluates blood flow and heart function, aiding diagnosis of coronary artery disease and assessment of myocardial viability ahead of interventions such as angioplasty or bypass surgery |
| Neurological conditions | Identifies areas of abnormal brain activity, supporting diagnosis of conditions such as Alzheimer's disease, epilepsy, and Parkinson's disease |
Role in treatment planning
- Personalized treatment plans — detailed data on a tumor's location, size, and metabolic activity helps oncologists design therapies tailored to each patient's specific cancer profile.
- Monitoring treatment response — changes in metabolic activity on follow-up scans show whether a tumor is responding to therapy, letting doctors adjust the treatment plan as needed.
- Detecting recurrence — after successful treatment, PET scans help monitor for signs that a disease has returned, so any recurrence can be caught and treated early.
PET usage by cancer type
| Cancer type | PET scan usage |
|---|---|
| Bladder | Not used for bladder cancer, because 18F-FDG collects in the bladder and urinary tract, making images very hard to interpret. |
| Brain | Limited use with brain cancer, since normal and cancerous brain tissue can have similar glucose uptake levels, making images difficult to interpret. Other radiotracers are being investigated. |
| Breast | Has shown strong potential in diagnosing primary and recurrent disease, staging, prognosis, and response to treatment, but is not as accurate as mammography for primary screening, so it can't replace mammography at this point. A 2021 study suggested PET may help identify women whose breast cancer will respond to hormonal treatment. |
| Cervical | Limited role in staging, but may be useful for assessing prognosis after treatment. |
| Colorectal | Useful in diagnosing recurrent colorectal cancer, with a sensitivity of 97% and specificity of 76%; other bowel conditions can also cause increased FDG uptake, which can complicate interpretation. |
| Endometrial | Able to detect recurrence during and after treatment, with a sensitivity of 96–100% and specificity of 78–88%. |
| Germ cell tumors | Still limited in early stages, but effective for evaluating residual disease after therapy. |
| Head and neck | Useful for diagnosis, assessing response to treatment, prognosis, and diagnosing relapse. Found to be more sensitive than CT (94–100% for PET vs. 77–91% for CT). |
| Lymphoma | Useful for disease detection, with higher sensitivity (94–100%) than CT (77–91%), as well as diagnosis, assessment of treatment response, prognosis, and diagnosis of relapse. |
| Lung | Useful for diagnosis, staging, prognosis, and radiotherapy planning; found to be comparable to fine needle aspiration (FNA) for diagnosis. |
| Melanoma | Not shown to be useful for primary staging, but can play a significant role in diagnosing relapse, with sensitivity, specificity, and accuracy all greater than 70%. |
| Non-Hodgkin's Lymphoma (NHL) | Capable of staging intermediate and high-grade NHL, and may also help predict treatment response. |
| Oesophageal | Found to be more accurate than CT and ultrasound for detecting metastatic disease — 82% accuracy for PET compared to 64% for CT and ultrasound. Also more accurate in nodal staging and may be useful for prognosis. |
| Ovarian | Useful for providing information about staging, especially when combined with CT. |
| Prostate | Imaging with PET alone has not been useful for prostate cancer imaging, though other options are currently under research. |
| Renal (kidney) | Shows nothing promising in the management of renal cancer, but has some limited ability to stage metastatic disease. |
| Thyroid | Found to be more accurate than MRI and CT in detecting metastasis, with a sensitivity of 82–95% and specificity of 83–95%. |