PET (positron emission tomography) scan


Can demonstrate increased utilization of glucose by activated neutrophils and macrophages in inflammatory reactions.

A tracer for glycolysis which is accelerated under aerobic conditions in cancer cells and is known as the Warburg effect.

The increased glucose consumption by cancer cells from the Warburg effect is the basis for tumor detection in a PET scan, in which an injected radioactive glucose analog is detected at higher concentrations in malignant cancers than in other tissues.

Fluorine-18 fluorodeoxyglucose (F-FDG-18) radiolabeled glucose analog is the most frequently used PET tracer.

When the glucose analog [18F]2-fluoro-2-deoxyglucose (FDG) is administered to a cancer patient it is transported into the cells and phosphorylated by the glycolytic enzyme hexokinase, producing [18F]FDG-6-phosphate which cannot proceed down the glycolytic pathway thereby accumulating and allowing imaging by the 511-keV photons released by the fluorine -18.

18F-FDG, a derivative of glucose Is transported into metabolically active cells where it is phosphorylated by hexokinase, , the first enzyme in the glycolytic pathway.

Once phosphorylated 18F-FDG cannot proceed into the process of glycol since itbecomes trapped inside the cell and accumulates radiotracer readily detected above background.

18F-fluorodeoxyglucose (FDG)-PET-CT is the most widely available technique and is based on the increased aerobic glycolysis of most intermediate and high-grade cancer lesions. 

Application of this imaging technique requires up regulation of glucose transporters and glycolytic enzymes( Warburg effect or hyperglycol lysis).

Increased uptake in benign lesions attributed to increased glycolytic activity in macrophages and neutrophils in inflammatory responses.

18F-FDG PET uptake is proportional to the rate of glucose metabolism and elevated uptake is related to enhance glycollysis.

SUV max is proportional to the rate of glucose metabolism.

(SUV) Standardized Uptake Value refers to the quantitative analysis of the PET scan, and is used to monitor the progress of disease during cancer therapy.

Provides combined functional and anatomic information in one scan.

Utilizes short-lived isotope that combines with normal tissue electrons to produce photons that can be detected by a camera.

Visualizes radiopharmaceuticals and produces high resolution whole body tomographic images.

Has the ability to examine the whole body for both primary malignancies and metastatic disease in a single procedure.

Overall sensitivity and specificity in evaluation of mediastinum in NSCLC is 67% and 73% respectively.

More reliable than CT scan imaging for staging the mediastinum in patients with NSCLC.

Malignant cells have increased rates of glucose metabolism.

Once FDG is uptaken into cells it is phosphorylated and trapped in the cell.

The FDG molecule is taken up in the cells in a manner analogous to glucose and when it undergoes radioactive decay with a half-life approximately 110 minutes and gives off a positron and is converted to a 0*-labeled glucose a non-radioactive compound.

The emitted positron encounters ian electron resulting in the annihilation of both the positron and the electron and simultaneous emits two gamma photons moving in opposite directions which can be measured by the detector.

The mean sensitivity and specificity of PET scan for detecting malignancy 93.9% and 85.8%, and the median sensitivity and specificity are 98% and 83.3% respectively.

For the current generation of scanners the spatial resolution is 7-8 mm for pulmonary nodules.

When used to detect bone metastases in lung cancer is has an accuracy of 96% compared to bone scans of 66%.

Meta analysis in accuracy of PET scanning of lung lesion has a sensitivity of 97% but a specificity of 78%.

More sensitive than bone scan in picking up metastases.

Maximal standardized uptake value (SUV) predicts overall survival in NSCLC treated with surgery or radiation.

Picks up osteolytic metastases better than osteoblastic lesions.

Not as accurate an indicator of occult axillary metastases in breast cancer as sentinel lymph node biopsy and immunochemistry.

Detection of breast cancer rates of 64-100%.

The routine use of PET imaging, CT scans in patients with DCIS and stage I and II BC are costly and not beneficial, and potentially harmful due to the need of for additional invasive procedures, overtreatment and unnecessary radiation exposure.

PET/CT versus CT has greatest sensitivity for the detection of regional nodal metastases and distant metastases and greater accuracy versus bone scan detection of bone metastases.

79-100% sensitivities in axillary staging.

Very accurate in evaluating residual masses seen on CT scan after treatment for lymphoma to discriminate between residual disease and fibrotic disease.

In Hodgkin’s disease and non-Hodgkin’s lymphoma focal tumor uptake after completion of therapy indicates a poor prognosis.

PET scans can be used to follow treatment success and modify treatment programs in Hodgkin disease.

FDG PET has more accurate delineation of lymphoma, than CT scans.

Lymphomas without FDG avidity includes CLL, Waldenstrom‘s macroglobulinemia, marginal-zone lymphoma and some T cell lymphomas.

In solid tumors including non-small cell lung cancer cervical cancer and esophageal cancer focal FDG uptake after completion of chemotherapy and radiation therapy associated with a poor prognosis.

Major drawback is that few anatomic landmarks can be seen on scans.

Integrated PET/CT scans provide improved diagnosis, and staging for malignancy with improved specificity and sensitivity.

The CT component provides increased anatomical information for optimal localization and characterization of tissue metabolic activity.

The combination of CT/PET results in decrease in the number of false positive and false negative PET findings.

PET/CT allows for a shorter PET scanning time since tissue density information derived from the CT is used for attenuation correction of PET photons.

Diffuse uptake of the bone marrow with the use of g-CSF, erythropoietin, anemia of chronic disease and leukemia can occur.

The addition of PET scanning to routine CT scans impacts therapeutic decisions in one of six cancer patients.

Useful in the evaluation of patients with fever of unknown origin, malignancy of unknown origin and to evaluate migratory thrombophlebitis, Trousseau’s disease.

Diffuse parenchymal uptake in lungs suggest Pneumocystis carinii pneumonia, radiation pneumonitis, bleomycin toxicity, acute respiratory distress syndrome and lymphangitic spread of carcinoma.

Increased activity maybe seen with subcutaneous and intramuscular injections due to needle induced damage.

Invasive thoracic procedures such as bronchoscopy and thoracoscopy can lead to positive scans.

Chest tube insertion may lead to a positive scan.

Use in adult sarcoma patients indicate that metabolic activity correlates with tumor differentiation and prognosis.

PET scans improve staging in pediatric sarcomas when compared to conventional imaging alone.

Has a sensitivity between 75-90% and a specificity of 90% in detecting thyroid malignancy.

Can detect primary malignancies including sarcomas, lymphomas, melanomas, diverse adenocarcinomas, brain tumors and may be a useful tool to screen for high risk groups such as families with the Li-Fraumeni syndrome.

Most lymphomas are FDG-avid.

The most common lymphomas including Hodgkin’s disease, diffuse large B-cell lymphoma, and follicular lymphoma are always avid at presentation.

FDG avidity related to tumor cells but also to environmental cells.

In Hodgkin’s disease the mass is about 1% of tumor cells and the FDG uptake is related to accessory cells amplify by cytokines produced by the Reed-Sternberg cells.

In diffuse large cell B-cell lymphoma the tumor cells makeup 90% of the mass and the FDG uptake results are from the tumor component.

Almost 100% of Hodgkin’s disease and diffuse large B-cell lymphomas are hypermetabolic with PET/CT.

The degree of avidity suggests the aggressiveness of lymphomas and a SUV of 14 or more is suggestive of transformed lymphoma.

PET-CT is the most accurate staging technique for Hodgkin’s and non-Hodgkin’s lymphomas, with increased sensitivity for nodal and extranodal disease without the loss of specificity compared to CT scan alone.

Using PET-CT scans in lymphoma with disease, compared with CT scan alone, patients will be upstaged 10-30%, resulting in a more accurate definition of stage and prognostic and therapeutic consequence for about 15% of patients.

PET-CT is not routinely recommended for lymphomas with low FDG-positive avidity-including CLL, marginal zone lymphoma, and subcutaneous and enteropathy type T-cell lymphomas.

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