Introduction:

• In this imaging modality, small biologically important molecules (sugars/ amino acids/ nucleic acids/ receptor binding ligands) tagged with positron emitting radionuclides are used.
• When positron emitting radionuclides undergo decay, their positions can be detected by PET scanner.
• Commonly used tracer in oncology practice is 18-F-2-Fluoro-2deoxy-D-glucose (FDG) which measures glucose metabolic rate. Malignant cells have higher rates of aerobic glycolysis and hence take up glucose which is administered.

Procedure:

• IV-FDG is given and allowed to circulate through the body for 60-90 minutes and then traced.
• Malignant tissue is detected as area of hypermetabolism or hot-spot.
• As PET images are of very low resolution, hence there is poor demarcation of lesion borders. Hence this study is often combined with CT scan.
• High glucose level competes with FDG for uptake by tumor. High level of insulin pushes FDG, along with glucose into skeletal muscle and myocardium and decreasing the availability of tracer. Hence patients have to be euglycemic, with 6-8hrs of fasting prior to starting the procedure.

Oncological indications for PET

• Lymphoma
• Melanoma
• Non-small cell lung cancer
• Solitary pulmonary nodule
• Breast cancer
• Colorectal cancer
• Esophageal cancer
• Head and neck cancer
• Thyroid cancer
• Cervical cancer

Other conditions where PET is positive

• Infections and inflammation
• Post radiotherapy (hence it is advisable to wait for 3 months following radiotherapy for doing a PET CT)
• Post chemotherapy (hence it is advisable to wait for 3 weeks following chemotherapy for doing a PET CT)
• All rapidly proliferating cells like bone marrow
• Patients who are anxious/ cold demonstrate activation of brown fat, which may be misinterpreted as mediastinal adenopathy.

Malignancies which are generally PET negative

• Carcinoma prostate
• Low grade sarcoma
• Low grade lymphoma
• Well differentiated adenocarcinoma of lung
• Neuroendocrine tumors
• Tumors less than 10mm diameter

Deauville scoring

• Grade 1- No activity
• Grade 2- Activity is less than mediastinum
• Grade 3- Activity more than mediastinum but less than liver
• Grade 4- Activity slightly/ moderately more than liver
• Grade 5- Markedly increased uptake