Review of PET/CT Images in Melanoma and Sarcoma: False Positives, False Negatives, and Pitfalls

Abstract

The most common cause of false-positive PET is inflammation, whether infectious or otherwise. Recent local therapy, such as surgery or radiation, will produce inflammation that can persist for months. Surgery can produce reactive uptake in local soft tissues and nodes, and the interpreter should be aware of changes in highly active tissues (such as a neobladder). Radiation can produce intense uptake, but is often in a linear pattern. Granulomatous inflammation (autoimmune or infectious) can produce false positives due to its intensity and nodularity. Access to the patient history is thus useful. False negatives may be seen in high-background organs such as the brain (necessitating a brain MR for full staging) or kidneys and bladder. Muscle metastases may be difficult to detect on CT, whereas bone metastases may be only detectable on CT. Brown fat in the neck is intense but has a characteristic pattern that should be recognized. Lesions below a centimeter cannot be reliably detected, though they are sometimes visible. A variety of motion artifacts, most commonly due to musculoskeletal motion or breathing, can cause errors in attenuation correction and/or anatomic correlation. Non-attenuation-corrected images may be useful here. Sites at the border of the imaging field may be poorly visualized. Intense physiologic uptake may obscure uptake in nearby metastases. Incidental findings are relatively common, particularly in the thyroid and colon, and should be noted as they may represent another primary malignancy. Finally, incidental findings on CT such as pneumothorax may be important as well.