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Tuesday, February 05, 2013

Childhood Tumors: How PET/CT Scans Can Help

When patients are evaluated for a possible tumor, the types of scans they hear about most often are CT (computerized tomography) scans and MRI (magnetic resonance imaging) scans. CT scanning is an advanced imaging technique that uses x-rays and computer technology to produce more clear and detailed pictures than a traditional x-ray. MRI scans produce extremely detailed pictures of the inside of the body by using very powerful magnets and computer technology.

CT scans and MRI scans are known as structural scans because they provide important information about the structure of the body. However, these scans do not provide information about how the area that is pictured actually functions.

There are other types of scans known as functional imaging scans which do provide this important information about function. One such imaging technique is known as a PET scan. A PET scan involves injecting the patient with a small radioactive chemical and being placed in a machine that detects and records energy given off by the substance. The computer translates the energy into 3D pictures which provides information about how cells in the body are functioning because normal cells react differently to the chemical than healthy cells. One type of chemical injected is known as FDG (fluorodeoxyglucose), which is a type of radioactive glucose. When a PET scan uses this substance, it is known as FDG PET.

One problem with functional imaging scans is that they do not provide good information about structure. This problem is solved by combining PET scan data with CT scan data, so that functional imaging data can be superimposed on structural imaging scans. When PET and CT scan data are combined, this is known as PET/CT and can be used to scan the entire body.

In the current issue of the American Journal of Roentgenology, researchers in Toronto, Canada, examined whether FDG PET/CT could reliably differentiate malignant (cancerous) tumors from benign (non-cancerous) tumors in 21 children (13 girls, 8 boys). Tumors are abnormal masses of tissue that form when cells in a certain area of the body reproduce at an increased rate.

The question of whether FDG PET/CT can do this is important because by itself, FDG accumulation can occur in benign masses as well as malignant masses during PET scans. To solve this problem, measuring cell uptake of FDG at two time points (instead of one) is performed, which is known as dual-time imaging. The reason this is done is because FDG is absorbed differently over time in benign and malignant processes. Essentially, malignant lesions will show an increase in FDG uptake (absorption) over time whereas benign areas will remain stable. The uptake of FDG glucose is known as the standardized uptake value (SUV).

In the study reference above, scan 1 was performed 60 minutes after FDG injection. Scan 2 was performed about 2 hours after the first scan. In children with malignant disease, the results showed that the average SUV increased from 7.3 to 10.9 between the two time points. In children with benign masses, the average SUV changed from 4.5 to 4.2 between the two time points. The researchers concluded that dual time point FDG/PET CT is useful in distinguishing malignant disease from benign processes in pediatric patients.

Suggested reading: Happily Hungry: Smart Recipes for Kids with Cancer

Related blog entry: Ovarian Tumors in Children: They Happen

Reference: Costantini DL, Vali R, Chan J, McQuattie S, Charron M. (2013). Dual-Time-Point FDG PET/CT for the Evaluation of Pediatric Tumors. AJR Am J Roentgenol. Feb;200(2):408-13

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