CT and PET Scans for Mesothelioma Diagnosis

Computed tomography involves a series of precisely calibrated X-rays to be taken and received by electronic means rather than photographic film. These images are enhanced by the use of contrast medium. Prior to the CT scan the patients is injected with contrast dye and if an abdominal ct is also requested oral contrast is provided. Patients are required to fast prior to an Abdominal CT scan but not for the Pleural CT scan. These images are then reconstructed by computer to offer a highly detailed view of the body in narrow slices, allowing a look inside the body at anatomical details not otherwise visible without surgical examination.

Figure h: pet/ct fusion showing combined images during mesothelioma diagnosis. photo courtesy k. brauchFigure H: PET/CT Fusion showing combined images. The red cross in the human image shows hot spot. The slices show (from left to right) CT scan, PET scan and Combined fusion scan. Click here for enlargement. Photo courtesy K. Brauch.

CT scans are excellent at showing topography but cannot distinguish between scar tissue or tumor. While they can detect pleural or peritoneal thickening, the results are not definitive enough to allow for a mesothelioma diagnosis with CT scan alone.

The Role of a PET Scan in a Mesothelioma Diagnosis

Another tool of importance in external exams is the Positron Emission Tomography scanner, or PET. PET is a process where radioactive glucose is injected into the bloodstream, where it is consumed in the greatest proportion by the most active parts of the body. This usually means the brain, heart, sex organs and tumors, if present, will consume the largest amount of this isotope, causing hot spots to appear on a scanned image of the body.

Tumors tend to be extremely active and therefore consume large amounts of the sugar isotope. This will cause hot spots to appear where they normally shouldn't, allowing the radiologist to identify possible indications of cancer.

PET Scans vs. CT Scans

Although PET scans can show activity, they are rather indistinct and cannot clearly identify the exact location of the activity. Simply put, CT scans can tell where a growth is located and PET scans can tell if there is excessive biological activity. The latest technology combines both the CT scan and the PET scan into a single computer image. Therefore, taken together, the PET/CT fusion scan produces a much more accurate image of activities inside the body. If an area of thickening is actively consuming sugar it may be cancerous. If the activity level is low or normal, it may simply be inflammatory response to injury, healing or other benign activity. PET scans are used as part of the initial work-up for surgery. It helps to identify areas of activity that might not be imaged well on CT scan. This may prompt a biopsy to rule out disease that is outside of the operative field. There are false positives whereas a PET may be positive but the biopsy reveals benign non malignant process but the opposite is also true. A negative PET scan does not always reflect accurately the extent of disease. It is not uncommon to discover more advanced disease during the planned operative procedure leading to technically unresectable disease.

Figure I: PET/CT slice showing tiny central hot spot and presence of EPP (removed lung) in left of image. For Enlargement click here. Photo courtesy K. Brauch

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