Tuesday, July 17, 2007

Nuclear Imaging in Cancer

Nuclear imaging uses low doses of radioactive substances linked to compounds used by the body's cells or compounds that attach to tumor cells. Using special detection equipment, the radioactive substances can be traced in the body to see where and when they concentrate. Two major instruments of nuclear imaging used for cancer imaging are PET and SPECT scanners.
The positron emission tomography (PET) scan creates computerized images of chemical changes, such as sugar metabolism, that take place in tissue. Typically, the patient is given an injection of a substance that consists of a combination of a sugar and a small amount of radioactively labeled sugar. The radioactive sugar can help in locating a tumor, because cancer cells take up or absorb sugar more avidly than other tissues in the body.
After receiving the radioactive sugar, the patient lies still for about 60 minutes while the radioactively labeled sugar circulates throughout the body. If a tumor is present, the radioactive sugar will accumulate in the tumor. The patient then lies on a table, which gradually moves through the PET scanner 6 to 7 times during a 45-60-minute period. The PET scanner is used to detect the distribution of the sugar in the tumor and in the body. By the combined matching of a CT scan with PET images, there is an improved capacity to discriminate normal from abnormal tissues. A computer translates this information into the images that are interpreted by a radiologist.
PET scans may play a role in determining whether a mass is cancerous (under clinical trial). However, PET scans are more accurate in detecting larger and more aggressive tumors than they are in locating tumors that are smaller than 8 mm and/or less aggressive. They may also detect cancer when other imaging techniques show normal results. PET scans may be helpful in evaluating and staging recurrent disease (cancer that has come back). PET scans are beginning to be used to check if a treatment is working - if a tumor cells are dying and thus using less sugar. I put together an example of a case where CT fails to detect cancerous nodules while PET scan detects them.

65 year old with recent right lower lobe resection referred for follow up.

Increased metabolic activity consistent with recurrent malignancy in the right hilar region, L4 vertebra and liver. None of these lesions are well visualized on the CT portion of the exam, however, these lesions are compatible with metastatic disease.

This is an unfortunate case of a man who underwent right lower lobe resection for a primary squamous cell carcinoma (non small cell). Six months later he was referred for PET•CT which showed at least 4 metastatic lesions. Unfortunately, this patient did not have a preoperation PET•CT which likely would have shown at least one of the metastatic lesions and made the patient a non-operative candidate. Interestingly, all four metastatic lesions were not able to be seen on the CT portion of the scan. The liver lesion wasn't well visualized on the CT portion of the exam either even with a good IV contrast enhanced exam.

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