question and answer
A PET is not a toy
August 2010
During an annual physical exam, a patient requested a PET scan to screen for cancer. The patient is healthy. He says he’s willing to pay for the test himself. What should I tell him? L.C. Cheung, MD, Toronto, ON

Nuclear medicine positron emission tomography (PET) scanning is a relatively new imaging modality, using radioactively labelled fluorine (18F-FDG), that can provide information about the location and metabolic activity of abnormal tissues such as cancer. It can potentially locate pathologic areas not detected by other more common imaging tests such as ultrasound, CT scans and MRI, and can be very useful in treatment planning for patients with specific cancers or advanced heart conditions.

It’s clear from the myriad of clinical trials and papers devoted to this imaging modality that PET and PET/CT can indeed detect subclinical cancers. However, to be detected on the scan, the abnormal cells must be 18F-FDG avid, which many cancer cells are not (i.e. certain prostate cancers, mucinous pancreatic and ovarian tumours, and bronchioloalveolar cell carcinoma of the lung don’t take up 18F-FDG). In other words, a negative PET screening study will give the patient a measure of reassurance, but some types of cancer may still be present (false-negative). Alternatively, a positive PET scan result may not mean cancer, as the labelled agent may be taken up in certain normal, infected or inflamed non-cancerous tissues (false-positive). Furthermore, a positive PET scan result will usually lead to extensive further clinical investigation, often at great cost and with additional radiation involved in various imaging tests, perhaps to discover a false-positive end result.

The radiation involved in the PET/CT study itself is far from insignificant. A whole body PET/CT study involves a patient dose of anywhere from 10 to 25 milliSieverts. Compare that dose to those of other common imaging procedures: chest x-ray (0.06 mSv), CT head (2 mSv), nuclear medicine bone scan (4 mSv) and a CT scan of the abdomen and pelvis (8-11 mSv), which may all be required in addition to investigate a potential positive finding on a PET scan test.

Given all this, the PET/CT can’t reasonably be applied to the general population in a cost and risk-effective manner as a screening study. It’s proven effective as an adjunctive surveillance tool in known high-risk cancer patients with 18F-FDG avid tumours. But it’s not the ultimate cancer-screening test as some private U.S. clinics have touted it to be. It may rather prove to be the opposite, opening the door to a variety of undesirable issues.

At the very least, your patient needs to be aware of these issues prior to making any decision on undergoing an elective screening procedure that may leave him thousands of dollars out of pocket, and saddled with a diagnostic dilemma to follow. Michael K. McLennan, MD

References

  1. Sarikaya I et al. World Journal of Surgical Oncology 2007;5:64-9.
  2. Schoder H, Gonen M. Journal of Nuclear Medicine 2007;48:4S-18S.
  3. Chang JM et al. Korean Journal of Radiology 2006;7:57-69.
  4. Kojima S et al. Journal of Clinical Oncology 2004;22(14S):6072.
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