GRACE :: Cancer Basics

PET Scans

Cancer 101 FAQ: Primer on PET Scans

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The following content is offered by the moderators and is adapted from text that appears in several different posts and discussion threads on this topic.

PET stands for “positron emission tomography”, and a PET scan is a procedure designed to identify abnormal cellular activity that might indicate cancer. A prominent use for the PET scan (or combination PET/CT) is in achieving accurate staging of cancer.

In a PET scan, the patient is injected with a glucose-based tracer substance: think of it as a “hot sugar” . The PET scan picks up where this sugar localizes in the body. The idea in use for oncology is that the cancer cells, because they’re very active, eat up more sugar than non-cancerous cells and so the cancer cells will collect this tracer and will appear brighter on the scan than normal tissue.

PET scan results are reported in “SUV” units, in which SUV stands for “standardized uptake value”. The SUV is just a measure that indicates how bright the tissue is on the scan; that is, how much cellular activity is occurring in that area. This activity can represent various things, from inflammation to infection to cancer. There is no threshold SUV number that distinguishes cancer from inflammation or infection, but higher numbers (especially in the high single digits or more) are most suggestive of cancer.

PET scans are very sensitive (finding abnormality when one exists), but not perfectly specific, meaning that they can be positive for things other than cancer. In reviewing the PET scan, medical personnel look for the shape and location of the abnormal activity as well as the SUV number.

Most cancer types (e.g., lung, breast, colon) show up well on PET scans. Certain cancers (e.g., renal cell) are not seen as well with the PET scan. Some areas of the body (e.g., heart, brain, sometimes the bowels) normally have a significant glucose uptake/metabolic activity anyway, soincreased SUV numbers in those areas are of less concern than high activity in other areas. Given this activity, other types of scans (e.g., MRIs) might be more useful than PET scans for imaging certain tumors (e.g., brain tumors).

Some studies have supported a correlation of cancers with higher uptake (maximum SUV) being associated with a more aggressive clinical behavior, or conversely, low PET uptake with a more indolent clinical behavior of the cancer. In addition, though PET scans are not a standard imaging test for assessing response to therapy (compared with the far more established CT scan), some research suggests that PET scans may provide early feedback about the clinical benefit of lack of benefit from a systemic therapy. However, PET scans also tend to pick up inflammation in the area of recent prior surgery or radiation and are therefore well known to be very difficult to interpret in that setting. Continue reading


GRACEcast on PET Scanning in Oncology

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PET scans have become a central component of oncology over the last decade, yet how best to use them remains controversial. The current podcast, with Dr. David Djang, the Director of Nuclear Medicine at Swedish Medical Center in Seattle, covers what a PET scan does, along with the strengths and limitations of this form of imaging.

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Using PET Scans to Predict Response to Chemo in Advanced NSCLC

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PET scans have become well established in initial staging of lung cancer and many other cancers, but another setting in which they may emerge as useful is in assessing response to treatment. Some oncologists and patients are already doing this, but the standard test most commonly used for measuring response is the CT scan, which is widely available and has the benefit of years and years of experience. We grade our work in oncology by looking for tumor shrinkage. The percentage of patients who have an “objective response” of tumor shrinkage by 50% is one of our key endpoints when we describe a treatment for cancer, but we now know that it is definitely very possible to have patients live longer without having major tumor shrinkage. Cancer without any treatment is definitely going to grow, so even keeping it stable in size can appropriately be considered a relative improvement. I’ll describe the evidence showing survival benefit in the setting of stable disease in a dedicated post soon.

But the issue here is the concept that either before a CT scan is able to detect changes in size, or in the event that there is no obvious change in the size of a cancer mass on CT, the PET scan may detect decreased or increased metabolic activity that can help us determine whether the treatment is helpful or futile. Even if nobody wants to learn that a treatment is ineffective, if it is causing side effects without the real promise of benefit, it makes sense to abandon that futile treatment earlier rather than later. And now that we have a growing number of treatments available for advanced NSCLC, we might consider it increasingly important to not waste our time, strength, and money on treatments that aren’t going to provide a benefit. Continue reading


Metabolic Imaging: How PET Scans Are Changing Oncology

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PET stands for positron emission tomography, and this generally uses a safe radioactive tracer molecule called 18F-fluoro-2-deoxy-D-glucose (FDG). Fortunately, that’s not on the quiz — you don’t need to know it. All that is worth knowing is that PET scans offer “metabolic imaging”, which detects differences in the metabolism of tissues. The most metabolically active tissues have the greatest needs for sugar from the bloodstream, and when the sugar is labelled, the scans detect these areas as “PET-avid”. The objective measurement is a number called a “standard uptake value”, or SUV, where higher numbers mean a higher metabolic rate. While metabolic activity can be increased with inflammation, infection, and normal body activities (organs like heart, brain, and bowel have uptake normally from regular ongoing activity — this is NOT a bad thing) the reason we all care is that PET scans tend to pick up cancer, because cancer cells generally have greater metabolic activity and are dividing faster than most normal tissue.

PET example slide (click to enlarge)

Some slower growing cancers, such as bronchioloalveolar carcinomas(BAC), are much less consistently reported to appear on PET scans than other NSCLC tumors. And there is some evidence that higher SUV activity of lung tumors correlates with faster progression and worse prognosis. Continue reading


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