DOS 511 - Week 3 Discussion
Research the use of PET scanning or PET/CT in radiation therapy. Is it useful enough in treatment planning to justify its expense?
Initial Post: PET is Worth the Price
The question of whether any technology justifies its expense, especially as applied to healthcare, is a sensitive subject. The people receiving the healthcare will say yes, the clinical staff delivering the healthcare will say yes, and the people paying for it will want to say no. Luckily, PET scanning is a technology that has proven its worth, and it is in widespread use in the US, Europe, and other parts of the world.1,2 In the US, it is estimated that there are about 2500 active PET scanners and around 2 million reimbursed procedures for oncology every year.1
The reason that PET scanning is so valuable, and so worth its price, is that it offers the ability to visualize metabolic activity.3 This is a complementary technique to anatomic imaging such as CT and MRI, which show anatomy. The two together can form a sort of weather map of the body, with PET showing what is happening, and CT or MRI showing where it is happening. PET scanners detect pairs of 511 keV photons produced when positrons interact with electron, annihilating and producing the paired gamma rays. Since the gamma rays are emitted 180 degrees from each other, coincident detections on a ring of detectors around a patient can provide data about the location of the pair's source by drawing a line between the two triggered detectors.4 Now that clocks and detectors are gaining sufficient temporal resolution (ability to measure tiny differences in time), it is actually becoming possible to gauge the difference in arrival time of these photons and make an assessment of the approximate point along that line, increasing the spatial resolution of the PET scan.
The positron emitter used in most PET scans in Fluorine-18, which is usually incorporated into a molecule called fluorodeoxyglucose (FDG).3 FDG is chemically very similar to glucose, which is the preferred fuel source for cancer cells.5 Because of its similarity to glucose, any cell that is using glucose will pull the FDG tracer in, but once it tries to metabolize it, the glucose-metabolizing enzymes won't work on it and the tracer will essentially get stuck in the cell. The half-life of F-18 is 110 minutes, and when the F-18 atom on the FDG molecule decays, it produces a positron which will interact with a nearby electron to create the aforementioned pair of 511 keV gamma rays, revealing the approximate location of that cell.
PET is particularly good at detecting small masses that are metabolically active but haven't grown to a size where they are noteworthy on an anatomic image yet.3 It is also very useful for tumor staging to judge whether disease is localized, spreading to the lymph nodes, or creating distant metastases. This information is especially valuable for tailoring treatment for patients. We use PET scanning regularly at our center. This week, we had a patient with breast cancer who had distinct FDG uptake in the axillary lymph nodes and internal mammary chain lymph nodes, but no involvement in the supraclavicular lymph nodes. We were able to tailor our dose to hit the affected areas, while delivering a much smaller dose to the brachial plexus than we would have if we had been unsure about supraclavicular lymph node involvement.
I am firmly in the camp of people who say that PET imaging is worth the expense.
- Farrell J. PET imaging market poised to grow with new compounds. Forbes Website. http://www.forbes.com/sites/johnfarrell/2014/01/10/pet-imaging-market-poised-to-grow-with-new-compounds/. Published January 10, 2014. Accessed November 19, 2014.
- Martinuk SD. The use of positron emission tomography (PET) for cancer care across Canada. TRIUMF Website. http://www.triumf.ca/sites/default/files/TRIUMF-AAPS-Martinuk-PET-Across-Canada-REPORT.pdf. Published 2011. Accessed November 19, 2014.
- Khan FM, Gerbi BJ. Treatment Planning in Radiation Oncology. Lippincott Williams & Wilkins; 2011.
- Cui JY, Grant A, Kim E, Lau F, Olcott P, Reynolds P, Spanoudaki V, Yeom, JY, Bieniosek M. Advanced time-of-tlight (ToF) PET photon detectors. Stanford Medical School Website. http://miil.stanford.edu/research/tofdetector.html. Accessed November 19, 2014.
- Basics of FDG. Harvard Medical School Website. http://www.med.harvard.edu/jpnm/chetan/basics/basics_scroll.html. Accessed November 19, 2014.