Patient Exposure

Define “S” number. Have you seen “S” numbers (other Exposure Indicators) manipulated or dose creep? If so how and why does this occur and what is your role as a professional to either continue or discontinue the practice? Do you feel this is an ethical practice?

The “S” number according to the FUJIFILM Medical Systems CR Users Guide is, “…the photostimulable luminescence (sensitivity) given off by the imaging plate while being scanned in the reader” (FUJIFILM, 2004, p. 21) The “S” number also indicates if the patient was underexposed or overexposed but this is not the only factor that affects the “S’ number value.

As a Nuclear Medicine Technologist, the closest thing I have seen to “dose creep” is injecting above the allowable dose but documenting that the dose was within range. This might occur if the patient shows up too early and the dose is calibrated for a later time. To avoid this, I manually remove some of the radiopharmaceutical into the hot lab shielded sharps in order to be within the correct window. For example, for Tc99m-MDP (skeletal imaging) the allowable dose is 25 mCi (millicuries is the unit for amount of radioactivity). We may inject +/- 10% of the target dose, so anywhere between 27.5 mci and 22.5 mCi is acceptable. It is to our discretion to use the higher end for a patient with a larger body habitus in order to compensate for increased attenuation and as a result produce images with optimal quality. It is unethical to go far above the allowable range and actually is considered to be a reportable incident. According to the Digital Radiography Image Quality article, “One effective way to eliminate dose creep is to develop validated radiographic technique charts for all performed examinations as a function of patient size” (Williams, 2007). In Nuclear Medicine we determine the dose target as part of a written protocol which the Radiologist and/or the RSO (Radiation Safety Officer) approves.

Considering the ALARA principle (As Low AS Reasonably Achievable), the NRC (Nuclear Regulatory Commission) is reevaluating its position on dose extravasations, stating, “The NRC staff is considering whether extravasations should be reported as medical events, but it has not made any conclusions” (NRC, 2020). The NRC is continually monitoring and regulating how radiopharmaceuticals are utilized, transported, wasted, disposed of, etc.

Dose extravasations rarely occur but will result in a great radiation exposure to the area injected. A dose extravasation of a PET tracer F-18 FDG (seen in picture below, left antecubital) will cause a technical repeat where the patient has to reschedule the PET/CT scan and be exposed again to not only the dose but also the whole body CT Topogram and CT scan portion of the PET/CT. Delayed treatment or diagnosis of cancer is the implication of dose extravasation.

References

FUJIFILM. (2004). FUJIFILM Medical Systems USA, Inc. Retrieved from https://blackboard.boisestate.edu/bbcswebdav/pid-10158214-dt-message-rid-52170920_1/xid-52170920_1

NRC. (2020, March). UPDATES TO INJECTION QUALITY MONITORING. Retrieved from NRC.GOV: https://www.nrc.gov/docs/ML2005/ML20050W303.pdf

Williams, M. e. (2007, June). Digital Radiography Image Quality: Image Acquisition. Journal of the American College of Radiology, 371-388. Retrieved from https://doi-org.libproxy.boisestate.edu/10.1016/j.jacr.2007.02.002