FDA recommends that facilities work with medical physicists to determine whether the radiation doses typically associated with their protocols
- fall below skin-injury thresholds, and
- correspond to values reported broadly in medical literature.1
1. Is the dose likely to lead to any deterministic effects, such as skin reddening, hair loss, or cataracts?
The highest radiation dose accruing acutely at a single site on a patient’s skin, referred to as the “peak skin dose” (PSD), is an important parameter in assessing risk of erythema (skin reddening) and epilation (hair loss).2 The threshold range for transient erythema and temporary epilation is 2-5 Gy PSD; prolonged erythema and permanent partial epilation have threshold ranges of 5-10 Gy PSD; and severe skin injury is associated with larger values of PSD.3
Cataracts are of particular concern if the eye falls within the directly irradiated region, and modern scanners are able to tilt to avoid direct exposure to the eye. Although otherwise authoritative sources4,5 cite doses to the eye lens of 2 Gy (in a single exposure) to 5 Gy (in fractionated exposures) as threshold values for the induction of cataracts, these values are questionable: They were based on studies6,7,8 with relatively limited amounts of epidemiological follow-up of radiation-associated cataract data from time of exposure. More recent studies suggest that the lowest cataractogenic dose in people is much less than these values, is statistically compatible with no threshold at all, and that increasing dose is associated with an increasing prevalence of cataracts.9,10,11,12
It is important to note that an estimate of PSD requires a “point-dose” evaluation, i.e., one that is spatially localized. Point-dose estimates are not equivalent to any of the standardized indices used to characterize dose in computed tomography, namely, CTDI100 peripheral, CTDI100 central, CTDI weighted (CTDIw), CTDIvol, or other indices, such as dose-length product (DLP) based on CTDI100 values. In typical brain-perfusion scanning the patient table is stationary or toggles back and forth over a range less than 100 mm. Also, the x-ray field along the central axis (z-axis) is collimated to a width of less than 100 mm. Under these circumstances, CTDI100 peripheral tends to overestimate the PSD by an amount that depends on the particular conditions of operation during scanning.13,14
2. Do dose-index values typically fall in a range that could be inferred from the medical and scientific literature?
While a range of dose-index values is reported in the literature,15 a value of 0.5 Gy can serve as a useful indicator of the order of magnitude of the average CTDIvol for brain-perfusion scans. Manufacturer-defined protocols should be used as starting points that may need revision to be appropriate for a particular patient or facility.
1 Anthony Siebert, 2009, "Considerations for Clinical CT Dose Estimations for Patients."
2 Donald L. Miller et al., “Radiation Doses in Interventional Radiology Procedures: The RAD-IR Study. Part II: Skin Dose,” J. Vasc. Interv. Radiol. Vol. 14, pp. 977-990, 2003.
3 Stephen Balter et al., “Fluoroscopically Guided Interventional Procedures: A Review of Radiation Effects on Patients’ Skin and Hair,” Radiology Vol. 254, No. 2, pp. 326-341, February 2010.
4 International Commission on Radiological Protection, ICRP Publication 60, 1990 Recommendations of the International Commission on Radiological Protection, Pergamon Press, Oxford UK, 1991.
5 National Council on Radiation Protection and Measurements, NCRP Report 116, Limitation of Exposure to Ionizing Radiation, NCRP, Bethesda MD, 1993.
6 M. Otake and W.Schull, “The relationship of gamma and neutron radiation to posterior lenticular opacities among atomic bomb survivors in Hiroshima and Nagasaki,” Radiat. Res. Vol. 92, pp. 574-595, 1982.
7 R.J. Miller, T. Fujino, and M.D. Nefzger, “Lens findings in atomic bomb survivors: a review of major ophthalmic surveys at the Atomic Bomb Casualty Commission (1949–1962), Arch. Ophthalmol. Vol. 78, pp. 697-704, 1967.
8 M.D. Nefzger, R.J. Miller, and T.Fujino, “Eye findings in atomic bomb survivors of Hiroshima and Nagasaki: 1963–1964, Am. J. Epidemiol. Vol. 89, pp. 129–138, 1969.
9 E. Nakashima, K. Neriishi, and A.Minamoto, “A reanalysis of atomic-bomb cataract data, 2000–2002: a threshold analysis,” Health Phys. Vol. 90, pp. 154–160, 2006.
10 Kazuo Neriishi et al., “Postoperative Cataract Cases among Atomic Bomb Survivors: Radiation Dose Response and Threshold,” Radiat. Res. Vol. 168, pp. 404-408, 2007.
11 Gabriel Chodick, “Risk of Cataract after Exposure to Low Doses of Ionizing Radiation: A 20-Year Prospective Cohort Study among US Radiologic Technologists,” American Journal of Epidemiology Vol. 168, No. 6, pp. 620-631, 2008.
12 E. A. Ainsbury et al., “Radiation Cataractogenesis: A Review of Recent Studies,” Radiation Research Vol. 172, pp. 1-9, 2009.
13 J. A. Bauhs, T. J. Vrieze, A. N. Primak, M. R. Bruesewitz, and C. H. McCollough, "CT dosimetry: comparison of measurement techniques and devices," RadioGraphics Vol. 28, pp. 245-253, 2008, available at http://radiographics.rsna.org/content/28/1/245.full.pdf+html.
14 For a discussion of the assessment of CT dose associated with a cone-beam irradiation geometry, see the American Association of Physicists in Medicine (AAPM) Report No. 111, Comprehensive Methodology for the Evaluation of Radiation Dose in X-Ray Computed Tomography, AAPM, February 2010 (http://www.aapm.org/pubs/reports/RPT_111.pdf).
15 See the following papers for examples of doses associated with CT brain-perfusion studies: Yoshimasa Imanishi et al., "Radiation-induced temporary hair loss as a radiation damage only occurring in patients who had the combination of MDCT and DSA," Eur. Radiol. Vol. 15, pp. 41-46, 2005; M Cohnen et al., "Radiation Exposure of Patients in Comprehensive Computed Tomography of the Head in Acute Stroke," AJNR Am. J. Neuroradiol. Vol. 27, pp. 1741-1745, September 2006; Sung Won Youn et al., "Perfusion CT of the Brain Using 40-mm Wide Detector and Toggling Table Technique for Initial Imaging of Acute Stroke," AJR Vol. 191, pp. W120-W126, September 2008; Anisa Mnyusiwalla, Richard I. Aviv, and Sean P. Symons, "Radiation dose from multidetector row CT imaging for acute stroke," Neuroradiology Vol. 51, No. 10, pp. 635-640, October 2009.