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Lens of the Eye Dosimetry

M Rehani

D Zhang

M Rehani1*, D Zhang2*, (1) European Society of Radiology, Vienna, (2) Toshiba America Medical Systems, Tustin, CA

TH-A-144-1 Thursday 8:00AM - 9:55AM Room: 144

International Commission on Radiological Protection (ICRP) issued a statement in 2011 decreasing the threshold for the lens of the eye to 0.5 Gy from earlier figure of 5 Gy for chronic and 0.5-2.0 Gy for acute exposures. Further, ICRP has reduced the dose limit for workers from150 mSv/y to 20 mSv/y, averaged over defined period of 5 years. Interventional radiology and cardiology are areas with high potential for risk to eye lens. If radiation protection is not practiced, the eye lens dose to workers in these areas can reach or exceed the regulatory limit. There are recent reports of eye lens injuries among interventionalists and support professionals in interventional suite.

As against well established personal whole-body dosimetry, eye lens dosimetry practically does not exist. Accurate assessment of eye lens dose is one of the most important aspects of correlating doses with observed lens opacities among workers in interventional suites. There are two approaches for practical dosimetry: a) passive dosimeters; and b) active dosimeters. Besides practical dosimetry, approaches are based either on retrospective dose assessment using scatter radiation dose levels or correlations between patient dose indices and eye doses to the operators. In cases when practical individual monitoring is not established or when it is not reliable, the later approaches are often the only possible.

At present, there is no clear consensus in the literature on the correlation between the patient dose and the dose to the eyes of the medical staff. Some authors provided a reasonable good relationship between PKA and dose to staff members, but there are also reports claiming that the establishment of the correlation between PKA and eye lens doses is associated with difficulties.

The retrospective assessment needs to be based on the reconstruction of the laboratory workload (types and numbers of procedures), usually with questionnaires and the application of many assumptions about past activity (procedures performed, corresponding doses based on previous dosimetric studies and the use of radiation protection tools). This approach is also related to a “typical” procedure in terms of fluoroscopy time and number of cine series, which may not always reflect the real clinical situation.

Accurate dose measurements are a prerequisite for investigation of low dose effects to the lens of the eye. This can be done only if dedicated and suitable calibrated dosimeters are available. With current state of technology and practice, only an approximate retrospective analysis can be accomplished. This is often not adequate for verification of compliance with regulatory dose limits. Future challenges include development of practical methods for regular monitoring of individual eye lens doses and development of better techniques to estimate eye dose from measurements at same reference points. While the situation with dosimetry is gloomy, it is very optimistic with eye protection.

"Eye lens radiation dose from brain perfusion CT exams".

CT perfusion imaging requires repeatedly exposing one location of the head to monitor the uptake and washout of iodinated contrast. The accumulated radiation dose to the eye lens can be high, leading to concerns about potential radiation injury from these scans. CTDIvol assumes continuous z coverage and can overestimate eye lens dose in CT perfusion scans where the table do not increment. The radiation dose to the eye lens from clinical CT brain perfusion studies can be estimate using Monte Carlo simulation methods on voxelized patient models. MDCT scanners from four major manufacturers were simulated and the eye lens doses were estimated using the AAPM posted clinical protocols. They were also compared to CTDIvol values to evaluate the overestimation from CTDIvol. The efficacy of eye lens dose reduction techniques such as tilting the gantry and moving the scan location away from the eyelens were also investigated.
Eye lens dose ranged from 81 mGy to 279 mGy, depending on the scanner and protocol used. It is between 59% and 63% of the CTDIvol values reported by the scanners. The eye lens dose is significantly reduced when the eye lenses were not directly irradiated. CTDIvol should not be interpreted as patient dose; this study has shown it to overestimate dose to the eye lens. These results may be used to provide more accurate estimates of actual dose to ensure that protocols are operated safely below thresholds. Tilting the gantry or moving the scanning region further away from the eyes are effective for reducing lensdose in clinical practice. These actions should be considered when they are consistent with the clinical task and patient anatomy.

Learning Objectives:
1. To understand current methods for estimation and measurement of eye lens dose for occupationally exposure
2. To understand correlation between patient dose indies and staff eye dose
3. To become familiar with method of eye dose estimation for patient in specific situation of brain perfusion CT

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