Cumulative Radiation Dose in Patients Admitted with Subarachnoid Hemorrhage: A Prospective Study Using a Self-Developing Film Badge

Discussion

Diagnostic imaging and endovascular treatments that use x-ray have improved outcomes for patients with SAH. The significant role of these tools in current management and immediate concerns about patient mortality and morbidity to some degree lessen consideration of the effects of radiation dose during a hospitalization for SAH. While the survivors may have a decreased overall life expectancy,¹ it is nevertheless important to consider both the immediate and long-term effects of radiation.

Recent publications have brought attention to the increased use of diagnostic CT. The number of CT scans in the United States alone has increased from 3 million per year in 1980 to 62 million in the past year.² There is reasonable concern about the overall effects of this x-ray exposure, with predictions of 4000 additional cancers from CT scans of the head performed in 2007 alone.³ The risks of radiation can be divided into long-term carcinogenic effects and immediate effects such as hair loss.

The unit of measure used to quantify the absorbed x-ray dose is the gray, equivalent to 1 joule of energy per kilogram. Temporary hair loss is expected at an absorbed dose of 3 Gy, and permanent loss, at 7 Gy. This unit should be considered with the understanding that a single head CT scan is expected to produce a skin dose of approximately 0.05 Gy.

Many neurointerventional procedures will frequently exceed 1 Gy locally. Two investigations reported average procedural skin doses of 1–1.5 Gy.^4,5 Reduction of radiation dose from each individual diagnostic and interventional exposure is critical for minimizing total patient and, thereby, population dose. While many publications suggest strategies to this end,^4,6,7 few address the issue of cumulative dose during the hospitalization.

Patients with SAH are exposed to x-ray repeatedly for a short time, and the potential for cumulative effects exists. For example, Imanishi et al⁸ investigated the unexpected occurrence of bandlike hair loss in 3 among 44 patients, all examined with the same CTP technique. They found that all 3 patients with hair loss also underwent ≥2 cerebral angiographies. Their implication was that CTP alone could not account for the hair loss experienced by these patients. While the absorbed dose from CTP is high relative to a diagnostic CT, under the usual circumstances, it would never approach the 3-Gy threshold for temporary hair loss. For example, even by using a high-dose technique of 120 kV/200 mA for CTP, skin dose would not be expected to exceed 2 Gy.⁹ The report of Imanishi et al of hair loss in patients who had both DSA and CTP examinations provides some evidence for a cumulative dose effect when multiple x-ray imaging tests are used for a short time.

The cumulative skin dose we report for the patients in this study we believe is conservative. For patients transferred from other hospitals or because of consent delays, some initial CTAs were not included. None of our patients had CTP examinations that are known to contribute substantially to local skin dose.

There are other reasons to believe that the maximum skin dose will be higher. The angle of incidence of the x-ray beam on the badge may influence its measurement on the film. Because many of the DSA runs used oblique angles, their contribution to dose may be undervalued by this approach. In addition, the study of DSA by Schueler et al¹⁰ found that the maximum skin dose during diagnostic angiography occurs in the back of the head, not on the side where we positioned the badges in this study.

The ACR committee, in their report on radiation in medicine, recommended, “The ACR should encourage radiology practices to define a surveillance mechanism to identify patients with high cumulative radiation dose due to repeat imaging.”¹¹ The estimated patient dose based on calculations or historic precedents has been used in some recent studies of radiation dose largely because there were no available data on a patient-specific dose.

That appears to be changing because most current CT scanners and angiography suites offer calculated dose estimates based on imaging parameters. For CT, this is the CT dose index, and for DSA, the dose-area product. With that information, it should be possible to calculate a running total of patient dose each day. Even when one incorporates known imaging factors, this is still a calculation and assumes that the x-ray tube performs as specified. In practice, it would require some process to record the data and keep a running total, which may prove possible with improved information systems.

Our DSA hardware did not provide a calculated dose at the time of imaging for this patient group, and for that reason, we made no comparison of the actual badge readings with a calculated dose. For the last patient who happened to have only CT studies, the badge read of 0.2 Gy closely matched the expected total dose of the 4 CT scans, however.

The rationale for a patient badge is that it provides an actual measured skin dose at least in 1 region rather than a calculated prediction. The advantages of the particular device we used are that it is relatively inexpensive, invisible, and unobtrusive and it provides an immediate visual indication of dose in 0.5-Gy increments. With the indicated total dose on the badge, this could be estimated daily during a hospital stay and in that way potentially influence imaging decisions.

However, while a patient-specific badge would seem easy to use in principle, because it requires participation of many staff in multiple locations, this project did require time and attention to train staff at the outset. Of course, other devices are available to measure skin dose. Thermoluminescent dosimeters and radiosensitive indicators attached to a hood have been used in other studies to measure the patient-specific x-ray dose.¹² These did not seem as readily integrated into the clinical scenario however.

There are benefits to monitoring patient cumulative dose actively. It amplifies any small increase or decrease of CT technique because the effects accumulate during multiple scans in a hospital stay. It would be trivial to add a suitable radiopaque marker to the badge to allow confirmation that the badge was included or excluded from the working view during interventional cases or CTP studies. A tool of this nature could have averted the problems encountered with an unexpected elevated dose from CTP that was recently widely reported by the press.¹³ While CT scans are usually performed with a standard technique, there may be a role for an especially low-dose follow-up examination in patients with high badge readings who require repeat studies. This seems particularly important for portable CT units in the ICU.

Whether a running total of calculated dose or an actual measure of skin dose as we have demonstrated, a metric for cumulative dose during hospitalization is not widely used. The greatest benefit of a real-time dose measurement may accrue from the Hawthorne effect. This phenomenon of human psychology means that the simple act of measuring a behavior can alter it in a beneficial way. Awareness of cumulative dose would provide positive feedback on efforts to reduce dose. For example, we decided to decrease the dose used for our portable CT scanner once the readings from our first 5 patients were available. As imagers are challenged with finding the right balance of dose and detail with x-ray techniques, cumulative dose should be considered along with dose from individual studies.