Cumulative Radiation Dose during Hospitalization for Aneurysmal Subarachnoid Hemorrhage

Risk of Injury

The exact risk to the patient associated with radiation exposure remains to be clarified. The deterministic effects of the radiation, ones that are dose-dependent, are more likely to be seen with increasing exposure and are more clearly understood.²⁰ Our data suggest that the patient with aneurysmal subarachnoid hemorrhage is at particular risk because of the high doses received during a short period of time. The stochastic effects of the radiation, ones that are dose-independent, are less understood compared with the higher doses seen for these patients. Long-term outcome studies have not explored the effects of radiation in the patient with subarachnoid hemorrhage, and reports of short-term complications of radiation exposure are limited.^6,21–23 Because the essential question that underlies this and other studies of radiation exposure for this patient group is one of short- and long-term risk, better controlled longitudinal studies are needed to follow patients for the development of skin, lens, thyroid, and cerebral injury or neoplasia.

The doses described here are comparable with doses known to result in severe late secondary skin injury and delayed neoplasia.²⁰ These clinical complications have not been seen in follow-up to date, and the minor side effects, including epilation and erythema, were not routinely documented. Significant injury may not have been witnessed in this population despite the high doses because the topographic dose distribution may not be accurately reflected by the total dose reported by the equipment. Furthermore, the actual dose may not be correctly represented by that reported by the equipment. A complementary longitudinal study will be important in specifically determining whether these patients develop any delayed radiation-induced injury. Nonetheless, the data suggest that special consideration be given to the patient with aneurysmal subarachnoid hemorrhage throughout the hospitalization, particularly because of the potential for cumulative exposures from repetitive studies.

Distribution of Dose

Fluoroscopy has a peak exposure at the skin next to the radiation source, and in biplanar fluoroscopy, the dose is maximally distributed at 2 points, typically posterior and left lateral (to minimize operator exposure). CT scanning evenly distributes the irradiation, while CT perfusion maximally irradiates a small cross-section. We estimated the exposure by combining the estimated entry-skin dose for each procedure and imaging study because it is most likely to reflect the cumulative radiation to the cranium. This is an understood limitation, in that the cumulative and effective doses received will overestimate exposure, particularly if the radiation-sensitive tissue is not directly in the field of the fluoroscope.

Reporting radiation dose for complex exposures, like fluoroscopy, is difficult because multiple metrics are available, including fluoroscopy time, cumulative dose, dose-area product, and entry-skin dose. The ideal measure is one that predicts actual injury and risk. While the best estimate may be entry-skin dose,²⁴ the cranium receives a nonuniform distribution of irradiation, making estimation of radiation exposure difficult. Entry-skin-dose measurements are possible with use of thermoluminescent detectors on the skin at the point of maximal radiation. This technique, however, does not afford an immediate on-line measure and is not universally available; these effects limit its utility. Additionally, use of thermoluminescent detectors assumes placement at the exact point of maximal dose, which is not known a priori.

Furthermore, patterns of irradiation are complex and vary among patients as well as procedures; these patterns prevent an accurate risk profile for each set of procedures and individual patients. For example, in the case of the lens, the radiation source during fluoroscopy is usually posterior to the head. Because entry doses are substantially higher than the exit doses, irradiation of the lens is overestimated by simple tissue-weighting of the total dose reported by the fluoroscopy equipment. Nonetheless, the estimation of cumulative doses allows an appreciation of the potential exposure accumulated during a brief hospitalization.

Doses from <1 to >6 Gy have been reported for an aneurysm coiling.^4–11,19 However, the doses received from individual embolizations in our patient group were higher, with a mean dose during treatment of the index aneurysm of 12.8 Gy. This high dose may be a reflection of the complexity of the aneurysms referred to our quaternary care institution, as well as from the use of biplanar fluoroscopy, as is standard during coiling of aneurysms. Additionally, treatment in a teaching institution inevitably results in extra dose from biplanar use when single plane imaging would be sufficient, filming through the venous phases of every run, extra runs between coil deployments, and so forth.

Another potential source of high exposure is the misunderstanding of the doses reported by the angiography equipment. Air kerma was reported by our equipment while skin-entry dose may be more important to monitor. Because the air kerma does not account for backscatter within the tissue itself, the dose is approximately increased by 1.4-fold to 1.7-fold (ICRU Report 47, Table A.3, page 25, 1992) depending on the kilovolt and energy spectrum of the beam. Understanding what is reported will lead to better patient monitoring.

The doses we identified are comparable with ranges of exposures known to correlate with mild-to-moderate skin injury, published by the US Food and Drug Administration¹ and in the literature.²⁰ However, the cumulative exposures we demonstrate are substantially higher. The distribution to these patients is wide (Fig 1), but most patients experience between 6 and 12 Gy. Unfortunately, the doses of one-fourth of patients exceeded 15 Gy, with few receiving >25 Gy.

The Joint Commission on the Accreditation of Healthcare Organizations, which monitors hospital practice, currently qualifies single-radiation exposure in excess of 15 Gy to a single field as an “overdose,” with potential for substantial harm.²⁵ No procedure identified in this study qualified as such a trigger. However, the single-event reporting mandate does not reflect the potential for a cumulative dose during the course of a hospitalization. Because patients who received these doses underwent a mean of 2.8 endovascular interventions and those receiving less radiation underwent a mean of 1.0 interventions, the physician is responsible for monitoring the cumulative radiation dose in the patients who require multiple procedures and are at potential risk for injury.

Mathematic Modeling

To identify imaging techniques that might reduce patient radiation exposure, dose was recalculated for each patient by using modified imaging techniques (Table). Calculations were made for the following: 1) replacing every portable CT scan with a fixed scan, 2) using the low-radiation acquisition mode of the portable CT, 3) replacing diagnostic angiography with CTA, 4) decreasing fluoroscopy rates from 15 to 10 frames per second, 5) replacing CT with MR imaging, 6 and 7) reducing the radiation dose during DSA from 3.6 μGy per frame to 2.7 μGy per frame or 1.8 μGy per frame, and 8) a combination of all the above changes. Simultaneous application of all changes included removal of all CT and diagnostic angiography, reducing the fluoroscopy rate to 10 frames per second, and reducing the radiation dose during DSA to 1.8 μGy per frame. Data for these changes were based on medical physics measurements as follows: Fluoroscopy at 10 frames per second decreased the dose by 33.3%, and the low-radiation acquisition mode of the portable CT decreased the dose by 50%.

These substitutions or modifications in imaging technique can reduce the cumulative radiation exposure (Table). Substitution of every portable CT scan for a fixed scan reduced exposure by 2.3%. Using the low-radiation acquisition mode of the portable CT reduced exposure by 1.9%. Replacing diagnostic angiography with CTA reduced exposure by 4.9%. Replacing CT with MR imaging reduced exposure by 6.6%. Decreasing fluoroscopy rates from 15 to 10 frames per second reduced exposure by 11.6%. Reducing angiography doses to 2.7 μGy per frame reduced exposure by 13.8% and to 1.8 μGy per frame reduced exposure by 29.1%. Applying all substitutions reduced exposure by 56.6%.

Incorporating these changes in neuroimaging to reduce the radiation exposure, however, has severe practical limitations. Fixed compared with portable CT scanning reduces radiation exposure but requires patient transport. MR imaging uses no radiation but may be impractical, requiring patient transport often far from patient care floors; is sensitive to patient motion; and is incompatible with certain metallic implants and/or generates potential image artifacts. Lower fluoroscopy rates, lower energy modes of CT scanning, and lower dose angiography do not involve these logistic considerations but trade radiation dose for image quality. The benefits of each imaging modification can be significant but should be carefully weighed against the risks to the patient. However, these suggestions are limited in that the potential dose reduction is at best a calculation and is not very likely exact for each individual situation. Furthermore, radiation-exposure reduction will be most practically and effectively made from the conscientious use of fluoroscopy and angiography.

Physician Awareness

This study demonstrates the substantial cumulative doses received by patients with subarachnoid hemorrhage during a complete hospital course. Understanding which patients are at even greater risk is important for procedure planning and in medical decisions. Increased length of hospitalization and higher Hunt and Hess grades and Fisher scores are frequently seen in sicker patients with the potential for vasospasm, hydrocephalus, and prolonged intensive care. Although each was anticipated to correlate with radiation dose, this correlation was not demonstrated in this study. The gradual rise in the cumulative dose for Hunt and Hess grades, Fisher scores, and aneurysm number may be suggestive. This effect may have been reduced by the wide variability in radiation exposure and higher mortality more commonly seen in higher grade patients.

The use of endovascular treatment for the primary aneurysm predicted a higher cumulative radiation dose. This result is expected in that the increased exposure is approximately 4 Gy, an average dose reported for a single aneurysm embolization. The decision to clip-versus-coil an aneurysm should not be made with this sole consideration. However, physicians should be aware of this significant contribution to the total radiation exposure.

Interventionalists should be aware that the long-term implications of substantial cranial irradiation may present in a delayed manner.²¹ Epilation, skin ulceration, or necrosis and neoplasms may not develop during the initial hospital course and are possible side effects of the radiation doses seen in our patient population. Aggressive screening for signs of radiation-induced injury and incorporation of early management should be part of the routine follow-up examination and care. Additionally, before the procedure, informed consent obtained for the intervention should incorporate the potential high doses of radiation and its associated complications. While mild epilation may be unavoidable and is often reversible, more severe forms of injury are possible and patients should be made aware of this possibility as a component of the preprocedural consent and in postprocedural education and screening.

Because many techniques are available to reduce radiation exposure, proper training highlighting radiation safety can substantially reduce exposure and potential injury to the patient. Maintenance of certification for the use of fluoroscopy equipment should similarly be emphasized to reinforce appropriate practice and encourage radiation-reduction techniques. Hospitals must similarly place particular emphasis on the quality and reliability of neuroimaging equipment. Antiquated fluoroscopy equipment can result in excess radiation through failure in shielding and use of analog rather than digital detection.⁵ The use of updated fluoroscopy equipment and software can minimize the radiation needed to acquire satisfactory images. While each interventionalist should individually decide which imaging changes can be made without compromising the quality of patient care, conscientious use of radiation for diagnostic and therapeutic purposes during prolonged procedures and hospitalizations should be a priority.

Suggestions to Lower the Cumulative Dose

We demonstrate that patients with subarachnoid hemorrhage can receive significant radiation exposure. Every maneuver that can reduce this exposure should be considered. Because the data demonstrated that most of the exposure is procedural, reducing exposure with appropriate techniques during interventions should be a focus. Such suggestions include reduction in DSA and fluoroscopy time; use of collimators, pulsed fluoroscopy, radiation filters, proper x-ray tube and intensifier placement; and limiting use of electronic magnification and so forth. These techniques should be used when possible.

Furthermore, the application of the recommended techniques to reduce radiation can positively impact this problem. Collimators effectively shield vital structures from direct exposure as well as reduce the tube current necessary to achieve the expected image quality. Using the minimum appropriate magnification reduces the dose but must be balanced against the image detail desired. Whenever anatomically reasonable, using an oblique or lateral imaging path that does not include the orbit can similarly reduce ocular exposure.