Published ahead of print on October 21, 2010
doi: 10.3174/ajnr.A2260

American Journal of Neuroradiology 31:E87-E88, November-December 2010
© 2010 American Society of Neuroradiology

A. Mamourian^a, M. O’Shea^a and A.D.A. Maidment^a
aDepartment of Radiology
Hospital of the University of Pennsylvania
Philadelphia, Pennsylvania

We applaud the efforts of Moskowitz et al to increase awareness of the risks of cumulative radiation dose in their article, “Cumulative Radiation Dose during Hospitalization for Aneurysmal Subarachnoid Hemorrhage.”¹ We certainly agree that it is essential to minimize radiation dose from all sources because the diagnosis and treatment of patients with subarachnoid hemorrhage may result in a substantial radiation exposure from multiple sources. At the same time, we were surprised at the magnitude of cumulative radiation doses reported. These are beyond those expected on the basis of the literature and our own experience. We believe that there are several contributing factors for these discrepancies.

There are 2 general types of radiation injuries: deterministic and stochastic. These need to be accounted for separately. In neuroradiology, the main organs susceptible to deterministic injury are the skin and the lens of the eyes. Skin erythema will generally be evident at 6–8 Gy. Doses exceeding 8 Gy will result in exudative and erosive changes to the skin, and doses exceeding 20 Gy will result in nonhealing ulceration.^2,3 Temporary epilation will occur at 3–5 Gy, and permanent epilation, at single doses exceeding 7 Gy.² Not all body areas are equally sensitive; however, the scalp and beard are among the most sensitive to radiation epilation. Irradiation of the eye will lead to cataract formation for single doses of 2 Gy and fractionated doses of 4 Gy.^3,4 The stochastic effects refer to the formation of future cancers. In this article, the authors refer to the cranial dose; we presume that the authors in fact are referring to the entrance skin dose.

In calculating and reporting the absorbed dose to the skin (an organ dose), one typically is interested in the peak dose toany 1 location on the skin. It is assumed that this region of peak exposure is the most likely to demonstrate injury. Maintaining that region at the lowest possible dose will, in general, reduce the severity of injury. One must, therefore, consider the orientation of the beam relative to the patient in such calculations. The relative skin dose at the entrance and exit surfaces of the patient typically varies by a factor of 30–100 in radiography and fluoroscopy. In CT scans, the skin dose is, to a first approximation, constant over all irradiated regions of the skin. In this article, the authors implicitly assume that the region of the skin exposed to the peak radiation in each procedure is the same and, thus, that the cumulative skin dose is equal to the sum of the procedural entrance skin doses; this is clearly an overestimation.

The result shown in this article for the mean cumulative radiation dose given to the cranium during the course of hospitalization was 12.8 ± 7.7 Gy (range, 2.4–36.1 Gy). This is a surprising number, especially because the authors report that even the patients who went to open surgical aneurysm clipping with no intervention accumulated doses in the 4 Gy range. Presuming a mathematic error, we recalculated the dose for patients without intervention from the data provided in the article. The Table is based on the doses indicated in their “Equipment and Radiation Dose” section of the article. We used their projected dose from C-arm intraoperative angiography alone because the authors indicate that routine digital subtraction angiography (DSA) was not part of their treatment algorithm. This rough approximation indicates that the result published in the article for this group (average, 4.6 Gy) is significantly higher than the estimated cumulative dose (1.2 Gy).

We have additional concerns with this work. The article does not indicate the neurointerventional procedure dose from thebiplane Axiom Artis dBA scanner (Siemens, Erlangen, Germany) but does state the dose by using a Siemens portable C-arm (Siremobil Iso-C). This dose of 310 mGy seems much higher than expected. The dose will depend on many factors, such as collimation, kilovolt, and milliampere settings and the magnification setting, which are not indicated in the article. If we assume a 1 R/min fluoroscopy rate and 100 mR/frame for the acquisition mode, then the dose for this procedure would be more like 90 mGy compared with 310 mGy.

Because the authors indicate that 87% of the cumulative dose could be accounted for by the neurointerventions, one wouldexpect that their experience could be benchmarked against other studies of radiation exposure during similar interventions.A study from 2007 by D’Ercole et al⁵ not only used the air kerma values but validated them against readings from a Gafchromic film (ISP, Wayne, New Jersey) placed on that patient. In their study of 21 procedures, the maximum absorbed dose was 3.20 Gy with mean of 1.1 Gy. Even assuming that all the patients in the article by Moskowitz et al had even more complex procedures, as the authors suggested, it is difficult to understand how their patients experienced doses that were 10-fold higher.

Because no comparative reference dosimetry method was used for the study of Moskowitz et al, it seems most likely that thenumbers reported are misinterpreted or misrepresented by the equipment as the authors of the Moskowitz paper themselves suggest. Adding support to this premise, the unit used to indicate cumulative dose that was correlated with length of hospitalization in Fig 5 is milligray, while Figs 3 and 4 use gray for the same patients. The absence of any reports of acute radiation injury in their patient population does not support the authors conclusions since at the doses cited, most of their patients should have demonstrated substantial skin injuries and cataract formation, depending on the proximity and/or inclusion of the orbits in the radiation field.

We think that is it important that the authors review their calculations and validate their equipment against another standard.If their patients are indeed receiving such doses, the authors should re-evaluate their interventional techniques. While thecumulative doses of CT, CT perfusion, and CT angiography (CTA) in addition to DSA and neurointervention can approach 3 Gy in some patients, we do not think that the high doses reported in this article are representative of the average radiationdose in this patient group. If this proves to be an overestimation, it illustrates the difficulties that may be encountered whenusing estimated doses and highlights the speculative nature of some articles that use dose estimates instead of the measured radiation dose.

References

1. Moskowitz SI, Davros WJ, Kelly ME, et al. Cumulative radiation dose during hospitalization for aneurysmal subarachnoid hemorrhage. AJNR Am J Neuroradiol 2010;31:1377–82[Abstract/Free Full Text]
2. Mettler FA J., Upton AC. Medical Effects of Ionizing Radiation. 2nd ed. Philadelphia: W.B. Saunders; 1985
3. International Commission on Radiation Protection 41. Non-Stochastic Effects of Ionizing Radiation. Oxford, United Kingdom: Pergamon; 1984
4. Merriam G, Szechter A, Focht E.. The effects of ionizing radiations on the eye. Front Radiat Ther Oncol 1972;6:346–85
5. D’Ercole L, Mantovani L, Thyrion FZ, et al. A study on maximum skin dose in cerebral embolization procedures.AJNR Am J Neuroradiol 2007;28:503–07[Abstract/Free Full Text]

Reply

Published ahead of print on October 21, 2010
doi: 10.3174/ajnr.A2323

American Journal of Neuroradiology 31:E89, November-December 2010
© 2010 American Society of Neuroradiology

S. Moskowitz^a and B. Davros^a
aCleveland Clinic Foundation
Cleveland, Ohio

We thank Mamourian et al for their comments about the radiation exposure in subarachnoid hemorrhage (SAH). We understand that there are significant limitations to the methods we applied. As we understand, there are 2 lines of comment that they are addressing.

The first relates to the fundamental measurements that we reported. Air kerma is a measurement of the exposure to air at a fixed distance from a radiation source. This is the value returned for all C-arm and biplanar equipment. Biologically relevant doses are, however, a related but different item. Air kermadoes not account for scatter, angle of inclination to the skin, 2D distribution over an area, and so forth. Because the head is, in fact, usually a complex nonspheric shape, the real skin entry dose seen from imaging studies is not equal to the air kerma reported by the software. Even with a conversion applied, the real life use of radiation is not perfectly represented. Nonetheless, for the purposes of a retrospective analysis, the best (and only) data available for exposure are those gathered by that measure, and hence we used it for our review.

The second concern relates to the measurements that we used to calculate exposure from the variety of sources. The measurements were obtained from regularly scheduled quality and safety checks from all equipment. The doses were consistent with prior measurements and have been since that time. The doses were within the acceptable range for each such study, so we are not concerned with the safety of the equipment or the accuracy of the numbers used for our calculations.

In summary, we have 2 fundamental comments about our article. The first reflects on the overall comments made by Mamourian et al. Air kerma is not a perfect metric to measure skin entry radiation dose. Many factors impact the absorbed dose and the biologic response to any radiation source, including scatter, shape of the head, distribution over the skin, angle of inclination, actual distance of the skin to the source, and so forth. The actual dose a patient receives may be many factors less or more than the air kerma reported by quality assurance studies and software included with the fluoroscopy equipment. A better measure would be direct analysis of the skin entry dose. The use of GafChromic film (Specialty Products, Wayne, New Jersey) would account for many of the variables, even if not a perfect solution itself. Already underway are future endovascular studies that use this to measure total and peak skin entry dose, with consideration of distribution over the skin and eyes. We anticipate that this will control for some of the distinctively high doses seen, and it will demonstrate that the peak skin dose will not be represented reliably by air kerma.

The second comment is that the purpose of this study was not to identify the highest skin dose possible. We were highlighting the fact that in a very sick population of neurovascular patients, repetitive radiation-based imaging studies can result in significant radiation exposure. We believe that great care must be applied to this patient population in this regard, and any technique to reduce unneeded exposure should be considered. We are aware that it is not possible today to expect no exposure at all through a prolonged hospitalization for SAH. However, we hope that the results of our study bring to light a need for conscientious use of imaging; they are not intended to define a standard by which individual patient care should be measured.