Ionizing radiation doses during lower limb torsion and anteversion measurements by EOS stereoradiography and computed tomography☆
Introduction
Lower limb torsion values (anteversion of femoral necks or torsion of the knees or legs) play a role in the functioning of these limbs and in the development of osteoarthritis of the hip [1]. These values must be measured before placement of hip or knee prostheses [2] but also before osteotomies for femoral or tibial correction of bone calluses or dysmorphisms. They are also useful before surgical revision of hip and knee prostheses in cases of poor positioning, given the frequent imprecision of surgeons’ intraoperative evaluation of prosthetic femoral anteversion [3].
Orthopedists and radiologists have long used computed tomography (CT) to take these measurements [4]. Its advantages include ease and speed of performance, combined with the wide availability of equipment. CT examination also provides a look at bone morphology and at the quantity and qualite of the bone matrix. Its greatest disadvantage is its irradiation, especially of the pelvis and genital organs.
Lower limb torsion is measured most often among the elderly (65 years or older) during work-ups before or after prosthesis placement. Nonetheless, work-ups before correction of acquired or congenital dysmorphisms involve a younger population (younger than 18 years). Some abnormalities even require regular follow-up (surgery to lengthen an unequal leg) with repeated lower limb measurements. These examinations are a source of irradiation and its risks are greatest for the youngest subjects [5], [6]. Population irradiation from medical sources has been rising quite substantially; in the US, the annual such dose received per person has climbed from 0.53 mSv in 1980 to 3.1 mSv, on average, with the mean overall dose rising from 3 mSv to 5.6 mSv [7], [8]. Some studies [8] suggests that the effective cumulative dose from radiologic procedures exceeds 20 mSv per year for approximately 4 million nonelderly American adults (from 18 through 64 years of age) and 30 to 40% of this subpopulation receiving high doses is younger than 50 years. CT scans are very largely responsible for this increase: the number of CT scans increased by more than 10% per year from 1993 to 2008 (although the US population has increased at less than 1% annually) [7]. Accordingly, in 2006, CT was responsible for 49% of the collective effective doses from radiology and nuclear medicine procedures in the US, even though these CT examinations accounted for only 17% of all procedures [7]. No data are currently available about the number of CT examinations performed annually to measure lower limb torsion. We can nonetheless suppose that it is rising, in view of the aging of the population in western countries and the consequent increase in the number of cases of osteoarthritis and joint replacement surgery [9], [10] even if presurgical assessment of lower limb torsion is not routine in all countries.
A 3D stereoradiography (SR) unit named EOS (EOS-Imaging, Paris, France), a full-body imaging system, has been used to model spinal scoliosis in 3D and thereby obtain precise measurements of the spine [11]. This system uses very sensitive ionization chamber detectors, based on Charpak's multiwire chamber technique, and an X-ray source collimated in a narrow beam that scans the area to be explored. It thus makes it possible to use very low doses of X-rays. The doses used in work-ups of scoliosis are 6 to 9 times lower than those from full spine radiography [12], the standard technique for studying the spine in standing position.
This SR system can also be used for the lower limbs to determine both morphologic (bone dimensions and torsion) and static (flexion, varus or valgus) parameters [13], [14] with the patient standing in his or her natural position. CT is the technique currently used to measure lower limb torsion. Two recent studies have shown that the SR system and CT perform similarly in measuring femoral and tibial torsion and essentially provide interchangeable measurements [14], [15]. Neither the X-ray dose delivered during this SR examination of the lower limbs nor the dose delivered during a helical CT examination for torsional measurements has ever been measured by direct dosimetry. Only several very old studies explored the doses from torsional measurements taken by the (now obsolete) sequential CT [16], [17], [18], [19].
The aim of this study is to compare the doses delivered to the lower limbs and pelvis by helical CT and SR during measurements of torsion of the lower limbs.
Section snippets
CT and EOS protocols
Dosimeters acquired the doses delivered by a CT scanner (Sensation 16 model, Siemens, Erlangen, Germany) and the EOS system (EOS-Imaging, Paris, France).
For the CT scanner, the protocol called for the helical acquisition of the anteroposterior (AP) scout (digital) view, focused on 3 areas of interest: (1) the coxofemoral joints and entire femoral necks; (2) the knees (femoral condyles and tibial plateaus); and (3) the ankles (malleoli). Table 1 reports the scanner's acquisition characteristics.
Results
All images were acquired with the techniques recommended by the manufacturers of these machines to obtain images of adequate diagnostic quality. The helical acquisitions and the scout views were summed to determine the CT doses to each organ.
The maximum surface entrance dose measured on the anterior side of the phantom pelvis was 0.57 mGy with SR, with a posterior exit dose of 0.15 mGy. The maximum surface dose measured with the CT scanner (combining entrance and exit doses because of the
Discussion
The effects of low doses (approximately 10 mGy) and very low doses (1 mGy or less) on the risk of radiation-induced cancer have been hotly debated for many years. During very low X-ray doses, the cell damage is the same as at low doses, but the number of cells affected diminishes linearly as the dose decreases. This is the argument for a linear no-threshold (LNT) model to estimate the effects of ionizing radiation on living tissue [24]. Nonetheless, a range of recently observed untargeted effects
Acknowledgments
We thank Michel Bourguignon for his advice in the preparation of this manuscript.
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