Kerma Area Product Method for Effective Dose Estimation During Lumbar Epidural Steroid Injection Procedures: Phantom Study
Abstract
OBJECTIVE. The purpose of this study was to derive from the kerma area product the dose conversion coefficient for estimating the effective dose for lumbar epidural steroid injection procedures.
MATERIALS AND METHODS. A mobile fluoroscopy system was used for fluoroscopic imaging guidance of lumbar epidural steroid injection procedures. For acquisition of organ dose measurements, 20 diagnostic metal oxide semiconductor field effect transistor detectors were placed at each organ in an anthropomorphic phantom of a man, and these detectors were attached to four mobile metal oxide semiconductor field effect transistor wireless bias supplies to obtain the organ dose readings. The kerma area product was recorded from the system console and independently validated with an ion chamber and therapeutic x-ray film. Fluoroscopy was performed on the phantom for 10 minutes for acquisition of the dose rate for each organ, and the average clinical procedure time was multiplied by each organ dose rate for acquisition of individual organ doses. The effective dose was computed by summing the product of each organ dose and the corresponding tissue weighting factor from International Commission on Radiologic Protection publication 60.
RESULTS. The effective dose was computed as 0.93 mSv for an average lumbar epidural steroid injection procedure (fluoroscopic time, 40.7 seconds). The corresponding kerma area product was 2.80 Gy·cm2. The dose conversion coefficient was derived as 0.33 mSv/(Gy·cm2).
CONCLUSION. The effective dose for lumbar epidural steroid injection can be easily estimated by multiplying the derived dose conversion coefficient by the console-displayed kerma area product.
Introduction
In the United States, low back pain is the second most common symptom-related ailment [1]. Approximately 80% of the U.S. population experiences low back pain in a lifetime [2]. One of the treatments routinely performed in pain clinics to reduce low back pain is injection of steroid medication into the epidural space between L4 and L5 [3]. Lumbar epidural steroid injection can reduce the inflammation and swelling caused by spinal disorders such as spinal stenosis, sciatica, radiculopathy, and herniated disk. In the middle of a lumbar epidural steroid injection procedure, epidurography (fluoroscopy) usually is performed to help the physician accurately place the steroid injection needle. To minimize the risk associated with use of ionizing radiation, monitoring of the absorbed organ doses and the effective dose according to the as low as reasonably achievable principle is recommended.
The dose conversion coefficient calculated from the kerma area product has been used to estimate the effective dose in interventional radiology procedures [4]. The kerma area product is usually measured with a radiation detector inside the x-ray tube housing. The dose conversion coefficient is typically derived as the ratio of the effective dose to the kerma area product. Because it is difficult to measure organ doses directly, the effective dose usually is estimated with Monte Carlo simulations [4–6]. In this study, the effective dose was estimated from physical measurements obtained with metal oxide semiconductor field effect transistor (MOSFET) technology, which emerged in the field of radiation dosimetry in the mid 1980s [7, 8]. Because of nearly real-time readout, the small dimensions of the detectors, and accuracy and reproducibility, MOSFET technology was first used in radiation therapy [9]. Thermoluminescence dosimeters have been used to validate the MOSFET method in the kilovoltage energy range of diagnostic radiology [10]. The purpose of our study was to determine the effective dose from organ doses measured with diagnostic MOSFET detectors in an anthropomorphic phantom and to derive the dose conversion coefficient from the effective dose and kerma area product values in lumbar epidural steroid injection procedures.
Materials and Methods
Our institutional review board granted a waiver of consent from human research subjects. We collected data on the tube energy, tube current, total fluoro scopy time, and kerma area product by retrospective review of the electronic records of patients who underwent lumbar epidural steroid injection.
Organ Dose Measurements
A mobile x-ray fluoroscopy system (OEC 9800 Super C-Arm, GE Healthcare) was used to measure the organ doses in an adult male anthropomorphic phantom (model 701-D, CIRS). The phantom had 39 sections, and each section had a slab thickness of 25 mm. The phantom was made from tissue-equivalent epoxy resins and represented an International Commission on Radiological Protection (ICRP) reference man (height, 173 cm; weight, 73 kg; thoracic anteroposterior by lateral dimensions, 23 × 32 cm) [11, 12]. The imaging system consisted of a rotating anode x-ray source (Varian RAD-99, Varian Medical Systems) with a trimode image intensifier (9/6/4-inch) attached to a C arm, which was easily controlled manually or with a motorized controller switch. The fluoroscopic parameters were 91 kVp and 4.03 mA. Beam quality was 5.27 mm aluminum-equivalent half-value layer.
Twenty high-sensitivity diagnostic MOSFET detectors (TN-1002RD, Best Medical Canada) were placed at each organ location in the phantom, and five detectors were attached to each of the four mobile MOSFET wireless reader modules (TN-RD-16, Best Medical Canada) to obtain a total of 20 organ-dose readings (Fig. 1A). The detectors were calibrated with an effective energy of 52.4 keV calculated with SRS-78 x-ray spectrum software, which is based on data in Institute of Physics and Engineering in Medicine report number 78 [13].
For dose measurements, the phantom was placed in the prone position to mimic a lumbar epidural steroid injection procedure (Fig. 1B). Fluoroscopy was performed for 10 minutes to obtain dose rates for individual organs, and the average lumbar epidural steroid injection procedure time of 28 seconds (Huh BK, personal communication, 2008) was used to obtain organ doses. The procedure was repeated three times, and the average dose was obtained for each of the 20 organs. The epidurogram acquired from this experiment is presented in Figure 2.
Effective dose was calculated by application of the tissue weighting factors from ICRP publication 60 to the equivalent doses, which were the same organ doses used for irradiation [14]. The effective dose was formulated as follows:
where WTi is the tissue weighting factor of an organ and Hi is the equivalent dose of an individual organ calculated as WRi · Di, where WRi is the radiation weighting factor (WR = 1 for x-ray) and Di is the individual organ dose.
The organ dose for the colon (Dcolon) was evaluated with the following weight fraction formula of ICRP publication 67 [15]:
where DULI is the average organ dose of the upper part of the large intestine and DLLI is the average organ dose of the lower part of the large intestine. DULI was calculated by weight fractioning of the organ dose of the transverse and ascending portions of the colon, and DLLI by fractioning of the descending and sigmoid parts of the colon and the rectum, on the basis of data in ICRP publication 23 [11]. For the red bone marrow, Cristy and Eckerman's percentage [16] distribution of active marrow data was adopted for estimation of the bone marrow distribution.
For the technically difficult direct measurements of bone surface doses according to the method of Kawaura and colleagues [17], the red bone marrow dose values were used to estimate the dose to the bone surface. The dry bone weight fractions and the ratios of cortical and trabecular bone in ICRP publication 70 [18] were used to derive the weight fractions of cortical bone in each bone location. This value was multiplied by the f-factor of cortical bone (4.675 for x-rays with effective energy of 52.4 keV) to derive the final bone surface dose. The equation for this procedure was as follows:
where Dbs is the bone surface dose, Drbm,i is the red bone marrow dose in each bone location i, Wdry,i is the weight fraction of dry bone in each bone location i, Wcort,i is the mass portion of cortical bone in each bone location i, (μen / ρ)bone cort is the mass energy coefficient for cortical bone, and (μen / ρ)tissue soft is the mass energy coefficient for soft tissue.
For the skin dose calculation, a detector was placed on the surface of the skin at the center of the irradiation field to measure a representative skin dose. We estimated the entire skin area of the phantom on the basis of information provided by the manufacturer, assuming that the phantom was elliptic. The estimated whole skin area was 1.63 m2, and the irradiated skin area was 0.0104 m2. Thus the area fraction of the skin was 0.0064, which was multiplied by the entrance skin dose reading to determine the skin dose. The skin dose in Table 1 is the dose after the area fraction was applied to the entrance skin dose.
| Structure | WT | D(mGy) | WT×D(mSv) | SDD(mGy) | SDWT×D(mSv) |
|---|---|---|---|---|---|
| Organs | |||||
| Gonads (male) | 0.20 | 0.0022 | 0.0004 | 0.0004 | 0.0001 |
| Red bone marrow | 0.12 | 0.0176 | 0.0021 | 0.0008 | 0.0001 |
| Colon | 0.12 | 0.1602 | 0.0192 | 0.0076 | 0.0009 |
| Lung | 0.12 | 0.0000 | 0.0000 | 0.0000 | 0.0000 |
| Stomach | 0.12 | 0.0036 | 0.0004 | 0.0002 | 0.0000 |
| Bladder and prostate | 0.05 | 0.0047 | 0.0002 | 0.0003 | 0.0000 |
| Breast (right) | 0.05 | 0.0000 | 0.0000 | 0.0000 | 0.0000 |
| Liver | 0.05 | 0.0010 | 0.0001 | 0.0009 | 0.0000 |
| Esophagus | 0.05 | 0.0001 | 0.0000 | 0.0003 | 0.0000 |
| Thyroid | 0.05 | 0.0000 | 0.0000 | 0.0000 | 0.0000 |
| Skin | 0.01 | 0.0060 | 0.0001 | 0.0125 | 0.0001 |
| Bone surface | 0.01 | 0.0080 | 0.0001 | 0.0005 | 0.0000 |
| Remaindersa | |||||
| Adrenals | 0.005 | 0.0004 | 0.0000 | 0.0006 | 0.0000 |
| Brain | 0.005 | NA | NA | NA | NA |
| Upper large intestine | 0.005 | NA | NA | NA | NA |
| Small intestine | 0.005 | 0.0398 | 0.0002 | 0.0010 | 0.0000 |
| Kidney | 0.005 | 0.0073 | 0.0000 | 0.0001 | 0.0000 |
| Muscle | 0.005 | NA | NA | NA | NA |
| Pancreas and kidney | 0.005 | 0.0073 | 0.0000 | 0.0001 | 0.0000 |
| Spleen | 0.005 | 0.0011 | 0.0000 | 0.0010 | 0.0000 |
| Thymus | 0.005 | 0.0000 | 0.0000 | 0.0000 | 0.0000 |
| Uterus | 0.005 | NA | NA | NA | NA |
Note—WT = tissue weighting factor of International Commission on Radiological Protection (ICRP) publication 60 [14], D = organ dose.
a
Remainders are the additional tissues and organs defined in ICRP publication 60 [14]. The organ dose measurements for brain and muscle were not measured and are listed as not assessed (NA) owing to dosimeter availability. The uterus is listed as NA because effective dose for men was evaluated. The organ dose for the upper part of the large intestine is considered in the colon dose calculation.
With the effective dose and kerma area product values read from the system console monitor, the dose conversion coefficient was derived as the ratio of effective dose to kerma area product.
Kerma Area Product Measurements
Kerma area product was independently validated by measurement of the air kerma with a 6-cm3 ion chamber (10x5-6, Radcal). The irradiation field was measured with radiographic film (X-Omat V, Kodak). The ion chamber was placed on the treatment table, and the film was placed on top of the chamber. A 5-inch-thick (12.7 cm) aluminum slab was placed 20 cm above the ion chamber to simulate patient body attenuation characteristics (Fig. 3A, 3B). Fluoroscopy was performed four times by activation of the x-ray tube for 30 seconds. For each irradiation, the kerma area product reading from the system console monitor was recorded and compared with the measured kerma area product.
Results
The individual organ doses and the products of tissue weighting factor and organ dose based on ICRP publication 60 [14] are shown in Table 1. The colon and entrance skin organ doses were higher than the other organ doses because of the location of the irradiation field (L4–L5). In this study, the effective dose calculated from the organ doses was 0.93 mSv (range, 0.33–2.20 mSv; n = 40 patients) and the kerma area product was 1.92 Gy·cm2 for the mean clinical fluoroscopic time of 40.7 seconds (range, 14.3–95.9 seconds) for the lumbar epidural steroid injection procedure. The dose conversion coefficient was derived as 0.33 mSv/(Gy·cm2). The kerma area product was measured directly and compared with the reading obtained from the system console unit (Table 2). The difference between measurements and readings from the system console was approximately 1%.
| Trial No. | Kerma Area Product from Console (Rcm2) | Ion Chamber (R) | Calculated Kerma Area Product (Rcm2) | Deviation (%) |
|---|---|---|---|---|
| 1 | 210.82 | 2.01 | 208.67 | -1.02 |
| 2 | 215.55 | 2.05 | 213.24 | -1.07 |
| 3 | 211.18 | 2.02 | 209.40 | -0.84 |
| 4 | 210.93 | 2.01 | 208.88 | -0.97 |
Note—R × 0.258 = mC/kg.
Discussion
Hart and Wall [19] in 2002 reported that the mean effective dose for a general lumbar spinal examination was 1.0 mSv and that the dose conversion coefficient was 0.21 mSv/(Gy·cm2). The United Nations Scientific Committee on the Effects of Atomic Radiation [20] also published medical radiation exposure data from a worldwide survey; the average effective dose for a lumbar spinal procedure was 1.0 mSv. Our finding of an effective dose of 0.93 mSv and dose conversion coefficient of 0.33 mSv/(Gy·cm2) agreed in general with the United Nations data. The slight differences may have been the result of the use of different clinical protocols at different centers and different equipment and technical parameters (tube voltage, beam quality, and irradiation geometry such as field size and focus-to-skin distance).
Our study had several technical limitations. First, the anthropomorphic phantom represented a reference man. The results of our experiment may vary depending on the size and weight of individual patients and the technical skills of individual physicians. Second, the total number of detectors in the current study was limited to 20 because of the available hardware. As a result, organ dose sampling was limited. In general, we placed one detector per organ at the center of each organ mass. Special care was taken, however, to place certain detectors (bone marrow, thoracic and lumbar spine, and upper large intestine) in the direct field of view during the lumbar epidural steroid injection procedure (Fig. 2). Although organ dose sampling was limited, scattered radiation contribution to organs outside the field of view was negligible owing to beam collimation. Therefore, we believe that our sampling of organ doses was adequate for our purpose. Third, care must be taken in applying the dose conversion coefficients because the coefficients can be influenced by specific exposure conditions and clinical protocols at other facilities [4].
We conclude that the effective dose for lumbar epidural steroid injection interventional radiologic procedures can be easily estimated by multiplying the derived dose conversion coefficient by the console-displayed kerma area product.
Acknowledgments
We thank Michael J. Cicchinelli of Duke Pain Clinic for technical assistance. We thank Richard Youngblood for editorial assistance.
Footnote
Address correspondence to T. Yoshizumi ([email protected]).
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Submitted: August 20, 2008
Accepted: November 23, 2008
First published: November 23, 2012
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