Journal ufClinicul Densitumerry, vol. 2, no. 4, 397401, Winter 1999 0 Copyright 1999 by Humana Press Inc. All rights of any nature whatsoever reserved. 0 169-41 94/99/2:39740 l/$l 1.25

Radiation Dose to the Patient and Operator from a Peripheral Dual X-Ray Absorptiometry System

Rajesh Patel, MSC, Glen M. Blake, PHD, and Ignac Fogelman,nm Department of Nuclear Medicine, Guy’s Hospital, London, UK

Abstract

Although peripheral dual X-ray absorptiometry (pDXA) scanners for measuring bone mineral density (BMD) in the forearm are known to produce an exceptionally low radiation dose to the patient, quantitative assessment of patient dose from pDXA procedures is important for reassuring patients about their safety. We have estimated the effective dose of radiation (ICRP-60) to the patient and also the scattered dose to the oper- ator from a forearm BMD examination performed on a DTX-200 pDXA system (Osteometer Meditech, Hoersholm, Denmark). Measurements were performed using thermoluminescent dosimeters attached to the forearm phantom supplied by the manufacturer. The effective dose to a patient was estimated to be 0.1 pSv. At a distance of 1 m from the center of the forearm, the time-averaged scattered dose rate to the operator assuming a throughput of five patients per hour was measured to be <0.1 pSv/h. The dose rate over the out- side surface of the DTX-200 in line with the primary X-ray beam was measured to be 1.4 pSvk. These fig- ures compare with a natural background radiation in the United Kingdom of 7 pSv/d. In conclusion, the radiation doses from forearm pDXA to both patients and operator were found to be truly trivial.

Dual X-ray absorptiometry; peripheral skeleton; effective dose. Key Words:

Introduction The increasing requirements for ready patient

access to bone densitometry scanning facilities has led to renewed interest in small, low-cost X-ray absorptiometry devices dedicated to measurements of the peripheral skeleton (1,2). The forearm makes a convenient choice for the assessment of a patient’s bone mineral status owing to its peripheral location and relatively small amount of surrounding soft tis- sue (3). Measurements of bone mineral density (BMD) at the forearm have been shown to predict

Received 02/25/99; Revised 05/21/99; Accepted 0611 1/99. Address correspondence to R. Patel, Department of Nuclear

Medicine, Guy’s Hospital, St. Thomas Street, London SE1 9RT, United Kingdom. E-mail: r.patel@umds.ac.uk

osteoporotic fractures at the forearm itself and also at the hip and the spine (4-6). Originally, bone densito- metry of the forearm was performed using the tech- nique of single photon absorptiometry (SPA), which utilized a radioactive 1251 source. However, following the technical advances in dual X-ray absorptiometry (DXA) devices for measuring the central skeleton, a new generation of peripheral DXA (pDXA) systems has been introduced in which an X-ray tube with a low-voltage generator has replaced the 1251 source. pDXA devices avoid the need for frequent source replacement and recalibration, which was a serious drawback of SPA, and the higher photon flux avail- able from X-ray tubes results in better precision, improved spatial resolution, and shorter scan times.

A further advantage of pDXA is the exceptionally low radiation dose to the patient. Despite the fact that

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the dose is so low, quantitative assessment of patient dose from pDXA procedures is important for reas- suring patients about their safety and for submissions to local ethical committees for research studies. Information on operator dose is important for clarify- ing what radiological precautions are necessary for the operation of equipment. Manufacturers of DXA equipment often quote radiation dose in terms of the entrance skin dose (ESD). However, the ESD is of limited relevance because it does not take into account the relative sensitivity of the affected organs to radiation-induced cancer or the volume of irradi- ated tissue. A better index of radiation risk is the effective dose (7), which relates directly to the prob- ability of carcinogenesis in patients or genetic injury in their offspring. Any exposure to X-rays entails a small risk to the patient that can be expressed in terms of the effective dose given the requisite information on organ doses and tissue-weighting factors (7).

This study was undertaken to assess the effective dose of radiation to the patient and the scattered dose to the operator from an examination of forearm BMD performed on the DTX-200 pDXA system (Osteometer Meditech, Hoersholm, Denmark).

Materials and Methods The effective dose is defined by the ICRP-60

report (7) as the sum of the absorbed dose to each irradiated organ weighted for the radiation type (the quality factor) and radiosensitivity of that organ (the weighting factor):

Effective dose = Z D B F W ,

in which DT = dose to organ Z QF = quality factor (1 for X-rays); and WT = weighting factor for organ T

Since the sum of the weighting factors for all the organs is unity, the effective dose is equal to the uni- form dose to the whole body which carries the same risk to the patient as the given examination. It is expressed in units of dose equivalent, usually either microsieverts or millisieverts.

Measurements of radiation dose were performed using thermoluminescent detectors (TLDs). The TLD- IOOH lithium fluoride chips (NE Technology, Reading, UK) used for this study contained trace

amounts of copper, magnesium, and phosphorous to increase their sensitivity and make measurements possible down to an absorbed dose of 1 to 2 pGy. The TLDs were calibrated by performing measurements of the primary radiation from a QDR-4500 DXA scanner (Hologic, Bedford, MA) that were compared with the readings from a calibrated ionization cham- ber (MDH-2025, Radcal, Monrovia, CA). Both mea- surements were performed using 5 cm of soft tissue-equivalent medium to provide backscatter.

To estimate the ESD to the patient, the TLDs were attached to either side of the forearm phantom sup- plied by the manufacturer (10 TLDs on each side). Ten scans of the forearm phantom were then per- formed. After establishing the direction of the X-ray beam, a depth dose curve was produced by scanning the TLDs between successive I-cm thick layers of a soft tissue-equivalent scattering medium.

The tissue-weighting factors allowing for the radiosensitivity of each type of tissue were obtained from ICRP-60 (7). The relevant radiosensitive tissues listed in ICRP-60 were bone surfaces, skin, muscle, and red bone marrow. The total volume of irradiated tissue in the forearm in a DTX-200 scan was esti- mated by performing measurements on the arm of a volunteer at I-cm intervals (width and depth) using a pair of callipers. DXA scans of the forearm and total body were performed on the QDR4500 DXA scanner and combined with information on the percentage of cortical and trabecular bone in the forearm to esti- mate the percentage of total bone surface area in the DTX-200 scan field. The percentage of total skin area in the scan field was estimated by normalizing the measured skin area in the scan field to the estimate of body surface area obtained from the Du Bois equa- tion (8). The percentage of muscle was estimated from the data on total body lean tissue mass provided by the whole-body DXA scan. The volume of bone marrow in the scan field was estimated from mea- surements of the cross-sectional areas of the radius and ulna on the forearm DXA scan after correction for the physical volume of bone tissue estimated from the forearm bone mineral content. The estimated forearm bone marrow was then expressed as a per- centage of the average adult mass of red bone marrow of 1500 g (9).

The scattered dose to the operator from a scan on the DTX-200 was measured by positioning the

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Table 1 Summary of Calculation of Effective Dose

for a DTX-200 Forearm Scana

Tissue ESD (pGy) Quality factor Attenuation factor Weighting factor Tissue (%) Effective dose (pSv)

Skin 80 1 0.7 0.01 1 .o 0.006 Bone surfaces 80 1 0.7 0.01 0.7 0.004 Muscle 80 1 0.5 0.005 0.7 0.002 Red marrow 80 1 0.7 0.12 1.3 0.087

Total 0.10

a The ESD is 80 pGy.

TLDs in a circular arc 15 cm from the center of the forearm phantom above the DTX-200. The phantom was then scanned 100 times. The surface dose in line with the primary X-ray beam was measured by attaching TLDs on the outside of the DTX-200 and then performing 50 scans of the phantom. The mea- surements were performed with soft tissue-equiva- lent material behind the TLDs to account for backscatter.

A RIALTO thermoluminescent reader (NE Technology) was used to measure the light emitted from the TLDs. The dose was then calculated by multiplying the counts obtained from the detectors by the relevant dose calibration factor and then sub- tracting the background (in dose units) obtained from undosed detectors.

Results The ESD for a DTX-200 forearm scan measured

using the TLDs was 80 pGy. For each of the four tis- sues irradiated, the fraction in the DTX-200 scan- ning field was estimated as approx 1%. Table 1 illustrates how the ESD, attenuation factors, weight- ing factors, and tissue fraction in the DTX-200 scan- ning field were used to establish the effective dose. The total effective dose for a forearm scan per- formed on the DTX-200 was estimated to be 0.1 pSv. This figure is likely to be a conservative esti- mate since the major contribution is from red bone marrow and since in middle-aged and elderly sub- jects this tissue is mainly restricted to the central skeleton. In such subjects, the effective dose may be as low as 0.01 pSv.

0

320 40

180

Fig. 1. Polar plot of DTX-200 scatter dose at 15 cm from the patient's forearm. The view is looking distally along the patient's left forearm and shows the time-aver- aged dose rate in microsieverts per hour assuming a throughput of five patients per hour. The outer circle is the controlled area limit of 7.5 pSv/h. The peak dose rate is 3.8 pSv/h at a position angle of 340". (- 0 -), DTX = 200; (- 0 -), controlled area.

Assuming a maximum patient throughout of five patients per hour (allowing 10,000 patients a year to be scanned), at a distance of 15 cm from the center of the phantom the time-averaged dose rate was measured to be <5 pSv/h (Fig. 1). By extrapolating this result using the inverse square law, the dose rate at a distance of 1 m from the center of the phantom was estimated to be <0.1 pSv/h (Fig.2).

Leakage radiation was estimated by measuring the dose rate over the outside surface of the DTX- 200. The mean radiation dose (including backscat- ter) measured using an array of 9 TLDs and assuming a throughput of five patents per hour was 1.4 ~ S v k .

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Pate1 et al. 400

10

h

f > I CT) 2 a, v

w s 8 0.1

g

0.01 50 100 150 200

Distance (cm) Fig. 2. A log-linear plot of DTX-200 scatter dose to

the operator in pSv/h vs distance from the patient’s forearm in centimeters. The solid circle shows the peak dose rate measured at 15 cm from the forearm as shown in Fig. 1. By extrapolation of this dose rate using the inverse square law (solid line) the dose rate at a distance of 1 m from the fore- arm (dashed line) is estimated to be <O. 1 pSv/h.

Discussion Several investigators have reported on the doses

involved in studies using axial DXA devices from both pencil- and fan-beam systems (10-19), but mea- surements of the radiation dose from pDXA systems have not previously been reported. Lewis et al. (11) estimated the effective dose of radiation to a typical female patient for a range of DXA studies and included data for the distal forearm. The effective dose to the forearm was estimated to be 0.07 pSv, which is comparable to the radiation dose of 0.1 pSv estimated for a f o r e m scan on the DTX-200 in the present study. This value is extremely low even when compared to the effective dose arising from a routine spine and hip DXA investigation of (depending on the model of equipment used) 0.2-40 pSv (18), which itself is considered to be a very small radiation burden for the patient. Other points of comparison that help put this result into context include an effec- tive dose of 20 pSv from a chest X-ray, 800 pSv from a spine X-ray, and 5000 pSv from a Tc-99m radionu- clide bone scan. Also, the daily natural background radiation in the United Kingdom is 7 pSv.

In general, the scattered radiation dose to the oper- ator in DXA investigations has not been considered a hazard (19). In the United Kingdom, the maximum annual dose for a nonclassified worker is 15 mSv, which corresponds to a time-averaged dose rate in the workplace of 7.5 pSvk (20). If dose rates approach this limit, the working area is defined as a controlled area and requires environmental monitoring and a written system of work. At lower dose rates of up to 5 mSv/yr (2.5 pSv/h), the lesser limit of a supervised area applies. In 1990 the International Commission on Radiological Protection recommended an annual dose limit of 1 mSv/yr for members of the public (7). Although not a limit applied to radiation workers, we note that this figure is equivalent to 0.5 pSv/h in the workplace. In the present study, the dose rate to the operator at a distance of 1 m from the forearm with maximum patient throughput was estimated to be <0.1 pSv/h, which is well within the ICRP-60 rec- ommendation. Under normal operating conditions, there is no requirement for the operator to be closer than 1 m from the patient since it is possible to view the progress of the scan and check for any movement using the image on a PC monitor screen. Applying the inverse square law suggests that the radiation dose limit for staff for a controlled area of 7.5 pSvk will be exceeded only when the distance from the center of the forearm is closer than 10 cm.

The dose rate measured over the outside surface of the DTX-200 (in line with the X-ray beam) was measured to be 1.4 pSv/h. This includes the effect of backscatter and assumes a throughput of five patients per hour. This result suggests that the DTX-200 is effectively shielded.

In the United Kingdom, there also is a recom- mended exposure limit to any single tissue (i.e., skin of forearm) of 150 mSv/yr (20). Although it would require approx 2000 scans of the forearm on the DTX- 200 to reach this limit, operators should not scan them- selves or place their hands in the scanning area.

In conclusion, the radiation doses from the DTX- 200 to both patients and staff were found to be truly trivial.

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