A survey of radiation dose to patients and operators during radiofrequency ablation using computed tomography
1 Biomedical Electronics Engineering Program,
University Malaysia Perlis, Arau, Malaysia
2 Department of Biomedical Imaging, University of Malaya, Kuala Lumpur, Malaysia
Computed tomography (CT) fluoroscopy is able to give real
time images to a physician undertaking minimally invasive procedures such as
biopsies, percutaneous drainage, and radio frequency ablation (RFA). Both
operators executing the procedure and patients too, are thus at risk of
radiation exposure during a CT fluoroscopy.
This study focuses on the radiation exposure present
during a series of radio frequency ablation (RFA) procedures, and used
Gafchromic film (Type XR-QA; International Specialty Products, USA) and thermoluminescent dosimeters (TLD-100H; Bicron, USA) to measure the radiation received by
patients undergoing treatment, and also operators subject to scatter radiation.
The voltage was held constant at 120 kVp and the current
70mA, with 5mm thickness. The duration of irradiation was between 150-638
Ultimately, from a sample of 30 liver that have undergone
RFA, the study revealed that the operator received the highest dose at the
hands, which was followed by the eyes and thyroid, while secondary staff dosage
was moderately uniform across all parts of the body that were measured. � 2010
Biomedical Imaging and Intervention Journal. All rights reserved.
Keywords: radiofrequency ablation, Gafchromic film,
thermoluminescent dosimeter, computed tomography fluoroscopy
While the field of view (FOV) is concentrated on a more
limited area in CT fluoroscopy, conventional CT uses 150-170 mA, and CT
fluoroscopy tube current is invariably reduced to 50 mA. Other dosimetry practices
are similar to the usual spiral CT scanning, along with tube voltage (kVp)
being at similar levels.
These real-time 2-D images are constantly updated while
the x-ray tube rotates around the subject. Dynamic axial imaging is provided,
with real-time observation of image reconstruction possible. Given this
advantage, it is an invaluable procedure for monitoring the passage of a biopsy
needle, or the tube during drainage.
A notable divergence in the two techniques is that CT
fluoroscopy is an interactive process, and as such requires the presence of an
operator at the location of the gantry. This necessarily invokes exposure to
scatter radiation, while conventional CT scanning allows the operator to
control the process remotely.
A non-surgical procedure, such as radio frequency ablation
(RFA), is a localised treatment that offers the patient freedom from side
effects without interfering with their general health, and a rapid return to
RFA uses the device of radiofrequency energy to which is
inserted into target tissue (invariably a tumour) via the tip of a needle
through the patient�s skin. On contact with the target tissue heat is produced,
which kills the target tissue while sparing tissue that is healthy. In the
event of the tumour causing bone matter to decay; this will not regenerate
however, the dead tissue of a tumour will reduce in size.
Materials and Methods
The University of Malaya Medical Centre provided the
opportunity to gather the data for this study from thirty RFA liver treatments.
The patients underwent sedation and the staff involved included a radiologist
and anaesthetist, medical officers, a radiographer, along with nurses and
support staff. The same radiologist and support staff participated in all 30
monitored treatments. Tube voltage was constant at 120 kVp, tube current at 70
mA and slice thickness was 5mm.
The use of film in dosimetry provides precise information
in respect of the location of the dose to the skin, and with calibration of the
film, it also allows for quantitative measurement of the dose. To this end,
Gafchromic� type XR-QA film (5" x 6") was placed at the rear of the
patient (Fig. 1), and the radiation received during treatment was measured.
Afterward, the film was converted to a digital format
using a Mikrotek ScanMaker 4800 flatbed scanner (Carson, USA) with ScanWizard5 Version 7 software. Measuring the extent of the blackened area on the digital
format allowed identification of the radiation dose received, and was made
possible by Adobe Photoshop version 6.0 (Adobe Systems, California, USA).
Thermo luminescent Dosimeters (TLD�s) were used to measure
the scatter radiation received by the operator and other staff. Each individual
had a sachet containing three TLD chips attached at the forehead, thyroid and
middle finger (Fig. 2), and the radiologist alone wore a thyroid shield. A TLD
reader (Harshaw 3500, Bicron, Ohio) then determined the average of these.
Result and Discussion
Radiation exposure during an RFA procedure that causes
blackening on the Gafchromic film is shown in Fig. 3. Due to the fact that the
x-ray tube has a constant orbit around the body of the patient, the darker
stripes signifying higher doses of radiation are found in the same central
location. Other alternating light and dark stripes occur below and above the
dark band at the centre due to scout and helical scanning that is used to
locate the target area prior to the insertion of the radio frequency probe into
The radiation dose received at the entry point on the skin
is provided as a function of time in Table 1, which shows that the dosage
received increases with time. While the highest dosage received was 326�2.12
�Gy for the duration 9<t<10 minutes, 326 �Gy is not considered particularly
harmful to a patient.
The World Health Organization (WHO) holds that skin
erythema and temporary epilation of the skin will occur at dosage levels of 2
and 3 �Gy, yet the basis for this study was conducted using a mere 32 �Gy/min
. Ordinarily, CT fluoroscopy uses 120 kVp tube voltage and 70 mA tuber
current for a typical RFA liver procedure, and with an exposure of
approximately 90 minutes, will invariably cause skin erythema and
temporary epilation of the skin to occur.
Given that the dosage received will increase as time
increases, the scatter radiation received by the radiologist and support staff
conform to this presumption (Tables 2 and 3).
The distance from the x-ray tube proved to be fundamental
to the extent of radiation received.
Accordingly, the radiologist being the closest to the
tube, received the greatest dose to the hand (354�4 �Gy), and less radiation to
the eye (260�4 �Gy) and the thyroid (73�7 �Gy). In order to insert the RF
needle into the tumour, the radiologists hand is quite close to the gantry, and
this provides sound basis of the results that emerged.
The rate of exposure also proved illuminating, as the
radiologist recorded higher levels to the hand (37�4 �Gy/min) and eye (31�
�Gy/min), yet only a mere 9�1 �Gy/min at the thyroid.
Of course, this is basis for the contention that a thyroid
shield will dramatically reduce exposure to radiation, with dose rates recorded
at a reduction of 24-29% compared to the unprotected eye and skin.
As one would expect, the supporting staff underwent far
lower levels of scatter radiation.
Here, the eye received the greatest dose (47�3 �Gy ) with
the thyroid (40�1 �Gy), and lastly the skin (37�3 �Gy). Due to the steep
increase in distance away from the gantry, and the negligible difference in
distance of the eye, thyroid and skin, the distribution of these results was
No thyroid shield was worn by supporting staff, and the
rate of dosage received also supported its use in order to reduce exposure to
scatter radiation. For the eye, thyroid and hand respectively, the data showed
rates of 5�2 �Gy/min, 4�1 �Gy/min, and 4�1 �Gy/min.
While careful calibration is required, the essential
element to this study was the use of Gafchromic film Type XR-QA. With it,
precise quantitative measurements are possible when measuring the distribution
of radiation from CT fluoroscopy.
Due to the x-ray beam in CT fluoroscopy orbiting along the
same channel, radiation is concentrated upon that area, the tissue surrounding
which may be at risk. Conversely, the preliminary diagnostic scanning process
did not record comparable results.
The variables that influence the radiation received by
patients and incidence of scatter radiation to operators are specifically, the
duration of the procedure, the distance away from the gantry, and the
Clearly, radiologists and supporting staff need to be
aware of the risks associated with exposure to scatter radiation, and adopt
occupational health and safety practices in wearing lead aprons, gloves and a
Figure 1 Diagram showing how Gafchromic film is positioned at the patient�s back.
Figure 2 The TLD sachets were taped at the forehead, thyroid and middle finger of the radiologist and supporting staff (a) Radiologist � the TLD sachet was taped under the thyroid shield (b) Supporting staff � TLD sachet was taped at the neck. Supporting staff did not wear thyroid shield.
Figure 3 The gafchromic film image after being exposed.
Table 1 Entrance dose in relation to exposure time.
Table 2 Dose received by the radiologist in relation to duration of procedures.
Table 3 Dose received by the supporting staff in relation to duration of procedures.
Table 4 Dose rate received by operators (radiologist and supporting staff) at eye, thyroid and skin of hand.
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|Received 3 May 2009; accepted 15 October 2009
Correspondence: School of Mechatronics, University Malaysia Perlis, Jalan Kangar-Arau, Jejawi, Perlis, Malaysia. E-mail: firstname.lastname@example.org (A. Saidatul).
Please cite as: Saidatul A, Azlan CA, Megat Amin MSA, Abdullah BJJ, Ng KH,
A survey of radiation dose to patients and operators during radiofrequency ablation using computed tomography, Biomed Imaging Interv J 2010; 6(1):e2