doi: 10.1161/hc2601.091710
(Circulation. 2001;104:58.)
© 2001 American Heart Association, Inc.
Clinical Investigation and Reports |
Accurate Assessment of Patient Effective Radiation Dose and Associated Detriment Risk From Radiofrequency Catheter Ablation Procedures
From the University of Crete, Faculty of Medicine, Iraklion, Crete, Greece.
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Abstract |
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Background—Radiofrequency (RF) cardiac catheter ablation procedures may require extended fluoroscopic exposure resulting in elevated radiation risk. The aim of the present study was to accurately establish RF ablation radiation risk levels and to provide means for accurate patient risk estimation from studies performed in any electrophysiology laboratory.
Methods and Results—Fluoroscopy required during cardiac ablation was classified into 4 types identified by beam orientation and irradiated tissue: (1) posteroanterior exposure during catheter advancing from the groin to the heart, (2) posteroanterior heart exposure, (3) left anterior oblique heart exposure, and (4) right anterior oblique heart exposure. The duration of each exposure was monitored in 24 patients undergoing RF cardiac ablation. Dose per minute of fluoroscopy was measured at 15 organs/tissues for each projection with the use of anthropomorphic phantom and thermoluminescence dosimetry. The effective dose rate was 219, 144, 136, and 112 µGy/min for groin-to-heart posteroanterior, posteroanterior, left anterior oblique, and right anterior oblique exposure, respectively. A typical ablation procedure results in a total effective dose of 8.3 mSv per hour of fluoroscopy. The average excess of fatal cancers was estimated to be 650 and 480 per million patients undergoing RF ablation requiring 1 hour of fluoroscopy for US and UK populations, respectively. The average risk for genetic defects was determined to be 1 per million births.
Conclusions—Radiation risk from RF cardiac ablation is moderate compared with other complications, but it may highly exceed radiation risk from common radiological procedures. Efforts should be made toward minimization of patient radiation risk from RF ablation procedures.
Key Words: catheter ablation • radiography • electrophysiology • risk factors
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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In less than a decade, radiofrequency (RF) catheter ablation has evolved from a theoretic concept to first-line therapy for patients with certain rhythm disorders.1 The procedure aims for localization and destruction of the arrhythmogenic foci or critical portions of an arrhythmia. Inserted commonly from the groin, electrode catheters are guided to the heart under fluoroscopic control in the posteroanterior (GHPA) projection, whereas fine catheter manipulations, within the heart, are commonly performed under fluoroscopic control in either posteroanterior (PA) or left or right anterior oblique (LAO or RAO, respectively) projections.2 The procedure is highly operator dependent and usually requires prolonged fluoroscopic exposure times.3 The cardiovascular complications, at the time of catheter ablation procedures and during short-term follow-up, have been described in the literature.4 5 6 Recently, the issue of potential harmful radiation effects from catheter ablation procedures has gained increased concern, given that patients are commonly young adults, and fluoroscopy occasionally exceeds 1 hour and even results in skin injuries in some extremely rare cases.7 8 9 10 The potential for radiation-induced cancer or genetic anomaly should not be disregarded in risk-benefit considerations, especially for individuals with many decades of expected life remaining. Therefore, patient radiation detriment risk levels should be established for the specific fluoroscopic system and technique used in every electrophysiology laboratory.9
The aims of the present study were to (1) accurately determine RF ablation organ dose data and patient effective dose from direct dose measurements, (2) provide data for the determination of patient radiation detriment risk from RF ablation procedures performed by use of different equipment and techniques, and (3) investigate the incidence of low-threshold deterministic effects, such as skin injuries, cataract formation, and parotiditis after RF ablation procedures.
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Methods |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The fluoroscopic exposures involved in a catheter ablation procedure were categorized into 4 types on the basis of beam orientation and tissue primarily irradiated: (1) GHPA exposure, involving fluoroscopy in the PA projection required for insertion and advancing the catheter from the groin to the heart, (2) PA cardiac fluoroscopic exposure, (3) 25° RAO cardiac fluoroscopic exposure, and (4) 45° LAO cardiac fluoroscopic exposure.
Patient Study
The duration of each fluoroscopic exposure was
monitored in a series of 24 consecutive patients (14 female and 10
male) undergoing catheter ablation procedures in the cardiac
electrophysiology laboratory of the University Hospital of Iraklion.
All patients gave informed consent. Mean patient age was 35±9 years
(range 21 to 53 years). All patients suffered from
symptomatic supraventricular
tachycardia. Fifteen patients were diagnosed with
atrioventricular nodal reentrant
tachycardia, and 9 were diagnosed with
Wolff-Parkinson-White syndrome. Three conventional quadripolar
catheters were inserted via the left and right femoral veins and
advanced to the right cardiac cavities. A special steerable catheter
for mapping and ablation purposes was also advanced to the right or
left cardiac cavity via the right femoral vein or artery, respectively.
Mean duration of the GHPA, PA, LAO, and RAO fluoroscopic exposures was
calculated.
All studies were performed by using a constant potential Philips BV300-R2 C-arm fluoroscopic unit (Philips Medical Systems) dedicated for cardiac catheterization procedures. The unit has an under-couch tube/over-couch image intensifier configuration providing the ability of last-image hold. Continuous fluoroscopy mode was used. The focus to image intensifier distance was 100 cm, and the input field size was 23 cm. Kilovoltage and tube current were selected by automatic brightness control. The measured half-value layer of the x-ray beam was 4.7 mm aluminum at 70 kV, corresponding to a total filtration of 9 mm aluminum. The x-ray tube output values at 70 cm from the tube were 1.29, 1.34, and 1.59 mGy/(min · mA) for tube voltages of 65, 67, and 70 kV, respectively.
Radiation Dose Measurements
Organ dose data were obtained separately for each of
the 4 identified fluoroscopic projections involved in catheter
ablation procedures from direct dose measurements on a Rando
anthropomorphic phantom (Alderson Research Labs). Rando phantom has
been broadly used for radiological dose
measurements.11 12 13 14 15
It is made by tissue equivalent material and simulates a human body
trunk from the upper third of the thighs to the vertex of an individual
73.5 kg in weight and 1.73 m in height. The internal organ built
into the phantom in addition to the skeleton is the lung. The lung
equivalent material is sculptured to a neutral respiratory volume, and
the left lung is smaller than the right to allow for the heart. The air
content of the trachea, stem bronchi, and esophagus is reproduced as
well. Thus, the phantom chest closely mimics a real human
chest.
Lithium (thermoluminescent dosimeter [TLD]-100) and calcium fluoride TLD-200 chips (Harshaw) were used to determine the dose at 546 different measuring points in the Rando phantom. Compared with TLD-100, TLD-200 is considered preferable for dose measurement at body sites far from the primarily irradiated volume, because it presents 300 times higher sensitivity.
The phantom was appropriately loaded with TLDs to allow determination of all organ/tissue doses required for the estimation of patient effective dose according to the recommendations of the International Commission on Radiological Protection (ICRP).16 In addition, the dose to eye lens and parotid gland was measured.
GHPA, PA, RAO, and LAO exposures were separately performed on the phantom. An experienced cardiologist positioned the x-ray tube for each projection aided by the fluoroscopic image and the internal phantom structure. The phantom was exposed for 20 minutes for each of the PA, RAO, and LAO projections, whereas GHPA fluoroscopic exposure was repeated 20 times to reduce the statistical error of dose measurements. The duration of all GHPA phantom exposures was taken equal to the average fluoroscopy time required for this projection estimated from the patient studies. A special effort was made to reproduce the table and x-ray tube movement commonly used in cardiac ablation procedures. Operating kilovoltage, tube current, and x-ray source to skin distance (SSD) for each phantom exposure were recorded. The effect of angulation on measured doses for LAO and RAO fluoroscopic exposures was evaluated by obtaining organ dose measurements for different angulations.
Organ Doses
Organ doses per minute of fluoroscopy were calculated
for each exposure from the following
formula:
 | (1) |
Patient Effective Dose
The effective dose per minute of fluoroscopy was
calculated for each exposure by using the following
formula:
 | (2) |
.
The patient effective dose (Etot) for
an ablation procedure requiring t1 fluoroscopic
time for GHPA, t2 for PA,
t3 for LAO, and t4 for
RAO projections may be determined as
follows:
 | (3) |
i is the effective dose per minute of
fluoroscopy for the exposure i given from Equation 2.
Detriment Risk Estimation
The radiation-induced fatal cancer risk was
determined by multiplying the effective dose by appropriate risk
factors. In the present study, age- and sex-related fatal cancer
risk factors provided by the Biological Effects of Ionizing
Radiations (BEIR) V Committee
report19 and National
Radiological Protection Board (NRPB) report
26020 were used. Derived by
US19 and
UK20 population mortality
rates,
Table 1
summarizes the estimates of lifetime risk for fatal
cancer stratified by age at exposure and sex. The risk for
radiation-induced hereditary effects was determined by multiplying mean
gonadal dose with an age- and sex-averaged risk factor of 0.01
Sv-1, recommended by
ICRP.16
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The minimum time of fluoroscopy required to induce skin
injuries was determined by dividing the threshold dose
reported21 by the entrance
skin dose rate obtained for LAO exposure, which was the maximum
observed. The fluoroscopic time required to induce cataract and
parotiditis was determined by dividing the corespondent threshold dose
by the mean organ dose per fluoroscopic minute resulting from a typical
ablation procedure. The fluoroscopy time threshold (T) for induction of
a deterministic effect, applicable to any other laboratory, may be
derived from the corresponding threshold T0
provided in the present
study:
 | (4) |
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Patient Study
Operating parameters and duration of the fluoroscopic exposures performed on patients undergoing RF ablation are shown in Table 2
. The mean total fluoroscopy obtained from the
patient measurements was 41±15 (range 15 to 67) minutes. The average
relative contribution of GHPA, PA, LAO, and RAO exposures to the total
fluoroscopy time was 1%, 59%, 27%, and 13%, respectively. The
ablation procedure was considered successful in 92% (22 of 24) of all
patients studied.
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Phantom Exposure
Parameters
The operating parameters and the duration
of GHPA, PA, LAO, and RAO fluoroscopic exposures performed on the
phantom are shown in
Table 2. The phantom SSD was 60, 64, 62, and 72 cm for
GHPA, PA, LAO, and RAO fluoroscopic exposures,
respectively.
Organ Doses and Effective Dose
The amount of absorbed radiation dose to various
organs/tissues and the total patient effective dose per minute of
fluoroscopy for each of the 4 projections involved in the catheter
ablation procedures are shown in
Table 3. Organ doses and total effective dose per minute of
fluoroscopy for a typical procedure requiring 1%, 59%, 27%, and 13%
of the total fluoroscopic time spent for GHPA, PA, LAO, and RAO
exposures, respectively, are also shown in
Table 3. The patient tissue receiving the greatest amount
of radiation dose is the skin area through which x-ray beam enters
patient’s body, whereas the organ receiving the greatest amount of
radiation from a typical ablation procedure is the lung. Significant
doses are received by esophagus, bone, stomach, breast, and red bone
marrow. Besides, the greatest contribution to patient effective dose is
the lung dose, followed by the dose absorbed by the esophagus, red bone
marrow, and stomach. The difference in effective dose per minute of
fluoroscopy for LAO and RAO projections was found to be <5% if
the angulation was changed by ±10°.
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Radiation Risk
In a patient, the age- and sex-averaged fatal cancer
risk from an RF ablation procedure requiring 60 minutes of fluoroscopy
is 0.065% for the US population and 0.048% for the UK population; the
genetic defect risk is 0.00012%. The dependence of radiation-induced
fatal cancer risk on patient age and sex is shown in the
Figure.
In general, compared with older patients, young patients are associated
with significantly higher risk. Compared with male patients of the same
age, young female patients are subjected to increased risk, whereas the
risk for older female patients is lower than the correspondent risk for
male patients. The minimum fluoroscopic time required for the induction
of various deterministic effects is shown in
Table 4.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The present study provides accurate risk levels for radiation-induced effects resulting from RF catheter ablation procedures. Data are also provided for the estimation of detriment risk for patients undergoing RF cardiac catheter ablation procedures in other electrophysiology laboratories using different equipment and techniques.
Direct Versus Indirect Estimation of
Effective Dose
Several methods have been developed for the
indirect estimation of patient effective dose and risk resulting from
medical exposures. With the use of such a method, organ/tissue doses
and effective dose are determined by using tabular data, given the beam
energy and entrance skin exposure or air kerma. Alternatively, direct
organ dose measurements can be obtained within anthropomorphic
phantoms. Such an approach enables direct measurement of the dose
distribution within the phantom and allows reliable estimates of mean
dose for any individual
organ.11 All studies found
in literature reporting risk levels from ablation
procedures2 3 7 10
have involved indirect organ dose estimation from measurements of
entrance skin dose or exposure. In the present study, the effective
dose was determined from the organ doses measured directly by TLDs
appropriately placed in a Rando phantom. An ablation procedure
requiring 1%, 59%, 27%, and 13% of the total fluoroscopy time for
GHPA, PA, LAO, and RAO exposures, respectively, was found to result in
a patient effective dose of 8.3 mGy per hour of
fluoroscopy.
Patient Radiation Risks
For the US population, the age- and sex-averaged
patient fatal cancer risk from an RF ablation procedure requiring 60
minutes of fluoroscopy was found to be
650x10-6; the genetic defect risk was
found to be 1x10-6. In the United States,
the spontaneous cancer risk is
20%,19 and the incidence
of serious birth defects is
6%.22 Thus, in 1
million patients undergoing typical RF ablation procedures requiring 60
minutes of fluoroscopy, 650 extra fatal malignancies are expected in
addition to the naturally occurring 200 000. Similarly, in 1 million
deliveries with 1 of the parents having undergone an RF ablation
procedure, 1 extra anomalous baby is expected in addition to the
naturally occurring 60 000.
The minimum fluoroscopic time required for induction of the lower threshold deterministic effect, namely, transient skin erythema, was 6.7 hours. Cataract formation and parotiditis cannot be observed as a result of RF cardiac catheter ablation procedures, because both effects require huge fluoroscopic times never occurring in clinical practice. Procedures requiring <6.7 hours of total fluoroscopy are not likely to induce deterministic effects in our laboratory. This threshold is high enough, inasmuch as a modern fluoroscopic system was used in the present study, which produces a heavily filtrated beam and an entrance exposure rate of <0.005 Gy/min at 70 kVp and 70 cm from the source.
Comparison With Other Studies
The mean total fluoroscopic time required during
catheter ablation procedures found in the present study (41±15
minutes) is similar to that reported by Lindsay et
al10 (50±31 minutes),
Calkins et al2 (47±40
minutes), and Rosenthal et
al3 (53±50 minutes). The
considerable difference between the mean fluoroscopic time reported in
the above studies and that reported by Kovoor et
al7 (94±44 minutes) may be
attributed to the small number of patients studied and the use of a
system not providing last-image hold. However, the large standard
deviations observed in the present as well as in all previous
studies demonstrate the significant variation in the total fluoroscopic
exposure received by patients undergoing catheter ablation
procedures.
The age- and sex-averaged radiation risks for fatal cancer
and hereditary effects per hour of fluoroscopy found in the present
study are presented in
Table 5, together with the correspondent risks found
in previous studies. Fatal cancer risk found in the present study
is lower than the correspondent values reported by Calkins et
al,2 Lindsay et
al,10 and Rosenthal et
al3 but 2-fold the risk
reported by Kovoor et al.7
Also, the genetic risk found in the present study is much lower
than that obtained by Calkins et al and Lindsay et al, whereas it is
similar to that reported by Kovoor et al. Inconsistencies may be
attributed to the (1) increased number of organs for which dose was
estimated in the present study, (2) use of modern equipment, and
(3) considerable variation of the irradiated tissue volume during each
fluoroscopic projection taken into account in the present
study.
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Data presented allow the accurate estimation of patient risk from procedures performed in any electrophysiology laboratory with different relative contribution of GHPA, PA, LAO, and RAO exposures to the total fluoroscopic time. Moreover, presented data may be used to estimate radiation risk from other cardiac catheterization procedures involving fluoroscopic projections similar to those in the present study.
Limitations of the Study
Inaccuracies in patient effective dose and associated
risk determination fall into 3 categories. First, there were
uncertainties related to the use of TLDs. Dosimeters were calibrated by
using a radiation spectrum similar to that used during measurements,
and the individual sensitivity of each TLD was used. Thus, the overall
uncertainty of TLD measurements was estimated to be <10%. Second,
there were inaccuracies due to the absence of direct dose measurements
during RF ablation procedures performed in patients. Thus, organ doses
of a patient undergoing RF ablation may differ from those measured in
the phantom because of the different body dimensions. The resultant
inaccuracy in effective dose and risk determination is expected to be
higher in overweight patients. Third, the fluoroscopic exposures
during a patient study may be performed with angulations that are
somewhat different from the corresponding phantom exposures. The
resultant uncertainty in risk estimations was evaluated to be
<5%.
Reduction of Patient Exposure
Even if radiation risks from an average RF
ablation procedure appear to be acceptable relative to the risks
associated with other therapeutic
approaches,4 5 6
these risks are significant compared with reported risks from common
radiological procedures. Therefore, efforts should be made to limit
radiation exposure. The fundamental approach to moderate patient
radiation exposure is to reduce the time that the beam is on. The
radiation field size should be minimized to include only the anatomic
region of interest. The SSD should be maintained at the maximum
permissible. The equipment used should be in concordance with
performance standards recommended by the US Food and Drug
Administration, Center of Devices and Radiological
Health.9 Fluoroscopic systems
providing a pulsed-fluoroscopy mode are preferable because of the
potential for delivering a lower dose to the patient. Periodic
inspection and quality control tests of the x-ray unit and image
intensifier of the fluoroscopic equipment should be conducted by a
medical physicist. It is noted that apart from reducing the patient
dose, all the above precautions also reduce the dose to the physicians
and nursing personnel involved in the
study.
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Footnotes |
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Reprint requests to K. Perisinakis, PhD, Lecturer of Medical Physics, Medical Physics Department, Faculty of Medicine, University of Crete, PO Box 1393, 71500 Iraklion, Crete, Greece.
Received December 7, 2000; revision received March 30, 2001; accepted April 10, 2001.
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