Radiation Exposure to the Surgeon During Fluoroscopically Assisted Percutaneous Vertebroplasty

SPINE Volume 30, Number 16, pp 1893–1898 ©2005, Lippincott Williams & Wilkins, Inc. Radiation Exposure to the Surgeon During Fluoroscopically Assiste...
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SPINE Volume 30, Number 16, pp 1893–1898 ©2005, Lippincott Williams & Wilkins, Inc.

Radiation Exposure to the Surgeon During Fluoroscopically Assisted Percutaneous Vertebroplasty A Prospective Study Roger Harstall, MD,* Paul F. Heini, MD,* Roberto L. Mini, PhD,† and Rene Orler, MD*

Study Design. A prospective case control study design was conducted. Objectives. The purpose of the current study was to determine the intraoperative radiation hazard to spine surgeons by occupational radiation exposure during percutaneous vertebroplasty and possible consequences with respect to radiation protection. Summary of Background Data. The development of minimally invasive surgery techniques has led to an increasing number of fluoroscopically guided procedures being done percutaneously such as vertebroplasty, which is the percutaneous cement augmentation of vertebral bodies. Methods. Three months of occupational dose data for two spine surgeons was evaluated measuring the radiation doses to the thyroid gland, the upper extremities, and the eyes during vertebroplasty. Results. The annual risk of developing a fatal cancer of the thyroid is 0.0025%, which means a very small to small risk. The annual morbidity (the risk of developing a cancer including nonfatal ones) is 0.025%, which already means a small to medium risk. The dose for the eye lens was about 8% of the threshold dose to develop a radiation induced cataract (150 mSv); therefore, the risk is very low but not negligible. The doses measured for the skin are 10% of the annual effective dose limit (500 mSv) recommended by the ICRP (International Commission on Radiologic Protection); therefore, the annual risk for developing a fatal skin cancer is very low. Conclusion. While performing percutaneous vertebroplasty, the surgeon is exposed to a significant amount of radiation. Proper surgical technique and shielding devices to decrease potentially high morbidity are mandatory. Training in radiation protection should be an integral part of the education for all surgeons using minimally invasive radiologic-guided interventional techniques. Key words: vertebroplasty, radiation exposure, radiation protection, fluoroscopy. Spine 2005;30:1893–1898

The ongoing trend towards less invasive surgical procedures in orthopaedics and especially in spine surgery has sparked an increasing interest and need for surgical navigation, which in most of the cases is performed by intraoperative fluoroscopy. Percutaneous reinforcement of osteopoFrom the *Department of Orthopaedic Surgery and the †Division for Medical Radiation Physics, Inselspital, University of Berne, Switzerland. Acknowledgment date: April 21, 2004. First revision date: September 22, 2004. Acceptance date: September 2, 2004. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and request for reprints to Paul F. Heini, MD, Department of Orthopaedic Surgery, Inselspital, University of Berne, CH-3010 Berne, Switzerland; E-mail: [email protected]

rotic vertebrae, the so-called vertebroplasty procedure, is one of these minimally invasive procedures. Vertebroplasty has gained popularity during the last years for the treatment of osteoporotic compression fractures, and several reports have documented its usefulness in addressing pain, prevention of further collapse, and preservation of posture. Rapid regression of pain has been found in 80 to 90% of treated patients with a long-lasting effect in a similar rate.1–10 The procedure is performed under biplanar fluoroscopic control. Although some authors have recommended computed tomography guidance during cannula placement11 or computer assisted surgery,12 cement injection can only be properly controlled by real-time fluoroscopy. With an increasing number of vertebroplasty procedures performed at our institution by the same spine surgeons and a general trend toward increasing intraoperative imaging in the majority of other spine procedures performed, the question of the radiation exposure for a spine surgeon developed. The purpose of this study was to determine the intraoperative radiation hazard to spine surgeons by occupational radiation exposure during percutaneous vertebroplasty and possible consequences with respect to radiation protection. Materials and Methods During a time period of 3 months, 32 consecutive vertebroplasty procedures were assessed. The prospective study includes 21 female and 9 male patients. The average age was 74 ⫾ 8.2 years. Twenty-nine patients had an established osteoporosis documented with a preoperative dual-energy x-ray absorptiometry scan. One patient sustained a traumatic fracture. Two patients also had multiple myeloma, and the possibility of a pathologic fracture could not be ruled out. Overall there were 88 radiologically diagnosed vertebral fractures. One hundred and thirty-six vertebrae were augmented in 30 patients via 156 pedicles (i.e., 4.5 vertebral bodies were reinforced per patient). The distribution of augmented levels is shown in Table 1. Two patients were treated with two separate sessions. All procedures were performed by two spine surgeons using standard fluoroscopic techniques in two planes as reported previously.13 The location of the surgeon with respect to the radiation source during fluoroscopy was standardized with the surgeon standing on the side of the source in the lateral view. In the anteroposterior view, the source was localized below the table and the image intensifier above the patient, respectively. Radiation exposure was registered with whole-bodydosimeters with lithium fluoride chips (Pedos AG; Muri, Switzerland). Six dosimeters were placed as depicted in Figure 1, and the cumulative radiation dose was measured for each location. The exposure of the hands was assessed with two special ring-dosimeters, placed on the ring-finger of each hand 1893

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Table 1. Distribution of Augmented Levels No. of Segments Augmented Level T5 T6 T7 T8 T9 T10 T11 T12 L1 L2 L3 L4 L5

Unipedicular

Bipedicular

2 5 6 5 4 6 10 13 15 16 15 15 4 116

1 0 0 2 0 0 1 3 6 2 3 1 1 20

Total

136

(Figure 2). All dosimeters were numbered and used at the same location on the surgeon during each procedure. The radiation dose was registered in millisievert (mSv). The surgical procedure was performed with a mobile C-arm fluoroscopy unit (Siremobil Iso-C; Siemens, Erlangen, Germany). The total filtration of the radiograph tube was ⱖ3 mm aluminum-equivalent, the focal-spot-to-image-intensifier distance was fixed at 90 cm, and the input field 27 cm in diameter. Exposure parameters were determined by an automatic brightness control. The radiograph unit was equipped with a permanent dose area product (DAP) meter and a beam-on timer, where the radiation time could be read directly from the

Figure 1. Dosimeter sites (front, thyroid, left arm, back; left and right ring finger, worn outside the apron/collar).

Figure 2. Whole-body dosimeter and special ring dosimeter containing LiF crystals to measure radiation dosage.

unit. The operative times and personal data of the patients were registered on a prospective protocol including diagnosis and augmented levels. The patient’s weight was not investigated because an average occupational exposure for the surgeon was to be assessed in a representative series of patients independently of the patient’s dimensions.

Results The cumulative radiation doses on the different dosimeters were the following: forehead 2.7 mSv, left upper arm 11.4 mSv, thyroid gland 7.1 mSv, back 0.3 mSv, left

Radiation Exposure During Percutaneous Vertebroplasty • Harstall et al 1895

Table 2. Radiation Dose after 32 Consecutive Procedures

Dosimeter Position

Cumulative Radiation Dose (mSv)

Extrapolated Radiation Dose Per Year (mSv)

Radiation Dose/ Vertebrae (mSv)

Radiation Dose/ Pedicle (mSv)

Thyroid gland Eye lens equivalent D4 left hand D4 right hand Left arm Back (Reference)

7.1 2.7 14.5 6.7 11.4 0.3

31.2 11.9 63.7 29.4 50 1.3

0.052 0.020 0.107 0.049 0.084 0.002

0.045 0.017 0.093 0.043 0.073 0.002

For the skin, we only reached 5.9% for the right hand, 12.7% for the left hand, and 10% for the left upper arm of this threshold dose. The calculated annual mortality is 0.0006 – 0.0013%. According to the risk catalogue, the mortality risk is very low but not negligible. Discussion

ring-finger 14.5 mSv, right ring-finger 6.7 mSv. The extrapolated annual radiation doses for each dosimeter location and radiation doses per augmented level are shown in Table 2. The radiation doses per pedicle punctured are slightly lower than per augmented vertebrae because of using a bipedicular augmentation technique in 20 vertebrae. The average operative time was 56.19 ⫾ 9.55 minutes. The average beam-on-time was 7.97 ⫾ 1.96 minutes. The average amount of vertebrae reinforced per operation was 4.25. The average beam-on-time per augmented vertebrae and per pedicle punctured was 2.23 ⫾ 0.89 and 1.79 ⫾ 0.53 minutes, respectively. The average dose area product, which was permanently measured by the DAP meter of the radiograph unit, was 2775 ⫾ 912 cGy cm2. The average DAP per augmented level and per pedicle punctured was 790 ⫾ 384 and 632 ⫾ 248 cGy cm2 respectively. The extrapolated value for the annual thyroid radiation dose in our series was 31.2 mSv, and the calculated annual mortality using the fatal cancer probability coefficient for the thyroid is 0.0025% in our unit. According to the risk catalogue by Fritzsche (Table 3),14 the annual mortality risk for a single operator is very low to low. Extrapolating further, the 10-year risk is low to medium and the life-time risk assuming a 20 to 25 years working period is medium to high. The annual morbidity associated with thyroid radiation is 0.025% in our unit, representing a low to medium one-year risk according to the risk catalogue. The life-time risk, however, is high to very high! By performing percutaneous vertebroplasty, we reach about 8% of the maximum allowed annual dose for the eye lens (150 mSv). Therefore, the annual risk for developing a radiation induced cataract is very low. Table 3. Mortality Risk Appraisal (14)

Extremely high Very high High Average Low Very low Negligible

Mortality Risk (Per 100,000)

Mortality Risk (%)

⬎1000 100–1000 30–100 10–30 3–10 0.3–3 ⬍0.3

1 0.1–1 0.03–0.1 0.01–0.03 0.003–0.01 0.0003–0.003 ⬍0.0003

Percutaneous vertebroplasty requires radiographic visualization in two planes (anteroposterior and lateral view) while inserting the filling cannulas, and real-time fluoroscopic monitoring is mandatory while injecting cement. In addition to proper surgical technique, visualization of cement flow is crucial to optimize the results of this procedure. As stated previously, “every-drop-has-to-bemonitored”,13 and a drawback of this technique is an important radiation exposure to the surgeon. With an increasing number of vertebroplasty procedures being performed, the question about the importance of this radiation exposure for a surgeon arose. Several studies have investigated the patient’s and the surgeon’s radiation during different surgical procedures in traumatology and orthopaedics requiring fluoroscopically navigated techniques.15–21 The current authors, however, are not aware of any well controlled study specifically focusing on vertebroplasty and exposure to various body areas of the surgeon. Theocharopoulos et al15 investigated the relative percent contribution of different procedures (hip, spine, kyphoplasty, and vertebroplasty) to the surgeon’s effective radiation dose and the associated risk for development of a fatal cancer when a protective apron and collar are used. Fluoroscopic screening using a mobile C-arm fluoroscopy unit was performed on an anthropomorphic phantom using four projections common in imageguided orthopaedic surgery. They simulated various common exposures for the spine and the hip and found an effective whole-body-dose of 96 mSv/patient for kyphoplasty and vertebroplasty procedures, 8.41 mSv/ patient for spine procedures, and 2.43 mSv/patient for hip procedures while wearing an apron, thyroid protection, and goggles. They concluded that 90% of the orthopaedic surgeon’s effective dose and risk is attributed to kyphoplasty and vertebroplasty procedures whereas another 8% was attributed to spine procedures. Amar et al22 assumed a decreased radiation exposure of the surgeons hand using a cement delivery system that is remote from the radiograph beam. They do not, however, provide any data of radiation exposure in their description. Kruger et al23 investigated the radiation dose reduction to the medical staff under two different conditions during vertebroplasty. They collected 12 months of occupational dose data for a single surgeon and compared the effect of lead sheets that were positioned directly on the patient in order to reduce scatter radiation to the operator’s hands, eyes, and upper extremities. Radiation was measured with a digital dosimeter that was placed approximately in the surgeon’s chest and hand area. Furthermore, they investigated the effect of different operational modes of the C-arm. They assessed 36 vertebro-

1896 Spine • Volume 30 • Number 16 • 2005

plasty procedures. The average whole body dose per vertebroplasty level was 1.44 mSv, and the hand dose was 2.04 mSv without lead sheets. With the sheets in place, the corresponding values were 0.004 mSv and 0.074 mSv. The effect of the lead sheets was much more important during lateral projections than during anteroposterior projections. In comparison to our own measurements of 0.052 mSv per level for the whole body exposure and 0.107 mSv for the hand dose, their values are at least 20 times higher. The higher doses may be caused by a longer beam-on-time per augmented level, but based on the data provided, it is not possible to explain these differences in more detail. When one considers the differences in technique between Kruger, who used an injection device connected with a tube and our own technique where the syringe is directly connected to the cannula, the discrepancy between measured radiation doses becomes even more perplexing. Kallmes et al24 measured the difference in radiation exposure for the direct cement injection and the use of an injection device with dosimeters placed at the left wrist during 39 vertebroplasty procedures. They compared the radiation dose for the cement injection with a 1-mL syringe in 19 procedures and with the injection device in 20 procedures. The radiation dose using a syringe was 1 mSv and using the device was 0.55 mSv, which were not significantly different from each other. The time per injection with the device was much longer, but the radiation exposure per minute was significantly smaller using the device. When compared with the current study, their exposure time per augmented level appears significantly longer (8.7 versus 2.23 minutes), especially considering that we reinforced 4.5 vertebral bodies during an average exposure time of 7.97 minutes including a investigation with the C-arm before draping. Theoretically the following technical aspects influence occupational radiation exposure of the surgeon: exposure time, distance between radiation source and patient, location of the radiation source, and the use of a modern fluoroscopic unit. Additionally, the patient’s weight and the anatomic location of the vertebra to be reinforced are leading to potentially higher occupational radiation doses for the surgeon because of increasing crosssectional area. Unfortunately, the latter aspects are given and cannot be influenced before surgery. The average radiation time per level is low with 2.23 minutes. However the direct injection of the cement with a syringe without any connecting tube brings the surgeons hands closer to the radiograph beam. The use of this direct injection technique allows for the injection of cement at a higher viscosity, which means a lower risk for cement leakage. For K-wire and cannula placement, a hands-off technique during radiation exposure has been used by means of a 25-cm clamp that holds the K-wire and allows for it to be navigated. The cannula is slid over the K-wire under lateral radiation with the hand remote from radiation. As stray radiation contributes largely to occupa-

tional exposure, total effective dose is lower while standing on the side of the image intensifier during lateral projection. This would be preferable for the surgeon; however, the construction of the C-arm does not allow standing on the opposite site. Comparable exposure times with vertebroplasty are seen in interventional cardiology procedures like percutaneous transluminal coronary angioplasty.25 It has been demonstrated that wearing goggles, a whole-bodyapron, and a thyroid shield, which is standard procedure during percutaneous transluminal coronary angioplasty, decreases effective whole body radiation dose by a factor of 28.26 This can reduce significantly the radiation hazard to the surgeon. Basic Information about Radiation Exposure Different tissues show a different sensitivity to radiation. In 1990, the International Commission on Radiologic Protection (ICRP) defined hypothetic mortality coefficients per radiation dose for fatal cancers, the so-called fatal cancer probability coefficient (Table 4).27 This coefficient can be used as an indicator of radiation sensitivity of different tissues. The detriment of each cancer type includes a nonfatal component weighted according to the lethality fraction k. Thus, if in a given tissue there is a number of fatal cancers F, the total number of cancers, including the nonfatal ones, is F/k. According to this, the annual mortality can be calculated either for the thyroid or the upper extremities, multiplying the fatal cancer probability coefficient with the annual radiation dose. The effect of ionizing irradiation and the induction of disease in the human thyroid gland is well known and described for different pathologies such as acute thyroiditis, hypothyroidism, and both benign and malignant thyroid tumors in an excellent review article by Maxon et al.28 The development of malignant tumors is of special interest. Dolphin et al29 and Raventos et al30 found that after external irradiation in childhood, cumulative frequency of thyroid cancer showed a rapid increase and

Table 4. Hypothetic Mortality in a Population of All Ages from Specific Fatal Cancer after Exposure to Low Doses (27)

Bladder Bone marrow Bone surface Breast Colon Liver Lung Esophagus Ovary Skin Stomach Thyroid

Fatal Cancer Probability Coefficient (10⫺4Sv⫺1) ICRP 1990

Proposed Lethality Fraction k

30 50 5 20 85 15 85 30 10 2 110 8

0.50 0.99 0.70 0.50 0.55 0.95 0.95 0.95 0.70 0.002 0.90 0.10

Radiation Exposure During Percutaneous Vertebroplasty • Harstall et al 1897

a plateau 15 to 25 years after exposure. The latent period for radiation-induced thyroid neoplasms may be longer than 30 years, and cases of thyroid cancers have been reported as long as 40 years after irradiation.31 Annual morbidity (the risk of developing a thyroid cancer, fatal or nonfatal) can be estimated by dividing the annual mortality rate by the lethality fraction k. Therefore, the annual morbidity for the thyroid in our unit is 0.025%, representing a low to medium 1-year risk. This risk can be decreased to a negligible level by wearing a standard 0.5-mm lead collar during vertebroplasty. The thyroid dose may be used to estimate the wholebody-dose. The annual effective whole-body-dose limit according to the ICRP and federal regulations concerning radiation protection is 20 mSv.32 Assuming that no whole-body-apron is worn throughout the procedures, the annual effective dose limit could easily be exceeded by only performing 100 vertebroplasty procedures. Therefore, the use of a whole-body-apron is mandatory. Opacification of the ocular lens is an important effect of exposure to ionizing radiation.33 The association of cataractogenesis to ionizing radiation was first described in 1897 by Chalupecky.34 The effect of radiation to the eye lens has been investigated in astronauts being exposed to relatively high doses of all types of radiation in space35,36 and patients undergoing total body irradiation before bone marrow transplantation.37,38 During protracted irradiation, repair of sublethal damage takes place. Therefore, the damaging effects of a dose administered at a low rate, as during fluoroscopically guided procedures, are less than of the same dose delivered at a high rate. This has also been shown by Merriam et al,39 who described the relationship among the total dose, exposure rate, and cataract formation. The rate of repair derived for cataract development is within the range of values reported for late responding tissues.37 Therefore, a deterministic risk for the eye lens to develop a radiation-induced cataract has been postulated at a threshold dose of 150 mSv per year. This dose is equal to the annual effective dose limit fixed by the ICRP and federal regulations concerning radiation protection.32 For skin tissue, a fatal cancer probability coefficient of 2 䡠 10⫺4Sv⫺1 and a lethality fraction k of 0.002 have been determined by the ICRP. Skin like the thyroid is a late responding tissue; however, its sensitivity to radiation is lower than the thyroids by a factor of 4. A high percentage (99.8%) of all radiation-induced cancers is nonfatal. Therefore, a deterministic risk to develop a fatal cancer has been postulated with a threshold dose of 500 mSv for upper and lower extremities, which represents the annual effective dose limit. Until a significant reduction in the radiation dose can be reached by the development of different techniques or new surgical tools, one should emphasize maximum radiation protection by the use of shielding devices such as lead aprons, thyroid collars, and goggles. Additionally, a properly maintained fluoroscopy unit and a surgeon properly trained in intraoperative imaging

techniques are mandatory in order to further limit radiation exposure. Conclusion Performing percutaneous vertebroplasty can lead to a significant radiation exposure for the spine surgeon. For effective radiation protection, operative techniques have to be optimized. Wearing a lead collar and a wholebody-apron is highly recommended to decrease the whole body radiation dose, which should decrease the occupational radiation exposure of radiation-sensitive tissues like the thyroid, the stomach, or the colon. Although the annual risk for developing a radiation induced cataract is very low, it is not negligible. Therefore, wearing special lead goggles is also recommended. The use of special lead gloves to decrease exposure of the hands must be weighed against the increased difficulty of handling surgical devices, which may lead to a longer beam-on-time with effectively increasing the radiation dose. Operators performing vertebroplasty need a fundamental understanding of radiation physics and radiation protection to minimize radiation exposure. Therefore, training in radiation protection for patients and staff should be an integral part of the education for those using interventional techniques. Risks and benefits, including radiation risks, should be taken into account when new interventional techniques are introduced. Maximum possible radiation protection should be performed until new techniques with lower radiation doses, shorter exposure time, and decreased total radiation dose are available. Key Points ● Fluoroscopic imaging during percutaneous vertebroplasty requires longer beam-on-times than in other orthopedic routine procedures. ● Occupational radiation exposure (e.g., by scatter radiation) lead to a high morbidity and mortality risk concerning the development of a thyroid cancer. ● International and federal radiation protection annual dose limits may be exceeded by a single operator performing percutaneous vertebroplasty if no radiation shielding devices are used. ● Radiation-reducing procedures and shielding devices such as a whole-body apron, lead-collar, and goggles lead to a significant decrease of mortality and morbidity of radiation-sensitive tissues such as colon, stomach, thyroid gland, and eye lens. ● Training in radiation protection as a part of education for all surgeons using fluoroscopically assisted techniques is mandatory.

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