Radiation Safety for the Chronic Pain Physician

February 2019 Issue

  1. Jonathan Hagedorn, MD Pain Medicine Fellow, Mayo Clinic Co-Author
  2. David A Provenzano, M.D. President, Pain Diagnostics and Interventional Care Co-Author


On November 8, 1895, physicist Wilhelm Conrad Roentgen discovered x-rays while working in his laboratory at Wurzburg University in Germany. A year later, Thomas Edison created the first fluoroscope. Radiation technology has subsequently undergone significant advancements and has become an increasingly important tool for the interventional pain management physician.

The number of interventional pain procedures performed annually has increased over the past 20 years, much of which can be attributed to advancements in imaging technology. The major advantages of fluoroscopy are visualization of the procedure area, confirmation of direct medication placement, and detection of unintentional intravascular injection, all of which increase clinical efficacy, decrease possible side effects, and enhance patient safety.[1] As more physicians use fluoroscopy, interest in radiation safety has grown. Unfortunately, that increased interest has not translated into additional or, in some circumstances, adequate in-depth training. The International Commission on Radiological Protection has called for improvements in training to ensure the health of both physicians and their patients.[2]


Interventional pain physicians are at risk for both types of radiation exposure injury because of the chronic, low-dose radiation exposure over a lifetime.


Terminology

Part of the difficulty in understanding radiation safety is its confusing terminology. Units used to define the biologic effects of radiation and enable calculations of effective absorbed dose are referred to as dose equivalents. Multiple units are often used to describe similar concepts, depending on whether the author is using imperial or international system of units (SI units). The radiation energy absorbed is described as radiation absorbed dose (rad, imperial unit) or gray (Gy, SI unit). To predict biologic effects of radiation during occupational exposure, rad is converted to radiation equivalent man (rem, imperial unit) or sievert (Sv, SI unit). One rem is equal to 0.01 Sv. Most articles describing radiation dose use millisievert (mSv) as the preferred unit.

Radiation Specifics

 But what exactly is radiation and how does it produce harmful effects? In a simple sense, radiation is a wave or particle of energy emitted from a source. X-rays are photons of energy that are part of the electromagnetic spectrum, along with gamma, ultraviolet, infrared, radar, microwave, and radio waves. Those waves and their associated physical properties are identified by their different wavelengths. As X-ray photons pass through the human body, the energy they impart can create ionized atoms and free radicals, which can produce harmful effects. Photons that interact with but do not ionize atoms are deflected in various directions, which is referred to as scatter radiation. Scattered photons place physicians and other staff in the room at risk for exposure to the same harmful effects as the primary x-ray beam photons. X-rays that do not interact with any atoms are transmitted to the image receptor and create the desired radiograph.

Most of the radiation dose pain physicians may be exposed to is in the form of scatter radiation, not the primary x-ray beam.2 When the x-ray source is located below a patient without any rotation, the maximum scatter is primarily in the form of backscatter projecting below the table and the majority of unintended exposure reaches the physician’s lower extremities and feet. However, when the x-ray tube is rotated toward the physician and the x-ray beam projects away from him or her, most of the scatter radiation will project back to the physician, occurring at the level of more radiosensitive structures such as the eyes, thyroid, and breast tissue.

Fluoroscopy Unit Characteristics

 The two main determinants of image quality are peak kilovoltage (kVp, tube voltage) and milliamps (mA, tube current). Typical tube currents are 1–5 mA and tube voltages are 75–125 kVp. The automatic brightness control (ABC) on the fluoroscopy unit adjusts the kVp and mA to enhance image quality (brightness and contrast) while balancing patient safety. If the fluoroscopy unit is used in manual mode, ideally the kVp should be increased prior to increasing the mA to limit the increase in radiation exposure. For an equivalent increase in exposure, the mA must be doubled whereas the kVp would have to be increased by only 15%.

Radiation Exposure Recommendations

The average annual background radiation exposure from natural sources for individuals living in the United States is estimated to be 3 mSv. A comparison of radiation doses from frequent sources and the amount of time that it would take to receive the same amount of radiation from nature are found in Table 1.

Table 1: Radiation Doses[3]

Source

Radiation Dose (mSv)

Background Equivalent Radiation Time

Background (yearly)

3

-

Round-trip coast-to-coast flight

0.03

4 days

Extremity x-ray

0.001

3 hours

Chest x-ray

0.08

10 days

Spine x-ray

1.5

6 months

Spine computed tomography

6

750 days

Abdomen or pelvis computed tomography

10

1,250 days

 The National Council on Radiation Protection and Measurements (NCRP) annual effective dose equivalent (EDE) limits for different organs or areas of the body are listed in Table 2. Initially, NCRP described maximal permissible dose (MPD) for occupational exposure. Although MPDs are often listed in textbooks, the terminology has been replaced by the EDE limiting system to indicate that “no dose is considered permissible.”

 Physician radiation exposure is cumulative over a lifetime. According to NCRP, adhering to the limits ensures that lifetime risk from radiation exposure remains acceptable, not negligible. Individuals ideally should not receive more than 10% of the EDE limits annually. An occupational worker’s lifetime effective dose should be limited to his or her age in years times one rem.

Table 2: Annual Effective Dose Equivalent Limits per Organ System[4]

Area/Organ

Annual Effective Dose Equivalent Limits (mSv)

Thyroid

500

Extremities

500

Gonads

500

Lens of the eye

150

Whole body

50

Pregnant women

5

Radiation Exposure During Procedures

Patient doses during fluoroscopically guided interventional pain procedures have been reported to range between 0.08 and 0.15 mSv per minute of fluoroscopy when pulsed fluoroscopy of 3–15 frames per second (fps) is used. Average patient radiation doses for some common interventional pain procedures are listed in Table 3. With proper planning and protection, these doses should be substantially lower to the physician.

Table 3: Patient Radiation Doses for Specific Pain Procedures[5],[6]

Procedure

Radiation Exposure (secs)

Radiation Dose (mSv)

Interlaminar epidural

5.9

0.008–0.015

Joint injection

7.5

0.01–0.019

Kyphoplasty-single level

228

0.304–0.57

Kyphoplasty-multiple levels (per level)

168

0.224–0.42

Lumbar medial branch block

5.7

0.008–0.014

Lumbar transforaminal epidural

10.9

0.015–0.027

Spinal cord stimulation trial

134

0.179–0.335

Physician exposure can be monitored quarterly with thermoluminescent dosimeter badges, which are are worn on the outside of the lead apron and submitted to the specific manufacturing company for processing to ensure the physician is staying below recommended radiation dose limits. If exposure limits are exceeded, it may trigger warnings and procedural modification recommendations.

Risks

Biologic effects of radiation exposure are divided into two categories: nonstochastic (ie, deterministic) and stochastic. Deterministic effects are seen after a certain threshold is passed. Above that threshold, injury occurs (eg, cataracts, erythema) and higher radiation doses will increase severity. The acute radiation doses required to cause nonstochastic effects should not be exceeded during routine interventional pain procedures as long as appropriate safety measures are followed.

Typically, interventional pain physicians are exposed to cumulative low-dose radiation. Significant previous radiation exposure lowers the dose threshold for a stochastic effect, which is defined by an increasing likelihood of effect with higher total radiation doses. Unlike deterministic effects, it is assumed that there is no threshold below which the effect does not occur, a concept represented in the popular linear no-threshold model. Additionally, the severity of stochastic effects is not determined by dosage. Cancers, both hematologic and tumors, are examples of stochastic effects.

Interventional pain physicians are at risk for both types of radiation exposure injury because of the chronic, low-dose radiation exposure over a lifetime. Most information on the biologic effects of radiation has come from epidemiologic studies of human populations that have been exposed to acute high-dose radiation such as the atomic bomb survivors in Hiroshima and Nagasaki and individuals near the Chernobyl power plant. The long-term adverse effects and biologic consequences of cumulative exposure to low-dose radiation remain unclear. Furthermore, the risk of developing cancer in people subjected to low-dose radiation is debated and uncertain.

Tips for Reducing Radiation Exposure

Lifetime radiogenic risk depends on annual operative workload, radiation protection measures, and years of occupational exposure. With appropriate safety measures and knowledge, physicians can drastically reduce their radiation exposure.

1. Distance From X-Ray Source to Physician

Radiation exposure is inversely proportional to the square of the distance from the x-ray source, which is known as the inverse square law. Therefore, if physicians double their distance from the source, the radiation exposure reduces by a factor of four. At one meter, the scatter radiation exposure level is approximately 0.1% of the patient skin entrance dose by the primary beam. Physicians should stand as far away from the radiation source as reasonably and safely as possible.

2. Distance From Image Intensifier to Patient

Skin entrance dose is lowest when the distance between the image intensifier and patient is small and the between the x-ray tube and patient is large. Those same principles will limit scatter radiation to the performing physician and operating room personnel. The shortest distance from the x-ray tube to patient should be 30 cm, and longer distances, if possible, should be sought.

 3. Collimation

A collimator is a device built into the C-arm that will narrow the x-ray beam surface area to the area of interest, which decreases the patient dose and scatter radiation to operating room staff and improves image quality. The intensity of scatter radiation is a function of exposed field size. Therefore, doubling the field of view doubles scatter radiation dose rates. Thus, physicians should seek maximum collimation.

4. Pulsation

Fluoroscopy is categorized into two types: pulsed and continuous. Framerate is the number of fps during the use of fluoroscopy. Continuous fluoroscopy obtains 30 fps, which approaches the maximum temporal resolution of the human eye, so image acquisition appears as one fluid, continuous motion (like a movie). In pulsed fluoroscopy, physicians manually select a lower fps setting that the human eye detects as separate image captures. The video appears “choppy”; however, the radiation dose is significantly reduced because fewer images are needed. Thus, seeking the lowest reasonable fps rate and avoiding continuous fluoroscopy when possible will help limit exposure.

5. Magnification

As the area of interest is projected toward a larger area of the image receptor (magnified), more x-rays are needed to maintain good spatial resolution, which increases a beam’s intensity. This can be detrimental to the patient’s skin at the entrance site and increases scatter radiation to operating room personnel. Physicians should minimize magnification.

6. Protection

Lead shielding protects through its density and its positive charge. Lead aprons come in 0.25 mm lead equivalent thicknesses with thicker amounts providing better protection. At 0.25 mm and 0.5 mm thick, lead aprons will decrease radiation exposure over 90% and 99%, respectively. The apron should hang to at least the knees, because most of the scatter radiation exposure is to the lower extremities. Physicians should also wear a thyroid collar (0.5 mm lead equivalent) and lead glasses. Follow proper care instructions: folding the apron or collar can create cracks in the shielding, thus allowing an area of decreased radiation protection. The clinic should have a protocol to test lead shielding devices periodically for defects.

Additional lead shields can be attached to the operating room table, hanging down and blocking scatter radiation to the physician and other staff. A mobile, clear lead shield can also be interposed between the physician’s upper body and the image intensifier, thus decreasing scatter radiation from the patient to the physician above the table.

7. Beam Orientation

Because of backscatter, radiation dose is always highest on the side of the patient or table facing the x-ray tube. Furthermore, x-ray beam orientation can have drastic effects on tube output, with anteroposterior views requiring the least amount of tube output and thus dose. Oblique and lateral views increase the total dose, and backscatter radiation to the physician is increased if the x-ray tube is moved closer to the physician in these projections. Therefore, when using oblique and lateral orientations, stand on the side of the image intensifier, where scatter radiation levels can be two to three times lower compared to the side of the x-ray tube.

Conclusion

With the increasing use of fluoroscopy in interventional pain management, physicians must be knowledgeable about radiation safety. Multiple health problems are linked to radiation exposure, so knowing how to reduce radiation is important for patient and operating room personnel safety. NCRP has created the ALARA Principle (As Low As Reasonably Achievable), which recognizes that no magnitude of radiation exposure is known to be completely safe. With each patient and every x-ray image obtained, remember the three cardinal principles of ALARA: increase distance, decrease time, and use shielding.

References

  1. Kim TW, Jung JH, Jeon HJ, Yoon KB, Yoon DM. Radiation exposure to physicians during interventional pain procedures. Korean J Pain. 2010;23(1):24–27. https://dx.doi.org/10.3344%2Fkjp.2010.23.1.24
  2. Vano E, Rosenstein M, Liniecki J, Rehani M, Martin CJ, Vetter RJ. Education and Training in Radiological Protection for Diagnostic and Interventional Procedures. Ann ICRP. 2009;39(5):113.
  3. "Radiation Dose in X-Ray and CT Exams". Radiological Society of North America. Published April 20, 2018. Accessed May 27, 2018. .
  4. "Information for Radiation Workers". United States Nuclear Regulatory Commission. Published June 19, 2018. Accessed June 23, 2018. .
  5. Manchikanti L, Cash KA, Moss TL, Pampati V. Radiation exposure to the physician in interventional pain management. Pain Physician. 2002;5(4):385–393.
  6. Wininger KL, Deshpande KK, Deshpande KK. Radiation exposure in percutaneous spinal cord stimulation mapping: a preliminary report. Pain Physician. 2010;13(1):7–18.

Tags: radiation, safety, chronic pain

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