Radiation doses in interventional procedures

Frequently asked questions about radiation doses in interventional fluoroscopy

» I see Gy∙cm2 and mGy on the monitor of my angiography machine. What do these represent? 
» How is KAP (DAP) measured and how can it be used to estimate effective dose? 
» How effective dose can be used to estimate risk of cancer? 
» What are the quantities that relate radiation risk to the skin and how are they measured? 
» How are the dose indices for fluoroscopy connected to patient skin dose and effective dose? 
» How to estimate staff eye dose? 
» What do I do in case the machine I use is not equipped with a dose monitoring device? 
» Are the above dose quantities applicable to children? 
 

» I see Gy∙cm2 and mGy on the monitor of my angiography machine. What do these represent?

These are two measured dosimetric quantities that can be used to estimate the radiation risks. Gy∙cm2 is used for estimating stochastic risk to patients while mGy relates to tissue reaction. 

Gy∙cm2 is a unit historically known as dose-area product (DAP) and currently named kerma-area product (KAP). The official notation recommended in ICRU report 74 is PKA. KAP represents the product of the dose (in mGy, cGy or Gy) at the center of a certain plane of the X-ray beam (e.g. the surface of the patient) multiplied by the area of the X-ray field at that plane (in cm2 or m2). Generally KAP is expressed as Gy∙cm2, cGy∙cm2, mGy∙cm2 or µGy∙cm2, however in 2010 the IEC selected Gy∙cm2 as the standardized unit. KAP provides a good index for estimating stochastic risk but is not directly useful for estimating tissue reactions. Skin injury is related to the peak skin dose (PSD). There is no currently available real-time method to measure or calculate PSD.

The dose at a defined reference point can be measured (Gy, cGy, mGy) and used to estimate PSD. For an iso-centric interventional fluoroscope, the reference point is located 15 cm from the isocenter toward the X-ray tube. The reference point moves with the gantry in such systems. Appropriate estimates of skin dose must account for gantry motion, patient size, and patient location relative to the gantry.

» How is KAP (DAP) measured and how can it be used to estimate effective dose?

KAP is frequently measured using a transparent ionization chamber mounted in the X-ray tube assembly between the collimators and the patient. In most fluoroscopic machines, the KAP chamber is hidden by the tube-housing cover. Some fluoroscopy machines calculate KAP using generator and collimator settings.

KAP does not depend on the distance of the measuring plane from the X-ray source because dose decreases according to the inverse square law and the area of the field increases with the square of the distance. This keeps the KAP value constant at any distance. KAP represents the total energy incident on the patent. KAP is combined with a coefficient depending on the irradiated portion of the body and protocol (irradiated organs) to estimate effective dose (E). The coefficients range from 0.028 to 0.29 (mSv/Gy∙cm2). They are derived from Monte-Carlo simulations using anthropomorphic digital phantoms. A summary of these coefficients adapted from NCRP report 160 is shown in the following table.

Data used to obtain effective dose estimates for fluoroscopically-guided diagnostic and interventional procedures.
(Adapted and reprinted with permission of the National Council on Radiation Protection and Measurements,
Report No. 160 - Ionizing Radiation Exposure of the Population of the United States (2009))

» How can effective dose be used to estimate risk of cancer?

It can be done by multiplying effective dose with a factor corresponding to a hypothetical standard person. Cancer risk cannot be directly measured or calculated for an individual patient. 

The cancer risk from the same amount of radiation is known to decrease with age. The risk of induced cancer should be put in perspective with the 1/3rd average life-time risk for cancer.  In most cases, the nominal risks are small relative to other risks of interventional procedures and should be balanced against the likely benefit. It is important that patients understand that there is a latent period ranging from years to decades between radiation causing a cancer and the appearance of clinical cancer. This is why stochastic risk is increasingly important for younger patients.

» What are the quantities that relate radiation risk to the skin and how are they measured?

The best quantity to assess the risk to skin is peak skin dose (PSD). The X-ray tube and gantry moves and therefore irradiates different portions of the skin during most interventional procedures. PSD can be directly measured using radiochromic films or a matrix of thermoluminescent dosimeters (TLDs). Unfortunately, this data is usually available only after the procedure is completed. Metal-oxide semiconductor field-effect transistor (MOSFET) detectors provide real-time information but are difficult to place at the correct location in advance. They can also influence radiation output and/or interfere with the visibility of critical anatomy. Modern fluoroscopy machines indicate the cumulative air kerma (CAK) at the reference point during the procedure as well as gantry angles and table positions. CAK in many (but not all cases) overestimates PSD. It is clinically useful as a real-time safety indicator. Efforts are underway to combine CAK and geometry with patient size to obtain real-time maps of skin dose distributions.

» How are the dose indices for fluoroscopy connected to patient skin dose and effective dose?

PSD is by definition equal to the maximum dose absorbed anywhere on the skin surface of the patient and is therefore directly related to the possibility and intensity of skin injury. According to current data, minimum prompt skin reactions may occur in sensitive patients within hours after an acute PSD exceeding 2 Gy. Medically important reactions occur in average patients several weeks later at PSDs exceeding 5 Gy.

If the reference point happens to be on the patient’s skin, and the beam does not move during the procedure, the PSD is the CAK multiplied by a backscatter factor. As noted above, in most cases, more complex calculations are needed to account for beam motion, patient position, and field overlap.

KAP can be used to estimate CAK. If the field size at the level of the reference point is 100 cm2, a KAP reading of 200 mGy∙cm2 is observed after one minute of fluoroscopy. The corresponding CAK rate is 2 mGy/min. The same KAP could be observed with a larger field such as 400 cm2. Under these conditions, the CAK rate is 0.5 mGy/min. Assuming that the same organs are irradiated in both cases, the total energy imparted to the patient, and the effective dose is approximately the same for both cases. However, the PSD will be a factor of four smaller for the larger field.

X-ray beam intensity is controlled by the automatic brightness control system to accommodate differences in patient thickness, projection angles, detector settings, and source to detector distance. Therefore PSD values based on KAP must be used with caution in most circumstances. Newer systems provide CAK at the reference point, incremental KAP, and geometry at an individual irradiation level in a radiation dose structured report. When such reports are available in real time they will be used to produce skin dose maps.

» How can staff eye dose be estimated?

A standard radiation monitor worn at collar level and above all radio-protective garments provides a reasonable estimate of eye dose. Unprotected eyes receive approximately the dose indicated by such a monitor. High-quality radio-protective glasses will reduce the eye dose to approximately 1/3 of the monitor reading. This is less than the nominal attenuation of the radio-protective lenses because radiation reaches the eyes through transmission around the glasses and through scatter in the worker’s head.

» What do I do in case the machine I use is not equipped with a dose monitoring device?

In case the fluoroscopy machine is not equipped with dose monitoring device, a portable KAP meter can be mounted at the X-ray tube assembly with the help of company’s engineer or mounting at the exit surface of collimator oneself.

If it is impossible to equip the machine with dose monitoring devices, a medical physicist can measure the dose output in typical settings (for both fluoroscopy and acquisition modes) at position of patient’s skin surface and provide rough figures for every projection. Multiplying this dose rate value with the duration of irradiation for each mode will provide a rough estimate. However, this method is prone to high uncertainty for patients of different physique, magnification and the various projections.
Whenever possible, potential high-dose procedures should only be performed using equipment with a dose monitoring device.

» Are the above dose quantities applicable to children?

Yes.

It must be stressed that children have smaller body dimensions than adults. This means that their radiosensitive organs are closer to other organs. The radiation field is likely to include the more sensitive organs in a child than for an adult. This is reflected in higher DAP-to-effective dose conversion coefficients that are used for children. In their study found that conversion coefficients for children are higher than those for adults ranging from 1.33 up to 14.6-16.4 times higher depending on projection and age of child.

Doses to children from fluoroscopically guided interventional procedures are of special concern because children are more radiosensitive than adults, their life expectancy is longer, and they may undergo repeated procedures.