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External Beam Radiotherapy

Radiography can be thought of as 'X ray photography'. The images that are formed by passing an X ray beam through some section of a patient’s body are recorded either on film or some form of digital media. Generally, the images recorded on film are viewed as transparencies on a lighted view-box or illuminator and the digital images are viewed on computer displays.

The general trend in the world is a transition from film-based to digital radiography. This is somewhat parallel to what is happening in other fields such as communications and entertainment where there are definite advantages to the use of digital technology.

Some of the specific advantages to digital radiography include:

  • elimination of chemical processing of films,
  • reduced space requirements for storage of images,
  • ability to apply digital image processing to optimize image quality and visibility of pathologic conditions,
  • rapid transmission of images to other locations for viewing by physicians.

While digital radiography offers definite advantages, it might not be the most appropriate choice at this time for many clinical facilities because of cost and other operational considerations.

It is important to recognize that the major factors of image information and radiation exposure to patients are not determined by the type of radiography (film or digital) but by the selection of the imaging system characteristics and operating factors for each clinical procedure.

Frequently asked questions

1. Can I reduce the radiation exposure to my patients by using low kV values?


No. In general reducing the kV would increase patient exposure. When the kV is reduced in radiography, both the output exposure of the X ray tube and the penetration of the radiation through the patient are reduced. This then requires an increase in the tube loading, PIt (in units of mAs) to obtain the same film exposure. The overall result is an increase in patient exposure.

The appropriate action is to select a kV value for a specific clinical procedure that provides the appropriate balance of image contrast and patient exposure. Established technique charts should be used to determine the optimum kV value for each procedure. Radiographers need to have the training and experience to evaluate patient thickness and other conditions to be considered in the selection of the appropriate kV values.

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2. Are there any precautions that should be observed with digital radiography to prevent unnecessary exposure to patients?

Yes, there is one characteristic of digital radiography that can cause unnecessary patient exposure. It is the wide dynamic exposure range of digital radiographic receptors. This is the characteristic that makes it possible to produce images that ‘look good’ and have appropriate contrast with a wide range of exposures. This is less of a problem with film radiography because ‘over-exposure’ produces a dark film and is recognized as such. However, an overexposed digital radiograph looks good and will not be recognized as one made with unnecessary exposure.

The appropriate action in digital radiography is to monitor the exposure used for each imaging procedure by observing the exposure index or other exposure indicator and train the staff to use appropriate exposure technique factors for each examination.

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3. Is it a good practice to select 'high speed' type intensifying screens that require relatively low exposure and use them for all radiographic procedures in my facility?

No. While high-speed intensifying screens are appropriate for many examinations, especially where patient exposure is a concern, they do not produce the necessary visibility of detail that is required for some types of radiography, such as of the chest and the skeletal system. The appropriate intensifying screens and film for each type of clinical examination should be specified in the procedure manual for the facility.

It is desirable to have a selection of intensifying screens available so that an appropriate one can be selected for a given procedure. Radiographers should have a good knowledge of the characteristics of the intensifying screens available in their facilities. The intensifying screens that provide optimal imaging for each examination with regard to image information and patient exposure should be identified and made known to the staff. Read more »

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4. What can I do to ensure that the X ray beam spectrum is optimized for the various clinical procedures that we perform in our facility?

An optimized X ray beam spectrum is one that provides the appropriate balance between image quality (contrast, detail, noise) and patient exposure.

For a specific X ray machine, the spectrum is determined by a combination of the filter in the beam and the kV value.

The first step is to have the machine periodically evaluated in the quality control programme for the equipment by a physicist or engineer to confirm appropriate filtration and to calibrate the kV control.

The second step is to ensure that the staff is using kV values and filtration that are optimum for the specific types of examinations.

The appropriate technique factors (kV, image size, etc) should be specified for each type of clinical examination in the procedure manual and technique charts for the facility.

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5. Does the use of automatic exposure control (AEC) always ensure the correct exposure to patients during radiography?

No, not always. The appropriate use of a properly calibrated and adjusted AEC is helpful in minimizing unnecessary exposure. However, if the AEC is not properly calibrated and the sensors are correctly located within the anatomical area being examined, it would result in incorrect exposures. Special attention has to be paid in installations used for paediatric patients since some exposure settings might be higher than necessary. Also, an improperly exposed film might mean repeating the radiography and subjecting the patient to additional exposure.

The appropriate action is to ensure that the AEC is correctly evaluated and calibrated in the equipment quality control programme and that the staff has acquired appropriate training and experience.

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6. How does performing a 'reject analysis' in radiography reduce unnecessary exposure to patients?

A reject analysis is performed by saving all ‘rejected’ films and then evaluating them periodically to determine the cause of the rejects and repeated exposures. The objective is to reduce the number of repeated examinations by correcting technical problems and improving the skills of the staff. A reject analysis is an important function in a quality assurance programme in radiology.

When using digital radiography, procedures should be established for identifying and analyzing all images that were repeated because of problems with quality, positioning, etc. This information should be used for training and guidance to reduce the need for repeated exposures. Read more »

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7. Does the quality of the chemical processing of radiographic film have any effect on the radiation exposure of a patient?

Yes, maintaining high-quality film processing reduces unnecessary patient exposure in several ways. First, if films are underdeveloped, and appear underexposed, the usual reaction is to increase the exposure of the patient and the film in an attempt to compensate. This results in unnecessary patient exposure. Second, if the processing is not properly controlled and fluctuates with time, some films might be incorrectly exposed and require repeating. This results in unnecessary exposure to patient. The appropriate action is to have a good film processor quality control programme that ensures both adequate and consistent processing.

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8. Is the radiation exposure to a patient affected by the size of the image (area covered by the X ray beam)?

Yes. It is not that reducing the size of the X ray beam (field of view) significantly reduces the absorbed dose to the tissues that are still in the X ray beam. The advantage is that reducing the size of the beam some tissues and organs are kept outside the direct beam. Significant dose reduction, especially to sensitive tissues and organs, can, therefore, be achieved with beam collimation and use of shielding devices placed on the patient’s body

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