English | Español

Radiation Safety in Brachytherapy

Important areas in relation to radiation safety in brachytherapy include:

  • All efforts made to ensure that protection in the treatment is optimized
  • All measures taken to prevent accidental exposures from occurring.

Optimization of brachytherapy

Preventing accidental exposures in brachytherapy

1. What advantages are there to using remote afterloading devices, compared with manual brachytherapy procedures?

Foremost, staff will be less exposed to radiation.

When circumstances allow remote afterloading to be used, there are some advantages from a radiation protection perspective and also from other practical points of view. There is reduced radiation exposure to staff, since radioactive sources do not have to be handled manually but are driven by the remotely controlled afterloading device. Furthermore, treatment can potentially be better optimized through enhanced reproducibility.

Page Top

2. Iridium-192 has proven to be an often-used source for high dose rate applications. Why is this, and why is it not used for all brachytherapy?

Brachytherapy

While iridium 192 (192Ir) has many good qualities, different radionuclides are good for different applications.

192Ir is a radionuclide with a high specific activity, or activity per unit mass, which means that a very small source can provide a very high dose rate (HDR) – essential for HDR-applications. The picture shows some typical dimensions for a HDR source of 192Ir . The effective photon energy of around 350 keV ensures a sufficient absorbed dose at a sufficient distance from the source to treat the target homogeneously. A drawback, however, is the short half-life of 74 days. This means that sources typically need to be replaced every three to four months in order to maintain an acceptable treatment time. In typical low dose sate (LDR) -applications, size is of somewhat less importance. An often-used radionuclide in gynaecological applications is caesium 137 (137Cs), which has a much longer half-life (30.2 years) than 192Ir and, thus, only needs to be replaced every 10-15 years, while the specific activity is only a hundredth of that of 192Ir . For permanent implants, an often-used source is iodine 125 (125I) due to its low photon energy making radiation absorbed within the patient.

There is no such thing as a perfect radionuclide for all brachytherapy applications. One has to look at the application in question and consider issues such as specific activity, half-life, type and energy of emission, and shielding requirements.

Page Top

3. Why are brachytherapy sources encapsulated?

To ensure containment of the radioactive material and sometimes to act as a filter of unwanted radiation.

Brachytherapy sources are usually sealed so that the radioactive material is contained fully encapsulated within a protective capsule. This capsule is designed to prevent leakage or escape of the radioactive source and it makes the source rigid. Furthermore, for photon emitting sources, the capsule can serve the purpose of absorbing alpha and beta rays produced through the source decay. A tiny brachytherapy seed (with a size roughly equivalent to a grain of rice) such as radioactive iodine 125 (125I) in such a seed is encapsulated in titanium.

Page Top

4. Is HDR brachytherapy better and safer than LDR brachytherapy in all aspects?

Brachytherapy

No.

While HDR brachytherapy may offer advantages such as more practical procedures with outpatient treatment, increased opportunities to optimize the absorbed dose and enhanced radiation protection of staff under normal conditions, there are still other factors where LDR brachytherapy has an advantage. LDR can be said to be less technically complex than HDR brachytherapy, where a large absorbed dose is given to the patient in a short time span. HDR treatment thus requires more training and advanced knowledge in terms of operating the equipment and optimizing protection in the treatment, including keeping the irradiating organs at risk to the minimum necessary to achieve the objective. While the potential for error might not be greater for HDR brachytherapy, the consequences of HDR error might be exacerbated due to the high activity of HDR brachytherapy sources.

Page Top

5. Does it matter what units I use to specify source strength in calculations?

Yes, you should aim towards following international standards and codes of practice.

Over the years, there have been many different ways of specifying source strength in brachytherapy. Examples of this: actual mass of radium, equivalent mass of radium, actual or apparent source activity in curie or bequerel, air-kerma strength and reference air-kerma rate (RAKR) [Williamson and Nath]. Accidents in brachytherapy have happened when source strength has been entered into a treatment planning system in units not requested by the system and when sources of a certain activity have been ordered while sources of another activity have been delivered with the same numerical of activity but other unit [IAEA Safety Reports Series No. 17].

In order to minimize the risk for this particular hazard, it is of value to strive towards meeting international standards and codes of practice on how to specify source strength in brachytherapy, and what units to use [ICRU Reports 38 and 58]. This applies to both end-users and to manufacturers. While this is not yet fully implemented internationally, care should be taken when specifying and verifying source strength.

References

Page Top

6. If we have a radiation room-monitor, is it really necessary to survey the patient with a portable monitor after treatment?

Brachytherapy

Yes, this should be done as part of safe practice.

In order to monitor for excessive post-treatment radiation levels in the treatment room (potentially indicating a source retraction failure), one of the most important safety measures is the use of radiation room monitor. In a fatal accidental exposure related to HDR brachytherapy, it was reported [ICRP 97] that the radiation room monitors had identified relatively high radiation level after treatment, but that these indications had been ignored due to a previous history of monitor malfunction. There had been a failure of the weld between the transfer wire and the source, which had left the source inside the patient. Had staff surveyed the patient with a portable monitor after treatment, they would have been able to confirm that the radiation room monitor had been correct. With both room monitors and portable monitors as part of the safety system, greater depth in the safety provisions is created. There are also many other safety systems that should be in place in order to be able to conduct safe and effective brachytherapy.

Page Top

7. What step size should be used when programming dwell position and dwell time in my centre?

Brachytherapy

Keep the step size constant within the centre.

When using stepping source remote afterloading technology, the dwell positions and dwell times are programmed so that target coverage and organ at risk avoidance are optimized. The selection of step size, which determines the distance between dwell positions, can vary, but it is recommended that in a particular centre the step size should be kept at a particular constant value e.g. 5 or 10 mm (or another value, as long as it is kept constant within the centre). The reason for this is that there are accident reports [ICRP 97] where steps have been introduced in the programming with an incorrect length causing an undesired dose distribution. There are also reports of dwell time data being introduced in reverse order, highlighting the necessity of independent check of this safety critical data.

Page Top

References

  • GERBAULET, A., et al. (Eds), The GEC ESTRO Handbook of Brachytherapy, European Society for Therapeutic Radiology and Oncology (ESTRO), Brussels (2002).
  • KUBO, H.D., GLASGOW, G.P., PETHEL, T.D., THOMADSEN, B.R., WILLIAMSON, J.F., High dose-rate brachytherapy treatment delivery: Report of the AAPM Radiation Therapy Committee Task Group No. 59, Med. Phys. 25 (1998) 375-403.


 
Copyright © 2013 International Atomic Energy Agency, Vienna International Centre, PO Box 100, 1400 Vienna, Austria