Preventing accidental exposure in brachytherapy

Frequently asked questions by the health professionals

» Does it matter what units I use to specify source strength in calculations? 
» If we have a radiation room-monitor, is it really necessary to survey the patient with a portable monitor after treatment? 
» What step size should be used when programming dwell position and dwell time in my centre?

» 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 these are 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). 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.

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. 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.

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

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 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.

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

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 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.

Read more:

• 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. 
• INTERNATIONAL ATOMIC ENERGY AGENCY, Lessons Learned from Accidental Exposures in Radiotherapy, IAEA Safety Reports Series No. 17, IAEA, Vienna (2000). 
• INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Dose and Volume Specification for Reporting Intracavitary Therapy in Gynecology, ICRU Report 38, ICRU, Bethesda, MD (1985). 
• INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Dose and Volume Specification for Reporting Interstitial Therapy, ICRU Report 58, ICRU, Bethesda, MD (1997). 
• INTERNATIONAL ATOMIC ENERGY AGENCY, Radiation Oncology Physics: A Handbook for Teachers and Students, STI/PUB/1196, IAEA, Vienna (2003). 
• INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Prevention of High-Dose-Rate Brachytherapy Accidents, ICRP Publication 97, Pergamon Press, Oxford and New York (2005). 
• WILLIAMSON, J.F., NATH, R., Clinical implementation of AAPM Task Group 32 recommendations on brachytherapy source strength specification, Med. Phys. 18 (1991) 439-448.