Biomed Imaging Interv J 2007; 3(2):e27
© 2007 Biomedical Imaging and
Accident prevention in radiotherapy
O Holmberg, PhD
Physics Department, Copenhagen University Hospital, Herlev, Herlev
Ringvej 75, 2730 Herlev, Denmark
In order to prevent accidents in radiotherapy, it is
important to learn from accidents that have occurred previously. Lessons
learned from a number of accidents are summarised and underlying patterns are
looked for in this paper. Accidents can be prevented by applying several safety
layers of preventive actions. Categories of these preventive actions are
discussed together with specific actions belonging to each category of safety
layer. � 2007 Biomedical Imaging and Intervention Journal. All rights reserved.
Keywords: Risk management, quality assurance, accident, incident, error
Preparation and execution of radiotherapeutic treatment is a
complex task with many inherent hazards. When considering the potential risks
in radiotherapy, it should, however, always be recognised that the treatment
has a potential substantial benefit to the patient.
In attempting to avoid accidents in radiotherapy, it is very
important to remember the lessons that can be learned from previous
radiotherapy accidents and to ensure that preventive actions are applied in a
clinical setting. A number of accidents have been thoroughly investigated and
the lessons learned have been disseminated [1-4] by the International Atomic
Energy Agency (IAEA). The International Commission on Radiological Protection
(ICRP) has summarised causes and contributory factors for radiotherapy
accidents in 2000 .
Prevention of accidents in radiotherapy involves applying
several layers of preventive actions, addressing this issue at several levels.
It is suggested  that these layers encompass:
Actions where potential deviations from intended dose and geometry can
be found before the first irradiation-fraction of the patient;
Actions where deviations can be found during or after the treatment
Application of safety-technology;
Application of safety procedures; and
Actions where contributing factors such as staffing-levels and
structure, training and communication are addressed.
The first objective of this review is to assess common
aspects of lessons learned from major radiotherapy accidents in order to
highlight patterns seen during accidents. This follows a review performed by
the author of the creation of an IAEA regional training course on prevention of
accidental exposure in radiotherapy. The second objective is to identify
actions within the preventive layers as suggested above.
Lessons learned from major radiotherapy accidents
Specific lessons learned from some of the major radiotherapy
accidents are presented below. Case histories are not presented in detail, as
they have been described in the literature. Finally, the lessons learned are
grouped under four headings, highlighting patterns seen in the lessons learned.
Incorrect decay data (USA) 
During a time period of two years, a physicist failed to
perform regular measurements [calibrations and quality assurance (QA)] on a
cobalt unit for radiotherapy but instead relied on estimations of the decay of
the source in order to predict the dose rate for calculation of the treatment
time. The dose rate was plotted on a graph paper and the dose rate was extrapolated
over time. This extrapolation was done incorrectly, resulting in the patients
receiving overdoses of 10% to 55%.
Some of the specific lessons learned from this accident
Independent check of a physicist�s work should be performed
Formal procedures for calibrating the treatment unit on a regular
schedule should exist and be followed.
A department should provide sufficient staff to handle the workload.
Records must accurately document the performance of accepted QA
Erroneous use of treatment planning system (UK) 
When a computerised treatment planning system (TPS) was
brought into clinical use, a hospital began treating with isocentric
techniques. The TPS correctly applied an inverse-square correction for these
treatments. Not aware of this, an additional distance correction factor was
applied manually by the persons calculating treatment time. A distance
correction factor was thus applied twice for all patients treated
isocentrically, causing patients to receive doses lower than prescribed. The
incorrect procedures were found to have been in place for approximately nine
years before they were discovered.
Specific lessons learned include:
Staff should be properly trained in the operation of the equipment and
understand the operating procedures.
Quality Assurance Programme procedures should include complete
commissioning of treatment planning equipment before first use, and procedures
for independent checking of patient treatment time calculations.
Accelerator software problems (USA and Canada) 
A specific type of accelerator relied on software for safety
interlocks (and not, as in other models, mechanical and electrical safety
interlocks). Several accidents occurred involving unintended carousel
positioning prior to treatment, resulting in extremely high electron energy
fluence directed towards the patient.
Some of the specific lessons learned from this accident are:
Patient reactions should be observed, reported and followed up, and all
reports of abnormal machine operation should also be investigated.
The Quality Assurance Program should include a review of procedures for
reporting unusual events.
Only the software for safety cannot be relied on.
Computer file not updated (USA) 
Data for treatment time calculations was updated at the
exchange of a cobalt source by a medical physicist, except data for treatment
with cobalt beam trimmer bars. It was stated by the oncologist that trimmer
bars would not be used for treatment anymore. Some time later, treatment with
trimmer bars was initiated again. The old computer file was used for
calculations, but this file contained the outdated source activity, leading to
patient treatment times that were too long and produced corresponding
Some specific lessons learned:
Develop procedures that clearly indicate the software commissioned for
clinical use, and software that has been removed from clinical service.
The Quality Assurance Program should include procedures for verifying
the correct function of software for patient calculations.
Perform manual calculations to confirm computer calculations of
treatment time (and use in vivo dosimetry).
Incorrect repair of accelerator (Spain) 
At the breakdown of a linear accelerator, a company
technician on another mission was called to the accelerator. Repair work was
started and a beam was recovered. However, a meter display indicated an energy
selection problem. Treatments were allowed to resume. Due to a transistor
having short-circuited, a full current was fed to the magnet system all the time,
making it possible to get a beam only when maximum electron energy was used.
The repair work had been incorrect, and the resulting beams led to severe
Some specific lessons learned:
The Quality Assurance Programme should include formal procedures for
returning medical equipment after maintenance, including making it mandatory to
report to the Physics group, before resuming treatment with patients.
There should be consideration of the need to verify the radiation beam
by the Physics group when the repair might have affected beam parameters.
There should be a procedure to perform a full review or investigation
when the radiotherapy equipment has unusual displays or behaviour.
Miscalibration of beam (Costa Rica) 
When a new cobalt source replaced an old one, the medical
physicist made an incorrect interpretation of 0.3 minutes as being 30 seconds
(as opposed to the correct interpretation of 18 seconds) during calibration
measurements. Consequently, the treatment times to be used were overestimated
by 66%, resulting in severe overdoses.
Some specific lessons learned:
Ensure there is a high level of training and competence in a clinic, to
ensure safe use of potentially hazardous sources.
Ensure there are provisions to stimulate working with awareness (e.g., a
new source is expected to require shorter treatment times).
Ensure there are written procedures for calibration of beams and for
independent verification of safety critical tasks before clinical
Error in TPS data entry (Panama) 
The TPS used in a clinic had limitations in the calculations
and presentation of results. To overcome these limitations, a new way of
entering data was devised locally. The TPS accepted this new data entry,
without giving a warning, but calculated incorrect treatment times. The result
was severe overdoses to several patients.
Some specific lessons learned:
Manufacturers should avoid ambiguity in instructions and perform
thorough testing of software, also for non-intended use.
The TPS is a safety critical piece of equipment.
Quality control should include TPS and a change in procedures should be
validated before being put into clinical use.
Computer calculation should be verified, at least through manual checks
for one point.
Awareness of staff for unusual treatment parameters should be stimulated
Accelerator interlock failure (Poland) 
After a power failure involving a clinic, an accelerator was
automatically shut down. At restoration of electrical power, the accelerator
was restarted. Some tests were completed, indicating a low dose rate, leading
to the filament current limitation being increased to a high level by staff so
that the remaining treatments could be completed. Unfortunately, there had been
a double fault: firstly a fault in a fuse of the power supply to the beam
monitoring system, leading to a high dose rate, and secondly a diode was broken
in the safety interlock chain. The combination of these faults, meant that no
problem was indicated, while the dose rate was in fact many times higher than
Some specific lessons learned:
There should be an immediate check upon power supply shutdowns or
unusual display of unit, and a written procedure to ensure that this check was
Patterns in the lessons learned
A report on several accidents in radiotherapy published by
the IAEA , reviewed together with the specific lessons learned from the
cases above, indicate that there are patterns in the lessons learned. It can be
argued that most of the reported accidents occurred when certain conditions
have been fulfilled. These conditions can be grouped as listed below:
Working with awareness and alertness: Accidental exposures have
occurred owing to inattention to details, and lack of alertness and awareness.
This could also be made worse if the personnel have to work in conditions prone
Procedures: Accidental exposures have occurred when there is a
lack of procedures and checks, or when they are not comprehensive, documented
or fully implemented.
Training and understanding: Accidental exposures have occurred
when there is a lack of qualified and well-trained staff, with necessary
educational background and specialised training.
Responsibilities: Accidental exposures have occurred when there
are gaps and ambiguities in the functions of personnel along the lines of
authority and responsibility. In these cases, safety critical tasks have been
Human errors should always be expected, leading to the
conclusion that there should be defences in place. When a hazard is realised,
it is due to weaknesses in this defence. These weaknesses can be seen as a
combination of two factors, with the first factor being active failures
(mistakes, lapses and procedural violations) and the second factor being latent
conditions (i.e., conditions built into the system such as understaffing, high
workload, and inadequate procedures or equipment). This approach follows Reason�s
model . Several layers of preventive actions should be put in place.
Actions where potential deviations from intended dose and
geometry can be found before the first irradiation-fraction of the patient
Independent verification of calculations has been seen to be
lacking in several of the accidents presented above. There are indications that
a recently reported accident in Glasgow  might have been prevented if a
truly independent calculation check had been used. The independency of the
check is vital to be able to find parameters that are not the same as intended.
Many mistakes in the calculation process are due to mistakes in the act of
transferring information. Another example of action in this safety-layer is
clinical peer review of treatment preparation (e.g., dose and volume to be
Actions where deviations can be found during or after the
In vivo dose measurement is a way of finding
deviations after one or a few treatment fractions. This is regularly performed
with diodes. Systematic dose deviations as low as about 1-2% that affected
large groups of patients, have been found by diode systems. Another action
belonging to this safety-layer is clinical monitoring of adverse effects in
Application of safety-technology
An example of safety-technology to serve as a safety layer
for the prevention of radiotherapy accidents is integrated radiotherapy
networking. This implies the automatic transfer of parameters and images as
well as the RV-system on linear accelerators. The most comprehensive level is
the full integration of images and parameters throughout the treatment chain,
without breaking the chain for manual transfer of information. However, a
department often has a mix of electronic and manual parameter transfer. It
should also be recognised that even if the full integration of equipment
decreases the likelihood of mistakes in transfer of information, it does not
necessarily remove the mistakes done in the creation of information. Video and
audio monitoring of patients are more examples from this safety-layer.
Application of safety procedures
There are many types of safety procedures to be put in place
in order to increase safety in radiotherapy. One example is the utilisation of
an incident reporting system. This has been successfully employed in a
non-medical setting for many years, enhancing safe practice. The objective is
for the organisation to learn from events within and outside the organisation.
Potential incidents (near misses) are important in this context. Another
example here is the use of documentation systems for procedures.
Actions where contributing factors such as
staffing-levels and structure, training and communication are addressed
Comprehensive training of all staff is mandatory. It is
important that staff have a full understanding of the equipment being used as
well as the data used. The department should also make sure that all
responsibilities are allocated and understood, and that the members of staff
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|Received 30 November 2006; accepted 30 December 2007
Correspondence: Physics Department, Copenhagen University Hospital, Herlev, Herlev Ringvej 75, 2730 Herlev, Denmark. E-mail: email@example.com (O. Holmberg).
Please cite as: Holmberg O,
Accident prevention in radiotherapy, Biomed Imaging Interv J 2007; 3(2):e27
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