Biomed Imaging Interv J 2006; 2(2):e34
doi: 10.2349/biij.2.2.e34
© 2006 Biomedical Imaging and
Intervention Journal
TUTORIAL
High dose rate (HDR) brachytherapy quality assurance: a practical
guide
DA Wilkinson,
PhD
Department of Radiation Oncology, Cleveland Clinic
Foundation, Cleveland, United States
ABSTRACT
The widespread adoption of high dose rate brachytherapy with
its inherent dangers necessitates adoption of appropriate quality assurance
measures to minimize risks to both patients and medical staff. This paper is
aimed at assisting someone who is establishing a new program or revising one
already in place into adhere to the recently issued Nuclear Regulatory
Commission (USA) regulations and the guidelines from the American Association
of Physicists in Medicine. © 2006 Biomedical Imaging and Intervention Journal.
All rights reserved.
Keywords: High dose rate brachytherapy, quality assurance
Introduction
Within five years of its discovery by the Curies in 1898, Ra-226
was being successfully used in brachytherapy [1].
For the next 50 odd years, radium was the isotope of choice
for brachytherapy applications, finally yielding to reactor-produced
nuclides such as cobalt, cesium, and iridium with much shorter
half-lives. These γ-ray emitters, known as “radium substitutes”,
were at first used in low dose rate (LDR) implants (< 200
cGy/hr; typically 40 to 80 cGy/hr). More recently, the ability
to produce high specific activity Ir-192 sources combined with
developments in computer controlled after loader technology
has led to widespread adoption of high dose rate (HDR) techniques,
i.e. > 1200 cGy/hr [1].
The advantages of HDR treatments include
● greater
ease and comfort for the patient (often as an out-patient),
● more
precise dose delivery,
● easier
dose shaping, and
● less
exposure to medical personnel.
However, because of the dangers of using a source with very
high activity (10 Ci), it is of utmost importance to have proper quality
assurance (QA) procedures in place along with the required dosimetric and
planning equipment, and appropriately trained staff. This guide focuses
primarily on the first (QA procedures) and the third (training) in this list. It
is intended to assist those who are in the process of establishing a program in
HDR brachytherapy.
Since each country regulates its own medical use of
radioactive material, it is the duty of the medical physicist to establish a
quality management program to satisfy those regulations. The focus of this
review is regulations of the US Nuclear Regulatory Commission, as contained in
10 CFR Part 10 (medical use of byproduct material) [2] and recommendations made
by the American Association of Physicists in Medicine (AAPM). The latter are intended
to provide the medical physicist in the USA (and it is hoped other nations as
well) the proper guidance to ensure that brachytherapy procedures are carried
out safely and with due attention to these rules. There are two AAPM task group
(TG) reports that are particularly relevant to HDR QA. They are TG 56: Code of
practice for brachytherapy physics [3], and especially TG 59: HDR brachytherapy
treatment delivery [4]. Another useful reference for brachytherapy quality
assurance has been published by ESTRO and is available on their website [5]. This
paper will provide details about HDR QA as it is performed in our radiation
therapy department (on a Nucletron Microselectron system) and how the QA
program is related to the NRC regulations and the task group recommendations.
APPLICATOR QA
Prior to the initial use of a new (or replacement)
applicator, it is necessary to verify that the source dwell positions
correspond to the radiographic marker positions used in simulation and
treatment planning. TG-56 recommends that coincidence of dummy and radioactive
sources be checked annually as well. There are many standard applicators;
photographs of several of those used in our institution are shown in Figure 1.
The method we employ to verify coincidence of dwell position
and radiographic marker is autoradiography. An applicator is taped securely to
a sealed film envelope (Figure 2) and the HDR after loader is programmed to
send the source to a few appropriately chosen dwell positions for less than 1
second (e.g. 0.3 s for 0.31 GBq source). Next, the film plus applicator is
transferred to a diagnostic X-ray source such as a simulator, the dummy source
markers are placed in the applicator and the film exposed and developed (e.g.
125 kVp, 125 mAs for Kodak XV film). An example of this is shown in Figure 3. TG-56
recommends that the coincidence of dummy and active sources be within 2 mm; the
NRC regulations call for ± 1 mm.
PERIODIC SPOT-CHECK
The new NRC regulations require a periodic spot-check of
each HDR unit prior to the first use on any given day that the after loader is in
operation and after each new source installation. These spot-checks need not be
done by the authorized medical physicist, but the latter must review the
results and notify the licensee in writing of his findings. Table 1 lists the
checks that must be performed at a minimum to assure proper operation of the
unit according to NRC Regulations 10 CFR Part 35.
A convenient way of implementing and recording the above
quality assurance is by using a checklist such as the one our clinic uses as
shown in figure 4.
Certain tests require only a simple inspection to ensure
that materials are present, viz. User manual, Removal kit, Emergency
instructions, Bailout pig, Radiation alarm setting, and Printer paper. Switching
on the system allows the tests in item 7 to be performed. The source activity
comparison can be made using a table generated by the medical physicist (Figure
5). This will also satisfy the requirement (see Full Calibration below) for
performing decay correction which must be done by the authorized medical
physicist. Agreement should easily be within 1 percent tolerances.
For the remaining tests, the active source will need to be
deployed. For this, the system can be programmed manually each time or a
standard program recalled from the system memory. A single dwell time of 20 to
30 s suffices to test the door interlock, the interrupt button, and the
emergency off button as well as to verify that the appropriate exposure
indicators and radiation monitors are functioning properly. The spot check form
requires testing of the functioning of the meter in the treatment room (Figure
6) under battery power alone. Its alarm setting of 4 mR/hr was established so
as to be above exposure levels in the room due to an adjacent linac therapy
suite. As an additional safety measure, we have a calibrated GM meter that is
carried by hand by personnel upon entering the treatment room. It is checked
using a 1 mCi Cs-137 source that yields a 10 mR/hr contact value. The remaining
item is an estimate of timer accuracy. For this, a stopwatch is used to time a
30 s dwell. Typical error estimates are well below 1 second.
FULL CALIBRATION
A “full calibration” is mandated for several different
circumstances, e.g. before first medical use, following a source change or any
major repair, etc. Since the source in most, if not all, modern HDR after
loaders is Ir-192 with a half-life of approximately 74 days, the requirement
for quarterly calibration [2] does not formally apply. However, it is usual to
replace an iridium source four times a year so as to maintain reasonable dose
rates and treatment times. Quarterly QA testing of HDR after loaders was
recommended in the report of TG 56. The components of a full calibration as
defined in NRC Regulations 10 CFR Part 35 are listed in Table 2.
The form we employ for the full calibration is on an Excel
spreadsheet (Figure 7) which allows convenient calculations of source activity
and positioning as well as timer accuracy and linearity. The activity of the
source is measured using a well chamber and electrometer having calibrations
traceable to the National Institute of Standards and Technology (within 2 years
as indicated by the dates on the form). The electrometer needs to be calibrated
in both current and charge (integral) modes. The source is programmed to go to
a series of positions within the well chamber and the maximum current reading
is used to calculate the activity in air kerma units. This value is then
compared to the manufacturer’s stated activity decayed to the day of
measurement. Agreement is typically within 2%. The regulations allow a 5%
range. If equipment calibrations traceable to a national standard are not
available, dosimetry system constancy checks can be performed using a
long-lived source such as Cs-137 [5]. This is not a desirable substitute for
proper calibration.
Positioning accuracy is measured using a special ruler
supplied by the manufacturer (Figure 8). The programmed position (e.g. 995) is
for the center of the source, hence the correction (one-half of the source
length, or 2.15 mm) for the leading edge. The one mm criterion may not be
satisfied if there is much curvature in the measuring system (see Figure 9). Thus,
some care must be taken to ensure that the transfer tube is reasonably straight
and horizontal.
We perform the battery back-up test by shutting off the AC
power to the after loader while a source has been deployed. This makes it a
somewhat different test compared to the one in the spot-check where the
emergency off button is pushed. That the source has been retracted is printed
out at the control console and is verified by the radiation monitor indicating
exposure rates below the set value (4 mR/hr).
Timer error and linearity are measured using a technique
established for teletherapy sources. Charge is collected and measured in the
well chamber for a set of predetermined times. The pass/fail criteria we
adopted seem both reasonable and reproducible and well within the capability of
the system.
We test the integrity of the transfer tube/applicator system
in three ways.
●
Once a program has been loaded into the control unit, a transfer tube +
applicator is attached to Channel 1 but the indexer ring is not locked.
●
The second test has the transfer tube removed from Channel 1 and the
ring locked.
●
The final test has the transfer tube inserted into Channel 1, the ring
locked, and an applicator with an obstruction in it attached. It should be
added that in the Nucletron system, failure to connect the transfer tube to the
applicator properly will usually generate the same error code as an
obstruction.
TREATMENT PLANNING QUALITY ASSURANCE
It is standard practice in external beam radiotherapy to
have a second, independent check of the treatment plan and monitor unit
calculations. This may take the form of a simplified algorithm using data from
phantom measurements or dose measurements inside a suitable phantom (especially
for IMRT plans). For brachytherapy, the independent check is also desirable
(but not mandated by NRC regulation), but there is no generally accepted method
for doing it. Some characteristic parameter(s) of the plan must be compared to
an expected value; however, what the characteristic parameters should be and
how to arrive at the expected value are left to each medical physicist or
institution. TG-59 addresses these issues and lists several approaches that
have appeared in the literature [6-8]. Typically, the dose is calculated at
representative points by the treatment planning system and then compared to the
results from a second independent system (perhaps a spreadsheet or nomogram). It
remains unclear what agreement is acceptable. Our method has been to use a plot
of treatment time x source activity/ dose versus the global parameter of
treatment volume for various applicator types. This is similar to the
Paterson-Parker tables from the days of radium sources. It has been described
previously [9] and will be summarized below.
The treatment volume (usually V100 in our experience) is
obtained from the dose volume histogram (DVH). Several plans were run on both
the Plato and a second treatment planning system and the respective DVH’s
compared to lend credibility to the use of this parameter. We, then, used data
from 20 to 30 patient plans for each of several applicator types (vaginal
cylinder, tandem/ring, endobronchial tube) to construct the T*A/D vs V100
plots. Several cases for each applicator were double planned with a second
treatment planning system (ROCS or Pinnacle) for verification purposes. The
data on each graph were then fitted to either a straight line or a second order
polynomial using statistical methods. The result was then used for checking new
patient plans to ensure consistency. A summary of the initial use of this
method for two types of applicators is shown in Table 3.
Clearly, the agreement is better when a polynomial is used
for fitting the reference data. A similar situation is found for other types of
applicators as well.
Perhaps an even more important aspect of treatment plan
quality assurance is to have a second trained person inspect the plan and
compare it with the written directive. The comparison should include such items
as the dose prescription (per fraction and per course of treatment), the step
size, dwell positions, etc. A more complete list is to be found in the report
of TG-59. A check-list that is part of the patient’s chart is a practical
method to ensure that this aspect of quality control is performed. Examination
of other input data such as simulator films and comparison with the treatment
plan is also essential. In our institution, specially trained radiation therapy
technologists and the authorized user physician check the treatment plan.
TRAINING OF PERSONNEL
The clinical personnel involved in an HDR program include
the authorized user physician, authorized medical physicist, radiation safety
officer, dosimetrist, nurse, and radiation therapy technologist. Some of these
roles may be combined into one. For example, the medical physicist may act also
as the radiation safety officer and do the treatment planning in lieu of a dosimetrist.
The authorized physician and medical physicist should be certified by the
appropriate medical specialties board and have had special training in brachytherapy.
Of prime importance is the radiation safety training that all personnel involved
in HDR treatments undergo. This is administered to new personnel and then
annually for all those in the HDR program. Included is training in the proper
response to a major emergency, in particular, failure of the source to be
retracted into the after loader safe upon completion of treatment or upon power
outage. The daily spot check should ensure that proper equipment (removal kit
and bailout pig) is in place and that simple emergency instructions are posted
so as to be readily available. If the source has to be retracted manually, the
standard precepts of radiation safety, viz. time, distance, and shielding,
should be followed. If operation of the hand crank is unsuccessful, then the
applicator containing the stuck source has to be removed from the patient. Once
again, speed is crucial as is having such items as long forceps and a
flashlight on hand. With the applicator plus source placed in the bailout pig
and the patient and medical personnel removed from the treatment room, the HDR
suite should be secured and a service engineer contacted for repair. We find it
useful at the time of the annual training to review and discuss in detail what
each member of our brachytherapy team would do in various emergency situations.
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Wilkinson DA, Clouser E, Fleming PA. A method for checking HDR brachytherapy treatment plans. Radiother Oncol 2004;71(supp 2):S47.
Received 9 March 2006; received in revised form 16 June 2006; accepted 19 June 2006
Correspondence:
Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, 44195, United States. E-mail: wilkina@ccf.org (David Wilkinson).
Please cite as: Wilkinson DA, High dose rate (HDR) brachytherapy quality assurance: a practical guide, Biomed Imaging Interv J 2006;2(2):e34
<URL: http://www.biij.org/2006/2/e34/>
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