REMOTE AFTERLOADING BRACHYTHERAPY UNIT
Remote afterloading of radioactive sources offers the advantages of reduced radiation exposure to medical personnel, increased patient treatment capacity, and potential improvements in treatment results by use of more consistent methods of treatment.
All Remote Afterloading Units (RAU) generally consists of six basic components:
1. A safe for the radioactive sources
2. The radioactive sources
3. A local or remote operating console (microprocessor)
4. A source control/drive mechanism to control, and move the sources in to specific configurations for specific time
5. Source transfer or guide tubes, and
6. An isodose computation arising from the selected source configuration.
A typical treatment requires data (planned source positions and treatment times) for entry into the operating console. As treatment begins, the RAU source selector mechanism (indexer, sorter) will select those sources desired for the treatment, and the sources are moved through the source transfer tubes to the applicators. Sources are monitored so that the sources are in the correct position.
QUALITY ASSURANCE IN REMOTE AFTERLOADING UNITS
Quality assurance of remote afterloading machines is essential, and an important aspect of the QA procedure will always
be the accurate calibration of the radionuclide sources. Quality assurance in remote afterloading equipment is ensures
that the machine functions meet user's specifications.
There are three principal quality assurance end points:
1. Accuracy of the source selection,
2. Spatial positioning, and
3. Control of treatment duration.
Specific QA procedures are dedicated by individual system design of which there are three major varieties:
a) Fixed source train devices. These system lacks the capability of composing arbitrary source trains from elemental
components (seeds, etc.). These machines do not have the ability to distinguish one source from another so that loading
the incorrect source into the intermediate safe is always a possibility.
b) Programmable source train devices. These systems allow the user to specify the order in which equal-strength radioactive
sources and geometrically-identical spherical spacers are loaded into each of the treatment catheters or "channels."
The system can program different treatment times.
c) Single stepping source devices. These machines consist of a single cable driven source which moves from each programmed
treatment position in a catheter after the position-specific treatment time has elapsed. After a treatment, the given
source is retracted into the machine, and re-injected in to the next treatment catheter by means of a selector. Both dwell
position and time in each catheter are independently programmable.
INTERLOCKS AND OPERATIONAL FEEDBACK SYSTEMS
Modern remote afterloading systems have many interlocks and sensory systems for verifying correct operation of the machine. Optical detectors are used to ensure that the radioactive source has been fully retracted into the afterloader safe during treatment interruptions. The interlock can detect accidental detachment of the source assembly from the drive cable.
Modern afterloading devices are equipped with battery-based backup power supplies that are intended to withdraw the sources in a event of a power outrage. These backup power supplies must be checked periodically.
SOURCE SELECTION AND SPATIAL ACCURACY
Verification of correct source positioning is easily achieved by autoradiography supplemented by external marking to define the position and orientation of the applicator on the film.
For fixed source-train systems, the relative optical density can be used to qualitatively distinguish different source strengths from one another allowing source-train selection to be verified.
For the programmable source-train system, the source-train composing and sorting functions are tested by auto-radiographing several source configuration, at commissioning and annually thereafter.
Remote Afterloader and Facility Functional Checks
(NRC Requirements & Recommendations)
1. The radiation monitor should be checked with a dedicated check source for proper operation each day before the
afterloader device is used. A record of the radiation monitor check described above should be maintained for a
period of three years.
2. If a dedicated treatment room relies upon a TV monitor to maintain constant observation, the TV monitor should be
checked to verify proper operation.
3. Intercom systems installed in dedicated treatment rooms should be checked to verify proper operation.
4. Electrical interlocks installed at each entrance to a dedicated treatment room for high and medium dose rate device
should be tested for proper operation. Records of each test should be maintained for a period of three years and should
include the date of the test, the results of the test, and the initials of the individual who performed the test.
5. The mechanical integrity of all applicators, source guide tubes, and connectors to be used should be determined by
visual inspection and/or radiographs. The presence and correct placement of any internal shields and other essential
internal components should be determined.
6. The afterloading device must be tested to determine the accuracy of source positioning. Source positioning within
the catheter guide tube should be accurate to within +/- 1 millimeter of the programmed position. A record of the test
should be maintained and shall include the date of the test, the programmed position, the actual position of the source
following activation of the device, and the initials of the individual who performed the test. (This record may include
the radiograph used to determine source position.) If the source position tolerance noted above (+/- 1 mm) is exceeded,
the authorized user and radiation safety officer must be notified prior to performing patient treatments. The licensee
should submit the procedure describing the test and specify the number of dwell positions used to conduct the test.
7. Timer accuracy and linearity.
8. For devices that use a cable and/or wire to transport the source(s), measurement of source guide tubes to confirm the
length to 1 mm accuracy.
9. The backup battery for the remote afterloading device will be tested, in accordance with the manufacturer's instructions,
to verify emergency source retraction capability upon power failure. At a minimum, this shall consist of function test with
the AC power disconnected.
10. A record of these checks should be maintained for a period of three years and will include the date of the check,
the results of the check, and the initials of the individual who performed the check.