HIGH DOSE RATE REMOTE AFTERLOADING BRACHYTHERAPY UNITS

High Dose Rate (HDR) remote afterloading units are routinely used in the delivery of intracavitary, intraluminal, and interstitial brachytherapy treatments of tumours. Because of the high radiation dose rate, it is critical that HDR brachytherapy be delivered accurately in terms of both source positioning and treatment time. The verification of HDR source positioning accuracy, relative nominal distance from a fixed point on the unit, or the indexer length, is a fundamental part of good HDR remote afterloading units, including the autoradiograph method, the double exposure radiograph method, and the closed-circuit TV camera method. 12

The high specific activity needed for HDR brachytherapy has limited the available sources to Cobalt-60 (for intracavitary procedures) and Iridium-192 (for intracavitary and interstitial procedures). The relatively large photon fluence from these HDR sources removes some of the sensitivity difficulties attending calibration of LDR sources. 13

The in-phantom methods use solid materials in order to achieve good mechanical reproducibility. One difficulty with the in-phantom procedures is determining the correction factors to be applied for the fluence gradient across the chamber and for the replacement of phantom material by the chamber.

The other basic technique of source calibration is to measure the air kerma or exposure from the source in air. HDR sources provide enough fluence to allow the measurements to be made with the same chambers routinely used for teletherapy calibrations.

The following machine functions should be periodically checked.

· First, the basic operation and safety of the HDR unit should be established using a dummy source. Check the operation of all interlocks, which can be tested, and also check the battery system. The accuracy of the source positioning can also be tested visually using the inactive source. This is also a good time to develop, test, and practice emergency procedures with all the staff who will be using the equipment.

· After the source installed, the radiation levels in the room with the source shielded, and outside the barriers with it exposed, should be checked. Some units are mobile and have a large range within the room, so the radiation levels should be checked with the unit at the extremes of its mobility.

· The positioning accuracy and reproducibility of the actual source can be checked visually, using transparent catheters and a TV system, with film, or with a scanning system using diodes or small chambers.

· Quality control of applicators used in HDR procedures is an important on-going requirement. Each should be periodically inspected with radiographs to determine the placement of the sources and any shields or screens within the applicator.

QUALITY ASSURANCE IN HIGH DOSE RATE (HDR) BRACHYTHERAPY

Quality assurance in HDR brachytherapy involves a number of components:
· Sources
- calibration
- uniformity
- decay correction

· Afterloading Equipment
- safety
- compliance with specifications
- accuracy of positioning
- reproducibility of positioning

· Applicators
- integrity
- position of sources
- correspondence of sources and of radiographic markers
- position of shields or screens

· Planning systems
- accuracy of source database
- accuracy of dose calculation algorithms
- effectiveness of optimization algorithms
- accuracy of radiographic localization
- transfer of programme to treatment unit

· Planning Procedures
- completeness of frequency of training
- existence and independence of calculation checks