Session Item

Brachytherapy QA Programme suppose verification accuracy of applicator library built-in into treatment planning system, which is provided by the manufacture. One method to create the user` own model of brachytherapy applicator is to measure mechanical characteristics of afterloader and applicators parts separately and based on that create simulation, which describe behavior of joined the afterloader-applicator connection. The purpose of this work is do all required measurements for applicator side of Flexitron (Elekta) system. Or, at this step of QA project, to measure the force braking down the source when it moves inside the brachytherapy applicator.

In figure you can see scheme of measurement setup to study strength in cable when dummy source 2 is moving inside brachytherapy applicator 1. Cable 6 attached to load cell 4 of digital scale 5 (Digital scales 0.01-200g, MH-200) pinned on chamber holder block of positioning assembly 7 of water phantom WP-3840 (CNMC). Movement of the system can be done manually/remotely 8. Reading of the scale in grams corresponds to the “resistance force” of dummy movement through brachytherapy applicator. Only care should be taken about “cable free space” A, where should be avoided creating the “snaking/arcing” of the cable.

Figure.   Scheme of setup for measurement of the force braking down the dummy source when it moves inside the applicator. 1- brachytherapy applicator; 2- dummy source; 3- holder for applicator; 4- load cell of digital scale; 5- Digital scales 0.01-200g, MH-200, fixated on chamber holder block of positioning assembly; 6- dummy cable; 7- depth positioning assembly of water phantom WP-3840 (CNMC); 8- manual/remote motor drive; 9- ruler; 10- pointer; A- “cable free space” zone.

On the table you can see measured braking force values for some popular brachytherapy applicators. Coordinate of dummy source, when it breaks against end of canal inside applicator, was chosen as “10 cm” coordinate. In this point resistance force starts dramatically increase.  Coordinate “0 cm” corresponds position of dummy source when it located on applicator entrance (for Ring applicator). If we compare applicators between each other, for the region 9-10 cm we have got unexpected results: braking force for new type applicator (Venezia) is ~2 times higher than for old type applicator (Ring), although, as it would seems that, the idea of developing new generation of applicators was to reduce resistance force inside applicator in order to reduce uncertainty in source position.

Table.   Measured braking force values for some brachytherapy applicators (Ring, Venezia)

Measured values for source cable strength inside applicator are essential basic data for calculation/creation user’ model of library applicator in quality assurance procedure for verification manufacture applicator library files in clinical QA programme. Unexpected results for Venezia applicator can stimulate introducing enhancements in construction of applicator.