ESTRO 2024 - Abstract Book

S329

Beachytherapy - Physics

ESTRO 2024

Conclusion:

The proposed applicator commissioning method demonstrates the required source position accuracy and has all the characteristics of the “gold standard” (human independent, film free, short time, 1 mm step size, Excel math) in this field. The described commissioning technique very well demonstrated the significance of the effects associated with the unpredictability of the friction force that stops the movement of the source inside the applicator, which we had previously only guessed about.

Keywords: Applicator commissioning, quality control

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Digital Poster

Model of the source movement in brachytherapy applicator based on the mechanical parameters

Vladimir Stserbakov

North-Estonia Medical Center Foundation, Radiotherapy, Tallinn, Estonia

Purpose/Objective:

The purpose of this work is to predict the position of the Ir-192 radiation source inside the applicator with known mechanical characteristics of the components of the treatment process. The first component is the effect of the cable delay (lagging effect) from the afterloader side, the second is the friction force inside the applicator when the source moves along the channel.

Material/Methods:

The Venezia 22 applicator (Flexitron, Elekta) was taken as an example to build the model. You can see the shape of the main curves used for calculations in Figure. A – The afterloader cable lagging value (in mm) reflects how far the source is from its nominal position (i.e. when there is no mechanical resistance to the source movement) as the mechanical resistance to its movement along the applicator increases. B - Friction force, i.e. mechanical resistance (in g) to the movement of the source (dummy) along the trajectory of the applicator channel. This value also increases as the source approaches the applicator tip.

We carry out the modeling in polar coordinates, and for the final correspondence, we use Cartesian coordinates. The Excel mathematical apparatus we used for calculations. We perform four processing steps:

1. Using curves from Figure, we assigned to dwell-positions of the source values of lagging effect (values of degrees in polar coordinate, which origin is located in a center of applicator) - curve lagg° (Table, A). 2. We create dwell-positions (in polar coordinate) of the source inside applicator with assumption that friction force is missing - curve DWpositionfr=0°. In this case, step size is 1 mm on the source pathway, but step size (in angles) is not constant, because it depends on the distance from applicator center.