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 separately and based on that to calculate parameters/behavior of joined the afterloader-applicator connection. The purpose of this work is do all required measurements for in clinical use Flexitron (Elekta) afterloader or, as first step/side of QA project, to investigate effect of radiation source position lagging from its nominal d-well place  caused by braking strength affected on radiation source movement inside brachytherapy applicator.

In figure you can see the idea of measurement setup based on using manufacture (Flexitron, Elekta) source position check ruler 5. Mechanical resistance to moving of the source 4 through source travelling path makes mandrel 3, inserted into source travel path, which is connected with elastic parts 2 (in this role  can be used, for example, rubber strips, springs or whatever) for producing needed increasing resistance force. Later, when source lag was measured, the breaking force scaled by strength measuring tool 1 (for this case we used Scale 500g 0.1g LCD Display Digital Weight Measuring Tool). To perform more accurate measurements for different strength ranges we use 3 types of elastic parts: „hard” – for 0-300 g force; „moderate” – for 0-100 g and „soft”- for 0-30 g. For all these elastic parts types breaking force should be produced in source position range of ruler from ”200” source dwell-position to “300” (i.e. for clinically most popular gynecologic applicators).

Figure.   Scheme of setup for measurement of source position lagging effect. 1- Strength measuring tool; 2 – elastic parts; 3 – needle mandrel; 4 – radiation source; 5 – source position check ruler; 6 – source cable; 7 – afterloader.

On the table you can see averaged value for Δ (radiation source position deviation/lag from its nominal location) measured with 3 different hardness of elastic parts, which correspond ≤30g, ≤100g and ≤300g braking force ranges. As you see, lag effect is more distinctive (Δ= 1.5 mm) for the first 20 g braking strength, and for the rest part its approximately linear up to “obstruction” source cable strength value ~300 g with rate 0.9 mm/100 g. Source cable strength range values (as we now preliminary estimate) for Ring (≤50g) and Venezia (≤120g) applicators are mentioned too in the table.

Table.   Measured Δ values (radiation source position deviation/lag from its nominal place) in mm. Ù 50 g,   Ù100 g - braking strength ranges of Ring and Venezia applicators correspondingly. 

Measured values for source position lag effect for known cable strength are essential basic data for calculation/creation user’ model of library applicator in quality assurance procedure for verification manufacture applicator library files.