Online

ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
A revisited recipe for targeted linac QA: precise, efficient, and tailored to current needs
Christian Clermont, Belgium
PO-1394

Abstract

A revisited recipe for targeted linac QA: precise, efficient, and tailored to current needs
Authors: Christian Clermont.(CHU UCL NAMUR - SAINTE ELISABETH, Radiotherapy, Namur, Belgium), Ann Van Esch.(7Sigma, QA-team in radiotherapy physics, Kasteeldreef 2- 3150 Tildonk, Belgium)
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Purpose or Objective

With the evolution of radiotherapy treatment techniques, physicists have gradually added more tests to their conventional machine QA protocol to assure correct delivery. In many centers, QA protocols are a time consuming patchwork of ancient and new tests with a variety of phantoms and tools. 
Commercial software packages that make use of the portal imager to automatically analyze such tests do exist. Unfortunately, they do not fully address our needs.
We have therefore developed a solution that is a) complete enough to perform all current mechanical machine QA tests, b) versatile enough to add possible future tests, c) intuitive enough to allow easy (geometric and dosimetric) interpretation of the test results with respect to clinical tolerances and d) independent of the practitioner.

Material and Methods

The QA procedure was developed on a Varian TrueBeam. In order to limit the amount of extra equipment required, we have made use of the MPC phantom, standard included with all TrueBeams.
We only added three small accessories to allow a comprehensive mechanical QA program: an IsoBall insert, a tungsten crosshair frame and a crossroad floor board. These should be used in combination with an appropriate series of test plans.
A number of tests are inevitably performed by the physicist in the treatment room (cross-hair, light field, lasers, ...), but most consist of irradiating the portal panel with pre-programmed test fields.
The obtained precision can be quantified off-line afterwards but a simple visual on-line assessment of the images during acquisition suffices to instantly check if the mechanical precision is within the set tolerance levels.

For the routine dosimetric QA, we rely on a 2D array as well as on two simple, yet versatile solid water phantoms (one for photons, one for electrons) and single ion chambers. Dosimetric QA requires setup of reference values by cross-validation to water phantom measurements.

Results

Our adapted QA program allows us to quickly assess the mechanical precision of the treatment unit and its on-board imaging with high precision. Although the idea sounded simple enough, it was challenging to work out a methodology that systematically assesses one parameter at a time, so that in case of failure the guilty component can be instantly identified.
During the actual development process itself, we encountered a number of issues that highlighted the importance of the order in which the test plans are executed to obtain unambiguous results. The quick on-line visual assessment during the image acquisition is enough to assess a pass/fail. Dosimetric QA is now also fast and efficient.

Conclusion

Our newly implemented treatment unit QA allows us to quickly assess the mechanical precision of the treatment unit and its on-board imaging with high precision through simple, quasi-instantaneous visual analysis of the images during acquisition. Dosimetric QA is now succinct yet complete.