ESTRO 2024 - Abstract Book

S4840

Physics - Quality assurance and auditing

ESTRO 2024

Due to the enclosed O-ring structure of the Halcyon treatment unit, mechanical measurements for gantry angles are difficult and as a result fewer angles are routinely checked. Additionally, the definition of isocenter size given by the MPC is not known.

In this work we obtain gantry angles and isocenter radius from a Star Shot pattern on a Gafchromic film (Image 1).

Material/Methods:

The Star Shot film analysis is performed with four Halcyon treatment units at HUCH, with 9 gantry angles, and repeated on three films for each machine.

A QA treatment plan is applied, with the collimator rotated 90 degrees to minimize the effect of gravity on the MLC at different gantry angles. The film is radiated with a 6X-FFF beam, with a 1 x 10 cm 2 MLC opening for 600 MU per gantry angle. A 30 x 30 x 10 cm 3 water equivalent phantom is used (Image 2), with a Gafchromic film in the middle along the lateral vertical plane. As a result, the isocenter is determined along the horizontal and vertical axes, and the error in the longitude direction is not determined [1].