ESTRO meets Asia 2024 - Abstract Book

S308

Physics – Detectors, dose measurement and phantoms

ESTRO meets Asia 2024

The measured dose distribution of the PRESAGE shows good agreement with the measured dose distribution of radiochromic film and treatment plan with a maximum difference of less than 3% for both IMRT and VMAT treatment. Most discrepancies in the dose distribution may be due to the dosimetry system. OCT is prone toring artefacts which can be due to light scattering and a mismatch in refractive index between the matching liquid and the PRESAGE®. However, having 3D volumetric dose points can compensate for these errors. The gamma analysis results of PRESAGE® in comparison to EBT film and Monaco TPS show pass rates of more than 95% for the criteria of 3% dose difference and 3 mm distance-to-agreement.

Conclusion:

This study demonstrated the capability of PRESAGE® and bespoke MAX-HD phantom in conjunction with the OCT system to measure 3D dose distribution for level III dosimetry audit.

Keywords: Anthropomorphic Phantom, 3D Radiochromic Dosimeter

362

Digital Poster

Apparent energy-independence of electron beam measurements achieved using ionisation chamber array

Tanya Kairn, Scott B Crowe

Cancer Care Services, Royal Brisbane and Women's Hospital, Brisbane, Australia

Purpose/Objective:

It is well known that ionisation chambers can be used to measure electron percentage depth-dose (PDD) profiles in liquid water by first measuring percentage depth-ionisation (PDI) profiles and then converting from relative ionisation to relative dose using energy- (and therefore depth-) dependent stopping-power ratios. It may therefore be assumed that if readings are made at a series of different depths in electron beams using a commercial ionisation chamber array dosimeter, stopping-power conversions will similarly be needed to obtain relative depth dose measurements from the resulting data. Testing this assumption, however, has produced an unexpected and important result, for a particular type of array dosimeter. As a preliminary step on the road to completing an electron radiotherapy end-to-end test using a pre-existing 3D printed face phantom [1], a set of depth-ionisation readings were made using an Octavius 729 detector (PTW Freiburg GmbH, Freiburg, Germany) using nominal 6 MeV, 9 MeV, 12 MeV, 15 MeV and 18 MeV electron beams from a Varian iX linac (Varian Medical Systems, Palo Alto, USA). This was achieved by first establishing the water equivalence of Virtual Water (Standard Imaging Inc, Middleton, USA) in the same electron beams, using conventional Advanced Markus ionisation chamber measurements with stopping-power corrections. Varying thicknesses of Virtual Water were then placed on top of the Octavius array, with appropriate position adjustments to maintain a consistent source-surface distance, and then the Octavius array was used to measure electron irradiations with a consistent 200 MU per reading. Central pixel values in the Octavius measurements were recorded and compared against depth-ionisation and depth-dose measurements made using the Advanced Markus chamber in a 3D scanning water tank. Material/Methods: