ESTRO meets Asia 2024 - Abstract Book

S310

Physics – Detectors, dose measurement and phantoms

ESTRO meets Asia 2024

Andrew DC Alves

ACDS, ARPANSA, Melbourne, Australia

Purpose/Objective:

We report the film dosimetry methods used by the ACDS in its end-to-end stereotactic radiosurgery (SRS) audit. SRS is a technique for precision treatment of small cranial lesions which was previously limited to specialist delivery systems. The availability of these technologies to patients has increased with the expansion of linear accelerator-based technologies such as volumetric modulated arc therapy (VMAT) significantly decreasing the treatment time compared to the previous techniques. An audit to assess SRS treatment quality and benchmark clinics is beneficial to our region and instructive to other regions where SRS has also been adopted. The ACDS audit uses Gafchromic film to assess the treatment’s accuracy.

Material/Methods:

Clinics apply their preferred technique and clinical protocols in three clinical audit cases:

Single met. (1 film), MR multi met. (3 films)

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Complex multi met (4 films)

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GTV/PTV and OAR dose constraints are defined in the audit requirements. Targets are either delineated from MR images or provided via DICOM RT structure set. An IMT MAX-HD TM (IMT, New York) cranial phantom with tissue equivalent materials and realistic bony anatomy suitable for IGRT is used. Clinical immobilization must be used to hold the phantom. Gafchromic EBT3 film (Ashland Inc., Bridgewater NJ, USA) was used for audit measurements. The machine specific reference (MSR) field is first measured using a Farmer-type PTW30013 ionisation chamber (PTW Freiburg, Germany). Reference films were irradiated in the MSR field to doses of 10, 20 and 30 Gy, providing means to adjust the film calibration in a single scan protocol of the reference films co-scanned with the audit case films. Additionally point dose measurements were performed using PTW60019 microdiamond detectors (PTW Freiburg, Germany). Audit dose map generation using the calibration films and linear scaling from reference films was performed with in-house Python software. Reference films returning greater than ±5% scaling factor failed the dosimetric quality control (QC) and were re-scanned. In-house MatLab software was used to locate the film data in the treatment frame of reference by matching the film edges to features visible in CT. Due to phantom immobilisation, according to patient protocols, the resulting film planes were often not congruent with 3D axes of the DICOM data so 3D image rotation was required to enable the necessary dose plane extraction. Film dose uncertainty was estimated to be ±2.5% and film position matching uncertainty was estimated to be ±0.3mm. A 5%/1mm gamma criteria was deemed clinically relevant for the SRS treatment modality with the dose and distance criteria suitably larger than the audit measurement uncertainty. A gamma pass rate of 90%, for doses above 10% of prescription dose, was required to achieve a passing audit result.