ESTRO 2024 - Abstract Book

S3261

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Ravikumar, Manickam & Ramamoorthy, Ravichandran. (2012). Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator. Strahlentherapie und Onkologie. 176. 223 228.

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Characterization of a solid water material for true end-to-end testing

Rune S Thing 1,2 , Bjarke Mortensen 1,2 , Martin Berg 1,2 , Liubov Nesterenko 1

1 University Hospital of Southern Denmark, Department of Oncology, Vejle, Denmark. 2 University Hospital of Southern Denmark, Radiotherapy Research Team, Vejle, Denmark

Purpose/Objective:

Phantom-based end-to-end (E2E) tests are typically designed with phantoms requiring density overrides in the treatment planning system (TPS) to allow for accurate dose calculations. The main reason for this is the lack of phantom materials that are water or tissue equivalent at both kV and MV photon energies. In this work, we characterize a known water equivalent plastic material designed for quality assurance in the kV energy range, for high energy photon beams in accordance with the IAEA TRS-483 protocol. This characterization is then applied to E2E measurements of a cylindrical phantom made of the same material, showing that with careful material selection, it is possible to design a “true” E2E test where the planned dose is calculated on the measured Hounsfield Units (HU) of the CT scan without the need for density override. We compare such planned dose with measurements to demonstrate the feasibility of using the material for E2E testing.

Material/Methods:

A set of 30x30 cm 2 slabs made from CT HE Solid Water was manufactured by Sun Nuclear (Sun Nuclear Corporation - A Mirion Medical Company, Melbourne, Florida, USA), in thicknesses varying from 1 to 50 mm. In one 20 mm slab, a cavity for the Sun Nuclear SNC125c ion chamber was drilled, allowing dose measurements to be performed. To determine the water equivalence of this material, we measured Percentage Depth Dose curves (PDDs), output factors (field sizes 3x3-20x20 cm 2 ), Tissue Phantom Ratios (TPR 20/10 ), and absolute dose measurements at 10 cm depth. Based on the absolute dose measurements, we also determined the phantom dose conversion factor, k w,plastic Q,msr , from the IAEA TRS-483. Measurements were performed on an Elekta Infinity linac (Elekta AB, Stockholm, Sweden) using beam energies of 6 MV, 10 MV FFF, 10 MV, and 18 MV. With the material properties determined from the slab phantom, we further designed a true E2E-test using the Sun Nuclear/Gammex Advanced Electron Density phantom model 463. 5x5 cm 2 fields were calculated from four different angles, incident on the small head cylinder with 10 cm diameter, measuring dose with the SNC125c ion chamber placed centrally in the phantom. Beam angles were selected to have a 28 mm diameter insert of either air, LN-450 Lung, CT HE Solid Water, or HE Cortical Bone in the beam path, upstream of the ion chamber. CT scanning of the cylindrical phantom was performed on a Siemens Somatom Definition AS scanner (Siemens Healthcare GMBH,