ESTRO 2024 - Abstract Book

S3600

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

to-valley dose ratio (PVDR), beam full width at half maximum (FWHM) and peak-to-peak distance, in the real scenario of mice irradiation.

Material/Methods:

Topas is an extension of Geant4 (Geometry and Tracking 4) that offers an advanced simulation environment for radiotherapy. In this study, the simulations are performed using Topas version 3.9 , based on Geant4 version geant4-10-07-patch-03 . The MC simulations are based on a small animal irradiator (SmART, PXI) available at the Preclinical Imaging Facility of the San Raffaele Scientific Institute. This irradiator has an X-ray source of 225 kV , 13 mA and was previously modelled in Topas. The size of the irradiator aperture is set to 14.4x14.4 mm . A 40x40 mm lead collimator with a thickness of 5 mm was previously built to generate minibeams: this collimator (that can be inserted at the exit of the irradiator) consists of 0.5 mm thick slits spaced by 0.5 mm of air, resulting in minibeams with an expected peak-to-peak distance of 1 mm . The agreement between dose profiles at several depths in acrylic was formerly verified by comparing MC prediction against measured profiles using Gafchromic films [5]. A calibrated Cone Beam Computed Tomography (CBCT) of a mouse with a subcutaneous tumour is imported in the MC simulation using the Topas built-in converter [6], to assign materials based on CT files containing Hounsfield Unit (HU) values. The mouse is positioned to ensure that the tumour is centrally located within the radiation beam at 2.5 mm from the collimator. The g4em-standard_opt4 physics module is activated and a 1 μm cutoff range is set for all particles. The dose is scored in the tumour region using voxel sizes of 0.1x0.1x0.1 mm . In total, 1x10 11 primary histories are simulated (within about 48h) using the Research Cluster of the San Raffaele Hospital. This cluster comprises 17 CPU nodes (17 x XL190r Gen10 blades), each equipped with 2 x Intel(R) Xeon(R) Gold 6240 CPU @ 2.60 GHz - 3.9 GHz and 384 GB of RAM.

Results:

Fig. 1 displays the axial, coronal and sagittal MBRT dose distribution and the dose profile at the centre of the tumour (along the yellow line in the figure). The peak-to-peak distance is equal to 1.03±0.01 mm , in good agreement with the simulated collimator geometry.