Development and dosimetric evaluation of a modulated intraoperative radiotherapy (mIORT) system using the Zeiss intrabeam device

Objective. Intraoperative radiotherapy (IORT) is a specialised radiotherapy technique that delivers a precise, single high-dose fraction to the tumour bed after surgical removal of the tumour, aiming to eliminate residual cancer cells. This study investigates the incorporation of novel applicators into an existing IORT system to enable dose modulation, performing Monte Carlo (MC) simulations, 3D printing, and experimental validation. The Zeiss Intrabeam IORT device, a low-kV IORT system capable of delivering x-rays nearly isotropically, with energies up to 50 kV, was used in this study. Approach. Applicators were modified to alter dose distributions, incorporating features such as shielding or changes to an ellipsoid shape. The EGSnrc MC code was employed to simulate the dose distributions of each applicator design, generating data such as dose maps, percentage depth dose (PDD) curves, per cent difference maps between shielded and unshielded regions, and energy spectra to characterise each applicator. Gafchromic EBT3 film measurements were performed on select 3D printed applicators, to verify the MC simulations, with dose distribution data extracted for comparison. Main Results. Visual comparisons of dose and percentage different maps indicate a high correlation between the MC simulations and film measurements. Most PDD points for spherical applicators showed deviations within 4%, while ellipsoid applicators had deviations of 14% for the unshielded and 5% for the shielded applicators. All Root Mean Square Error (RMSEs) were below 0.05 for spherical and 0.18 for ellipsoid designs. Based on film data, shielded ellipsoid applicators reduced the dose by ∼99%, 48%, 22%, and 8% at 0.3, 1, 2, and 3 cm, respectively, while shielded spherical applicators achieved ∼83%, 35%, 14%, and 7% reductions at the same distances. Energy spectra for photons exiting shielded regions were also generated. Significance. Results of this study may be used in the development of patient-specific IORT techniques, or the development of a treatment planning system involving mIORT.