Development and validation of a Monte Carlo model of a mobile accelerator for intraoperative radiation therapy

Intraoperative electron radiation therapy (IOERT) relies on accurate and precise dose delivery to the tumor or tumor bed using mobile accelerators and interchangeable applicators, while critical organs are typically displaced or shielded during surgery. Treatment planning and linac commissioning are often based on water measurements, Monte Carlo (MC) simulations of the accelerator head and applicator system provide detailed insights into dose distributions and beam characteristics, offering additional support for clinical evaluation.

This study develops an MC model of the Liac HWL mobile accelerator using a hypothetical linac head geometry, due to the limited availability of detailed information on its internal components resulting from manufacturer disclosure policies. The model is optimized by adjusting three geometric parameters of the linac head and the initial beam energy spectrum to match experimental data. Additionally, it provides a set of Phase Space Files (PSFs) to support research and clinical applications.

The MC code PENELOPE, integrated with the penEasy framework, was used to simulate the Liac HWL. The hypothetical head geometry was defined by parameters such as the inner diameter of the head, the thickness of the scattering foil, and the thickness of the exit window. Output factors (OFs), percentage depth doses (PDDs), and off-axis ratios (OARs) were calculated in a virtual water phantom for different applicator sizes, bevel angles, and energies. Gamma analysis was employed to validate the model by comparing calculated and measured dose distributions. PSFs were made available in the IAEA PHSP format at four energies (6, 8, 10, and 12 MeV).

The model matched measured OFs within 2.5%. PDDs and OARs met the gamma analysis criteria (2% dose difference and 1 mm distance-to-agreement) in more than 93% of the studied cases, with the worst-case scenario occurring for the smallest applicator (3 cm diameter) with a 45° bevel angle at 6 MeV, resulting in OAR gamma passing rates of 85.7% at $z_{max}$ and 86.1% at $R_{90}$ .

Despite the use of a hypothetical geometry, the model offers accurate dosimetric data and practical guidance for IOERT commissioning and treatment planning. It highlights potential dosimetric issues, particularly the lack of homogeneity in OARs for large-diameter applicators, and allows fine-tuning based on real-world data. Additionally, the PSFs generated in this study provide a reliable resource for simulating IORT dose distributions and analyzing the characteristics of IOERT beams.