Author: Paolo Pellicioli, Mohammad Rezaee, Ehsan Tajikmansoury, John W. Wong 👨🔬
Affiliation: Institute of Anatomy, University of Bern, Baltzerstrasse, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine 🌍
Purpose: Half-value layer (HVL) is a critical parameter for commissioning and quality assurance of preclinical x-ray irradiators. However, measuring HVL in cabinet systems presents challenges due to space limitations, significant scatter from cabinet walls, and broad beam geometries. This study introduces a novel methodology for accurate HVL measurement by minimizing scatter contributions.
Methods: HVL measurements were performed on multiple cabinet systems, including FLASH-SARRP, CIX-FLASH, SARRP, and CIXD. Scatter components were analyzed based on field size(broad vs. narrow beams) and distances between the focal spot and diaphragm (FDD), ion-chamber(FCD), and downstream cover(FWD). Narrow beam geometry was achieved by progressively reducing the field size from 20mm to 10mm, 5mm, 2mm, and 1mm diameter at upstream level and 20mm to 8mm diameter at the level of the chamber. A calibrated pinpoint ion-chamber(≤5 mm diameter of sensitive volume) mounted on an adjustable holder was used for beam output measurements. Copper foils(99% purity, 0.01–0.02 mm) served as attenuators. Radiographic films were used to ensure precise chamber alignment. Scatter radiation components were characterized, and their contributions to the HVL measurement were quantified. Monte Carlo simulations(Geant4) and analytical software(SpecPy) were utilized to validate the results.
Results: Scattered radiation had significant impacts on the HVL measurements, particularly for the second HVL, resulting in measurement variations up to 1.8 times. Narrow beam geometry reduced the scatter by up to 90%. Under narrow beam geometry, HVL measurement varied by 50% due to the scatter from cabinet wall, which was sensitive to the FDD distance. The scatter effects were minimized by decreasing FDD and increasing FWD. Using the optimized methodology, the first and second HVLs for FLASH-SARRP were measured as 0.13mmCu and 0.39mmCu, respectively. These measurements were within 10% of the simulated values.
Conclusion: Optimizing beam geometry and measurement setup significantly improves the accuracy of HVL measurement in preclinical cabinet systems.