Air density dependence of the response of the PTW SourceCheck 4pi ionization chamber for $^{125}$I brachytherapy seeds

TL;DR

This study investigates how air density affects the response of the PTW SourceCheck 4pi ionization chamber used in brachytherapy, revealing a linear dependence that can cause significant overestimation at high altitudes.

Contribution

It provides the first detailed analysis of air density dependence for the SourceCheck 4pi chamber, demonstrating a linear correction model for accurate dosimetry in varying atmospheric conditions.

Findings

Residual air density dependence observed after standard correction.

Linear function describes the residual dependence accurately.

Overestimation of up to ~4% at high altitudes.

Abstract

Purpose: To analyze the air density dependence of the response of the new SourceCheck 4pi ionization chamber, manufactured by PTW. Methods: The air density dependence of three different SourceCheck 4pi chambers was studied by measuring 125I sources. Measurements were taken by varying the pressure from 746.6 to 986.6hPa in a pressure chamber. Three different HDR 1000 Plus ionization chambers were also analyzed under similar conditions. A linear and a potential-like function of the air density were fitted to experimental data and their achievement in describing them was analyzed. Results: SourceCheck 4pi chamber response showed a residual dependence on the air density once the standard pressure and temperature factor was applied. The chamber response was overestimated when the air density was below that under normal atmospheric conditions. A similar dependence was found for the HDR 1000 Plus chambers analyzed. A linear function of the air density permitted a very good description of this residual dependence, better than with a potential function. No significant variability between the different specimens of the same chamber model studied was found. Conclusion: The effect of overestimation observed in the chamber responses once they are corrected for the standard pressure and temperature may represent a non-negligible ~4% overestimation in high altitude cities as ours (700 m AMSL). This overestimation behaves linearly with the air density in all cases analyzed.