Quantifying measurement uncertainties in diode-based in-vivo dosimetry for Cobalt-60 high dose rate brachytherapy

Abstract High-dose-rate (HDR) brachytherapy provides a highly conformal cancer treatment modality by exploiting steep dose gradients, and achieving excellent tumour control while minimising radiation exposure to healthy tissues. In-vivo dosimetry (IVD) serves as an essential quality assurance tool, offering independent verification of delivered dose. However, its accuracy can be affected by several measurement-related uncertainties. This study aimed to characterise diode-based IVD for Co-60 HDR brachytherapy and quantify the uncertainties influencing detector performance. A Co-60 HDR afterloading system (SagiNova ® ) was used along with a diode-based in-vivo detector. Calibration was performed using a Polymethyl Methacrylate PMMA phantom. A custom-designed acrylic phantom was fabricated to ensure reproducible detector positioning and fixed geometry during irradiation. Detector linearity and uniformity were assessed by delivering known doses from 1 to 8 Gy in 1 Gy increments. Since conventional brachytherapy treatment planning systems do not account for tissue heterogeneity, additional measurements were performed by placing materials simulating bone (Teflon), lung (cork), and soft tissue (acrylic) of 1–3 cm thickness between the source and detector. The diode exhibited excellent stability, with repeatability showing < 2% relative standard deviation. Sensitivity across cumulative absorbed doses demonstrated < 2.5% variation, confirming strong consistency. A linear response was observed throughout the tested dose range. Heterogeneity analysis revealed notable dose perturbations: as expected, bone-equivalent material produced the highest attenuation, while lung-equivalent material resulted in the least, underscoring the importance of accounting for tissue density variations in IVD measurements. Although IVD offers valuable real-time dose verification in HDR brachytherapy, its widespread clinical adoption remains limited by challenges such as detector size and the steep dose gradients surrounding the source. Comprehensive commissioning—including evaluation of linearity, reproducibility, geometric dependence, and heterogeneity effects—is critical for understanding detector behaviour under clinical conditions. It is concluded that accurate characterisation of these uncertainties enhances the reliability of diode-based IVD systems and supports their integration into routine brachytherapy practice for improved patient safety and treatment precision.