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

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.