High dose-rate (HDR) brachytheraphy (BT) is radiation therapy (RT) which temporarily inserts a highly radioactive source into the planning target volume (PTV) with the use of an afterloader, applicator, transfer tubes, needles and planning software. The dose delivered to a point in the tumour depends on the type and activity of the source, the distance to the source and the time the source dwells at a predetermined dwell position. Current therapies have proven to be capable of tumour control. However, in contrast with other RTs there is no suitable integrated dose verification method available for HDR BT which does not disturb the clinical workflow. The focus of this research is on the dose verification for the most commonly used form of HDR BT. The one with the Ir-192 source. An optical in vivo dosimetry (OIVD) sensor assembly consists of a capsuled scintillator coupled into an optical fibre, which connects to a photodiode capable of processing the light signal generated in the scintillator. These systems are currently only available in experimental set-ups and not jet integrated with the afterloader, making them clinically unusable. Furthermore, the system needs to be coupled with an optical fibre of 100-300μm, give a linear output signal to remodel the delivered dose, be non-hygroscopic, be suitable at treatment temperatures, and have low production costs. By adding a OIVD sensor to the current clinical BT workflow, better treatment can be achieved. This sensor assembly needs to be made with a suitable scintillator and shape to have a high coupling efficiency in the OIVD sensor system to give reliable dose verification. Both a literature study and simulations are executed as part of my master thesis to provide a context for the project and to find the best-suited shape and scintillator, This report provides an overview of the current status of knowledge of OIVD from the available experiments and clinical studies derived from the published data, so knowledge for choosing the scintillator material, fibre diameter and the coupling geometry can be obtained. Special attention is given to identifying why scintillators excite light of a specific wavelength and how to achieve an efficient light coupling. The inorganic scintillator ZnSe : O has favourable scintillation characteristics and is has proven itself to be helpful in a OIVD sensor for Ir-192 BT with its high intensity, good energy resolution, slight afterglow and low production cost. The experimental part of this research is the verification of the OIVD sensor assembly with simulations executed within the COMSOL Multiphysics®ray-tracing package, which compares multiple similar OIVD sensor assembly heads. The OIVD model is inspired by the experimental set-up from E.Andersen and simulates for multiple configurations on a range of parameters (5). The simulations of the OIVD sensor assembly can conclude the following: 1. The most favourable scintillator is ZnSe :O, this scintillator has the highest light yield and low absorption. 2. With a fibre diameter of 200μ m, sufficient light yield reaches the detector to verify the dose delivery. 3. Couple the fibres off the centre from the central axis of the scintillator will negatively influence the coupling efficiency. The outcome of this research contributes to the design of a OIVD sensor assembly for HDR BT for with Ir-192 . These simulations lead to an optimal design that will fit the current afterloader system. Next to this, the OIVD sensor assembly head can be made with simple manufacturing steps so that it can be produced at a minimal cost. The combination of the COMSOL model with the scintillation properties from the literature shows that a sensor assembly of a ball coupled ZnsSe :O scintillation is has a higher light throughput than the prototype from E.Andersen (5), this shows that it is favourable to verify the dose delivery of Ir-192 HDR BT (5).