Brachytherapy (BT) is a type of interventional radiotherapy that is advantageous due to high absorbed dose conformity and possibility to deliver high dose in few fractions. It is often used for prostate and gynecological tumors as monotherapy or a boost alongside external beam radiotherapy (EBRT). However, there is a number of things that can compromise treatment delivery, starting from incorrect source data in a treatment planning system to malfunctioning of a treatment delivery unit. None of the established quality assurance (QA) procedures emulate treatment delivery where the planned dose could be compared with the experimentally determined value. While such practices are employed in EBRT, BT suffers from the lack of detectors that would be water-equivalent and convenient to use for regular measurements. First-choice thermoluminescence dosimeters are water-equivalent but have passive readout. Sporadic attempts to use other detectors have not led to any established practices at clinical sites. Stepping ahead, the safety of treatment delivery could be further evaluated using real-time in vivo dosimetry. If detectors were characterized with high-accuracy, a reliable error detection level could be set to terminate treatments if needed. Contrary to in-phantom QA, there are detectors suitable for such applications but their characterization is incomplete. In this thesis we address both problems.

Focusing on high-dose-rate ¹⁹²Ir remote afterloading treatments, which are among the most common in BT, we investigate and propose a direct readout synthetic diamond detector for in-phantom QA of treatment units. The detector was designed for small-field high-energy EBRT dosimetry but our findings demonstrate its suitability for BT dosimetry. Additionally, due to detector calibration with traceability to absorbed dose to water primary standards of high-energy photon beams and combined experimental and Monte Carlo (MC) characterization, the uncertainties in absorbed dose to water were comparable to passive readout detectors and lower than for other direct readout detectors. We complemented detector investigation with a theoretical study on diamond material properties and which values (mass density, mean excitation energy, number of conduction electrons per atom) shall be used for the most faithful description of ionizing radiation interactions in diamond for MC simulations and calculations of mass electronic stopping power. The findings improve diamond dosimetry accuracy, and subsequently, experimental dosimetry of not only BT but all radiotherapy beam qualities where the detectors are used.

Aiming to further contribute to experimental BT dosimetry, we focused on high atomic number inorganic scintillators used for in vivo dosimetry: ZnSe, CsI, and Al₂O₃. These are already existing dosimeters exhibiting promising luminescence properties, but until now, their investigation has been solely experimental. MC simulations are not subject to detector positioning uncertainties which are high due to steep dose gradients and other detector response artifacts, thus we used the method to investigate the absorbed-dose energy response of detectors, its dependence on radial distance and polar angles, scatter conditions, as well as detector design. We clarified how error-prone high atomic number detector characterization might be if experimental and MC methods are not combined. Both have certain limitations and have to complement each other.

Though the thesis addresses two different types of detectors for two different applications, the underlying theme is to understand the detector at hand. The use of MC simulations allowed introducing a new synthetic diamond detector into BT field and improving accuracy of in vivo dosimetry systems using inorganic scintillators. We also raised awareness to the lack of unified detector calibration and characterization practice in BT dosimetry.