Progress in radiotherapy has led to increased cancer patient survival rates. To achieve further improvements in patient survival rates, identifying the exact dose delivered to tumour sites and minimising damage to surrounding healthy tissue and organs, ideally through implantable dosimeters, is considered the next frontier. However, the lack of a low-cost, flexible, tissue-equivalent direct conversion semiconductor diode continues to impede further developments in this area. Compared to commercial solid-state detectors, hybrid inorganic-in-organic (IOH) detectors offer several benefits, such as biocompatibility, high sensitivity under clinically relevant X-ray energies, and lower cost of manufacture. This thesis addresses direct X-ray detection with ultrathin hybrids based on P3HT:PC70BM:Bi2O3 and P3HSe:PC70BM, translated from rigid platforms and ~55 µm flexible stacks to ~1 µm flexible stacks. The work discussed here integrates four elements: thickness down-scaling to ultrathin implantable dimensions of ~1 µm; understanding the in-vivo operando of ultrathin detectors under laboratory and clinical X-ray irradiation; pixel down scaling and integration of polyimide (PI) as a hole blocking layer, and clinical-beam validation under a 6 MV LINAC. On investigations related to the influence of thickness reduction, ultralow dark-current densities of <0.5 pA mm⁻² at −10 V were maintained, with box-shaped X-ray photocurrent response. Under 40 kV exposure during 0–96 h immersion in PBS, sensitivity was observed to increase from ~3.58×103 to ~6.91×103 pC mGy⁻¹ mm⁻² without any noticeable increase on charge transport properties, consistent with electrostatic screening at the PBS/parylene intersection. Clinical-beam tests confirmed dose and dose-rate linearity following ~1140 hours of immersion in PBS. P3HT:PC70BM:Bi2O3 NP devices showed a slight decrease in sensitivity from 2.34×10³ to 2.22×103 pC mGy-1 mm-2 while P3HSe:PC70BM devices demonstrated an unchanged sensitivity of 1.32×103 pC mGy-1 mm-2 between 0 and 1440 hours with. Downscaling of pixel areas (from 3.75 to 0.09 mm2 ) indicated an increase in dark currents from 0.16 to 5.27 pA mm-2 under a bias of -10 V and a gain in sensitivity from ~0.17 to 6.42 pC mGy-1 mm-2 . Introducing a PI hole blocking layer at the cathode contact, yielded ultra-low dark currents of 0.48 and 0.27 pA mm ² and high sensitivities of 70.2 and 36.6 pC mGy⁻¹ mm⁻² with 66-68% retention after 100 bend cycles for P3HT:PC70BM:Bi2O3 NP and P3HSe:PC70BM blends, respectively. Together, these results demonstrate that these low noise ultra-thin IOH detectors are capable of quantitative low perturbation real-time dosimetry in clinically relevant biomimicking environments.