Intracavitary electron radiotherapy with dynamic shielding for rectal cancer: applicator design and dosimetric...

Intensity-modulated brachytherapy (IMBT) can improve dose conformity for rectal cancer, but current 192Ir-based systems are limited by isotropic photon emission, sequential shield motion, treatment-time penalties, and radioisotope logistics. This study designed, optimized, and characterized a dynamically shielded applicator-based intracavitary electron radiotherapy system intended to improve dose localization and enable ultra-high dose-rate delivery without a radioactive source. Approach. A cylindrical electron-beam applicator was designed with a high-density shield containing an emission window, a low-Z scatterer, and a polycarbonate guide tube. Applicator geometry was optimized using TOPAS Monte Carlo simulations to minimize radial and distal transmission factors (TFradial and TFdistal). Dosimetric performance was benchmarked against a modified Monte Carlo model based on a commercial 192Ir shielded rectal applicator (Varian GM11004160). A 3D-printed prototype with a stainless-steel shield and resin components was experimentally evaluated using a custom linear accelerator (5 MeV nominal energy, 60 mA peak current, 4 μs pulse width). Dose distributions were measured with EBT-XD radiochromic film. Main results. In the modeled homogeneous benchmark geometry, the optimized tungsten applicator achieved a radial transmission factor of approximately 1% at 10 mm from the applicator surface, compared with approximately 20% for the modeled 192Ir reference system. Distal leakage remained below 5% beyond the applicator distal end. Under modeled accelerator conditions, the electron-beam system reached approximately 100 Gy/s at 100 Hz. Superposition of single-dwell dose kernels produced both uniform fields and spatially fractionated radiotherapy patterns with valley-to-peak dose ratios ≤ 0.2. Prototype measurements demonstrated localized directional dose delivery and ultra-high mean dose rates under the reported pulse conditions, while identifying leakage pathways that caused order-of-magnitude differences from simulation. Significance. The optimized Monte Carlo model demonstrated improved radial dose localization, radioisotope-free operation, and ultra-high dose-rate capability in homogeneous geometry. Further work is needed for accelerator integration, motion control, inverse planning, and validation in anatomically realistic settings.