A novel X-ray detector diode, optimized for angular independent (isotropic) dose response, is presented. The diode is designed as a silicon cube with p-n junctions on all six sides, which creates a device that is close to being symmetrical in 3D. The cube edge is 300 μm or 410 μm. Its manufacturing process is based on micromachining, featuring deep reactive ion etching (DRIE) of silicon-on-insulator (SOI) substrates, doping of vertical walls from gas phase dopants and re-fill of etched trenches with polysilicon. The variation in detector response to 6 MV X-rays, in a ±30° beam angle range, was at best ±0.5% for a cubic diode compared to ±3.3% for conventional diodes, which indicates improvement by a factor 7.