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