In external radiotherapy, complex and sophisticated treatment techniques such as Intensity-Modulated Radiation Therapy (IMRT) and stereotactic radiotherapy are becoming widely used in clinics. These techniques are designed to fit tightly the target volumes and deliver complex and non-uniform dose distributions with steep dose gradients. However, the dosimeters currently available in clinics only measure point or planar dose distributions, or can be assembled in 2D or 3D arrays to provide 2D or sparse 3D dose distributions with a low spatial resolution.