The results of photon radiation testing of various microcontroller devices are described. This testing was useful to select the microcontroller in a 6DOF MEMS-based INS. This system is being developed for the in-vivo monitoring of tumour position during clinical radiotherapy treatments. This application requires a radiation-tolerant processor in order to perform appropriately in a radiotherapy environment. A phantom has been built to replicate the working conditions that the microcontroller devices are required to endure. Each time, a number of identical microcontrollers have been exposed, in turn, to X-ray doses in excess of 50Gy from a clinical radiotherapy LINAC.