To investigate the application of a recently developed metallic oxide semiconductor field effect transistor(MOSFET)detector for use in vivo desimetry.Methods The MOSFET detector was calibrated for X-ray beams of 8 MV and 15 MV,as well as electron beams with energy of 6,8,12 and 18 MeV.The dose linearity of the MOSFET detector was investigated for the doses ranging from 0 up to 50 Gy using 8 MV X-ray beams.Angular effect was evaluated as well in a cylindrical PMMA phantom by changing the beam entrance angle every 15?clockwise.The MOSFET detector was then used for a breast cancer patient in vivo dose measurement, after the treatment plan was verified in a water phantom using a NE-2571 ion chamber,in vivo measurements were performed in the first and last treatment,and once per week during the whole treatment.The measured doses were then compared with planning dose to evaluate the accuracy of each treatment.Results The MOSFET detector represented a good energy response for X-ray beams of 8 MV and 15 MV,and for electron beams with energy of 6 MeV up to 18 MeV.With the 6 V bias,Dose linearity error of the MOSFET detector was within 3.0% up to approximately 50 Gy,which can be significantly reduced to 1% when the detector was calibrated before and after each measdurement.The MOSFET response varied within 1.5% for angles firm 270?to 90?.However,maximum error of 10.0% was recorded comparing MOSFET response between forward and backward direction.In vivo mea surement for a breast cancer patient using 3DCRT showed that,the average dose.deviation between measurement and calculation was 2.8%,and the maximum error was less then 5.0%.Conclusions The new MOSFET detector,with its advantages of being in size,easy use,good energy response and dose linearity,can be used for in vivo dose measurement.