A metal-oxide-semiconductor field effect transistor (MOSFET) has been used as a radiation monitor to map the profile of a variety of X-ray radiation microbeams. The present work studies the role of the topology of the MOSFET gate oxide when mapping radiation microbeams. The edge-on MOSFET has been introduced for microbeam mapping and the spatial resolution of the edge-on MOSFET is investigated. Comparison is made with other mapping techniques including, Gafchromic film, radiographie film, ionization chamber. The results clearly demonstrate the superiority of the edgeon MOSFET when applied to mapping of narrow radiation beams. The spatial resolution of the "edge-on" MOSFET is estimated to be I pm and appears to be limited by the width of the gate oxide thickness. © 1999 IEEE.