A miniature X-ray source device for effecting radiation therapy at least comprising a vacuum tube containing a cathode and an anode spaced apart at some distance from each other; emitting means for emitting free electrons from the cathode; electric field generating means for applying during use a high-voltage electric field between the cathode and the anode for accelerating the emitted free electrons towards the anode, as well as an exit window for X-ray radiation being generating at the anode. The present invention provides an improved miniature X-ray source device, that can also properly be used in treating skin cancer and which is easy to handle. The anode is provided with a flat X-ray emitting surface. In particular, the cathode exhibits a concave shaped surface having a center part surrounded by an upright circumferential edge, wherein the center part of the concave shaped surface is provided with an electron emitting material.