This paper proposes a new method for the automatic contrast enhancement of fiducial markers in low-radiation Electronic Portal Images. It is shown that the proposed approach significantly enhances the contrast of the fiducial markers and produces results where these markers are clearly visible. The main theoretical contribution consists in designing an algorithm that enhances the contrast of small structures in noisy images; the parameters of this algorithm are not empirically selected, but determined via a maximum search over a contrast metric. From a practical standpoint, the proposed method has direct applications in the current clinical workflow involving manual marker detection. It is also able to significantly improve the performances of automatic marker detection reported in literature.