A collimator (1) primarily adapted for usage in a narrow scanned pencil beam radiation therapy system (100) includes adjacent pairs (5) of collimator leaves (10, 20). An inner portion (12) of a collimator leaf (10) facing the opposite leaf (20) of a pair (5) is made of a first material having high linear radiation attenuation. The remaining, major portion (14) of the leaf (10) is made of a second material having a comparatively low density, weight and radiation attenuation. The collimator (1) provides effective penumbra trimming of a radiation beam (60), while simultaneously protecting healthy tissue around a tumor in an irradiated patient (80) from the radiation. The new design results in a significantly more compact, lighter and less expensive collimator (1) as compared to traditional collimators.