Complex sentence: His expertise involving brain disease led directly to the first successful removal of a brain tumor, which was performed by R. Köhler, and with Fedor Krause (1857-1937), he reported the first successful removal of a pineal tumor.
Split into simpler sentences: His expertise involving brain disease led directly to the first successful removal of a brain tumor, an operation that was performed by physician named R. Köhler. With surgeon Fedor Krause (1857-1937), he reported the first successful removal of a pineal tumor.