The present invention relates to a crystalline form of 1-{4-[1-(2,6-difluorobenzyl)-5-dimethylaminomethyl-3-(6-methoxypyridazin-3-yl)-2,4-dioxo-1,2,3,4-tetrahydrothieno[2,3-d]pyrimidin-6-yl]phenyl}-3-methoxyurea that exhibits an X-ray powder diffraction pattern having 2-theta (2¸) peaks at approximately 8.932°, 16.607°, and 17.328° or peaks expressed in d values (Å) at approximately 9.893, 5.334, and 5.114.