Radiation usually causes posterior subcapsular cataract which has their onset 1-2 years after completion of radiation therapy. A radiation dose of 15Gy is associated with a 50% risk of cataract formation. Christmas tree cataract is a rare form of age related cataract. In this type, the highly retractile multicolored needle shaped lenticular inclusions are seen within the deep perinuclear coex of the lens. Posterior polar cataract is a rare form of congenital cataract with a round, discoid opacity located in the central posterior pa of the lens adjacent to and merging with the posterior capsule. REF: Cataract Surgery from Routine to Complex: A Practical Guide By George J. C. Jin PAGE 21.