All of the following are true regarding retinoblastoma, except:

Retinoblastoma is the most common primary intraocular malignancy of children. Cell of origin of retinoblastoma is a neuronal progenitor. In approximately 40% of cases, retinoblastoma occurs in individuals who inherit a germline mutation of one RB allele In the sporadic cases, both RB alleles are lost by somatic mutations Retinoblastomas arising in those with germline mutations are often bilateral. In addition, they may be associated with pinealoblastoma ("trilateral" retinoblastoma), which is associated with a dismal outcome. Tumors may contain both undifferentiated and differentiated elements. The undifferentiated components appear as collections of small, round cells with hyperchromatic nuclei. In well-differentiated tumours there are Flexner-Wintersteiner rosettes and fleurettes reflecting photoreceptor differentiation. Zones of dystrophic calcification are characteristic of retinoblastoma. Retinoblastoma tends to spread to the brain and bone marrow and seldom disseminates to the lungs. Prognosis is adversely affected by extraocular extension and invasion along the optic nerve and by choroidal invasion.