Is there any mature data about cognitive outcomes in 12 Gy vs 18 Gy in the setting of prophylactic cranial irradiation for ALL in children?  

There is some data on AML showing some differences between 12 vs. 18 Gy, yet still quite a low incidence.https://www.ncbi.nlm.nih.gov/pubmed/214804694/134 (3%) of patients treated with 12 Gy versus 8/108 (7%) of patients treated with 18 Gy demonstrated behavioral disorders and cognitive impairment. ...