lauren hill was diagnosed with an inoperable form of brain cancer aged 18 .
despite illness , she achieved dream to play college basketball in cincinnati .
also raised a huge $ 1.5 million for diffuse intrinsic pontine glioma research .
she died in hospital on friday , aged 19 , after defying doctors ' expectations .
now , lebron james has penned a touching farewell letter to the teenager .
in letter , he praises lauren for the ` leadership ' and ` strength ' she showed .
basketball star tells her : ` you time spent on earth will never be forgotten '