I'm not an expert in that specific field, but my understanding is that hereditary retinoblastomas are in fact associated with an increased risk of other cancers.

https://www.cancer.org/cancer/retinoblastoma/after-treatment/second-cancers.html

As for sporadic retinoblastoma, that is where individual cells in the retina mutate so is a localised issue.

This is commonly occurred retinal tumour in children. Few studies show that In children with the heritable genetic form of retinoblastoma there is a mutation in the RB1 gene on chromosome 13.