The idea that selected changes in Bilaterian's bodies would cause an increase in juvenile cancer rates has been part of my theory from its inception; it was clearly expressed in the 1984 Journal of Theoretical Biology Letter as it was in manuscripts submitted to other journals as early as 1979.
Here is a relevant paragraph from the 1984 Letter:
Adaptive pro-oncogenes are those that imparted some survival benefit to the germ line in spite of a likely increase in juvenile deaths from cancer following their selection. Increased somatic complexity, greater body size, extended pre-reproductive life and migration to more mutagenic habitats occurred in so many Bilaterian lineages that they can be confidently judged to have been adaptive. It is, however, most probable that selection of such characters was followed by increases in the incidence of somatic mutational events in juveniles and resulted in increased losses of genetic material to cancer.