Just look at this figure in order to understand that NOT ALL TUMOURS decrease with a ketogenic diet!

10.1038/458713a 

" Finally, in mammals and other species, different dietary-restriction regimens can all extend lifespan. It would be interesting to test whether different regimens could also reduce PI3K-dependent cancer expansion, as only some regimens might be realistically applicable to humans — for instance, feeding every other day rather than a 40% restriction every day. "