Suggested intelligence correlates are in the form of convolutions, interconnections between the corpus collosum and neocortical structure.
Furthermore, you haven't specified what type of cancer? I have heard of a paracrine/endocrine mimicking tumour (Australian spelling) which correlates to neoplasms acting as paraendocrine glands. How would that structurally have an effect on the neuroplasticity?
Lets have a look at neurological changes in patients with paraneoplastic syndrome.
Under 'Neurological' changes we can see,
Which are ion-channelopathies and auto-immune pathologies.
Tell me, where does the plasticity occur to boost brain function?
How are you measuring '40%'
Unless, lets say the Cancers don't elicit a neuroplastic response. Rather, some metabolite of a cancer system is acting upon, quotient based equilibria which would eventually de-regulate existing homeostatic body chemistry. I can foresee such a chain of events predominately leading to neurodegeneration.
Is this what is happening OP? Gimmie some peer reviewed articles bro, don't post conjecture otherwise no scientist will believe you!
(Nota bene - http://www.cmaj.ca/content/166/7/887.full?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=cannabis&searchid=1025853946146_44&stored_search=&FIRSTINDEX=0&journalcode=cmaj
an example of how cannabis utilised the existing endocannabinoid system to elicit neuroplasticity).