Hmmm,
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2770698/
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.
http://en.wikipedia.org/wiki/Paraneoplastic_syndrome
Under 'Neurological' changes we can see,
http://en.wikipedia.org/wiki/Lambert%E2%80%93Eaton_myasthenic_syndrome
http://en.wikipedia.org/wiki/Paraneoplastic_cerebellar_degeneration
http://en.wikipedia.org/wiki/Encephalomyelitis
http://en.wikipedia.org/wiki/Limbic_encephalitis
http://en.wikipedia.org/wiki/Opsoclonus_myoclonus_syndrome
http://en.wikipedia.org/wiki/Anti-NMDA_receptor_encephalitis
Which are ion-channelopathies and auto-immune pathologies.
Tell me, where does the plasticity occur to boost brain function? How are you measuring '40%'
http://webspace.ship.edu/cgboer/iku.html
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!
an example of how cannabis utilised the existing endocannabinoid system to elicit neuroplasticity).