@Sqad. Tumours of cartilaginous tissues are rare, but not unheard of. Chondrosarcoma, for instance.
There is work and research to suggest that cartilaginous tissue = from any source= has anti-angiogenic properties.And depriving a nascent tumour of its blood supply by inhibiting its ability to grow new blood vessels aids the fight.
Problem here is that the notion that taking pills comprised of cartilaginous shark tissue can cure or help fight cancer are not just unproven - they are disproven, with a variety of studies showing no clinical benefit.
And whilst sharks are indeed comprised of cartilaginous tissue, the fact remains that sharks do indeed develop cancer. It should not be surprising to anyone that the nature of their cancer may differ from a bipedal land dwelling air-breathing primate :)
You might be interesting in reading a paper published not that long ago in The New Scientist, which discussed this very issue, Sqad...
http://blogs.scientificamerican.com/science-sushi/2011/09/01/mythbusting-101-sharks-will-cure-cancer/