CE Mark Odds, page-110

Sirtex uses embolisation which tries to block as much blood supply to the tumour as possible. It also releases radioactivity into the remaining blood stream so as to irradiate the cancer cells. However after passing through the tumour, one would expect the irradiated blood has to exit the tumour into the rest of the body's circulation system causing side effects. I have not found any documentation to confirm this but I am just drawing on logical deduction.

Oncosil on the other hand injects radiated sand into the centre of tumour and held in place amongst cancer cells and NOT directly in the blood stream. Therefore the radioactivity leakage would be significantly less than that of Sirtex.

Both are aiming at destroying the cancer cells by radiation. But the cancer cells would attempt to beat embolisation by inducing blood vessel growth (angiogenesis) by secreting various growth factors (e.g. VEGF) and proteins. That's why cancer cell could multiply much faster than human cells.