Why Does Cancer Recur?

While walking through the indoor eco park,an Indian aunty that became my friend at the counter was telling me about how bad was her sister's oncologist.She said within 3 years after chemotherapy,the cancer reappeared on the opposite side.

Tumour grows under chaotic condition and in order to grow,they need to form new blood vessels.Their blood vessels are usually abnormal and not fully matured nor functional where holes and shunt can present in it.

Since blood vessels carries oxygen and nutrition to the cell,tumour cells tend to have area deficient of oxygen,known as hypoxic area.

In classical cancer fashion,people believe it's good but the truth is,lack of oxygen won't kill the tumour cells.It activate hypoxic inducible  factor which influence 100s of genes,many are  pro-survival, pro-proliferative,anti-apoptotic, increase invasive capabilities of the tumour and also reprogramming it's metabolism.

Radiation and chemotherapy works by causing free radical production.So hypoxic area will not response to treatment because it don't have the substrate to produce free radicals which is oxygen.This will limit the treatment and such hypoxic region escape the therapy.

So after treatment, it repopulate  the tumour with time and become more aggressive and resistance to previous chemo meds due to those hypoxic factor enhancement.
When they recurs,they not just come again,but they come back with vengeance.