Preeti H.

Great question and quite an important one for all of us teachers to remember as well :).
As you may have seen, the definition of yog means to be connected (especially to connect with oneself). The main benefits of yog are many:
1. Potential to build strength and build it each day.
2. No doubt increases strength and stamina.
3. Relieves you from general aches and pains. Brings about mobility in our sedentary lifestyles. (Chronic and extreme cases may also have adverse effects if done without supervision.)
4. Weight Management. The agility with which you practice determines the rate at which you would lose weight (If that is your goal). 
5. No doubt mental clarity and calmness are bonuses that you achieve.
All of this though comes with regular practice.

If this is a general query then the answer is below. On the contrary, If a loved one is suffering from this disease, I'm sorry to hear, and my prayers are with you and your loved ones.
In simplest terms Cancer is a condition where cells divide and grow exponentially. These cells then tend to harm the body from inside. The cancer found as recently as 50-100 years ago were genetic conditions; however, off late, the kind of cancer that humankind faces is emerging more and more from lifestyle and eating abnormalities. The intake of sugar has increased over the years and research suggests that it is this sugar that these cells feed on the most. Hence, lifestyle changes would be the most immediate steps post-chemo (As adviced by the doctor).

Can Yoga prevent cancer? There is no scientific study that proves that it does but Yoga can help manage the symptoms of cancer and help generate a ratio of mental calm. A paper produced by the National Center for Biotechnology Information provides further details for interested candidates. I can share if required
Hope this helps.