I'm so sorry about your father-in-law.
When my grandfather was diagnosed with cancer, he had all kinds of treatments...radiation for sure, maybe chemo. It made him so much sicker; he couldn't eat because he couldn't stand the taste of most foods, and even smelling food would make him nauseous. He could no longer stand to eat the foods he used to love. It was awful seeing him like that, and I wondered the same thing...is all that worth it?
In my mom's case, I think it was worth it because they caught her cancer early enough to where she's fine now, but they recommended radiation just in case they hadn't removed all the cancerous tissue. The treatments made her sooooooo tired, she couldn't even take me shopping for a prom dress, but now she's all right and I'm really glad for that.