Chemotherapy does affect voice quality. If you know Marie Fredriksson of Roxette (they have hits like The look, listen to your heart, Joyride, Fading like a flower, It must have been love, Wish I could fly) her voice was really very beautiful. She suffered from cancer and underwent Chemotherapy, in their latest albums, her voice is changed. Though you can recognize her soft voice but if she tries to sing high note, her voice starts to feel distorted.
For example Youtube Milk and toast and honey by Roxette (song before cancer) and then Youtube Only when I dream by Roxette (in the ending of the song) you would find that she can't sing with that flow.
Likewise people who attended Roxette worldwide tour in 2011-2012, they also noticed that Marie doesn't sound the same live as she used to. In studio albums, her voice is still good but live, now her voice seems very changed.
Dr. Wasif
http://coolbluez.t15.org