Dose Response Curves
Dose response curves can be generated for tumour control or normal tissue complications. If compared, the ratio between the two curves at a particular dose is the therapeutic index.
If the curves can be shifted (eg. by drugs, oxygenation etc) then the therapeutic index is altered, possibly providing a beneficial effect.
Chemotherapy and Dose Response Curves
Chemotherapy agents which sensitise cells to radiation will usually shift the tumour control and normal tissue complication curves to the left - ie: a greater effect for a similar dose. If chemotherapy widens the gap between tumour control and normal tissue complications for a particular dose, then the therapeutic index will be improved and chemotherapy may be beneficial.
This explains why, for some treatments, a reduced fraction size or dose is used when concurrent treatment is used instead of radiation alone.