When a chemotherapeutic agent is stated to have caused a partial response this implies what degree of reduction in measurable tumor volume?

d. 50–99%

A partial response is regarded as a 50%–99% reduction in all bidimensionally measurable disease, lasting at least four weeks or one treatment cycle. A complete response is defined as a total disappearance of all signs and symptoms associated with all malignant lesions, and complete resolution of all laboratory parameters associated with the disease, lasting at least four weeks or one treatment cycle. A minor response is a reduction of less than 50% of all bidimensionally measurable disease, or a reduction of 50% or greater which does not last at least four weeks or a treatment cycle. A minor response is not considered significant, though it may indicate potential biologic activity. Neither a minor nor a partial response is curative, though they may be associated with palliation of symptoms. The presence of a residual mass does not always indicate viable disease, as it may be composed of necrotic or fibrotic tissues.