Over the last two decades, enormous progress has been achieved in characterizing the genetic and molecular changes that occur in cancer. The drug approval procedure, on the other hand, has not kept up with advances in tumor biology. Measurement of tumor diameters before and after treatment and classification of tumor reduction according to criteria published more than 25 years ago are still used to assess treatment responses. The initial WHO criteria were based on dimensional tumor measurements and defined response as a reduction of at least 50% in the product of two perpendicular tumor diameters.
