On the basis of 1,015 entrance and 863 exit dose in vivo measurements, 863 calculations of midline dose were done, and the average deviation and ranges of its value were estimated. Data of 710 advanced larynx cancers were reviewed in order to achieve dose-response relationship. Patients data were fitted directly to L-Q model using maximum likelihood estimation. In 16.5% of measurements the deviation of midline dose was larger than -5.2%. A steep dose response relationship for TCP was found. Considering -5.2% deviation of 2 Gy fraction and 72 Gy of total dose, the 17% (from 48 to 31%) decrease of TCP was found. It shows that deviations of delivered dose influence the tumor control probability and that after systematic error finding during fractionated radiotherapy the value of remaining fraction size and total dose should be modified to compensate the change of TCP.