Uncertainties in dosimetric data and beam calibration.

Recent studies indicate that the calibration of therapeutic beams is one of the main sources of uncertainty in the mean absorbed dose to the target volume in radiotherapy. Interaction coefficients and data used through the different steps in the calibration are pointed out as the main contribution to this uncertainty. Procedures used to select dosimetric data, that is, input parameters used in the specification of the quality of the beam, cause another contribution. In this paper the actual status of the data used for the dosimetry of photon and electron beams is introduced first. Uncertainties along the dosimetric chain are analyzed according to the procedure and data used in recent publications. Uncertainties in stopping-power ratios, considered the main contribution, are discussed in detail starting from the basic electron stopping-power data. Overall uncertainties in the presently available set of stopping-power ratios are analyzed. Recent developments in the dosimetry of electron beams, related to the effect of energy and angular spread and electron and photon contamination, are discussed in connection with the procedure to select stopping-power ratios for clinical dosimetry. Uncertainties along the dosimetric chain are evaluated in terms of the present knowledge of error sources.