en Radiation Biology Radiation Biology تحقيق بديع Original Research <span style="font-weight: bold">Background:</span> Absolute dosimetry of external beam radiotherapy is carried out by the use of ionization chambers. These chambers must be calibrated at a standard dosimetry laboratory before any use in clinical dosimetry. The secondary standard dosimetry laboratory of Iran (SSDL) has the duty of calibrating the ionization chambers used in radiotherapy centers in Iran. <span style="font-weight: bold">Materials and Methods:</span> The present work has described traceability of SSDL radiation measurement standards to relevant international standards, and calibration of therapy level ionization chambers in terms of air kerma and absorbed dose to water against 60Co gamma radiation, as well as uncertainty evaluation of calibration coefficients.<span style="font-weight: bold"> Results:</span> The expanded uncertainties in the determination of air kerma and absorbed dose to water are estimated to be 2% and 2.3% at approximately 95% confidence level, respectively. <span style="font-weight: bold">Conclusion: </span>In order to maintain the requirement of ±5% accuracy in the dose delivery, the combined standard uncertainty of the other factors in the dose delivery i.e., dose measurement set-up, dose calculations, treatment planning, patient set-up, etc, should be less than 2.3%. Iran. J. Radiat. Res., 2010 8 (3): 195-199 Dosimetry, ionization chamber, calibration, uncertainty. 195 199 http://ijrr.com/browse.php?a_code=A-10-1-368&slc_lang=en&sid=1 A. Solimanian 790031947532846001832 790031947532846001832 No M. Ghafoori mghafoori@nrcam.org 790031947532846001981 790031947532846001981 Yes