The IROC Houston on-site audit consists of dosimetry measurements, quality assurance review, and benchmark cases. The benchmark cases are downloaded by the institution and completed prior to the audit. The quality assurance review is performed during the on-site visit and consists of reviewing the procedures and documentation of the institutionís QA protocol. The dosimetry measurements performed by IROC Houston during the on-site audit are like a mini-annual. IROC Houston physicists travel with their own water tank and dosimetry equipment and perform independent verification of the machine parameters.
The dosimeter used to measure external beams is a 0.6 cc Farmer-type ionization chamber, the charge is integrated in a Standard Imaging Max-4000 electrometer. The collecting potential to the chamber is 300 V. The absorbed dose to water calibration factors for the chambers are obtained from the ADCL and verified against an intercomparison with a transfer quality chamber whose factor traceable to NIST. The IROC Houston correction factor (coulomb/rdg) for the electrometer is obtained from the M.D. Anderson ADCL.
The institution is customarily requested to perform a spot check of the machine output immediately prior to the IROC Houston's measurements. A correction can then be applied to the IROC Houston measured dose rates to account for minor variations from the nominal clinical dose rates. Thus, the IROC Houston/Institution ratios stated in an IROC Houston Report are intended to be representative of the institution's system and not reflect a small output aberration that may have existed on the day of the visit.
Monitor end effect, ő, (also known as timer or shutter error) is determined by the IROC Houston using the technique described by Orton and Seibert (Refs. 1 and 5). A positive timer error indicates that net irradiation time is greater than that set on the machine. It should be noted that this method gives a sign opposite to that obtained when end effect is measured by the linear regression and extrapolation method.