QUALITY ASSURANCE OF CENTRAL AXIS DOSE DATA FOR PHOTON BEAMS BY MEANSOF A FUNCTIONAL REPRESENTATION OF THE TISSUE PHANTOM RATIO

A recently proposed four-parameter functional representation for the t issue phantom ratio (TPR) in the domain of electronic equilibrium was tested for accuracy and applied to quality assurance of central axis d ose data. The four parameters are energy dependent and are found for e ach beam by a minmax search. For photon energies of 4 MV and greater, the functional representation was accurate to within 1% of measured da ta for all depths and field sizes exceeding 10 cm. The representation was also found to be robust. With only nine measurements of the TPR (a t the extremes and middle of the electronic equilibrium domain) used t o determine the four parameters, the representation reproduced the TPR value of any depth and field size to within 1% of measured data for p hotos energies of 6 MV or greater. The representation was insensitive to random measurement errors of a magnitude comparable to that expecte d in clinical practice. Other findings indicated that the representati on degrades gracefully as the domain is extended into regions not in e lectronic equilibrium. When used with a QA program, the functional rep resentation of TPR provides a means of detecting and correcting errors of measurement and data transcription of central axis dose data.