Quality assurance of central axis dose data for photon beams by means of a functional representation of the tissue phantom ratio.
A recently proposed four-parameter functional representation for the tissue phantom ratio (TPR) in the domain of electronic equilibrium was tested for accuracy and applied to quality assurance of central axis dose data. The four parameters are energy dependent and are found for each beam by a minmax search. For photon energies of 4 MV and greater, the functional representation was accurate to within 1% of measured data for all depths and field sizes exceeding 10 cm. The representation was also found to be robust. With only nine measurements of the TPR (at the extremes and middle of the electronic equilibrium domain) used to determine the four parameters, the representation reproduced the TPR value of any depth and field size to within 1% of measured data for photon energies of 6 MV or greater. The representation was insensitive to random measurement errors of a magnitude comparable to that expected in clinical practice. Other findings indicated that the representation degrades gracefully as the domain is extended into regions not in electronic equilibrium. When used with a QA program, the functional representation of TPR provides a means of detecting and correcting errors of measurement and data transcription of central axis dose data.
Xiao
Y
Y
Department of Radiation Oncology, School of Medicine, University of Pennsylvania, Philadelphia 19104, USA.
Altschuler
M D
MD
Bjärngard
B E
BE
D004581:Electronics; D006801:Humans; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use*; D011785:Quality Assurance, Health Care; D011786:Quality Control; D011878:Radiotherapy / Q000379:methods* / Q000592:standards*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods / Q000592:standards*; D015203:Reproducibility of Results; D012984:Software