RECONSTRUCTION OF 6 MV PHOTON SPECTRA FROM MEASURED TRANSMISSION INCLUDING MAXIMUM ENERGY ESTIMATION

Photon spectra from a nominally 6 MV beam under standard clinical cond itions and at higher and lower beam qualities have been derived from n arrow-beam transmission measurements using a previously published thre e-parameter reconstruction model. Estimates of the maximum photon ener gy present in each spectrum were derived using a reduced number of mod el parameters. An estimate of the maximum contribution of background, or room, scatter to transmission measurements has been made for this s tudy and is shown to be negligible in terms of the quality index and p ercentage depth-dose of the derived spectra. Percentage depth-dose dat a for standard beam conditions derived from the reconstructed spectrum were found to agree with direct measurements to within approximately 1% for depths of up to 25 cm in water. Quality indices expressed in te rms of TPR1020 for all spectra were found to agree with directly measu red values to within 1%. The experimental procedure and reconstruction model are therefore shown to produce photon spectra whose derived qua lity indices and percentage depth-dose values agree with direct measur ement to within expected experimental uncertainty.