On the existence of low-energy photons (<150 keV) in the unflattened x-ray beam from an ordinary radiotherapeutic target in a medical linear accelerator

Abstract

Low-energy photons (<150 keV) are essential for obtaining high quality x-ray radiographs. These photons are usually produced in the accelerator target, but are effectively absorbed by the flattening filter and, at least partially, by the target itself. Experimental proof is presented for the existence of low-energy photons in the unflattened x-ray beam produced by a 6 MeV electron beam normally incident on the thinner of the two existing ports of the all-Cu radiotherapeutic target of a Clinac 18 (Varian Associates) linear accelerator. A number of one-shot absorption measurements were carried out with 12 foils of Pb absorbers with thicknesses varying from 0.25 to 3 mm in steps of 0.25 mm arranged symmetrically around the central axis on a 7.2 cm radius circumference. A Kodak ECL film-screen-cassette combination was used as a detector in the absorption measurements, in which optical density was measured as a function of the thickness of the Pb absorbers. Two sets of absorption measurements were carried out: the first one with the Clinac 18 6 MV unflattened beam and the second one with the Clinac 600C 6 MV therapeutic counterpart beam. There is a striking difference between the two sets: the optical density versus Pb-absorber thickness curve shows a sharp increase in optical density at small absorber thicknesses in the case of the unflattened 6 MV x-ray beam as compared with a gently sloping dependence in the case of the 6 MV therapeutic beam. A semi-quantitative assessment of the low-energy photon contribution to the whole optical density/contrast is presented. A 0.85 mm thick Pb absorber intercepting the 6 MV unflattened x-ray beam eliminates almost totally the sharp peak in the optical density curve at small Pb-absorber thicknesses. This constitutes additional evidence for the existence of low-energy photons (<150 keV) in the unflattened 6 MV beam from the Cu therapeutic target.

Dosimetric properties of a beam quality‐matched 6 MV unflattened photon beam

“…Various other advantages, such as a reduction in head scatter and electron contamination, were also reported. (7,9,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) Removal of the flattening filter increases the weight of low-energy photon fluence and results in decreased UF beam energy. (7,16,22) The decreased UF beam energy could be increased to the original WF beam energy by matching the percent depth dose (%dd) to that of the WF beam.…”

Flattening filter removal for improved image quality of megavoltage fluoroscopy

“…Measurements of unflat versus flat beams have demonstrated the existence of low-energy photons (<150 keV) in an unflat 6 MV beam that are available for increased image contrast. 20,21 FFF MV portal imaging has been shown to improve contrast and overall portal image quality. 4,16,22 Imaging with a low-Z target and FFF system has been shown to improve MV portal image quality, 23,24 gold marker contrast, 25 and image contrast and resolution in MV cone beam CT. 15,24,26 However, the effects of a FFF beam alone (without a low-Z target) on MV fluoroscopic images have not been reported.…”

Unflattened photon beams from the standard flattening filter free accelerators for radiotherapy: Advantages, limitations and challenges

“…A number of Monte Carlo and experimental studies dealing with characteristics, dosimetric aspects, and radiation protection issues of FFF photon beams generated by mechanically removing the flattening filter of existing standard linacs of different makes and models have been reported in the recent past. [2–15] Studies related to treatment planning and dose delivery of various clinical cases using FFF beams demonstrate their clinical suitability and superiority over FF photon beams. A review of the properties of FFF photon beams summarizing the findings of different investigators has also been published recently.…”