A thin target approach for portal imaging in medical accelerators

Abstract

A new thin-target method (patent pending) is described for portal imaging with low-energy (tens of keV) photons from a medical linear accelerator operating in a special mode. Low-energy photons are usually produced in the accelerator target, but are absorbed by the target and flattening filter, both made of medium- or high-Z materials such as Cu or W. Since the main contributor to absorption of the low-energy photons is self-absorption by the thick target through the photoelectric effect, it is proposed to lower the thickness of the portal imaging target to the minimum required to get the maximum low-energy photon fluence on the exit side of the target, and to lower the atomic number of the target so that predominantly photoelectric absorption is reduced. To determine the minimum thickness of the target, EGS4 Monte Carlo calculations were performed. As a result of these calculations, it was concluded that the maximum photon fluence for a 4 MeV electron beam is obtained with a 1.5 mm Cu target. This value is approximately five times less than the thickness of the Cu target routinely used for bremsstrahlung production in radiotherapeutic practice. Two sets of experiments were performed: the first with a 1.5 mm Cu target and the second with a 5 mm Al target (Cu mass equivalent) installed in the linear accelerator. Portal films were taken with a Rando anthropomorphic phantom. To emphasize the low-energy response of the new thin target we used a Kodak Min-R mammographic film and cassette combination, with a strong low-energy response. Because of its high sensitivity, only 1 cGy is required. The new portal images show a remarkable improvement in sharpness and contrast in anatomical detail compared with existing ones. It is also shown that further lowering of the target's atomic number (for example to C or Be) produces no significant improvement.

Low‐dose 2.5 MV cone‐beam computed tomography with thick CsI flat‐panel imager

“…The thinner thickness of the target also decreases the amount of self‐absorption and any further attenuation of these low‐energy photons can be eliminated by removing the flattening filter. This beamline configuration subsequently improves the image contrast as demonstrated by several previous studies 10 , 12 , 13 , 14 , 15 …”

A Monte Carlo investigation of low‐Z target image quality generated in a linear accelerator using Varianˈs VirtuaLinaca)

“…Monte Carlo simulation of low atomic number (Z) targets in conventional linear accelerators (Linacs), such as the Varian Clinac series [1][2][3][4][5][6] (Varian Medical Systems, Inc., Palo Alto, CA), Elekta SL25 and Precise Treatment Systems [7][8][9] (Elekta Oncology Systems, Crawley, UK) or Siemens Primus and Oncor Linacs [10][11][12] (Siemens Medical Solutions, Concord, CA), have all used geometric and material specifications of the Linac provided by the manufacturer to help construct accurate models in BEAMnrc. 13 However, this technical information for Varian's TrueBeam Linac platform (Varian Medical Systems, Inc., Palo Alto, CA) has not been made available to the research community for proprietary reasons.…”

“…13 However, this technical information for Varian's TrueBeam Linac platform (Varian Medical Systems, Inc., Palo Alto, CA) has not been made available to the research community for proprietary reasons. Currently, International Atomic Energy Agency (IAEA) format phase space files for individual clinical photon beams (4,6,8,10, and 15 MV, plus 10 and 15 MV flattening filter free) are available from the manufacturer (at myvarian.com/montecarlo), generated using the Geant4 Monte Carlo code 14 and geometry input from computer aided designs. 15,16 While the phase space files have been shown to produce accurate dose distributions compared to measurement, 17 they are not useful for investigating novel target designs in TrueBeam since they correspond to a location just above the secondary collimation (jaws).…”

Kilovoltage energy imaging with a radiotherapy linac with a continuously variable energy range

“…The first improvement has resulted in the ability to acquire cone beam computed tomography images using the therapy beam. 21,35 The second improvement has largely focussed on the use of low atomic number (Z) targets 3,[7][8][9][11][12][13][14]26,29,37 or a combination of medium Z materials and electron absorbers 30 to increase the low energy component 60 of the beam.…”