A thin target approach for portal imaging in medical accelerators.

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.