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-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.

An analytical method to generate portal film charts for 10-MV photon beams, which takes into account the presence of the cassette front screen is presented. The selection of the best film-screen combination was based on the new AAPM recommendations for radiotherapy portal imaging [AAPM Rep. No. 24, AAPM Task Group No. 28 (1987)]: 2-g/cm2-thick copper front screen and 0.4-g/cm2-thick copper rear screen, and X-OMAT TL Kodak unwrapped film. Doses at the film position were measured as a function of patient thickness, field size, air gap, and the results compared well with the doses derived from the analytical method: within +/- 15%. An optical density of 1.6 was selected for construction of the portal film charts. The application of the method for routine treatment planning quality assurance allows a quick and precise determination of the best exposures to the portal films.

A small personnel neutron dosimeter composed of a single polycarbonate foil and four loB layers, used as CY and 'Li radiators, is described. A certain combination of the number of particles emerging from these radiators, found theoretically, is Rem equivalent in the 1 eV-1 MeV energy range. The region of the foil not masked by ' OB layers gives a measure of the fast neutron dose above 1 MeV. Theoretical and experimental studies of this dosimeter are described which show that it can also be used to extract the effective energy of the incident neutrons. The electrochemical etching conditions for revealing simultaneously the damage sites produced by a, 'Li and heavy ion recoil particles (carbon and oxygen ions due to high energy neutrons) are reported.