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AlignRT3C can be used as a nonionizing IGSPS with accuracy comparable to current image/marker-based systems. IGSPS and CBCT can be combined for high-precision positioning without the need for patient-attached localization devices.

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Purpose: Accurate modeling of beam profiles is important for precise treatment planning dosimetry. Calculated beam profiles need to precisely replicate profiles measured during machine commissioning. Finite detector size introduces perturbations into the measured profiles, which, in turn, impact the resulting modeled profiles. The authors investigate a method for extracting the unperturbed beam profiles from those measured during linear accelerator commissioning. Methods: In-plane and cross-plane data were collected for an Elekta Synergy linac at 6 MV using ionization chambers of volume 0.01, 0.04, 0.13, and 0.65 cm 3 and a diode of surface area 0.64 mm 2 . The detectors were orientated with the stem perpendicular to the beam and pointing away from the gantry. Profiles were measured for a 10ϫ 10 cm 2 field at depths ranging from 0.8 to 25.0 cm and SSDs from 90 to 110 cm. Shaping parameters of a Gaussian response function were obtained relative to the Edge detector. The Gaussian function was deconvolved from the measured ionization chamber data. The Edge detector profile was taken as an approximation to the true profile, to which deconvolved data were compared. Data were also collected with CC13 and Edge detectors for additional fields and energies on an Elekta Synergy, Varian Trilogy, and Siemens Oncor linear accelerator and response functions obtained. Response functions were compared as a function of depth, SSD, and detector scan direction. Variations in the shaping parameter were introduced and the effect on the resulting deconvolution profiles assessed. Results: Up to 10% setup dependence in the Gaussian shaping parameter occurred, for each detector for a particular plane. This translated to less than a Ϯ0.7 mm variation in the 80%-20% penumbral width. For large volume ionization chambers such as the FC65 Farmer type, where the cavity length to diameter ratio is far from 1, the scan direction produced up to a 40% difference in the shaping parameter between in-plane and cross-plane measurements. This is primarily due to the directional difference in penumbral width measured by the FC65 chamber, which can more than double in profiles obtained with the detector stem parallel compared to perpendicular to the scan direction. For the more symmetric CC13 chamber the variation was only 3% between in-plane and cross-plane measurements. Conclusions:The authors have shown that the detector response varies with detector type, depth, SSD, and detector scan direction. In-plane vs cross-plane scanning can require calculation of a direction dependent response function. The effect of a 10% overall variation in the response function, for an ionization chamber, translates to a small deviation in the penumbra from that of the Edge detector measured profile when deconvolved. Due to the uncertainties introduced by deconvolution the Edge detector would be preferable in obtaining an approximation of the true profile, particularly for field sizes where the energy dependence of the diode can be neglected. However, an averaged respons...

The cross section for double bremsstrahlung differential in the radiated photon energies and angles has been measured for 70-keV electrons on targets of Al, Cu, Ag, Tb, and U for photons radiated at ±45° to the incident beam for photon energies in windows from 10 to 30 keV. In contrast with previous experiments at ±90°, the results are in reasonable agreement with the relativistic first Born approximation at lower Z. However, the results exhibit a Z dependence which disagrees with the first Born Z^ dependence, suggesting the need for consideration of a second Born approximation.PACS numbers: 34.80.-i Double bremsstrahlung is a quantum electrodynamic process in which two photons are radiated simultaneously in the scattering of an electron by an atom. This process was first mentioned by Heitler and Nordheim [I] who estimated that the cross section would be about 137 times smaller than that of single bremsstrahlung, or essentially smaller by a factor of the fine-structure constant due to the emission of the second photon. While the two-photon process is too small to make much contribution to the production of radiation, it is nevertheless especially interesting as an example of a quantum radiative process for which, unlike single bremsstrahlung, there seems to be no prescription for a classical calculation of the cross section. Although recently there has been much interest in two-photon and multiphoton processes, the radiative two-photon process has been studied in only two experiments [2,3], both in a ±90° geometry. In both experiments, a significant discrepancy, as large as 2 orders of magnitude, with theories has been observed.The first measurement of the cross section was made in 1985 by Altman and Quarles [2]. They measured the cross section for two-photon emission for 75-keV electrons on thin targets of silver, terbium, gold, and uranium. The electron was not observed, so a differential cross section integrated over the unobserved electron was measured. Altman and Quarles also evaluated the theoretical cross section by numerical integration of the very complicated formula for the cross-section differential in the two photon energies and angles and the electron angles worked out by Smirnov [4] in the relativistic first Born approximation. The experimental result for gold was about 300 times the computed theoretical value. It was not clear whether the discrepancy was due to an error in the theoretical formula, the numerical integration of the formula, or the experimental data. An independent evaluation of the theory in the nonrelativistic Coulomb approximation was then provided by Veniard, Gavrila, and Maquet [5] and also by Florescu and Djamo [6]. The result for the particular geometry of Ref. [2] was larger by about a factor of 3 than the relativistic Born approximation but was still about a factor of 100 smaller than the observed cross section. These nonrelativistic calculations when evaluated in the Born rather than the Coulomb approximation were close to the relativistic result computed in Ref. [2], sug...