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The Enhanced Dynamic Wedge (EDW) presents many advantages over the physical wedge. However, in order to calculate monitor units (MUs) necessary to deliver a certain dose at a certain point, EDW factors (EDWFs) need to be determined. In this work, based on analysis of the golden segmented treatment table (GSTT) and the MU fraction model, an empirical analytic formula has been developed to calculate EDW factors for symmetric and asymmetric fields. This formalism is an extension of the MU fraction model. However in comparison with previous studies [J. P. Gibbons, Med. Phys. 25, 1411-1418 (1998) and M. Miften et al., Med. Dosim. 25, 81-86 (2000)], this formula is simpler, and easier to use. It is applicable to EDW fields of different sizes, wedge angles and different photon energies. For 6 and 18 MV beams from a Varian 21EX accelerator with 7 EDW angles (Varian Oncology Systems, Palo Alto, CA), more than 250 measured EDWFs for symmetric and asymmetric fields with different off-axis distances and field sizes were compared with model calculations. Results show that 80% and 98% of calculated EDWFs match corresponding measured values to within 0.5% and 1.0%, respectively, the maximum deviation being 1.3%.

We evaluated a commercial three-dimensional (3D) electron beam treatment planning system (CADPLAN V.2.7.9) using both experimentally measured and Monte Carlo calculated dose distributions to compare with those predicted by CADPLAN calculations. Tests were carried out at various field sizes and electron beam energies from 6 to 20 MeV. For a homogeneous water phantom the agreement between measured and CADPLAN calculated dose distributions is very good except at the phantom surface. CADPLAN is able to predict hot and cold spots caused by a simple 3D inhomogeneity but unable to predict dose distributions for a more complex geometry where CADPLAN underestimates dose changes caused by inhomogeneity. We discussed possible causes for the inaccuracy in the CADPLAN dose calculations. In addition, we have tested CADPLAN treatment monitor unit and electron cut-out factor calculations and found that CADPLAN predictions generally agree with manual calculations.

Transmitted intensity through water was measured in a narrow-beam geometry for different energy x-ray beams from commercial accelerators. In order to accurately obtain the attenuation coefficient of the incident beam using transmission data, a novel formula was developed based on consideration of beam hardening in phantom. The value of the attenuation coefficient obtained by fitting transmission data to this formula was found to be independent of the absorber thickness used in experiments, whereas the attenuation coefficient obtained from the traditional formula, I(x) = I0 exp(-mux), changed by up to 7% with absorber thickness for a given beam. The beam hardening coefficient obtained from our formula indicates that the attenuation coefficient in water changes by about 0.33% per cm near the surface for the high-energy photon beams studied. Variations in beam quality with off-axis distance were subsequently investigated using the new formula. Results show that the attenuation coefficient at the water surface increased by about 15% for 15 and 18 MV beams, and by 11%-13% for 6 MV beams, when the off-axis distance at 100 cm from the source was changed from 0 to 18 cm. Consideration of the physics of bremsstrahlung production suggests that these variations should be mainly determined by the shape of the flattening filter, i.e., by the path length of rays traversing the filter in different directions. This expectation was confirmed by observing that the attenuation coefficient at the phantom surface can be related to the ray path of the beam in the flattening filter using the new transmission formula.

An analytic representation proposed for the relative intensity distribution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a closed-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15-MV photon beams from Varian 600C and 2300CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

An analytic representation proposed for the relative intensity distribution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a closed-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15-MV photon beams from Varian 600 C and 2300 CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

Head-scatter factors of symmetric square and rectangular fields (field center on the central beam axis) defined by the upper (Y) and lower (X) jaws for 6 and 15 MV photon beams from 2300CD and 600C accelerators (Varian Associates, Inc., Palo Alto, CA) were measured, as well as those for fields shaped by the Y jaws and the multileaf collimator (MLC) of the 2300CD. For rectangular fields, the head-scatter factor for the field (x = a and y = b) was different from that for the field (y = a and x = b). This difference was 2% -3% for fields defined by conventional collimators when [formula, see text] was large, and became 4%-5% when the MLC and YT jaws were used to shape the fields with the X jaws retracted. In order to calculate values for head-scatter factors of rectangular fields accurately using an equivalent square formalism, the side of the equivalent square should be obtained with different weights for lower and upper jaws, as proposed by Vadash and Bjärngard [Med. Phys. 20, 733-734 (1993)]. Our measurements demonstrate that the relative weight (G) of upper and lower jaws is strongly dependent on their distances from the x-ray source, while the beam energy has little effect on the value of G. We further show that G can be calculated simply from these distances. An analytical representation for head-scatter factors of square and rectangular fields is also developed in this paper. The quality of this representation was judged by the root-mean-square (rms) deviation from measured head-scatter factors, which ranged from 0.11%-0.27%.