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%.
The acquisition and processing of the Jaszczak phantom is a recommended test by the American College of Radiology for evaluation of gamma camera system performance. To produce the reconstructed phantom image for quality evaluation, attenuation correction is applied. The attenuation of counts originating from the center of the phantom is greater than that originating from the periphery of the phantom causing an artifactual appearance of inhomogeneity in the reconstructed image and complicating phantom evaluation. Chang's mathematical formulation is a common method of attenuation correction applied on most gamma cameras that do not require an external transmission source such as computed tomography, radionuclide sources installed within the gantry of the camera or a flood source. Tomographic acquisition can be obtained in two different acquisition modes for dual-detector gamma camera; one where the two detectors are at 180° configuration and acquire projection images for a full 360°, and the other where the two detectors are positioned at a 90° configuration and acquire projections for only 180°. Though Chang's attenuation correction method has been used for 360° angle acquisition, its applicability for 180° angle acquisition remains a question with one vendor's camera software producing artifacts in the images. This work investigates whether Chang's attenuation correction technique can be applied to both acquisition modes by the development of a Chang's formulation-based algorithm that is applicable to both modes. Assessment of attenuation correction performance by phantom uniformity analysis illustrates improved uniformity with the proposed algorithm (22.6%) compared to the camera software (57.6%).
The Medical Physics departments of the Tom Baker Cancer Center (TBCC) and the Cross Cancer Institute (CCI) independently performed preliminary evaluation of the new Analytical Anisotropic Algorithm (AAA) implemented in Varian's Eclispe (v. 6.0) treatment planning system (TPS). The TPS was pre‐commissioned with “Golden Beam Data” from the vendor. We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beams; wedged square and half‐blocked beams; square fields at three SSDs; open and wedged oblique beams; irregular field defined by MLC and cerrobend blocks. All measurements were performed on Varian 2100EX linear accelerators. Measurements were made to assess the dose in heterogeneous media at both the CCI (CIRS Thorax IMRT phantom) and at the TBCC (TLDs in a Rando phantom). Profiles were evaluated in the buildup, penumbra, inner and outer beam regions as per AAPM Task Group 53. Measured and calculated profiles agreement was very good in all regions except for the inner beam region at the CCI, attributed a difference in interpolation schemes at the two institutions and the large volume ion chamber used for measurements. The AAA penumbra was also found to be steeper than measured penumbra since AAA was pre‐commissioned using diode measurements. Total scatter factors for most measurements differed by less than 2% from the calculated ones except for the hard wedges where differences up to 4% were found. Anthropomorphic phantoms measurements differed from AAA by as much as 5.6%. Funding provided by Varian.
Purpose: Evaluation of the accuracy of Eclipse AAA TPS when commissioned with Golden Beam or measured data. Method and Materials: Two cancer centers independently performed preliminary evaluation of the Analytical Anisotropic Algorithm (AAA) implemented in Eclipse TPS. The AAA photon algorithm was commissioned with vendor supplied “Golden Beam Data”(GBD). We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beam; wedged square and half‐blocked beam; square fields at three different source to surface distances; open and wedged beam at oblique incidence; beams shaped using cerrobend blocks and MLC. The measurements were performed on the Varian 2100 EX linear accelerators installed at the two institutions. After the initial tests, Eclipse was recommissioned with measured data from one of the machines. Results: The evaluation of the profiles was performed in the buildup, penumbra, inner and outer beam regions as per AAPM TG53. With the GBD the agreement of the measured and calculated profiles at the two institutions was very good in all regions except for the inner beam region on one machine. The tests that had a significant number of failures in the inner portion of the beam were mainly those cases where the TG53 tolerances are very tight. In these cases a significant number of points were just beyond the tolerance, and some of the off axis scans had 100% fail. On one particular test case where 16 profiles were measured for a particular geometry, 59% of points passed in the inner section with GBD, while 91% of points in that region passed once the beam was commissioned. Conclusion: When Eclipse is commissioned with GBD it was quite accurate, however, commissioning with measured data can improve the overall match. Conflict of Interest: Funding provided by Varian.
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