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Purpose: Standard CT dosimetry phantoms are currently perfectly circular cylinders. Lately elliptical cylinders have been proposed as an alternative because they better represent the shapes of the human body and allow the evaluation of tube current modulation systems. The purpose of this study was to extend the concept of CTDIw to elliptical phantoms of various aspect ratios. Methods: Based on published adult and pediatric data, eight body aspect ratios were chosen from the full range between 1 (perfectly circular) and 1.72 (extremely elliptical). For each value, two elliptical cylinders were created digitally to represent adult and pediatric bodies. They had the same cross‐sectional areas as the standard 32‐cm and 16‐cm CTDI phantoms. For each digital phantom, CTDI₁₀₀ at central and peripheral locations were simulated for tube voltages between 70–140 kVp using a Monte Carlo program previously validated for a clinical CT system (SOMATOM Definition Flash, Siemens Healthcare). The simulation also output the average dose over the cross‐sectional area, Dxsec, the quantity CTDIw is intended to represent. Values of Dxsec and CTDI₁₀₀ allowed linear systems of equations to be established, from which central and peripheral weighting coefficients were solved. Results: All elliptical phantoms had the same Dxsec as the standard circular phantoms. Regardless of phantom shape, only two weighting coefficients (w1 and w2) were needed: w1 for central CTDI₁ ₀₀ and w2 for the average of the four peripheral CTDI₁₀₀'s. For perfectly circular phantoms, w1 and w2 were 0.37 and 0.63, respectively, agreeing well with the conventional weighting coefficients of 1/3 and 2/3. Over the full range of aspect ratios, w1 increased linearly from 0.37 to 0.46, whereas w2 decreased linearly from 0.63 to 0.54. Conclusion: CTDI₁₀₀ weighting coefficients are linear functions of phantom aspect ratio, allowing the concept of CTDIw to be easily extended to elliptical phantoms of various shapes. This research is supported in part by a Faculty Startup Fund from Cleveland State University.

Purpose: SymmetryTM 4D IGRT system of Elekta has been installed at our institution, which offers the 4D CBCT registration option. This study is to evaluate the accuracy of 4D CBCT system by using the CIRS 4D motion phantom and to perform a feasibility study on the implementation of 4D‐CBCT as image guidance for SBRT treatment. Methods: The 3D and 4D CT image data sets are acquired using the CIRS motion phantom on a Philips large bore CT simulator. The motion was set as 0.5 cm superior and inferior directions with 6 seconds recycle time. The 4D CT data were sorted as 10 phases. One identifiable part of the 4D CT QA insert from CIRS phantom was used as the target. The ITV MIP was drawn based on maximum intensity projection (MIP) and transferred as a planning structure into 4D CBCT system. Then the 3D CBCT and 4D CBCT images were taken and registered with the free breath (3D), MIP (4D) and average intensity projection (AIP)(4D) reference data sets. The couch shifts (X, Y, Z) are recorded and compared. Results: Table 1 listed the twelve couch shifts based on the registration of MIP, AIP and free breath CT data sets with 3D CBCT and 4D CBCT for both whole body and local registration. X, Y and Z represent couch shifts in the direction of the right‐left, superior‐inferior and anterior‐posterior. The biggest differences of 0.73 cm and 0.57 cm are noted in the free breath CT data with 4D CBCT and 3D CBCT data registration. Fig. 1 and Fig. 2 are the shift analysis in diagram. Fig. 3 shows the registration. Conclusion: Significant differences exist in the shifts corresponding with the direction of target motion. Further investigations are ongoing.