The purpose of this study is to establish a comprehensive set of dose measurements data obtained from the X-ray Volumetric Imager (XVI, Elekta Oncology Systems) and the On-Board Imager (OBI, Varian Medical Systems) cone-beam CT (CBCT) systems. To this end, two uniform-density cylindrical acrylic phantoms with diameters of 18 cm (head phantom) and 30 cm (body phantom) were used for all measurements. Both phantoms included ion chamber placement holes in the center and at periphery (2 cm below surface). For the XVI unit, the four standard manufacturer-supplied protocols were measured. For the OBI unit, the full bow tie and half bow tie (and no bow tie) filters were used in combination with the two scanning modes; namely, full-fan and half-fan. The total milliampere x seconds (mA s) setting was also varied for each protocol to establish the linear relationship between the dose deposited and the mA s used (with all other factors being held constant). Half-value layers in aluminum (Al) were also measured for beam characteristic determination. For the XVI unit, the average dose ranged from 0.1 to 3.5 cGy with the highest dose measured using the "prostate" protocol with the body phantom. For the OBI unit, the average dose ranged from 1.1 to 8.3 cGy with the highest dose measured using the full-fan protocol with the head phantom. The measured doses were highly linear as a function of mA s, for both units, where the measurement points followed a linear relationship very closely with R2 > 0.99 for all cases. Half-value layers were between 4.6- and 7.0-mm-Al for the two CBCT units where XVI generally had more penetrating beams at the similar kVp settings. In conclusion, a comprehensive series of dose measurements were performed on the XVI and the OBI CBCT units. In the process, many of the important similarities and differences between the two systems were observed and summarized in this work.

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Purpose: Volumetric modulated arc therapy (VMAT) is a relatively new treatment modality characterized by variable angular dose rate arc delivery. The purpose of this study was to compare treatment plans for multileaf collimators (MLCs) with different leaf widths and different dose calculation grids, in order to determine the optimal planning quality and delivery efficiency for VMAT of GBM boost plans. Method and Materials: CT images of 10 GBM patients were used for this study on an ERGO++ Treatment planning system ((ELEKTA; Crawley, UK). Two dose grids were used. The first grid was 2mm and the second 3mm. We used the Elekta Beam Modulator (4mm leaf) and Elekta MLCi (1cm leaf). The same dose constraints and beam parameters were used for optimization. Tissue inhomogeneity corrections were applied during optimization and dose calculation. Plans were optimized such as that the CTV received 14 Gy in 7 fractions. Dose distributions to the target and normal structures were evaluated. The number of monitor units (MU) and delivery times were used to evaluate delivery efficiency. Results: The doses to the CTV, eyes, optic nerves, tracts, brain stem, pituitary gland and optic chiasm were calculated. The 3mm grid with the 4mm leaf width used 303±18 MU and 0.66±0.068 conformity index (CI). The 3mm grid with the 1cm leaf width used 274±19 MU and 0.67±0.064 CI .The 2mm grid with the 1cm leaf width used 280±25MU and 0.67±0.064CI. The 2mm grid with the lcm leaf width used 264±20MU and 0.64±0.060CI. Conclusion: Treatment plans generated with the 2mm dose grid and the 4mm leaf width seem to be optimal for both plan quality and delivery efficiency. While this study was specifically designed for the ERGO++ Treatment planning system combined with the ELEKTA linear accelerator/MLC and IMPAC R&V system, the conclusions drawn have ramifications for other environments as well.

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