Accuracy of Electron Density Mapping of a Novel kVCBCT System Designed for Planning

PURPOSE/OBJECTIVE(S)

A recently developed advanced kVCBCT system, designed for simulation and planning, includes improvements to the imaging panel and reconstruction technique, enabling more accurate electron density mapping than a typical CBCT. This study investigates the accuracy, limitations, and dosimetric impact of this new system.

MATERIALS/METHODS

CBCT images were taken of both large (pelvis) and small (head) electron density phantoms with inserts ranging from lung (0.29) to cortical bone (1.69). Images were taken with several pre-set protocols with energies of 125 kVp and 140 kVp. The effects of longitudinal buildup (for scatter due to the cone-beam geometry) and blade position were also investigated. The HU values of each insert were measured over a small region of interest. Several electron density curves were generated - pelvis protocol on the large phantom, head protocol on the small phantom, and average - and imported into a treatment planning system. Dose calculation was performed using each curve and differences were observed.

RESULTS

HU to electron density mapping was sensitive to the use of longitudinal buildup and blade position, with differences in the curves observed at both low and high electron densities, due to imager saturation effects not found in typical clinical scenarios. Subsequent studies used both longitudinal buildup and fully opened blades. Under these conditions, mapping was not sensitive to technique or phantom size at low electron densities. At higher values, the curves diverged, with the head protocol showing higher HU values and the pelvis protocol showing lower. The average curve matched very closely with the curve from our standard CT simulator. Dose calculation showed little dependence on the curve chosen, with max point dose differences of 1.2% between the pelvis and average scans, though most of the plan was <0.5% different. Results for the head protocol were similar.

CONCLUSION

The system provides reliable HU values comparable to a CT simulator, though it is important to consider the effects of imager saturation by choosing appropriate scatter and collimation. The calibration is slightly sensitive to the choice of phantom size and beam energy, though these differences led to negligible (<0.5%) differences in dose calculation accuracy.

ACR Accreditation of a Novel Linac-Based kV-CBCT System on a High-Speed Ring-Gantry

PURPOSE/OBJECTIVE(S)

An advanced kV-CBCT imaging system mounted on a high-speed ring-gantry Linac has recently been introduced, offering improved imaging hardware and reconstruction software to produce high quality CBCT images usable for treatment planning. Imaging performance of the system was assessed using the American College of Radiology (ACR) accreditation designed for diagnostic CT systems, additionally comparing its results to pre-existing diagnostic CT and Linac-based CBCT systems.

MATERIALS/METHODS

All imaging protocols on the novel imaging system were scanned with the ACR head phantom and evaluated using ACR recommended testing and passing criteria. ACR image quality parameters include contrast-to-noise ratio (CNR), spatial resolution, HU accuracy, image scaling, and HU uniformity. CNR ≥1.0 for adult head and body, ≥0.7 for pediatric head, and ≥0.4 for pediatric body imaging protocols pass ACR criteria. Spatial resolution ≥6 line-pairs/cm for head and ≥5 line-pairs/cm for body protocols pass ACR criteria. HU accuracy passing criteria includes acrylic (110 to 135 HU), air (-1005 to -970 HU), bone (850 to 970 HU), polyethylene (-107 to -84 HU), and water (-7 to 7 HU). Image scaling measurements with errors ≤5% and HU uniformity maximum differences ≤5 HU pass ACR criteria. For machine cross-comparisons, adult Head and Pelvis imaging protocols were acquired on an existing diagnostic CT and 2 Linac-based CBCT systems (1 traditional C-arm and 1 ring-gantry) and analyzed with the same ACR methodology and passing criteria, with passing rates compared.

RESULTS

On the novel imaging system, all standard patient size imaging protocols using 125 kVp (Head, H&N, Thorax, Thorax Slow, Breast, Abdomen, Pelvis, Pediatric Head, and Pediatric Abdomen) passed all ACR criteria; while 2 larger patient focused imaging protocols using 140 kVp (Abdomen Large and Pelvis Large) produced minor deviations on HU uniformity (maximum differences of 5 - 7 HU) but passed all other ACR criteria. The novel system matched passing rates of a diagnostic CT simulator, and outperformed pre-existing Linac based CBCT imaging systems.

CONCLUSION

This newly developed advanced imaging system produces high quality images, meeting diagnostic CT ACR recommendations and far surpassing the CBCT image quality currently available on pre-existing Linacs.

Differentiating Tumor Progression from Pseudoprogression in Patients with Glioblastomas Using Diffusion Tensor Imaging and Dynamic Susceptibility Contrast MRI

BACKGROUND AND PURPOSE

Early assessment of treatment response is critical in patients with glioblastomas. A combination of DTI and DSC perfusion imaging parameters was evaluated to distinguish glioblastomas with true progression from mixed response and pseudoprogression.

MATERIALS AND METHODS

Forty-one patients with glioblastomas exhibiting enhancing lesions within 6 months after completion of chemoradiation therapy were retrospectively studied. All patients underwent surgery after MR imaging and were histologically classified as having true progression (>75% tumor), mixed response (25%-75% tumor), or pseudoprogression (<25% tumor). Mean diffusivity, fractional anisotropy, linear anisotropy coefficient, planar anisotropy coefficient, spheric anisotropy coefficient, and maximum relative cerebral blood volume values were measured from the enhancing tissue. A multivariate logistic regression analysis was used to determine the best model for classification of true progression from mixed response or pseudoprogression.

RESULTS

Significantly elevated maximum relative cerebral blood volume, fractional anisotropy, linear anisotropy coefficient, and planar anisotropy coefficient and decreased spheric anisotropy coefficient were observed in true progression compared with pseudoprogression (P < .05). There were also significant differences in maximum relative cerebral blood volume, fractional anisotropy, planar anisotropy coefficient, and spheric anisotropy coefficient measurements between mixed response and true progression groups. The best model to distinguish true progression from non-true progression (pseudoprogression and mixed) consisted of fractional anisotropy, linear anisotropy coefficient, and maximum relative cerebral blood volume, resulting in an area under the curve of 0.905. This model also differentiated true progression from mixed response with an area under the curve of 0.901. A combination of fractional anisotropy and maximum relative cerebral blood volume differentiated pseudoprogression from nonpseudoprogression (true progression and mixed) with an area under the curve of 0.807.

CONCLUSIONS

DTI and DSC perfusion imaging can improve accuracy in assessing treatment response and may aid in individualized treatment of patients with glioblastomas.