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.

Optimization of photon beam energies in gold nanoparticle enhanced arc radiation therapy using Monte Carlo methods

As a recent area of development in radiation therapy, gold nanoparticle (GNP) enhanced radiation therapy has shown potential to increase tumour dose while maintaining acceptable levels of healthy tissue toxicity. In this study, the effect of varying photon beam energy in GNP enhanced arc radiation therapy (GEART) is quantified through the introduction of a dose scoring metric, and GEART is compared to a conventional radiotherapy treatment. The PENELOPE Monte Carlo code was used to model several simple phantoms consisting of a spherical tumour containing GNPs (concentration: 15 mg Au g^-1 tumour, 0.8 mg Au g^-1 normal tissue) in a cylinder of tissue. Several monoenergetic photon beams, with energies ranging from 20 keV to 6 MeV, as well as 100, 200, and 300 kVp spectral beams, were used to irradiate the tumour in a 360° arc treatment. A dose metric was then used to compare tumour and tissue doses from GEART treatments to a similar treatment from a 6 MV spectrum. This was also performed on a simulated brain tumour using patient computed tomography data. GEART treatments showed potential over the 6 MV treatment for many of the simulated geometries, delivering up to 88% higher mean dose to the tumour for a constant tissue dose, with the effect greatest near a source energy of 50 keV. This effect is also seen with the inclusion of bone in a brain treatment, with a 14% increase in mean tumour dose over 6 MV, while still maintaining acceptable levels of dose to the bone and brain.

A method for converting dose-to-medium to dose-to-tissue in Monte Carlo studies of gold nanoparticle-enhanced radiotherapy

Gold nanoparticles (GNPs) have shown potential in recent years as a means of therapeutic dose enhancement in radiation therapy. However, a major challenge in moving towards clinical implementation is the exact characterisation of the dose enhancement they provide. Monte Carlo studies attempt to explore this property, but they often face computational limitations when examining macroscopic scenarios. In this study, a method of converting dose from macroscopic simulations, where the medium is defined as a mixture containing both gold and tissue components, to a mean dose-to-tissue on a microscopic scale was established. Monte Carlo simulations were run for both explicitly-modeled GNPs in tissue and a homogeneous mixture of tissue and gold. A dose ratio was obtained for the conversion of dose scored in a mixture medium to dose-to-tissue in each case. Dose ratios varied from 0.69 to 1.04 for photon sources and 0.97 to 1.03 for electron sources. The dose ratio is highly dependent on the source energy as well as GNP diameter and concentration, though this effect is less pronounced for electron sources. By appropriately weighting the monoenergetic dose ratios obtained, the dose ratio for any arbitrary spectrum can be determined. This allows complex scenarios to be modeled accurately without explicitly simulating each individual GNP.

Monoclonal antibodies to human osteosarcoma-associated antigen(s)

Monoclonal antibodies (MoAbs) against human osteosarcoma cells were obtained by the production and cloning of hybrids resulting from the fusion of mouse myeloma cells P3 X 63Ag8.653 with spleen cells from partially purified, osteosarcoma-associated antigen (OSAA)-immunized BALB/c mice. OSAAs were isolated from the spent culture medium of a human osteosarcoma cell line (TE-85). Five hybrid clones were established and designated as OSA1, OSA2, OSA3, OSA4, and OSA5. OSA1 and OSA2 had similar activity. All 5 MoAbs reacted strongly with most osteosarcoma cell lines and with all osteosarcoma tissues tested but not with 10 tumor cell lines and 2 tumor tissues from other cancers. OSA3, OSA4, and OSA5 cross-reacted with a fibrosarcoma cell line, a colon cell line, and fibrosarcoma, respectively, as well as with a melanoma cell line. None of the MoAbs were reactive with activated normal human peripheral blood mononuclear cells (PBMC). Immunoprecipitation of membrane protein isolated from LM cells and TE-85 cells with the MoAbs OSA1 and OSA2 conjugated with Staphylococcus aureus yielded a molecule with molecular weight of approximately 92,000. No detectable membrane protein was precipitated when 125I-labeled membrane protein from pooled activated human PBMC and tumor cells of other histologic types were used in the immunoprecipitation.

Cell flow cytometry of fixed and unfixed bursal and thymic cells

Analysis in a cell flow cytometer (Ortho Spectrum III, Ortho Inst.) of single cell suspensions from the bursa and thymus of 20-day embryos revealed two distinct cell clusters. The two clusters were less apparent after fixation of the cells in paraformaldehyde and assumed a comet-like appearance at 30 min fixation in ethyl alcohol (EA). The G (postmitotic), S [deoxyribonucleic acid (DNA) synthesis], and G2/M (premitotic and mitotic) phases of the life cycle were visualized in two cell flow cytometers (Ortho Spectrum III and FACS IV, Bectin Dickinson) after treating the cells with EA, ribonuclease (RNase), and propidium iodide (PI, a fluorescent dye). Bursal cell suspensions exposed to the EA-RNase-PI protocol and stored for 3 weeks in phosphate-buffered saline showed minor changes in the G1 coefficient of variation, G1, and S percentages, but marked changes in these parameters occurred after 6 weeks of storage. Thymic cells treated in a similar fashion could not be maintained for 3 weeks. Bursal and thymic cells may remain in the EA for one day and possibly as long as 7 days prior to preparing them for DNA life cycle analysis. Paraformaldehyde was not a satisfactory cell fixative for assessing a cell's DNA life cycle.

Flow cytometric analysis of bursal, thymic, and splenic cells from normal and cyclophosphamide-treated embryos

Cell flow cytometry was used to assess the life cycle of embryonic lymphocytes. Cells were prepared for flow cytometry by fixation for 30 min in a final concentration of 70% ethyl alcohol, treated with ribonuclease (50 to 70 Kunitz/ml) and incubated for 30 min. in propidium iodide. Our data demonstrated that bursal lymphocytes from 20-day embryos (DE) exhibited significantly fewer cells in pre-DNA (deoxyribonucleic acid) and more cells in DNA synthesis (S) than thymic cells from 20 DE, and thymic lymphocytes from 16 DE expressed a more active S than bursal cells. Lymphocytes were separated from granulocytes by a Ficoll-hypaque double density gradient. The DNA cycle of bursal lymphocytes was more easily disrupted with cyclophosphamide (Cy) than the DNA cycle of thymic lymphocytes. However, unfractionated splenic cells from embryos treated with Cy revealed a greater percentage of cells in S and post-DNA synthesis and mitosis than splenic cells from control embryos. Because the splenic cells of Cy-treated embryos were predominantly immature granulocytes, it was concluded that Cy stimulated myelopoiesis in the spleen.