Varian Novalis Tx Linear Accelerator Research Articles

Characterization and comparison of imaging contrast enhancement with PEG-functionalized gold nanoparticles in kV cone beam computed tomography and computed tomography imaging

The purpose of this study is to define a simplified method to accurately predict and characterize kV cone beam computed tomography (kV CBCT) and computed tomography (CT) image contrast enhancement from gold nanoparticles (GNPs). Parameters of the kV CBCT of a Varian Novalis Tx linear accelerator and of a GE LightSpeed 4 Big Bore CT machine were modeled using the MCNP 6.2 Monte Carlo code. A 0.25 × 0.25 cm2 source, defined with a 100 kVp energy spectrum with appropriate filtration, was implemented in the MCNP6.2 model for kV CBCT, which also contained x- and y-blades and a full bowtie filter. A 1 cm3 cube of GNP solution (modeled as a mass percentage of gold in water) was placed 100 cm below the source. For the CT-simulator model, a source was defined with energy spectra for 80 and 140 kVp x‐rays with appropriate filtration and angular spectrum. A 1 cm3 GNP solution was modeled as before and a detector was placed 40 cm below that. Attenuation coefficients of four GNP solutions were computed and Hounsfield unit (HU) values were calculated. The computed HU values were compared against experimentally measured values obtained by scanning batches of GNPs of various sizes and concentrations using a GE LightSpeed 4 Big Bore CT scanner at 80 kVp and 140 kVp energies, as well as the kV CBCT capability of a Varian Novalis Tx linear accelerator. HU analysis was carried out using Velocity Medical Solutions clinical CT image analysis software. The MCNP calculated HU values matched the measured values to within ± 5%. Image contrast enhancement analysis showed a total increase in HU of up to 223. The sample having the highest gold mass percentage tested showed the greatest increase in HU number compared to water.

P170] Influence of detector selection for VMAT patient QA on a gamma analysis result

Purpose The aim of this study was to investigate how the selection of detector for VMAT patient QA influences on a gamma analysis result. Methods Twenty RapidArc (10 prostate, 10 head&neck) plans were made in an Eclipse (ver. 10.0, AAA algorithm) treatment planning system (Varian Medical Systems) were measured with Delta 4PT, Delta 4+ (ScandiDos) and MatriXX MultiCube (IBA Dosimetry) detectors. Most prostate plans consisted of 2 full arc fields while some of the head&neck plans had only 2 half arc fields. The measurements were performed on a Varian Novalis Tx linear accelerator with use of a 6 MV energy. With one device we have measured the entire number of plans on a single day in order to avoid the error of the daily dose output variation and the daily phantom setup error. Before the start of the RA plan measurements we performed a warm up and measured the daily correction factor for the linear accelerator output as well as the temperature correction. Based on the measurements we have applied a phantom position correction to eliminate the error caused by alignment of the phantom to laser system. Gamma evaluations using the same global settings (DTA 3 mm, dose difference 3%, gamma 95% Results In total 41 fields were measured with one device. The biggest difference between the results of gamma analysis for different detectors was detected in a dose difference parameter. The Delta 4+ and Delta 4PT had an average difference between the dose difference parameter 2%, while the average differences regarding to MatriXX MultiCube were 7.5% and 9.5%. Conclusions Based on the results we can conclude that both Delta4 systems are more suitable for VMAT patient QA than the MatriXX MultiCube. The MatriXX is designed for verification where the device is perpendicular to radiation field. Nevertheless the decision of the appropriate equipment to be used for the patient QA is upon the medical physicist.

The Application of Image Guided Radiotherapy on the Precision of Intensity Modulated Radiotherapy in Patients with Locally Advanced Rectal Cancer

Objective: To investigate the effect of onboard image (OBI) system-based image guided radiotherapy (IGRT) on the precision of fractionated intensity modulated radiotherapy (IMRT) for patients with locally advanced rectal cancer. Methods: The IGRT validation images of the 12 patients with rectal cancer were obtained after initial setup by the OBI system of Varian Novalis TX linear accelerator, and registered to the planning CT image system. Subsequently, the setup deviations on three translational directions [ventral-dorsal direction (VD), cranial-caudal direction (CD) and lateral direction (LD)] for the three-validation phase including Pre-treatment (Pre-RT1), repositioning (Pre-RT2) and Post-treatment (Post-RT) were obtained and comparatively analyzed. Results: The frequency of setup deviation of ≤2.0 mm in the lateral, cephalocaudal and ventral direction was 83.01%, 65.71%, and 68.91%, respectively for Pre-RT1; 100%, 98.72% and 100%, respectively for Pre-RT2; 100%, 97.76%, and 99.68%, respectively for Post-RT. Compared with the Pre-RT1 phase, the ranges of setup deviation on Pre-RT2 and Post-RT phases possessed a significant contraction trend. The absolute values of setup deviations on the three translation directions between the Pre-RT1 and Pre-RT2 or Post-RT were statistically significant (p Conclusion: Application of OBI-based daily IGRT may help improve the precise delivery of fractionated IMRT by decreasing the inter- and intra-fractionated setup deviation in the ventral-dorsal direction, cranial-caudal direction and lateral direction for patients with locally advanced rectal cancer.

Evaluation of Imaging Dose From Different Image Guided Systems During Head and Neck Radiotherapy: A Phantom Study.

This work evaluated and compared the absorbed doses to selected organs in the head and neck region from the three image guided radiotherapy systems: cone-beam computed tomography (CBCT) and kilovoltage (kV) planar imaging using the On-board Imager® (OBI) as well as the ExacTrac® X-ray system, all available on the Varian Novalis TX linear accelerator. The head and neck region of an anthropomorphic phantom was used to simulate patients' head within the imaging field. Nanodots optically stimulated luminescent dosemeters were positioned at selected sites to measure the absorbed doses. CBCT was found to be delivering the highest dose to internal organs while OBI-2D gave the highest doses to the eye lenses. The setting of half-rotation in CBCT effectively reduces the dose to the eye lenses. Daily high-quality CBCT verification was found to increase the secondary cancer risk by 0.79%.

Direct reconstruction of the source intensity distribution of a clinical linear accelerator using a maximum likelihood expectation maximization algorithm

Direct determination of the source intensity distribution of clinical linear accelerators is still a challenging problem for small field beam modeling. Current techniques most often involve special equipment and are difficult to implement in the clinic. In this work we present a maximum-likelihood expectation-maximization (MLEM) approach to the source reconstruction problem utilizing small fields and a simple experimental set-up. The MLEM algorithm iteratively ray-traces photons from the source plane to the exit plane and extracts corrections based on photon fluence profile measurements. The photon fluence profiles were determined by dose profile film measurements in air using a high density thin foil as build-up material and an appropriate point spread function (PSF). The effect of other beam parameters and scatter sources was minimized by using the smallest field size ( cm2). The source occlusion effect was reproduced by estimating the position of the collimating jaws during this process. The method was first benchmarked against simulations for a range of typical accelerator source sizes. The sources were reconstructed with an accuracy better than 0.12 mm in the full width at half maximum (FWHM) to the respective electron sources incident on the target. The estimated jaw positions agreed within 0.2 mm with the expected values. The reconstruction technique was also tested against measurements on a Varian Novalis Tx linear accelerator and compared to a previously commissioned Monte Carlo model. The reconstructed FWHM of the source agreed within 0.03 mm and 0.11 mm to the commissioned electron source in the crossplane and inplane orientations respectively. The impact of the jaw positioning, experimental and PSF uncertainties on the reconstructed source distribution was evaluated with the former presenting the dominant effect.

Sci-Thur AM: Planning - 08: Validation of a commercial Monte Carlo code used for stereotactic radiosurgery and stereotactic body radiation therapy.

Our project consisted of validating the BrainLab iPlan Monte Carlo algorithm, used in conjunction with the stereotactic radiosurgery (SRS) mode of the Varian Novalis TX linear accelerator, for clinical use. Our approach was to "benchmark" the iPlan algorithm by comparing dose distributions with those obtained using a BEAMnrc model of the Novalis SRS mode. The BEAMnrc model was obtained by modifying an existing accelerator model to include the SRS flattening filter and source characteristics of the Novalis TX, and by reprogramming a component module to model the high definition 120-leaf multi-leaf collimator. The free parameters of interleaf air gap and leaf density were adjusted by matching to interleaf leakage profiles measured with EBT2 film. The BEAMnrc model was used to perform comparisons of depth dose curves and planar distributions for fields in homogeneous and heterogeneous slab phantoms between both MC codes and film. The source parameters of electron beam energy, size and angular spread were determined to be 6.6 MeV, 0.7 mm and 0.8 mm (cross and in-plane), and 1.27°, respectively. Comparisons between iPlan and EGSnrc MC codes show agreement within 2% for PDD curves, and a high pass rate (>98%) on gamma analysis (3%/3mm) for planar distributions, when the scored quantity is dose to medium. Discrepancies between both MC codes and film measurements were seen near bone inhomogeneities, where the film trend agrees somewhat with iPlan MC reporting dose-to-water. Further work is being performed to understand these differences and how film is used to measure dose near bone.

TU‐A‐BRB‐09: Ion Recombination in a Liquid‐Filled Ionization Chamber in High‐Energy Photon Beams

Purpose: To characterize the ion recombination effect of a liquid‐filled ionization chamber (LIC) in high‐energy photon beams. Methods: The ion recombination effect on the response of a PTW micro liquid ion chamber model 31018 (microLion) was investigated with a 6 MV photon beam in normal and SRS modes produced from a Varian Novalis Tx linear accelerator. Repetition rates were set to 100, 400 and 1000 MU/min, which correspond to pulse repetition frequencies of 60, 240 and 600 Hz, respectively. Polarization voltages applied to the microLion were +800 V and +400 V. The collection efficiency of the microLion as a function of dose per pulse was experimentally measured and theoretically calculated with changing polarization voltage and repetition rate. Results: The relative collection efficiency of the microLion decreased with increasing dose per pulse or pulse repetition frequency from the linear accelerator, or with decreasing polarization voltage to the microLion because of more ion recombination in the liquid. For repetition rates of 100, 400 and 1000 MU/min, the collection efficiency decreased by up to 0.62%, 0.71% and 1.9%, respectively, in a range of dose per pulse from 0.062 to 0.429 mGy/pulse with the polarization voltage of +800 V. For +400 V, the collection efficiency decrease was by up to 0.92%, 2.0% and 5.8% at 100, 400 and 1000 MU/min, respectively. For pulsed radiation with high repetition rate, the experimental relative collection efficiency for all polarization voltages tends to the theoretically calculated efficiency for continuous beams because of low ion mobility in the liquid. We proposed a correction technique for differences in recombination between different radiation fields. Conclusions: Ion recombination effects in a LIC were quantified and a practical correction method was proposed thereby enabling the use of the LIC for accurate measurements of output factors and relative doses in small and nonstandard fields.

Comparison of four commercial devices for RapidArc and sliding window IMRT QA

For intensity‐modulated radiation therapy, evaluation of the measured dose against the treatment planning calculated dose is essential in the context of patient‐specific quality assurance. The complexity of volumetric arc radiotherapy delivery attributed to its dynamic and synchronization nature require new methods and potentially new tools for the quality assurance of such techniques. In the present study, we evaluated and compared the dosimetric performance of EDR2 film and three other commercially available quality assurance devices: IBA I'MatriXX array, PTW Seven29 array and the Delta 4 array. The evaluation of these dosimetric systems was performed for RapidArc and IMRT deliveries using a Varian NovalisTX linear accelerator. The plans were generated using the Varian Eclipse treatment planning system. Our results showed that all four QA techniques yield equivalent results. All patient QAs passed our institutional clinical criteria of gamma index based on a 3% dose difference and 3 mm distance to agreement. In addition, the Bland‐Altman analysis was performed which showed that all the calculated gamma values of all three QA devices were within 5% from those of the film. The results showed that the four QA systems used in this patient‐specific IMRT QA analysis are equivalent. We concluded that the dosimetric systems under investigation can be used interchangeably for routine patient specific QA.PACS numbers: 87.55.Qr, 87.56.Fc