Varian Clinac Research Articles

Effect of Magnetic Field During Radiotherapy on Double-Strand DNA Breaks and Cell Proliferation on Prostate, Cervical, and Breast Cancer Cells

MR-guided radiotherapy (RT) using MR-Linacs is an emerging technology. It allows for real-time tracking of targets and organs-at-risk to deliver RT with high precision. However, the biological effect of a strong magnetic field on cancer cells during RT is not well known, with conflicting data reported in literature. In this study, the effects of a magnetic field during RT on resulting DNA double-stand breaks (DSBs) and cell viability were investigated. Human cancer cells from prostate (PC3), cervix (HeLa), and breast (MCF-7, MB 231, T47D) were cultured used DMEM media with 10% FBS. A Varian CLINAC (Palo Alto, CA, USA) 6 MV beam was used to deliver RT to a 96 well plate. The plate was mounted in a custom 0.21 T magnetic solenoid coil with the magnetic field oriented parallel to the RT beam. DSBs were assessed via γ- H2AX assay: cells were fixed 24 hours post 2 Gy of RT and immunostained with fluorescent γ- H2AX antibody probe, and the number of foci per cell was determined using a Molecular Imaging MetaXpress High Content Imaging Platform (San Jose, CA, USA). Cell viability was determined using Alamar Blue fluorescence assay at 72 hours post RT using calibrated dilution of cells per cell line at 2, 4, 6, and 8 Gy dose. Each RT experiment was done with and without the magnetic field turned on (RT and MRT, respectively). For PC3, fewer cells expressed >6 foci of DSBs under MRT than RT (58% vs 63%, p<0.01. In contrast, more cells demonstrated >6 DSB foci under MRT than RT for HeLa (18% vs 16%, p<0.01), MCF-7 (2.7% vs 0.1%, p<0.001), MB 231 (8.2% vs 1.5%, p<0.001), and T47D (2.3% vs 1.4%, p=0.027). Cell viability following 2 - 8 Gy of radiation for all cell lines demonstrated no significant difference between MRT or RT. Preliminary results suggest differences in the quantity of DSBs were observed under MRT vs RT at 24 hours post-2Gy RT with a 0.21 T field, with a less proliferative cell line (PC3) demonstrating lower DSBs under MRT than RT, and the other highly proliferative cell lines showing the opposite trend. This did not manifest in altered cell viability under the doses of RT or magnetic field strength tested. Future studies aimed at exploring differing biological effects under different magnetic field strengths and cell lines should be explored.

Modeling scattered radiation from multi-leaf collimators (MLCs) to improve calculation accuracy of in-air output ratio

This study aims to model an extra-focal source for the scattered radiation from multi-leaf collimators (MLCs), namely an MLC scatter source, and to correct in-air output ratio (Sc) calculated using the conventional dual source model (DSM) to achieve better accuracy of point dose calculation. To develop the MLC scatter source, a 6 MV photon beam from a Varian Clinac® iX linear accelerator with millennium 120 MLCs was used. It was assumed that the position for the MLC scatter source was located at the center of the MLC, consisting of line-based and area-based sources to consider the characteristics of the scattered radiation from the MLCs empirically. Based on the measured Sc values for MLC-defined fields, optimal parameters for the line-based and area-based sources were calculated using optimization process. For evaluation of proposed method, measurements were taken for various MLC-defined square and irregular fields. The Sc values calculated using the proposed MLC scatter source and conventional DSM were compared with the measured data. For MLC-defined square fields, the measured Sc values showed better agreement with those calculated using the MLC scatter source (the mean difference was − 0.03%) compared with those calculated using the DSM (the mean difference was 0.18%). For MLC-defined irregular fields, the maximum dose differences between measurements and calculations using the MLC scatter source and DSM were 0.54% and 1.45%, respectively. The developed MLC scatter source could improve the accuracy of Sc calculation for both square and irregular fields defined by MLCs.

A feasibility study for high-resolution silicon array detector performance in the magnetic field of a permanent magnet system.

Magnetic field effects on dose distribution and detector functionality must be well understood. The detector utilized to investigate these magnetic field effects was the DUO silicon array detector; the performance of this high spatial resolution detector was assessed under these conditions. The results were compared to Gafchromic EBT3 film to highlight any intrinsic magnetic field effects in the silicon. The results were also compared to previously published MagicPlate-512 (M512) data. The DUO has an improved spatial resolution (200µm) over the M512 (2mm). A permanent magnet named Magnetic Apparatus for RaDiation Oncology Studies (MARDOS) paired with a standard linear accelerator (linac) enables either transverse (1.2T) or inline (0.95T) orientations of the magnetic field with respect to the radiation beam. A 6MV Varian 2100C Linac provided the radiation component for the measurements. The DUO detector has 505 sensitive volumes (each volume measuring 800×40×100µm3 ) organized in two orthogonal, linear arrays. The DUO was embedded in a solid water phantom in the first set-up and then a solid lung phantom in the second set-up and placed between the magnet cones. Beam profiles were compared under the magnetic field conditions and 0T. Small field sizes from 0.8×0.8cm2 up to 2.3×2.3cm2 were investigated. The size of the air gap above the sensitive volumes of the DUO was investigated in the transverse orientation to assess the anticipated magnetic field effects. Full width at half maximum (FWHM), 80-20% penumbral widths and maximum dose differences between detectors and between the presence/absence of a magnetic field were investigated. Symmetry was also assessed for investigation of profile skewness under the transverse field. The penumbral widths measured by the DUO detector demonstrated good agreement with film and the M512 to within an average of 0.5mm (within uncertainty: ±1mm). The static inline magnetic field had minimal effect on the profiles in solid water. As expected, the lower density of solid lung meant that this material was more susceptible to demonstrating magnetic field effects in the dose deposited. The greatest penumbral narrowing due to the inline field (0.7mm) occurred in lung. Central axis dose increase was greatest in lung (maximum: 9%). The transverse field widened penumbra, most notably in the solid lung phantom, by a maximum of 2.3mm. The largest asymmetry due to the transverse field (4.6%) was also in solid lung. When the air gap above the DUO was filled with bolus, the dose maximum measured by the DUO was within 1.4% of film. The DUO detector has been shown to be successful in accurately describing the dose changes for small field sizes to within a 200-µm resolution in an environment resembling that of an MRI-linac. The DUO measurements were in agreement with both film and the M512 measurements, and therefore the DUO was found to be an appropriate alternative to the M512, with improvement in terms of its higher spatial resolution. MARDOS provided a suitable environment for these preliminary tests before progressing to the MRI-linac.

Measuring the amplitude-time characteristics of a pulsed high-intensity gamma radiation accelerator Varian Clinac 600C with a CdTe detector

Measuring the amplitude-time characteristics of a pulsed high-intensity gamma radiation accelerator Varian Clinac 600C with a CdTe detector

BMAG interlock troubleshooting of Varian clinac IX linear accelerator

Through the analysis of 3 maintenance of Bmag interlocking fault of Varian Clinac IX Linear Accelerator, it is found that the full wave rectifier diode breakdown in the bend magnet power supply, the improper setting of the bend magnet interlock, and the failure of the modulator CB3 air switch can cause the bmag interlocking. Through the replacement of accessories, adjustment parameters, etc., so that the fault is resolved. Key words: Linear accelerator; Bend magnet power; Current alarm; Voltage alarm

Cherenkov emission-based external radiotherapy dosimetry: II. Electron beam quality specification and uncertainties.

Cherenkov emission (CE) is ubiquitous in external radiotherapy. It is also unique in that it carries the promise of 3D, micrometer-resolution, perturbation-free, in-water dosimetry with a beam quality-independent detector response calibration. Our aim is to bring CE-based dosimetry into the clinic and we motivate this here with electron beams. We Monte Carlo (MC) calculate and characterize broad-beam CE-to-dose conversion factors in water for a clinically representative library of electron beam qualities, address beam quality specification and reference depth selection, and develop a preliminary uncertainty budget based on our MC results and relative experimental work of a companion study (Paper I). Broad electron beam CE-to-dose conversion factors include CE generated at polar angles θ±δθ on beam axis in water. With modifications to the EGSnrc code SPRRZnrc, factors are calculated for a total of 20 electron beam qualities from four BEAMnrc models (Varian Clinac 2100C/D, Clinac 21EX, TrueBeam, and Elekta Precise). We examine beam quality, depth, and detection angle dependence for (4π detection), , , and . As discussed in Paper I, 4π detection offers the strongest CE-dose correlation and with small δθ is most practical. The two additional configurations are considered as a compromise between these two extremes. We address beam quality specification and reference depth selection in terms of the electron beam quality specifier , obtained from the depth of 50% CE , and derive a best-case uncertainty budget for the CE-based dosimetry formalism proposed in Paper I at each detection configuration. The factor was demonstrated to capture variations in the beam spectrum, angle, photon contamination, and electron fluence below the CE threshold (∼260keV in the visible) in accordance with theory. The root-mean-square deviation and maximum deviation of a second-order polynomial fit of simulated values in terms of were 0.05 and 0.11mm at 4π and 0.20 and 0.33mm at detection, respectively. The fit performance on experimental data in Paper I was in agreement with these values within experimental uncertainties (±1.5mm, 95% CI). A two-term power function fit of in terms of at a reference depth resulted in total -dependent dose uncertainty contribution estimate of 0.8% and 1.1% and preliminary best-case estimate of the combined standard dose uncertainty of 1.1% and 1.3% at 4π and detection, respectively. The results and corresponding uncertainties with the two intermediate apertures were generally of the same order as the 4π case. In addition, a theoretically consistent downstream shift of the percent-depth CE (PDC) by the difference between and improved the depth dependence of the 4π conversion by an order of magnitude (±2.8%). Therefore, a large aperture centered on a θ value between and combined with a downstream PDC shift may be recommended for beam-axis CE-based electron beam dosimetry in water. By delivering -based CE-to-dose conversion data and demonstrating the potential for dosimetric uncertainty on the order of 1%, we bring CE-based electron beam dosimetry closer to clinical realization.

Quality assurance of intensity modulated radiation therapy treatment planning using head and neck phantom

AbstractPurposeThe purpose of this study is the verification of intensity modulated radiation therapy (IMRT) head neck treatment planning with one-dimensional and two-dimensional (2D) dosimeters using imaging and radiation oncology core (IROC) Houston head &amp;amp; neck (H&amp;amp;N) phantom.MethodThe image of the H&amp;amp;N phantom was obtained by computed tomography scan which was then transferred to Pinnacle@3 treatment planning system (TPS) for treatment planning. The contouring of the target volumes and critical organ were done manually and dose constraints were set for each organ according to IROC prescription. The plan was optimised by adoptive convolution algorithm to meet the IROC criteria and collapse cone convolution algorithm calculated the delivered doses for treatment. Varian Clinac 2110 was used to deliver the treatment plan to the phantom, the process of irradiation and measurement were repeated three times for reproducibility and reliability. The treatment plan was verified by measuring the doses from thermoluminescent dosimeters (TLDs) and GafChromic external beam therapy 2 films. The agreement between the planned and delivered doses were checked by calculating the percentage dose differences, analysing their isodose line profiles and 2D gamma maps.ResultsThe average percent dose difference of 1·8% was obtained between computed doses by TPS and measured doses from TLDs, however these differences were found to be higher for organ at risk. The film dose profile was well in agreement with the planned dose distribution with distance to agreement of 1·5 mm. The gamma analysis of the computed and recorded doses passed the criteria of 3%/3 mm with passing percentages of &amp;gt;96%, which shows successful authentication of delivered doses for IMRT.ConclusionIMRT pre-treatment validation can be done with IROC anthropomorphic phantoms, which is essential for the delivery of modulated radiotherapies. It was concluded that films and TLDs can be used as quality assurance tools for IMRT.

Development of a Geant4‐based independent patient dose validation system with an elaborate multileaf collimator simulation model

Despite the improvements in the dose calculation models of the commercial treatment planning systems (TPS), their ability to accurately predict patient dose is still limited. One of the limitations is caused by the simplified model of the multileaf collimator (MLC). The aim of this study was to develop a Monte Carlo (MC) method‐based independent patient dose validation system with an elaborate MLC model for more accurate dose evaluation. Varian Clinac 2300 IX was simulated using Geant4 toolkits, after which MC commissioning with measurements was performed to validate the simulation model. A DICOM‐RT interface was developed to obtain the beam delivery conditions including the hundreds of MLC motions. Finally, the TPS dose distributions were compared with the MC dose distributions for water phantom cases and a patient case. Our results show that the TPS overestimated the absolute abutting leakage dose in the closed MLC field, with about 20% more of the maximum dose than that of the MC calculation. For water phantom cases, the dose distributions inside the target region were almost identical with the dose difference of less than 2%, while the dose near the edge of the target shows difference about 10% between Geant4 and TPS due to geometrical differences in MLC model. For the patient analysis, the Geant4 and TPS doses of all organs were matched well within 1.4% of the prescribed dose. However, for organs located in areas with high ratio of leaf pairs with distances less than 10 mm leaf pair (LP (<10mm)), the maximum dose of TPS was overestimated by about 3% of the prescribed dose. These dose comparison results demonstrate that our system for calculating the patient dose is quite accurate. Furthermore, if the MLC sequences in treatment plan have a large ratio of LP (short), more than 3% dose difference in normal tissue could be seen.

Analysis of Effect of Change Source to Surface Distance (SSD) and the Field Size to Distribution Dose Using Monte Carlo Method-EGSnrc

Research was conducted to analyze the effect of changes in the distance radiation source to the surface it is called the medium Source to Surface Distance (SSD) and wide exposure field (Field Size) on the distribution of the dose in linear air Accelerator (LINAC) using Monte Carlo - EGSnrc. Monte Carlo simulation is used for modeling and simulation head linac at BEAMnrc. Virtual model of the linac is made based on the data characteristics of the aircraft Linac Varian Clinac iX then its output obtained information characteristic photon beam using BEAMDP, while modeling and simulation phantom done on DOSXYZnrc with the size of the phantom is (40x40x40) cm3 , with the material in the form of water, using a beam of photons 6 MV, testing variation SSD at 80 cm, 90 cm, 100.1 cm, 110 cm, 120 cm and variation field size is (6x6) cm2, (10x10) cm2, (20x20) cm2 to obtain disribution of dosage form of curves Percentage Depth dose (PDD) and Profile dose. The results showed that the smaller distance radiation source to the surface of the medium (SSD) and the greater the broad field (field size), then increasing the dose distribution is obtained. In the SSD and Field Size variation, the radiation dose will continue to rise significantly from the surface of the medium 0 cm to a depth of maximum dose (Dmax), then the dose began to decline after passing the depth Dmax.

Estimation of Backscatter from Internal Shielding in Electron Beam Radiotherapy Using Monte Carlo Simulations (EGSnrc) and Gafchromic Film Measurements.

Purpose:The purpose of the study was to estimate the backscatter electron dose in internal shielding during electron beam therapy using Monte Carlo (MC) simulations and Gafchromic film measurements.Materials and Methods:About 6 and 9 MeV electron beams from a Varian 2100C linac were simulated using BEAMnrc MC code. Various clinical situations of internal shielding were simulated by modeling water phantoms with 2 mm lead sheets placed at different depths. Electron backscatter factors (EBF), a ratio of dose at tissue-shielding interface to the dose at the same point without the shielding, were estimated. The role of 2 mm aluminum in reduction of backscatter was investigated. The measurements were also performed using Gafchromic films and results were compared with MC simulations.Results:For particular beam energy, the EBF value initially increased with depth in the buildup region and then decreased rapidly. The highest value of EBF for both the energies is nearly same though at different depths. Decreased EBF was observed for 9 MeV beam in comparison to the 6 MeV beam for the same depth of shielding placement. Two millimeter aluminum reduced the backscatter by nearly 25% at maximum backscatter condition for both the energies, though the effectiveness slightly decreased at higher energy. The range of backscatter electrons was varying from 5 to 12 mm in the upstream direction from the interface. The Gafchromic film-measured EBF and MC-simulated EBF were matching well within the clinically acceptable limits except in close vicinity of tissue-lead interface.Conclusions:This study provides an important clinical data to design internal shielding at the local clinical setup and confirms applicability of MC simulations in backscatter dose calculations at interfaces where physical measurements are difficult to perform.

Factor 10 Expedience of Monthly Linac Quality Assurance via an Ion Chamber Array and Automation Scripts.

Purpose:While critical for safe and accurate radiotherapy, monthly quality assurance of medical linear accelerators is time-consuming and takes physics resources away from other valuable tasks. The previous methods at our institution required 5 hours to perform the mechanical and dosimetric monthly linear accelerator quality assurance tests. An improved workflow was developed to perform these tests with higher accuracy, with fewer error pathways, in significantly less time.Methods:A commercial ion chamber array (IC profiler, Sun Nuclear, Melbourne, Florida) is combined with automation scripts to consolidate monthly linear accelerator QA. The array was used to measure output, flatness, symmetry, jaw positions, gated dose constancy, energy constancy, collimator walkout, crosshair centering, and dosimetric leaf gap constancy. Treatment plans were combined with automation scripts that interface with Sun Nuclear’s graphical user interface. This workflow was implemented on a standard Varian clinac, with no special adaptations, and can be easily applied to other C-arm linear accelerators.Results:These methods enable, in 30 minutes, measurement and analysis of 20 of the 26 dosimetric and mechanical monthly tests recommended by TG-142. This method also reduces uncertainties in the measured beam profile constancy, beam energy constancy, field size, and jaw position tests, compared to our previous methods. One drawback is the increased uncertainty associated with output constancy. Output differences between IC profiler and farmer chamber in plastic water measurements over a 6-month period, across 4 machines, were found to have a 0.3% standard deviation for photons and a 0.5% standard deviation for electrons, which is sufficient for verifying output accuracy according to TG-142 guidelines. To minimize error pathways, automation scripts which apply the required settings, as well as check the exported data file integrity were employed.Conclusions:The equipment, procedure, and scripts used here reduce the time burden of routine quality assurance tests and in most instances improve precision over our previous methods.

27 CBCT-based dose calculation for head-and-neck tumors treated with VMAT in order to approach adaptive radiotherapy

Introduction Head-and-neck (H&N) treatment delivery time is divided in 35 fractions of 2 Gy and takes 7 weeks. Anatomical modifications can occur during treatment and introduce uncertainties in VMAT dose distribution. Cone Beam Computed Tomography (CBCT) is used for patients positioning but might be considered for dose calculation. The purpose of this study is to improve the image quality and accuracy of CBCT-based dose calculation through the use of an optimized CBCT acquisition mode to finally build a patient-specific H&N deformation model to approach adaptive radiotherapy (ART). Methods A calibration was performed with the view to achieve a higher accuracy in a 25 cm field-of-view (FOV) CBCT ORL , ART (Varian Clinac® OBI 1.6) image-based dose assessment by varying the source-to-imager distance (SID), tube voltage (kVp) and mAs. Then, the Hounsfield unit (HU) vs relative electron density curves were drawn for the CIRS (062 M) phantom associated to the QuasarTM (Modus QA) using the post-treatment software Syngo.via (Siemens Healthinners) with the intention of comparing them to the planning CT curve ( CT plan ) . The optimized CBCT ORL , ART acquisition mode dose has been assessed as described in the AAPM report n. 111 [1] with the use of a 0.3 cm3 ionization chamber inside a 26 cm PMMA cylindrical phantom. This retrospective cohort study includes 38 H&N cancer patient cases who underwent VMAT treatment with one CBCT ORL , ART per week in 2017. For each patient, a deformable image registration algorithm (DIR) has been applied on the CT plan and on the 7 CBCT ORL , ART using Syngo.via. A comparative study between deformation vector fields (DVF), planned target volume (PTV) and organ-at-risk (OAR) contours similarity measurements and dose-volume histogram (DVH) has been done with the aim of assessing the feasibility of CBCT-based dose calculation. Results The optimized CBCT ORL , ART mode chosen had the following acquisition parameters: 100 kVp, 145 mAs and a 150 cm SID. A 3.83 mGy CTDIw has been assessed for the CBCT ORL , ART mode with a + 1.8% relative gap between measured and constructor data. Due to the limited field of view (FOV), truncation of the CBCT caused incorrect deformations when the DIR algorithm was applied and led to significant errors in dose calculation while deforming the CT plan image to match the CBCT ORL , ART was considered suitable for dose calculation based on the 2 mm gamma-test on over 95% of the voxels and DVH. Conclusions This first adaptive radiotherapy approach verifies the feasibility of CBCT-based dose calculation. Following this study, a dosimetric alert protocol was set up to evaluate the need of performing a re-plan during the treatment.

Comparison between two different methods used for IMRT plans QA of Prostate cancer

Introduction: The aim of this study is evaluation of two Quality Assurance methods in sliding window IMRT technique and Determination of gamma index in both methods Materials and Methods: In this study two tools named phantom and software has been used to perform QA on the treatment plan before the actual treatment by Varian linear accelerator Clinac 600C. For this purpose, 20 patient’s treatment plan were assessed with the 3% and 3mm criteria. Results: Each treatment field demonstrated that the gamma index in the delta4 phantom was more than the Epiqa software for all patients. The most uncertainty for a treatment plan in delta4 phantom was 0.88% and for Epiqa software was 1.53%. (p- value <0.05) Conclusion: In spite of these variations in results and also studying similar researches, these tools can be used as a reliable QA method in radiotherapy centers. Delta4 phantom is preferred to Epiqa software because it is a dosimeter. However, Epiqa software is economical to install. therefore can be used for those centers which cannot afford delta4 phantom due to economic consideration.

175. A preliminary study on the correlation of modulation plan parameters with dosimetric accuracy of VMAT treatment plans performed in two centres with different linear accelerators and treatment planning systems

PurposeTo evaluate a correlation between plan parameters and Γ passing rate that could predict dosimetric accuracy of VMAT treatment plans. MethodsTen pelvis treatment plans (irradiation of prostate, seminal vesicles and pelvis nodes with simultaneous integrated boost technique) have been performed on the same CT series and contoured structure sets in Istituti Clinici Scientifici Maugeri (ICSM) and in Azienda Ospedaliera Ospedali Riuniti Marche Nord (AORMN). Two 6 MV VMAT beams have been planned with Philips Pinnacle3 v16 (ICSM) and Elekta Monaco v 5.11(AORMN) for Varian Clinac iX® with Millenium MLC® 120-leaves and Elekta Synergy® with MLCi2 80 leaves respectively. The total prescribed dose has been 70.2 Gy in 26 fractions. The same planning criteria have been followed in both centres; the dose constraints on OARs (in particular peritoneal cavity) had been always achieved and preferred to the PTV coverage. RTPlan files were analyzed with Matlab® to obtain MUs and plan parameters used to calculate MCS, average Leaf Travel (LT) and their combination (LTMCS) whose definition is found in literature [1]. Local, global and normalized to dose prescription Γ index (3%, 3 mm) enabled the comparison between calculated and measured dose on the PTW Octavius 4D® volume phantom. The Pearson coefficient between Γ passing rate values and all the considered parameters has been computed to looking for a correlation. ResultsPearson coefficients are shown in Table 1 for all the parameters and Γ passing rate considered. The correlation is always strong for MU and moderate for all Γ indexes against MCS for ICSM and for the two Γ global indexes for Pesaro against MCS and LTMCS (Table 1). ConclusionsThe results show a strong correlation against MU for both centres that could be useful to predict VMAT dosimetric accuracy expressed as Γ passing rate. [Display omitted]

Determination of an Effective Wedge Angle by Combination of Two Arbitrary Universal Wedge Fields in Radiation Therapy of Cancer Patients with Megavoltage Photon Beams.

Introduction: Wedge filters are commonly used in radiation oncology for eliminating hot spots and creating a uniform dose distribution in optimizing isodose curves in the target volume for clinical aspects. These are some limited standard physical wedges (15°, 30°, 45°, 60°), or creating an arbitrary wedge angle, like motorized wedge or dynamic wedge, ... The new formulation is presented by the combination of wedge fields for determining an arbitrary effective wedge angle. The isodose curves also are derived for these wedges. Materials and Methods: we performed the dosimetry of Varian Clinac 2100C/D with Scanditronix Wellhofer water blue phantom, CU500E, OmniPro - Accept software and 0.13cc ionization chamber for 6Mv photon beam in depth of 10cm (reference depth) for universal physical wedges (15°, 30°, 45°, and 60°) and reference field 10x10cm2. By combining the isodose curve standard wedge fields with compatible weighting dose for each field, the effective isodose curve is calculated for any wedge angle. Results: The relation between a given effective wedge angle and the weighting of each combining wedge fields was derived. A good agreement was found between the measured and calculated wedge angles and the maximum deviation did not exceed 3°. The difference between the measured and calculated data decreased when the combined wedge angles were closer. The results are in agreement with the motorized single wedge appliance in the literature. Conclusion: This technique showed that the effective wedge angle that is obtained from this method is adequate for clinical applications and the motorized wedge formalism is a special case of this consideration.

160. Kilovoltage rotational radiotherapy of breast cancer with synchrotron radiation: A phantom study with 100 keV and 6 MV VMAT dose delivery

Purpose We proposed a new technique for breast cancer rotational radiotherapy using kilovoltage synchrotron radiation (SR) [1] . Here, we compared 2D dose distributions in a breast phantom at 6 MV in circular orbit Volumetric Arc Therapy (VMAT) with respect to 100 keV irradiation in rotational radiotherapy with SR. Methods The dose distribution was evaluated with calibrated EBT3 radiochromic films inserted at midplane in a 14-cm diameter PMMA cylindrical phantom, simulating the pendant breast. We irradiated a cylindrical target (15 × 20 mm2 (W × H)) at the centre of phantom. Measurements were performed at the Imaging and Medical Beamline (Australian Synchrotron, Clayton, Australia) at 100 keV, with a SR beam of size 15 × 20 mm2 (W × H) continuously irradiating the phantom during a 360 deg rotation around the cylinder axis with a central dose to the target of 7 Gy. Megavoltage measurements were performed at 6 MV using a Varian Clinac iX System Linac, with a target dose of 6 Gy. Results The principle of dose summation along the axis of rotation produced a dose distribution peaked at the centre of rotation for small fields. The target-to-surface dose ratio was 20:1 at 6 MV and 9:1 at 100 keV, for a target at the center of the phantom. Conclusions We performed a feasibility test of kilovoltage rotational radiotherapy dedicated to the breast showing a target-to-surface dose ratio higher than with MV radiotherapy but still acceptable for skin tissue sparing.

Beam characteristic of P.E collimators Add-on multileaf collimator

Introduction: Multileaf collimators (MLCs) are computer-controlled devices which are Useful and cost-effective tools for conformal therapy and intensity modulation radiotherapy (IMRT). Nowadays MLCs are important and standard configuration of the new medical linear accelerators which are substitute of conventional Blocks and has eliminated the difficulties of working with blocks. MLCs are available from many manufacturers and their designs differ in the way they couple with conventional jaws, number and width of leaves and in their physical and dosimetric characteristics, well understanding on the mechanics and dosimetric characteristics of MLC is the basic task of medical physicist. Materials and Methods: In this study some mechanical and dosimetric characteristics of a P.E. Collimators Add-On MLC which is installed on VARIAN CLINAC 2100 C/D are presented. Scatter factors (Sc, Sp, Scp), central axis depth dose and beam profiles were measured before and after installation of MLC. mechanical and Leaf Leakages for 6MV and 18MV beams were measured with Gafchromic EBT3 films. Results: radiation and light fields were match below the 2mm acceptable level based on AAPM reports. Average leaf leakage was noted 1.79% for 6MV and 1.98% for 18MV beams, respectively. No significant differences were found in beam profiles, collimator (Sc), phantom (Sp) and total (Sc, p) scatter factors after MLC installation. The percent depth dose data for all fields and both beam energies were within 1.6 % of the original data. Conclusion: Based on the Mechanical and dosimetric results which have been achieved from our different standard tests, we found no significant differences between before and after MLC installation Values on Clinac 2100 C/D Varian LINAC.

303. Radiomic feature changes on cone-beam CT during RT for rectal cancer

Purpose The study of the change of radiomic features extracted from cone-beam CT acquired before each fraction for IGRT opens a new, promising scenario for assessing tumor response to radiotherapy during the treatment [1] . In this preliminary study, we investigated if radiomic features of rectal cancer treated with VMAT extracted from CBCT change significantly in the course of the radiotherapy treatment. Methods and materials 8 patients underwent VMAT for rectal cancer with 54 Gy in 25 fractions. For each patient, the GTV was manually contoured on three CBCT images acquired for IGRT at the first fraction, middle (12th) and last fraction of RT. All the CBCT images were acquired with 125 kVp and 80 mA with a Varian Clinac Trilogy, and the intensity levels of CBCT images were equalized using the mean intensity level of a ROI in the bladder as reference. Radiomic features were extracted after image denoising and smoothing from 3D tumor regions, and can be classified into: shape-based, histogram-intensity and higher-order texture features. The feature changes were evaluated between last and mid-fraction (Delta1), and between last and first fraction (Delta2). Wilcoxon signed-rank test was used to test for significant changes of features. Results 18 features had significant Delta1 and 23 had Delta2, while 12 had both changes of statistical relevance (p Conclusions The results exhibit significant changes of radiomic features extracted from daily CBCT images at different treatment times, showing the possibility for studying early response of tumor during therapy and develop a radiomics-adapted radiotherapy approach. The correlation of feature changes with tumor response at the end of therapy will be subject of ongoing investigation.

Neutron Contamination Unexpected Dose in Varian LINAC

Introduction: Neutron contamination is very harmful for the patients receiving radiation therapy due to the high relative biological effect of neutrons. Medical linear accelerators head are made up of materials with high atomic numbers, and High-energy photons interaction with heavy materials can produce neutron contamination. In this study, neutron equivalent dose, neutron spectrum, and the effect of field size on neutron contamination dose is calculated by the FLUKA Monte Carlo code system. Materials and Methods: In this study, all parts of the Varian Clinac 2100 accelerator head’s elements, such as target, flattening filter, jaws, MLC, shields, primary and secondary collimators are simulated by the FLUKA code system. LINAC performance energy was considered 18MV and a water phantom with dimension of 140×140×35 cm3 was simulated for neutron dose calculation. To investigate the effect of field size on neutron contamination, larger treatment fields such as 10×10, 20×20, 30×30 and 40×40 was formed by the jaws, and the smaller ones like 1×1, 3×3 and 5×5 was formed by the MLC. Results: The results show that the Neutron contamination dose can be up to 2.54 mSv per 1Gy photon dose at isocenter. Neutron to photon dose ratio is 0.004 at isocenter and more than 8.6 in out-of-field areas. Furthermore, the neutron dose changes at the surface of the water phantom is only 15% that show the protective elements of the LINAC head are not able to control the neutron contamination flux. The most of neutron dose distribution is at the surface of the water phantom, So it can increases the secondary cancer risk in patients, including skin cancer. The effect of treatment field size on neutron contamination shows that the larger field size formed by the jaws contribute to higher in-field neutron dose, and using of smaller field size formed by the MLC, cause the lower in-field neutron dose. Conclusion: The neutron contamination dose can not be neglected, and the effective parameters in neutron contamination exasperation should be thoroughly investigated. Furthermore, the harmful effects of the neutron contamination should be minimized by the appropriate protective shields

MONITORING OF HIGH-ENERGY PHOTON FLUENCE IN CLINAC USING Zn AND Cr PHOTOACTIVATION.

Monitoring of high-energy photon dose in radiation therapy is crucial for radiation protection, as well as to estimate the radiation effects in the operating microelectronic devices. The aim of this work was to investigate the possibility of using the photoactivation technique to monitor the high-energy photon fluence in the Varian 21EX 23MV CLINAC. The 52Cr(γ,n)51Cr and 66Zn(γ,n)65Zn reactions were used. It was found that 51Cr and 65Zn can be used successfully to monitor the fluence for short term (80 days) and long term (110 days), respectively.