kV Beam Research Articles

Radiation Physics of a New Electronic Brachytherapy System for Treating Skin Lesions

To commission and analyze a new low energy high dose rate electronic brachytherapy system. Half-value layer (HVL), absolute dose output, beam profiles, and insert factors of an electronic brachytherapy system (EBT) were measured and analyzed. A thimble-shaped parallel-plate ionization chamber was utilized for HVL, absolute output, and insert factor determinations. HVL measurements of the 69.5 keV beam of EBT were conducted using thin aluminum foils in narrow beam geometry. In this setup, the aluminum filters were positioned at a distance of 30 cm from tip of the EBT device and 30 cm from the ion chamber. Absolute calibration of this X-ray beam was carried out based on the AAPM Task Group 61 (TG61) report. Beam profiles and depth ionization curves were measured and analyzed using radiochromic films, a professional scanner, and a scientific software package. The relative output factors of the five standard inserts of this device (10, 15, 20, 25, and 30 mm in diameter) were determined employing the ion chamber in air. Finally, a secondary dose calculation system was developed in a spreadsheet. HVL of the X-ray beam of EBT was measured to be 1.61 mmAl. For TG61-based calibration of this low energy kV beam employing the ion chamber at the SSD of 6 cm, it was determined that for this system Pion, Ppol, backscatter factor, ratio of mass energy absorption coefficient, inverse square correction from the surface to the effective point of measurement, and end-effect are 1.009, 0.995, 1.121, 1.017, 1.061, and 0.997, respectively. Measuring the PDD of this beam was proved difficult. Per TG61 recommendations, PDD values from BJR supplement #25 were employed for translating the measured output at the surface to the depth of 3 mm in tissue. The PDD values from BJR were in agreement with those supplied by the manufacturer to within <0.5%. The absolute dose at the depth of 3 mm was therefore established to be 3.1 Gy, for when the EBT system was programmed to deliver 3 Gy. This agreement level was deemed acceptable for the low-energy and small SSD of this device. Beam profiles showed a relatively flat beam (<5%) with small (1 mm) penumbra. Insert factors varied from 0.95 to 1.01 for the five standard inserts of this EBT device in relations to the 30 mm insert output value. Radiation physics parameters of an EBT X-ray unit were measured to match those claimed by the manufacturer to an accuracy of better than 5%, except HVL. Our measured HVL was 1.61 mm Al, whereas that indicated by the manufacturer is 1.83 mm Al. This discrepancy in HVL has a 0.3% effect on the NK value used for this chamber and less than 2% impact on the PDD employed for determining the absorbed radiation dose at depth. Reproducibility of the radiation beam of this device was on the order of less than 0.3%.

Design and Test of a Thermal Measurement System Prototype for SPIDER Experiment

SPIDER, the full-size prototype of the ITER heating neutral beams (HNBs) radio frequency (RF) ion source, has been designed and is under construction at Consorzio RFX in Padua, Italy. Several thermocouples will be installed in both SPIDER and HNB source to characterize and monitor the possibly severe thermal load on mechanical components. The presence of about 1 MW RF power ionizing the gas and frequent electrical breakdowns on the 100 kV beam accelerator represent a very harsh environment for thermal measurements, both in terms of possible sensors damage and of disturbances induced on the signals. A complete measurement chain prototype, composed of two sensors, conditioning electronics and data acquisition system, was realized to test and validate technologies and design solutions. The prototype was installed and tested inside the BATMAN RF ion source at IPP-Garching, where measurement conditions are very similar to those expected in SPIDER, and in particular high RF power and high voltage breakdowns are present. A careful installation was carried out, with extensive optimization of grounding and shielding configuration, to achieve sufficient ElectroMagnetic Interference immunity. Analog and digital filtering were also implemented on signals to eliminate the residual noise. Results produced during a dedicated experimental campaign on BATMAN have been very successful: 0.5 °C measurement global accuracy can be obtained during RF plasma pulses and no damage occurred on sensors or electronic equipments. The paper describes the prototype design detailing the technical solutions implemented, in particular regarding sensors design and installation, signal conditioning, and noise reduction. Hardware and software characteristics of the data acquisition system are presented and discussed. Experimental results obtained are also described and their significance for future work is analyzed.

Radiation dose enhancement of gadolinium-based AGuIX nanoparticles on HeLa cells

Radiation dose enhancement of high-Z nanoparticles is an active area of research in cancer therapeutics. When kV and MV energy photon beams interact with high-Z nanoparticles in a tumor, the release of secondary electrons can injure tumor cells, leading to a higher treatment efficacy than radiation alone. We present a study that characterizes the radiation dose enhancing effects of gadolinium-based AGuIX nanoparticles on HeLa cells. Our in vitro clonogenic survival assays showed an average dose enhancement of 1.54× for 220 kVp radiation and 1.15× for 6 MV radiation. The sensitivity enhancement ratio at 4Gy (SER4Gy) was 1.54 for 220 kVp and 1.28 for 6 MV, indicating that these nanoparticles may be useful for clinical radiation therapy. From the Clinical EditorThis study characterized the radiation dose enhancing effects of gadolinium-based AGuIX nanoparticles on HeLa cells, showing clear effects at 220kV as well as 6MV, suggesting that after additional studies, these nanoparticles may be beneficial in human radiation therapy.

SU‐E‐I‐72: First Experimental Study of On‐Board CBCT Imaging Using 2.5MV Beam On a Radiotherapy Linac

Purpose:Varian TrueBeam version 2.0 comes with a new inline 2.5MV beam modality for image guided patient setup. In this work we develop an iterative volumetric image reconstruction technique specific to the beam and investigate the possibility of obtaining metal artifact free CBCT images using the new imaging modality.Methods:An iterative reconstruction algorithm with a sparse representation constraint based on dictionary learning is developed, in which both sparse projection and low dose rate (10 MU/min) are considered. Two CBCT experiments were conducted using the newly available 2.5MV beam on a Varian TrueBeam linac. First, a Rando anthropomorphic head phantom with and without a copper bar inserted in the center was scanned using both 2.5MV and kV (100kVp) beams. In a second experiment, an MRI phantom with many coils was scanned using 2.5MV, 6MV, and kV (100kVp) beams. Imaging dose and the resultant image quality is studied.Results:Qualitative assessment suggests that there were no visually detectable metal artifacts in MV CBCT images, compared with significant metal artifacts in kV CBCT images, especially in the MRI phantom. For a region near the metal object in the head phantom, the 2.5MV CBCT gave a more accurate quantification of the electron density compared with kV CBCT, with a ∼50% reduction in mean HU error. As expected, the contrast between bone and soft‐tissue in 2.5MV CBCT decreased compared with kV CBCT.Conclusion:On‐board CBCT imaging with the new 2.5MV beam can effectively reduce metal artifacts, although with a reduced softtissue contrast. Combination of kV and MV scanning may lead to metal artifact free CBCT images with uncompromised soft‐tissue contrast.

Low‐cost flexible thin‐film detector for medical dosimetry applications

The purpose of this study is to characterize dosimetric properties of thin film photovoltaic sensors as a platform for development of prototype dose verification equipment in radiotherapy. Towards this goal, flexible thin‐film sensors of dose with embedded data acquisition electronics and wireless data transmission are prototyped and tested in kV and MV photon beams. Fundamental dosimetric properties are determined in view of a specific application to dose verification in multiple planes or curved surfaces inside a phantom. Uniqueness of the new thin‐film sensors consists in their mechanical properties, low‐power operation, and low‐cost. They are thinner and more flexible than dosimetric films. In principle, each thin‐film sensor can be fabricated in any size (mm2 – cm2 areas) and shape. Individual sensors can be put together in an array of sensors spreading over large areas and yet being light. Photovoltaic mode of charge collection (of electrons and holes) does not require external electric field applied to the sensor, and this implies simplicity of data acquisition electronics and low power operation. The prototype device use for testing consists of several thin film dose sensors, each of about 1.5 cm×5 cm area, connected to simple readout electronics. Sensitivity of the sensors is determined per unit area and compared to EPID sensitivity, as well as other standard photodiodes. Each sensor independently measures dose and is based on commercially available flexible thin‐film aSi photodiodes. Readout electronics consists of an ultra low‐power microcontroller, radio frequency transmitter, and a low‐noise amplification circuit implemented on a flexible printed circuit board. Detector output is digitized and transmitted wirelessly to an external host computer where it is integrated and processed. A megavoltage medical linear accelerator (Varian Tx) equipped with kilovoltage online imaging system and a Cobalt source are use to irradiate different thin‐film detector sensors in a Solid Water phantom under various irradiation conditions. Different factors are considered in characterization of the device attributes: energies (80 kVp, 130 kVp, 6 MV, 15 MV), dose rates (different ms × mA, 100–600 MU/min), total doses (0.1 cGy‐500 cGy), depths (0.5 cm–20 cm), irradiation angles with respect to the detector surface (0°‐180°), and IMRT tests (closed MLC, sweeping gap). The detector response to MV radiation is both linear with total dose (~1‐400 cGy) and independent of dose rate (100‐600 Mu/min). The sensitivity per unit area of thin‐film sensors is lower than for aSi flat‐panel detectors, but sufficient to acquire stable and accurate signals during irradiations. The proposed thin‐film photodiode system has properties which make it promising for clinical dosimetry. Due to the mechanical flexibility of each sensor and readout electronics, low‐cost, and wireless data acquisition, it could be considered for quality assurance (e.g., IMRT, mechanical linac QA), as well as real‐time dose monitoring in challenging setup configurations, including large area and 3D detection (multiple planes or curved surfaces).PACS number: 87.56.Fc

Modality comparison for small animal radiotherapy: A simulation study

Small animal radiation therapy has advanced significantly in recent years. Whereas in the past dose was delivered using a single beam and a lead shield for sparing of healthy tissue, conformal doses can be now delivered using more complex dedicated small animal radiotherapy systems with image guidance. The goal of this paper is to investigate dose distributions for three small animal radiation treatment modalities. This paper presents a comparison of dose distributions generated by the three approaches-a single-field irradiator with a 200 kV beam and no image guidance, a small animal image-guided conformal system based on a modified microCT scanner with a 120 kV beam developed at Stanford University, and a dedicated conformal system, SARRP, using a 220 kV beam developed at Johns Hopkins University. The authors present a comparison of treatment plans for the three modalities using two cases: a mouse with a subcutaneous tumor and a mouse with a spontaneous lung tumor. A 5 Gy target dose was calculated using the EGSnrc Monte Carlo codes. All treatment modalities generated similar dose distributions for the subcutaneous tumor case, with the highest mean dose to the ipsilateral lung and bones in the single-field plan (0.4 and 0.4 Gy) compared to the microCT (0.1 and 0.2 Gy) and SARRP (0.1 and 0.3 Gy) plans. The lung case demonstrated that due to the nine-beam arrangements in the conformal plans, the mean doses to the ipsilateral lung, spinal cord, and bones were significantly lower in the microCT plan (2.0, 0.4, and 1.9 Gy) and the SARRP plan (1.5, 0.5, and 1.8 Gy) than in single-field irradiator plan (4.5, 3.8, and 3.3 Gy). Similarly, the mean doses to the contralateral lung and the heart were lowest in the microCT plan (1.5 and 2.0 Gy), followed by the SARRP plan (1.7 and 2.2 Gy), and they were highest in the single-field plan (2.5 and 2.4 Gy). For both cases, dose uniformity was greatest in the single-field irradiator plan followed by the SARRP plan due to the sensitivity of the lower energy microCT beam to target heterogeneities and image noise. The two treatment planning examples demonstrate that modern small animal radiotherapy techniques employing image guidance, variable collimation, and multiple beam angles deliver superior dose distributions to small animal tumors as compared to conventional treatments using a single-field irradiator. For deep-seated mouse tumors, however, higher-energy conformal radiotherapy could result in higher doses to critical organs compared to lower-energy conformal radiotherapy. Treatment planning optimization for small animal radiotherapy should therefore be developed to take full advantage of the novel conformal systems.

A method for sizing submicrometer particles in air collected on Formvar films and imaged by scanning electron microscope

Abstract. A method was developed to systematically investigate individual aerosol particles collected onto a polyvinyl formal (Formvar)-coated copper grid with scanning electron microscopy. At very mild conditions with a low accelerating voltage of 2 kV and Gentle Beam mode aerosol particles down to 20 nm in diameter can be observed. Subsequent processing of the images with digital image analysis provides size resolved and morphological information (elongation, circularity) on the aerosol particle population. Polystyrene nanospheres in the expected size range of the ambient aerosol particles (20–900 nm in diameter) were used to confirm the accuracy of sizing and determination of morphological parameters. The relative standard deviation of the diameters of the spheres was better than ±10% for sizes larger than 40 nm and ±18% for 21 nm particles compared to the manufacturer's certificate. Atmospheric particles were collected during an icebreaker expedition to the high Arctic (north of 80°) in the summer of 2008. Two samples collected during two different meteorological regimes were analyzed. Their size distributions were compared with simultaneously collected size distributions from a Twin Differential Mobility Particle Sizer, which confirmed that a representative fraction of the aerosol particles was imaged under the electron microscope. The size distributions obtained by scanning electron microscopy showed good agreement with the Twin Differential Mobility Sizer in the Aitken mode, whereas in the accumulation mode the size determination was critically dependent on the contrast of the aerosol with the Formvar-coated copper grid. The morphological properties (elongation, circularity) changed with the number of days the air masses spent over the pack-ice area north of 80° before the aerosol particles were collected at the position of the icebreaker and are thus an appropriate measure to characterize transformation processes of ambient aerosol particles.

Characterisation of rare earth minerals with field emission scanning electron microscopy

Rare earth elements are becoming increasingly in demand, due to their prevalence in both renewable energy devices and high end electronics. The characterisation of the composition and morphology of the various phases that have valuable rare earth elements in the ores are needed in conjunction with the study of their physicochemical properties to optimise industrial process to extract the minerals containing the rare earth elements. Rare earth bearing minerals contain many elements with overlapping X-ray peaks (L- and M-lines) with an energy dispersive spectrometry detector, requiring a high degree of X-ray energy resolution. A program was developed to obtain the intensity of each peak by deconvolution of the X-ray spectrum. Low accelerating voltage of less than 5 kV and small beam diameter of less than 10 nm of a cold field emission scanning electron microscope allow x-ray microanalysis on the nanometre scale. A 100 nm wide phase was observed at 4 kV on a backscattered electron micrograph. Furthermore, a small beam diameter of less than 10 nm was used for identification of small phases of a few micrometres.Les éléments de terre rare sont de plus en plus en demande, grâce à leur prévalence tant dans les dispositifs d’énergie renouvelable que dans l’électronique de haute gamme. On a besoin de caractériser la composition et la morphologie des différentes phases qui contiennent des terres rares de valeur dans les minerais, en combinaison avec l’étude de leurs propriétés physico-chimiques, afin d’optimiser le procédé industriel d’extraction des minéraux contenant les terres rares. Les minéraux porteurs de terre rare contiennent plusieurs éléments ayant des pics de rayons x qui se chevauchent (lignes L et M) avec un détecteur de spectroscopie à dispersion d’énergie, nécessitant une haute résolution énergétique. On a développé un programme visant à obtenir l’intensité de chaque pic par déconvolution du spectre de rayons x. Un faible voltage d’accélération de moins que 5 kV et un faisceau à faible diamètre de moins que 10 nm d’un microscope électronique à balayage par cathode froide permettent la microanalyse des rayons x à l’échelle du nanomètre. On a observé une phase de 100 nm de largeur à 4 kV sur une micrographe d’électrons rétrodiffusés. De plus, on a utilisé un faisceau de faible diamètre de moins que 10 nm pour l’identification de petites phases de quelques μm.

Radiation exposure to patients from image guidance procedures and techniques to reduce the imaging dose

PurposeTo compare imaging doses from MV images, kV radiographs, and kV-CBCT and describe methods to reduce the dose to patient’s organs using existing on-board imaging devices. Method and materialsMonte Carlo techniques were used to simulate kV X-ray sources. The kV image doses to a variety of patient anatomies were calculated by using the simulated realistic sources to deposit dose in patient CT images. For MV imaging, the doses for the same patients were calculated using a commercial treatment planning system. ResultsPortal imaging results in the largest dose to anatomic structures, followed by Varian OBI CBCT, Varian TrueBeam CBCT and then kV radiographs. The imaging doses for the 50% volume from the DVHs, D50, to the eyes for representative head images are 4.3–4.8cGy; 0.05–0.06cGy; 0.04–0.05cGy; and, 0.12cGy; D50 to the bladder for representative pelvis images are 3.3cGy; 1.6cGy; 1.0cGy; and, 0.07cGy; while D50 to the heart for representative thorax images are 3.5cGy; 0.42cGy; 0.2cGy; and, 0.07cGy; when using portal imaging, OBI kV-CBCT scans, TrueBeam kV-CBCT scans and kV radiographs, respectively. The orientation of the kV beam can affect organ dose. For example, D50 to the eyes can be reduced from 0.12cGy using AP and right lateral radiographs to 0.008–0.017cGy when using PA and right lateral radiographs. In addition, organ exposures can be further reduced to 15–70% of their original values with the use of a full-fan, bow-tie filter for kV radiographs. In contrast, organ doses increase by a factor of ∼2–4 if bow-tie filters are not used during kV-CBCT acquisitions. ConclusionCurrent on-board kV imaging devices result in much lower imaging doses compared to MV imagers even taking into account of higher bone dose from kV X-rays. And a variety of approaches are available to significantly reduce the image doses.

SU-D-141-02: KV OBI Spectroscopy: Measurement Vs Monte Carlo Simulation

Purpose: With IGRT there has been a dramatic increase in the use of kV based on-board imaging (OBI). Although, numerous Monte Carlo (MC) simulation studies on the OBI kV source exist, the only validation of the actual OBI photon energy spectrum has been in the form of half-value-layers. Realistic MC OBI simulation requires correct modeling of the kV beam spectrum. Here, we present for the first time, actual experimentally measured OBI spectra together with their MC simulated counterparts. Methods: The spectra produced by a commercial LINAC OBI were measured using a thermoelectric cooled CdTe detector and a multichannel analyzer. To reduce detector over saturation, the kV radiation source photon flux was reduced by the use of collimating disks placed before the CdTe surface and by setting the kV source to its minimum mAs. Preliminary measured spectra for 60 kVp, 80 kVp, 100 kVp, and 125 kVp beams for the OBI were compared to MC results based on a model of the OBI. For the 125 kV beam, a percent depth dose (PDD) curve was measured with a diamond detector, and compared to a PDD generated by MC based on the simulated spectrum. Results: The corrected measured spectra correspond roughly to the MC generated spectra, with the MC spectra generally showing more high energy photons and fewer low energy photons. The MC PDD and measured PDD agree to within 2% of absolute dose. Conclusion: We present new measurements of kV OBI spectrometry to advance kV dosimetry and more sophisticated modeling of kV imaging systems. The discrepancy between measured and simulated spectra is small, and the dosimetric effect of the discrepancy is also small.

SU-E-T-72: Influence of Chamber Wall Material On Ionization Chamber Absorbed Dose Energy Response: A Numerical and Experimental Study

Purpose: To study the energy response of ionization chambers with different wall materials. Methods: The dose inside the cavity of an accurately modeled Exradin A12 ionization chamber was scored with Monte Carlo user code egs++/egs_chamber. The C552 plastic chamber wall material was changed in the simulations with materials of higher atomic number Z (aluminum, copper, molybdenum and tungsten) and simulations were carried out with five beam energies ranging from 120 kVp to 18 MV. The dose scored inside the cavity for each wall material was normalized to that from a C552 plastic wall. The mean secondary electron energy for each beam Ee‐ was also calculated at the level of chamber cavity using MC user code FLURZnrc. The simulations were experimentally verified by replacing the Exradin A12 chamber C552 wall with Al or Cu walls of identical dimensions. AAPM TG51 and TG61 setups were followed for high and low energy beams, respectively. Large attenuation of kilovoltage photons by high Z wall materials was accounted for by correcting the readings with a CAVRZnrc‐calculated chamber wall attenuation and scatter correction Awall. Results: The relative readings obtained showed that with the use of higher Z wall materials, the chamber signal increased by up to a factor of 2.96 for MV photons, and 54.71 at 120 kVp. Higher Z walls Result in larger contribution of photoelectrons, and as such changing the wall material significantly affects the absorbed dose energy‐dependence of the chambers. Experimental results agree with simulations to within 9.8 %. The discrepancy is largest at kV beams and can be mitigated if the impurities found in each wall material were considered in the MC simulations. Conclusion: The change in ionization chamber absorbed‐dose energy dependence is studied (numerically and experimentally), by replacing the original wall chamber with walls of different Z material, while keeping the wall dimensions identical.

WE‐A‐134‐02: Combined MV+kV Beam Optimization for Enabling Real‐Time KV Tumor Tracking

Purpose: Despite the existence of real‐time kV intrafractional tumor tracking strategies for many years, clinical adoption has been held back by concern over excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This work aims to solve this specific problem by performing a first time investigation into the use of convex optimization tools to best integrate this excess kV imaging dose into the MV therapeutic dose as to make real‐time kV tracking clinically feasible. Methods: Phase space files for both a 125 kVp kV beam and a 6 MV treatment beam were generated with BEAMNRC, and used to make dose influence matrices in DOSXYZNRC for a lung cancer patient. The dose optimization problem for IMRT, formulated as a quadratic problem, was modified to include additional kV imaging constraints as required for real‐time kV fluorescent tracking. The MOSEK optimization toolkit was used to solve the modified optimization problem. Results: Compared to conventional IMRT optimization, combined MV+kV optimization leads to a 2.4–5.2% reduction in the total number of monitor units assigned to the MV beam due to inclusion of the kV dose. When using an open kV aperture, combined MV+kV optimization allows for a reduction of up to 50% of the excess kV dose compared to standard IMRT with kV fluoro tracking. For all kV field sizes considered, combined MV+kV optimization provided PTV coverage equal to standard IMRT without kV imaging and superior to standard IMRT plus kV fluoro tracking. Skin dose with combined MV+kV imaging was only slightly higher than the no‐imaging case. Conclusion: The use of MV+kV optimization allows for greatly reduced imaging dose to the skin, especially in cases of large kV aperture imaging. Combined MV+kV optimization demonstrates potential for real‐time tumor tracking without excessive imaging dose, paving the way for clinical implementation.

SU-D-144-01: Implementation of a Clinical Treatment Planning System for Use with a Small Animal Irradiation System

Purpose: Current commercially available small animal irradiators provide limited software tools for treatment planning. We report on the implementation of an in-house treatment planning system, Metropolis, for micro-irradiator and compare its capabilities with those of the vendor-supplied system, TPS(XRAD). Methods: The 225 kV beam of the micro-irradiator was commissioned using both ion chamber and radiographic film (EBT3). Dose calculation in Metropolis was verified by comparing with measured dose in phantom. Starting from a 3D CT image of a tumor-bearing mouse, the same treatment plans were designed in the two systems. In TPS(XRAD), a treatment point was selected on a CT slice, two orthogonal beams were defined based on tumor size on the CT slice, and isocenter depth was computed from a body contour determined by image thresholding. In Metropolis, structures (outer, tumor, and lung) were contoured on CT images, beams were defined based on tumor volume coverage on beams-eye-view (BEV), 3D dose calculation was performed, and plan was evaluated based on dose distribution and dose-volume histogram (DVH). Results: Measured dose in phantom agreed with Metropolis calculation within 3% for all collimators. The beam-on-times calculated by the two systems agreed within 1% at isocenter. Whereas TPS(XRAD) provides only beam-on-time for each beam, Metropolis capabilities include various image segmentation tools, multimodality image registration, verification of target coverage and normal tissue sparing using BEV, 3D isodose distribution overlaid on CT images, and DVH. DVH inspection of the two-beam treatment plan reveals a tumor dose variation (D95 : 388 and D05 : 401 cGy) and dose to lung (Dmean: 84 cGy) for a prescribed isocenter dose of 400 cGy. Conclusion: By implementing a clinical TPS for a small animal irradiator system, both efficient planning and precise plan evaluation become possible, allowing the full potential of advanced micro-irradiator radiation treatment planning to be conducted for pre-clinical experimentation.

SU-E-T-99: From Thermal Response to CT Dose - a Calorimetric Journey in HDPE

Purpose: : CT dose, normally measured with an ionization chamber, can be directly determined by measuring temperature increases in the sub‐mK levels in a material, as shown in a recent proof‐of‐principle study using the proposed AAPM TG200 high‐density polyethylene (HDPE) phantom. The present study set out to investigate radiation‐induced chemical transformation (heat defect) that impacts the dose estimates. Methods: The calorimeter employs two small thermistors (wired into a Wheatstone bridge and monitored via lock‐in detection), embedded in a removable “core” element (1.5 cm dia. × 2 cm cylinder) inserted into a small channel of a cylindrical HDPE phantom (26 cm dia. × 10 cm) along the axis. To improve upon the earlier core which was made of PE, the present one is made of polystyrene (PS), which is expected to have no heat defect. Comparison measurements were made using the two cores and with a calibrated ionization chamber. The typical experimental run consisted of 30 2.8‐s axial scans (16‐slice CT scanner, 120 kV beam, 250 mA, 16x1.5 mm collimation), yielding an aggregate dose at the center of the HDPE phantom of ∼0.5 Gy. Results: The PS core data show a thermal response much closer to the nominal dose than the PE core data, suggesting that much of the difference could be attributable to excess heat from the larger thermistor beads in the PE core plus the heat defect in PE (5 to 10% by literature). However, variability in the traces due to heat transfer distortions limits the precision of the estimate at present. Conclusion: The effect of heat defect mixed in with the excess heat produced from the non‐phantom materials is observable in the present study. Further work is planned to address heat transfer within the phantom, which limits the precision of estimating heat defect corrections.

High throughput film dosimetry in homogeneous and heterogeneous media for a small animal irradiator

PurposeWe have established a high-throughput Gafchromic film dosimetry protocol for narrow kilovoltage beams in homogeneous and heterogeneous media for small-animal radiotherapy applications. The kV beam characterization is based on extensive Gafchromic film dosimetry data acquired in homogeneous and heterogeneous media. An empirical model is used for parameterization of depth and off-axis dependence of measured data. MethodsWe have modified previously published methods of film dosimetry to suit the specific tasks of the study. Unlike film protocols used in previous studies, our protocol employs simultaneous multi-channel scanning and analysis of up to nine Gafchromic films per scan. A scanner and background correction were implemented to improve accuracy of the measurements. Measurements were taken in homogeneous and inhomogeneous phantoms at 220 kVp and a field size of 5 × 5 mm2. The results were compared against Monte Carlo simulations. ResultsDose differences caused by variations in background signal were effectively removed by the corrections applied. Measurements in homogeneous phantoms were used to empirically characterize beam data in homogeneous and heterogeneous media. Film measurements in inhomogeneous phantoms and their empirical parameterization differed by about 2%–3%. The model differed from MC by about 1% (water, lung) to 7% (bone). Good agreement was found for measured and modelled off-axis ratios. ConclusionsEBT2 films are a valuable tool for characterization of narrow kV beams, though care must be taken to eliminate disturbances caused by varying background signals. The usefulness of the empirical beam model in interpretation and parameterization of film data was demonstrated.

EP-1305: Management of the interruptions in the conventional treatment of prostate cancer upon a database analysis

Purpose/Objective: The delivery of high quality stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments requires knowledge of the position of the isocentre to sub millimetre accuracy. The deviation between the radiation and mechanical isocentres must be less than 1 mm. The use of an add-on micromultileaf collimator (μMLC) in SRS and SRT is an additional challenge to the anticipated high-level geometric and dosimetric accuracy of the treatment. The aim of this work was to quantify the gantry excursions during rotation with and without an add-on μMLC attached to the gantry head. In addition, the shift in the position of the isocenter and its correlation to the kV beam centre of the cone-beam CT system was included in the study. Materials and Methods: The quantification of the gantry rotational performance was done using a pointer supported by an inhouse made rigid holder attached to the gantry head. The pointer positions were measured using a digital theodolite. The displacement of the isocentre due to an add-on μMLC of 50 kg was investigated. In case of the pointer measurement the μMLC was simulated by weights attached to the gantry head. A method of least squares was applied to determine the position and displacement of the mechanical isocentre. Additionally, the displacement of the radiation isocentre was measured using a Winston-Lutz phantom and the electronic portal image device (EPID) system. These measurements were based on eight MV photon beams irradiated onto the ball from the four cardinal angles and two opposed collimator angles. The measurements and analysis of the data were carried out automatically using software delivered by the manufacturer. Results: The displacement of the mechanical isocentre caused by a 50 kg heavy μMLC was found to be (-0.01±0.06, -0.10±0.03, -0.26±0.05) mm in lateral, longitudinal and vertical direction, respectively. Similarly, the displacement of the radiation isocentre was found to be (0.00±0.03, -0.08±0.06, -0.32±0.02) mm. Good agreement was found between the displacement of the two isocentres. A displacement of the kV cone-beam CT beam centre due to the attached weight of 50 kg could not be detected. By comparing the CW and CCW data the presence of the effect of backlash appears. The effect of backlash was found to be < 0.14 mm and < 0.10 mm in the lateral and vertical direction, respectively. Conclusions: General characteristics of the gantry arm excursions and displacements caused by an add-on μMLC have been reported. A 50 kg heavy add-on μMLC results in an isocentre displacement downward of 0.26 to 0.32 mm. We recommend that the beam centre of the kV cone-beam CT image system should be matched to the isocentre related to the weight of the μMLC. Consequently, the imperfections in isocentre localizations are transferred to the conventional radiotherapy where the clinical consequences of uncertainties in the sub millimetre regime are negligible.

Gantry and isocenter displacements of a linear accelerator caused by an add‐on micromultileaf collimator

The delivery of high quality stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments to the patient requires knowledge of the position of the isocenter to submillimeter accuracy. To meet the requirements the deviation between the radiation and mechanical isocenters must be less than 1 mm. The use of add-on micromultileaf collimators (μMLCs) in SRS and SRT is an additional challenge to the anticipated high-level geometric and dosimetric accuracy of the treatment. The aim of this work was to quantify the gantry excursions during rotation with and without an add-on μMLC attached to the gantry head. In addition, the shift in the position of the isocenter and its correlation to the kV beam center of the cone-beam CT system was included in the study. The quantification of the gantry rotational performance was done using a pointer supported by an in-house made rigid holder attached to the gantry head of the accelerator. The pointer positions were measured using a digital theodolite. To quantify the effect of an μMLC of 50 kg, the measurements were repeated with the μMLC attached to the gantry head. The displacement of the isocenter due to an add-on μMLC of 50 kg was also investigated. In case of the pointer measurement the μMLC was simulated by weights attached to the gantry head. A method of least squares was applied to determine the position and displacement of the mechanical isocenter. Additionally, the displacement of the radiation isocenter was measured using a ball-bearing phantom and the electronic portal image device system. These measurements were based on 8 MV photon beams irradiated onto the ball from the four cardinal angles and two opposed collimator angles. The measurements and analysis of the data were carried out automatically using software delivered by the manufacturer. The displacement of the mechanical isocenter caused by a 50 kg heavy μMLC was found to be (-0.01 ± 0.05, -0.10 ± 0.03, -0.26 ± 0.05) mm in lateral, longitudinal, and vertical direction, respectively. Similarly, the displacement of the radiation isocenter was found to be (0.00 ± 0.03, -0.08 ± 0.06, -0.32 ± 0.02) mm. Good agreement was found between the displacement of the two isocenters. A displacement of the kV cone-beam CT beam center due to the attached weight of 50 kg could not be detected. General characteristics of the gantry arm excursions and displacements caused by an add-on μMLC have been reported. A 50 kg heavy add-on μMLC results in a isocenter displacement downward of 0.26-0.32 mm. The authors recommend that the beam center of the kV cone-beam CT image system should be matched to the isocenter related to the weight of the μMLC. Consequently, the imperfections in isocenter localizations are transferred to the conventional radiotherapy where the clinical consequences of uncertainties in the submillimeter regime are negligible.

Dosimetric impact of monoenergetic photon beams in the small-animal irradiation with inhomogeneities: A Monte Carlo evaluation

This study investigated the variations of the dose and dose distribution in a small-animal irradiation due to the photon beam energy and presence of inhomogeneity. Based on the same mouse computed tomography image set, three Monte Carlo phantoms namely, inhomogeneous, homogeneous and bone-tissue phantoms were used in this study. These phantoms were generated by overriding the relative electron density of no voxel (inhomogeneous), all voxel (homogeneous) and the bone voxel (bone-tissue) to one. 360° photon arcs with beam energies of 50–1250kV were used in mouse irradiations. Doses in the above phantoms were calculated using the EGSnrc-based DOSXYZnrc code through the DOSCTP. It was found that the dose conformity increased with the increase of the photon beam energy from the kV to MV range. For the inhomogeneous mouse phantom, increasing the photon beam energy from 50kV to 1250kV increased about 21 times the dose deposited at the isocenter. For the bone dose enhancement, the mean dose was 1.4 times higher when the bone inhomogeneity was not neglected using the 50kV photon beams in the mouse irradiation. Bone dose enhancement affecting the mean dose in the mouse irradiation can be found in the photon beams with energy range of 50–200kV, and the dose enhancement decreases with an increase of the beam energy. Moreover, the MV photon beam has a higher dose at the isocenter, and a better dose conformity compared to the kV beam.

Evaluation of Different External Radiation Therapy Techniques for the Treatment of Malignant Pleural Mesothelioma After Extrapleural Pleuropneumonectomy

ratio and beam output. Finally, a hypothetical treatment of a 2-cm diameter target located at a depth of 11 cm and near the pancreas was simulated on patient CT images using an optimized kV x-ray source and compared to a dose distribution generated with a 6 MV beam. Results: MC simulated depth-dose curves of the existing large-area x-ray source agreed with film measurements to within 5%. Beam energy and filtration, target thickness, collimator hole geometry, and treatment SSD had the largest effect on the quality of dose distributions in terms of both target-to-skin ratio and beam output. An optimized kV x-ray source design for the 4-cm diameter target resulted in target-to-skin ratio of 5.2 and D50 of 19.5 Gy delivered in 30 minutes. For the 5 mm-diameter target, the target-to-skin ratio increased to 11.2 and D50 in 30 minutes exceeded 35 Gy. The MC models of treatment coverage for a target near the pancreas demonstrated that optimized kV and 6 MV treatment plans delivered comparable doses to critical structures. The integral dose delivered by the kV x-ray source was similar to the integral dose delivered by the 6 MV beam due to the fast fall-off of kV beams. Conclusion: A novel radiation therapy kilovoltage x-ray source has been modeled and optimized using Monte Carlo simulations. Due to its unique design, the source delivers conformal doses of radiation to deep-seated tumors with dosimetric parameters and treatment times acceptable for clinical use. A hypothetical clinical scenario showed that treatment plans with the novel cost-effective kV x-ray source have the potential to match treatment plans generated with commonly used 6 MV beams. Author Disclosure: M. Bazalova: None. E. Graves: None. B. Wilfley: None. M. Weil: P. Ownership Other; yes. Q. Leadership; yes.

Monte Carlo model of the scanning beam digital x-ray (SBDX) source

The scanning-beam digital x-ray (SBDX) system has been developed for fluoroscopic imaging using an inverse x-ray imaging geometry. The SBDX system consists of a large-area x-ray source with a multihole collimator and a small detector. The goal of this study was to build a Monte Carlo (MC) model of the SBDX source as a useful tool for optimization of the SBDX imaging system in terms of its hardware components and imaging parameters. The MC model of the source was built in the EGSnrc/BEAMnrc code and validated using the DOSXYZnrc code and Gafchromic film measurements for 80, 100, and 120 kV x-ray source voltages. The MC simulated depth dose curves agreed with measurements to within 5%, and beam profiles at three selected depths generally agreed within 5%. Exposure rates and half-value layers for three voltages were also calculated from the MC simulations. Patient skin-dose per unit detector-dose was quantified as a function of patient size for all three x-ray source voltages. The skin-dose to detector-dose ratio ranged from 5–10 for a 20 cm thick patient to 1 × 103–1 × 105 for a 50 cm patient for the 120 and 80 kV beams, respectively. Simulations of imaging dose for a prostate patient using common imaging parameters revealed that skin-dose per frame was as low as 0.2 mGy.