Thimble Chamber Research Articles

In vivo dose estimations through transit signal measured with thimble chamber positioned along the central axis at electronic portal imaging device level in medical linear accelerator in carcinoma of the middle-third esophagus patients undergoing three-dimensional conformal radiotherapy.

This study presents a method to estimate midplane dose (Diso, transit) in vivo from transit signal (St) measured with thimble ionization chamber in cancer of the middle-third esophagus patients treated with three-dimensional radiotherapy (RT). This detector is positioned at the level of electronic portal imaging device in the gantry of a medical linear accelerator. Efficacy of inhomogeneity corrections of three dose calculation algorithms available in XiO treatment planning system (TPS) for planned dose (for open fields) (Diso, TPS) was studied with three heterogeneous phantoms. Diso, transit represents measured signal at transit point (St) far away correlating to dose at isocenter. A locally fabricated thorax phantom was used to measure the in vivo midplane dose (Diso, mid) which was also estimated through St. Thirteen patients with carcinoma of the middle-third esophagus treated with three-dimensional conformal RT were studied. St was recorded (three times, with a gap of 5-6 fractions during the treatment) to estimate Diso, transit, which was compared with the doses calculated by TPS. The dose predictions by superposition algorithm were superior compared to the other algorithms. Percentage deviation of Diso, transit, Diso, mid with Diso, TPS combined all fields was 2.7 and -2.6%, respectively, with the thorax phantom. The mean percentage deviation with standard deviation of estimated Diso, transit with Diso, TPS observed in patients was within standard deviation -0.73% ±2.09% (n = 39). Midplane dose estimates in vivo using this method provide accurate determination of delivered dose in the middle-third esophagus RT treatments. This method could be useful in similar clinical circumstances for dose confirmation and documentation.

Implementation of dosimetry equipment and phantoms at the MedAustron light ion beam therapy facility.

To describe the implementation of dosimetry equipment and phantoms into clinical practice of light ion beam therapy facilities. This work covers not only standard dosimetry equipment such as computerized water scanners, films, 2D-array, thimble, and plane parallel ionization chambers, but also dosimetry equipment specifically devoted to the pencil beam scanning delivery technique such as water columns, scintillating screens or multilayer ionization chambers. Advanced acceptance testing procedures developed at MedAustron and complementary to the standard acceptance procedures proposed by the manufacturer are presented. Detailed commissioning plans have been implemented for each piece of dosimetry equipment and include an estimate of the overall uncertainty budget for the range of clinical use of each device. Some standard dosimetry equipment used in many facilities was evaluated in detail: for instance, the recombination of a 2D-array or the potential use of a microdiamond detector to measure reference transverse dose profiles in water in the core of the primary pencil beams and in the low-dose nuclear halo (over four orders of magnitude in dose). The implementation of dosimetry equipment as described in this work allowed determining absolute spot sizes and spot positions with an uncertainty better than 0.3mm. Absolute ranges are determined with an uncertainty comprised of 0.2-0.6mm, depending on the measured range and were reproduced with a maximum difference of 0.3mm over a period of 12months using three different devices. The detailed evaluation procedures of dosimetry equipment and phantoms proposed in this work could serve as a guidance for other medical physicists in ion beam therapy facilities and also in conventional radiation therapy.

Optimal orientation for ionization chambers in MRgRT reference dosimetry

Abstract The interest in hybrid systems combining magnetic resonance imaging and medical linear accelerator (MR-Linac) is rapidly increasing due to the clinical availability of different systems. Reference dosimetry is a critical issue for integrating these devices into clinical practice. However, the response of ionization chambers changes according to the distinct orientation of the chamber with respect to the magnetic field. In this study, we have carried out Monte Carlo simulations to identify an optimal orientation for thimble type chambers in MRgRT reference dosimetry. Our findings suggest that an orientation where the chamber axis is parallel to the magnetic field axis should be preferred.

Computation of the electron beam quality [formula omitted] factors for the NE2571, NE2571A and NE2581A thimble ionization chambers using PENELOPE

The quality correction factor kQ,Q0 for electron beams was calculated for three thimble ionization chambers, namely, NE2571, NE2571A and NE2581A. The Monte Carlo code PENELOPE was used to estimate the overall correction factor fc,Q of these chambers for electron beams with nominal energies ranging between 6 and 22MeV, corresponding to a Varian Clinac 2100 C/D. A 60Co beam was used as reference quality Q0. Also eight monoenergetic electron beams reproducing the quality index R50 of the Clinac beams were considered. The kQ,Q0 factors were calculated as the ratio between fc,Q and fc,Q0. Those obtained for the NE2571 ionization chamber show a nice agreement with those calculated by Muir and Rogers with EGSnrc. As it occurred to other ionization chambers analyzed in previous works, the kQ,Q0 factors found for the monoenergetic beams are larger (smaller) than those corresponding to the Clinac beams at low (high) R50 values, the differences being slightly above 0.5%. Finally, the kQ,Q0 factors obtained in the case of the NE2571A chamber are systematically ∼0.5% below those of its predecessor chamber, the NE2571.

A preliminary study of test and clinical application of a physical model based on the three-dimensional dose verification system using electronic portal imaging device

Objective To establish a physical model and optimize its physical parameters using the electronic portable imaging device (EPID) three-dimensional dose verification system, and to prepare it for clinical application. Methods EPID was used to acquire images of 3, 5, 10, 15, 20, and 25 cm square fields for construction of a physical model. The parameters of the physical model were optimized based on the percentage depth dose, total scatter factor, and off-axis ratio at a depth of 10 cm in a homogeneous water phantom. A thimble ionization chamber and radiochromic films were used to measure the point and planar doses for single fields, combined fields, and IMRT plans in a homogenous phantom and a human phantom. The results were compared with those in the three-dimensional reconstruction. In the human phantom and 10 intensity-modulated radiotherapy (IMRT) plans for tumors in different sites, the passing rates under the criteria of 5%/3 mm and 3%/3 mm were compared between three-dimensional reconstruction and treatment planning system (TPS) calculation. The dose and volume analyses were performed on target volume and organs at risk (OARs) in patients. Results For the single fields, combined fields, and IMRT plans, the mean deviations of point dose were less than 0.5% between three-dimensional reconstruction and ionization chamber measurement, and less than 2.0% between three-dimensional reconstruction and TPS calculation. In the homogenous phantom, the human phantom, and patients, the mean passing rates of both two-and three-dimensional doses were higher than 95% under the criteria of 5%/3 mm and 3%/3 mm. In patients, however, OARs with small volume had relatively large dose deviations. Conclusions A series of pre-clinical tests show that the three-dimensional dose verification system is an effective approach for clinical dose verification and holds promise for clinical application. Key words: Electronic portal imaging device; Three-dimensional dose verification; Physical model; Intensity modulated radiation therapy

Feasibility study of a lead monoxide-based dosimeter for quality assurance in radiotherapy

Lately, cancer has been treated using high-energy radiation, and this requires highly reliable treatment plans. Therefore, a dosimeter with excellent performance, which is capable of precise dose measurement, is critical. In current clinical practices, an ionization chamber and diode utilizing the ionization reaction mechanism are widely used. Several studies have been carried out to determine optimal materials for the detector in a dosimeter to enable diagnostic imaging. Recently, studies with lead monoxide, which was shown to have low drift current and high resolving power at a high bias, were reported with the dosimeter exhibiting a fast response time against incident photons. This research aims to investigate the feasibility of a lead monoxide-based dosimeter for QA (quality assurance) in radiotherapy. In this paper, we report that the manufactured dosimeter shows similar linearity to a silicon diode and demonstrates similar characteristics in terms of PDD (percent depth dose) results for the thimble ionization chamber. Based on these results, it is demonstrated that the lead monoxide-based dosimeter complies with radiotherapy QA requirements, namely rapid response time, dose linearity, dose rate independence. Thus, we expect the lead monoxide-based dosimeter to be used commercially in the future.

A Comparative Study on Phytochemical And Biological Activities of two Grewia Species

Background Medicinal plants are currently in considerable significance view due to their special attributes as a large source of therapeutic phytochemicals that may lead to the development of novel drugs. Herbs are staging a comeback and herbal ‘renaissance’ is happening all over the globe. The herbal products today symbolize safety in contrast to the synthetics that are regarded as unsafe to human and environment. It has been estimated that in developed countries such as United States, plant drugs constitute as much as 25% of the total drugs, while in fast developing countries such as China and India, the contribution is as much as 80%. Thus, the economic importance of medicinal plants is much more to countries such as Nepal than to rest of the world. Inventorisation of herbal drugs used in traditional and modern medicines for a country like Nepal, appears to be a stupendous task, where a number of well established indigenous or traditional systems, including Ayurveda, Unani, Siddha, Homoeopathy, Tibetan, Amchi, Yoga and Naturopathy are practiced along with modern medicine for the management of total health care system.Method Classic techniques for the solvent extraction of active constituents from medicinal plant matrices are based on the choice of solvent coupled with the use of heat. Soxhlet extraction is a general and well-established technique, which surpasses in performance other conventional extraction techniques. : In this method, finely ground sample of Grewia species were placed in a porous bag or “thimble” made from a strong filter paper or cellulose, which is place, is in thimble chamber of the Soxhlet apparatus. Extraction solvents was heated in the bottom flask, vaporizes into the sample thimble, condenses in the condenser and drip back. When the liquid content reaches the siphon arm, the liquid contents emptied into the bottom flask again and the process is continued. The resultant extracts were used for qualitative phytochemical analysis by color reactions, antibacterial effects by disc diffusion method and Human RBC membrane stabilization method was used to test the anti-inflammatory potential of the phytoconstituents.Findings Two species Grewia asiatica and Grewia optiva (family: Tiliaceae) from genus Grewia were studied for their phytochemical and biological properties. The result showed that the extractive value of Grewia optiva was found to be greater (2.56 % in hexane and 7.17 % in methanol) than Grewia asiatica (n-hexane 2.24% and methanol 5.04%) respectively. The test for the phytoconstituents for both species showed the presence of glycosides, steroids, tannins, flavonoids, terpenoids and saponins. No any extract of both species at dose of (0.25mg, 0.5mg and 1mg per cup) possessed a zone of inhibition against tested bacterial strains. The potency of n-hexane extract and methanol extract of both species for Human RBC membrane stabilization test were compared with standard Diclofenac potassium. HRBC stabilization and resulted a good membrane stabilization of n-hexane extract of Grewia asiatica (40.89%, 73.57%, 78.23%, 80.07%, 80.91% and 51.95%) at (50 μg/ml, 100μg/ml, 200μg/ml, 400μg/ml, 600μg/ml and 800μg/ml) than methanol extract of both species. The result of the both species possessed similar phytochemicals and showed some membrane stabilization property which supports the traditional use.Conclusion Alkaloids were absent in the Grewia species extracts. The tested extracts have no any antibacterial effects against the tested bacterial stamps. The human RBC membrane stabilization tests were compared with standard Diclofenac potassium showed the good membrane stabilization of n-hexane extract of Grewia asiatica indicating the potent anti-inflammatory effect.Journal of Manmohan Memorial Institute of Health Sciences Vol. 2 2016 p.53-60

Verification of gamma knife based fractionated radiosurgery with newly developed head-thorax phantom

ObjectivePurpose of the study is to verify the Gamma Knife Extend™ system (ES) based fractionated stereotactic radiosurgery with newly developed head-thorax phantom. MethodsPhantoms are extensively used to measure radiation dose and verify treatment plan in radiotherapy. A human upper body shaped phantom with thorax was designed to simulate fractionated stereotactic radiosurgery using Extend™ system of Gamma Knife. The central component of the phantom aids in performing radiological precision test, dosimetric evaluation and treatment verification. A hollow right circular cylindrical space of diameter 7.0 cm was created at the centre of this component to place various dosimetric devices using suitable adaptors. The phantom is made of poly methyl methacrylate (PMMA), a transparent thermoplastic material. Two sets of disk assemblies were designed to place dosimetric films in (1) horizontal (xy) and (2) vertical (xz) planes. Specific cylindrical adaptors were designed to place thimble ionization chamber inside phantom for point dose recording along xz axis. EBT3 Gafchromic films were used to analyze and map radiation field. The focal precision test was performed using 4 mm collimator shot in phantom to check radiological accuracy of treatment. The phantom head position within the Extend™ frame was estimated using encoded aperture measurement of repositioning check tool (RCT). For treatment verification, the phantom with inserts for film and ion chamber was scanned in reference treatment position using X-ray computed tomography (CT) machine and acquired stereotactic images were transferred into Leksell Gammaplan (LGP). A patient treatment plan with hypo-fractionated regimen was delivered and identical fractions were compared using EBT3 films and in-house MATLAB codes. ResultsRCT measurement showed an overall positional accuracy of 0.265 mm (range 0.223 mm–0.343 mm). Gamma index analysis across fractions exhibited close agreement between LGP and film measured dose with ≥90% (max 93%) pixel pass rate at 1 mm of spatial and 1% of dosimetric tolerances. The focal precision test showed the variation of 0.465 mm between radiological and planned iso-centre. ConclusionsThe study demonstrated the suitability of newly developed head-thorax phantom for dosimetric verification of fractionated stereotactic radiosurgery using Extend™ system of Gamma Knife.

SU‐F‐T‐137: Out‐Of‐Beam Dose for a Compact Double‐Scattering Proton Beam Therapy System

Purpose:The out‐of‐beam dose is important for understanding the peripheral dose in radiation therapy. In proton radiotherapy, the study of out‐of‐beam dose is scarce and the treatment planning system (TPS) based on pencil beam algorithm cannot accurately predict the out‐of‐beam dose. This study investigates the out‐of‐beam dose for the single‐room Mevion S250 double scattering proton therapy system using experimentally measured and treatment planning software generated data. The results are compared with those reported for conventional photon beam therapy. However, this study does not incorporate the neutron contribution in the scattered dose.Methods:A total of seven proton treatment plans were generated using Varian Eclipse TPS for three different sites (brain, lung, and pelvis) in an anthropomorphic phantom. Three field sizes of 5×5, 10×10, and 20×20 cm2 (lung only) with typical clinical range (13.3–22.8 g/cm2) and modulation widths (5.3–14.0 g/cm2) were used. A single beam was employed in each treatment plan to deliver a dose of 181.8 cGy (200.0 cGy (RBE)) to the selected target. The out‐of‐beam dose was measured at 2.0, 5.0, 10.0, and 15.0 cm from the beam edge in the phantom using a thimble chamber (PTW TN31010).Results:The out‐of‐beam dose generally increased with field size, range, and volume irradiated. For all the plans, the scattered dose sharply fell off with distance. At 2.0 cm, the out‐of‐beam dose ranged from 0.35% to 2.16% of the delivered dose; however, the dose was clinically negligible (<0.3%) at a distance of 5.0 cm and greater. In photon therapy, the slightly greater out‐of‐beam dose was reported (TG36; 4%, 2%, and 1% for 2.0, 5.0, and 10.0 cm, respectively, using 6 MV beam).Conclusion:The measured out‐of‐beam dose in proton therapy excluding neutron contribution was observed higher than the TPS calculated dose and comparable to that of photon beam therapy.

TH-CD-BRA-10: Towards Reference Dosimetry of MR-Linacs Using a Clinical Probe-Format Calorimeter

Purpose: To evaluate the influence of a 1.5 T magnetic field (B-field) on the response of a small-scale graphite calorimeter probe (GPC) developed for use as a novel clinical reference dosimeter. Characterization of the GPC was also assessed in a hybrid MRI-linac (MRL) clinical prototype by performing absolute dosimetry in multiple detector orientations. Methods: B-field influence was characterized using a variable-strength electromagnet system located 280 cm from the source of a clinical linac. The GPC was used to perform a total of 160 absolute dose measurements (6 MV, 920 MU/min) in a water phantom placed between the poles of the electromagnet. The magnitude of the B-field between the poles was varied in the range of 0 – 1.5 T. The relative response of the GPC was determined and compared to that of a thimble type ionization chamber (Exradin A1SL, Standard Imaging). Next, 65 dose measurements were performed using the GPC in a clinical MRL field (7 MV, 620 MU/min) to quantify the rotational dependence of the detector in the presence of a 1.5 T B-field. The GPC was rotated in steps of 90° inside a graphite phantom (SSD 140 cm, depth 2.5 cm) for two detector orientations (parallel and perpendicular to the B field). Results: Relative to the zero B-field condition, the A1SL chamber exhibited an average overresponse of +1.2 % ± 0.03 % at a B-field of 1.5 T, while the GPC under-responded on average by −0.5 % ± 0.9 %. For the MRL measurements, no significant differences were observed between the parallel and perpendicular orientations. In both cases, a rotational dependence of approximately ±1 % was measured. Conclusion: This work suggests that the B-field has minimal influence on the response of the GPC, making it a potentially attractive solution for clinical MRL reference dosimetry. This work has been supported in part by the CREATE Medical Physics Research Training Network NSERC grant RGPIN 432290, as well as NSERC grants RGPIN 298191 & 435608. JR is a scholar from The Terry Fox Foundation Strategic Training Initiative for Excellence in Radiation Research for the 21st Century (EIRR21).

SU‐F‐T‐552: A One‐Year Evaluation of the QABeamChecker+ for Use with the CyberKnife System

Purpose:By attaching an adapter plate with fiducial markers to the QA BeamChecker+ (Standard Imaging, Inc., Middleton, WI), the output of the CyberKnife can be accurately, efficiently, and consistently evaluated. The adapter plate, known as the Cutting Board, allows for automated alignment of the QABC+ using the CK's stereoscopic kV image‐based treatment localization system (TLS). Described herein is an evaluation of the system following a year of clinical utilization.Methods:Based on a CT scan of the QABC+ and CB, a treatment plan is generated which delivers a beam to each of the 5 plane‐parallel ionization chambers. Following absolute calibration of the CK, the QA plan is delivered, and baseline measurements are acquired (and automatically corrected for temperature and pressure). This test was performed at the beginning of each treatment day for a year. A calibration evaluation (using a water‐equivalent slab and short thimble chamber) is performed every four weeks, or whenever the QABC+ detects a deviation of more than 1.0%.Results:During baseline evaluation, repeat measurements (n=10) were performed, with an average output of 0.25% with an SD of 0.11%. As a test of the reposition of the QABC+ and CB, ten additional measurements were performed where between each acquisition, the entire system was removed and re‐positioned using the TLS. The average output deviation was 0.30% with a SD of 0.13%. During the course of the year, 187 QABC+ measurements and 13 slab‐based measurements were performed. The output measurements of the QABC+ correlated well with slab‐based measurements (R2=0.909).Conclusion:By using the QABC+ and CB, daily output was evaluated accurately, efficiently, and consistently. From setup to break‐down (including analysis), this test required 5 minutes instead of approximately 15 using traditional techniques (collimator‐mounted ionization chambers). Additionally, by automatically saving resultant output deviation to a database, trend analysis was simplified.Spectrum Medical Physics, LLC of Greenville, SC has a consulting contract with Standard Imaging of Middleton, WI

Quantitative performance characterization of image quality and radiation dose for a CS 9300 dental cone beam computed tomography machine

This paper aims to characterize the radiation dose and image quality (IQ) performance of a dental cone beam computed tomography (CBCT) unit over a range of fields of view (FOV). IQ and dose were measured using a Carestream 9300 dental CBCT. Phantoms were positioned in the FOV to imitate clinical positioning. IQ was assessed by scanning a SEDENTEXCT IQ phantom, and images were analyzed in ImageJ. Dose index 1 was obtained using a thimble ionization chamber and SEDENTEXCT DI phantom. Mean gray values agreed within 93.5% to 99.7% across the images, with pixel-to-pixel fluctuations of 6% to 12.5%, with poorer uniformity and increased noise for child protocols. CNR was fairly constant across FOVs, with higher CNR for larger patient settings. The measured limiting spatial resolution agreed well with 10% MTF and bar pattern measurements. Dose was reduced for smaller patient settings within a given FOV; however, smaller FOVs obtained with different acquisition settings did not necessarily result in reduced dose. The use of patient-specific acquisition settings decreased the radiation dose for smaller patients, with minimal impact on the IQ. The full set of IQ and dose measurements is reported to allow dental professionals to compare the different FOV settings for clinical use.

High spatial resolution dosimetric response maps for radiotherapy ionization chambers measured using kilovoltage synchrotron radiation

Small circular beams of synchrotron radiation (0.1 mm and 0.4 mm in diameter) were used to irradiate ionization chambers of the types commonly used in radiotherapy. By scanning the chamber through the beam and measuring the ionization current, a spatial map of the dosimetric response of the chamber was recorded. The technique is able to distinguish contributions to the large-field ionization current from the chamber walls, central electrode and chamber stem. Scans were recorded for the NE 2571 Farmer chamber, the PTW 30013, IBA FC65-G Farmer-type chambers, the NE 2611A and IBA CC13 thimble chambers, the PTW 31006 and 31014 pinpoint chambers, the PTW Roos and Advanced Markus plane-parallel chambers, and the PTW 23342 thin-window soft x-ray chamber. In all cases, large contributions to the response arise from areas where the incident beam grazes the cavity surfaces. Quantitative as well as qualitative information about the relative chamber response was extracted from the maps, including the relative contribution of the central electrode. Line scans using monochromatic beams show the effect of the photon energy on the chamber response. For Farmer-type chambers, a simple Monte Carlo model was in good agreement with the measured response.

A new technique to characterize CT scanner bow‐tie filter attenuation and applications in human cadaver dosimetry simulations

To present a noninvasive technique for directly measuring the CT bow-tie filter attenuation with a linear array x-ray detector. A scintillator based x-ray detector of 384 pixels, 307 mm active length, and fast data acquisition (model X-Scan 0.8c4-307, Detection Technology, FI-91100 Ii, Finland) was used to simultaneously detect radiation levels across a scan field-of-view. The sampling time was as short as 0.24 ms. To measure the body bow-tie attenuation on a GE Lightspeed Pro 16 CT scanner, the x-ray tube was parked at the 12 o'clock position, and the detector was centered in the scan field at the isocenter height. Two radiation exposures were made with and without the bow-tie in the beam path. Each readout signal was corrected for the detector background offset and signal-level related nonlinear gain, and the ratio of the two exposures gave the bow-tie attenuation. The results were used in the geant4 based simulations of the point doses measured using six thimble chambers placed in a human cadaver with abdomen/pelvis CT scans at 100 or 120 kV, helical pitch at 1.375, constant or variable tube current, and distinct x-ray tube starting angles. Absolute attenuation was measured with the body bow-tie scanned at 80-140 kV. For 24 doses measured in six organs of the cadaver, the median or maximum difference between the simulation results and the measurements on the CT scanner was 8.9% or 25.9%, respectively. The described method allows fast and accurate bow-tie filter characterization.

Computed tomography dosimetry with high-resolution detectors commonly used in radiotherapy - an energy dependence study.

New methods of dosimetry in computed tomography (CT) X‐ray fields require the use of high‐resolution detectors instead of pencil‐type ionization chambers typically used for CT dose index (CTDI) measurements. This paper presents a study on the suitability of a wide range of ionization chambers, diodes, and a two‐dimensional detector array, used primarily in radiation therapy, for CT and cone‐beam CT dosimetry. Specifically, the energy dependence of these detectors from 50 kVp up to 125 kVp is reported. All measurements were performed in reference to a calibrated diode for use in this energy region. The radiation quality correction factors provided by the manufacturer were used, depending on the measured half‐value layer (HVL) for the particular X‐ray beam. Our study demonstrated the general usability of thimble ionization chambers. These thimble ionization chambers showed a maximum variation in energy response of 5%. Ionization chambers with even smaller sensitive volume, and which exhibit similar variation in energy dependence, can be used if higher spatial resolution is required. Furthermore, the investigated detectors are better suited for dosimetry at CT and CBCT units than conventional large volume or flat detectors, due to their rotational symmetry. Nevertheless, a flat detector can be used for certain measurement tasks, such as the acquisition of percent depth‐dose curves or beam profiles for nonrotating beams, which are important for beam characterization.PACS numbers: 87.57.uq, 87.56.Da, 87.57.Q‐

On the nuclear halo of a proton pencil beam stopping in water

The dose distribution of a proton beam stopping in water has components due to basic physics and may have others from beam contamination. We propose the concise terms core for the primary beam, halo (see Pedroni et al Phys. Med. Biol. 50 541–61) for the low dose region from charged secondaries, aura for the low dose region from neutrals, and spray for beam contamination.We have measured the dose distribution in a water tank at 177 MeV under conditions where spray, therefore radial asymmetry, is negligible. We used an ADCL calibrated thimble chamber and a Faraday cup calibrated integral beam monitor so as to obtain immediately the absolute dose per proton. We took depth scans at fixed distances from the beam centroid rather than radial scans at fixed depths. That minimizes the signal range for each scan and better reveals the structure of the core and halo.Transitions from core to halo to aura are already discernible in the raw data. The halo has components attributable to coherent and incoherent nuclear reactions. Due to elastic and inelastic scattering by the nuclear force, the Bragg peak persists to radii larger than can be accounted for by Molière single scattering. The radius of the incoherent component, a dose bump around midrange, agrees with the kinematics of knockout reactions.We have fitted the data in two ways. The first is algebraic or model dependent (MD) as far as possible, and has 25 parameters. The second, using 2D cubic spline regression, is model independent. Optimal parameterization for treatment planning will probably be a hybrid of the two, and will of course require measurements at several incident energies.The MD fit to the core term resembles that of the PSI group (Pedroni et al ), which has been widely emulated. However, we replace their T(w), a mass stopping power which mixes electromagnetic (EM) and nuclear effects, with one that is purely EM, arguing that protons that do not undergo hard single scatters continue to lose energy according to the Beth-Bloch formula. If that is correct, it is no longer necessary to measure T(w), and the dominant role played by the ‘Bragg peak chamber’ vanishes.For mathematical and other details we will refer to Gottschalk et al (, arXiv: 1409.1938v1), a long technical report of this project.

TH‐EF‐BRA‐10: Monte Carlo Simulation of CT Dose Index and Equilibrium Dose‐Pitch Product: Effect of Ion Chamber Model On Simulated Dose Response

Purpose:In Monte Carlo simulations of patient dose from a CT scan, accurate knowledge about the x‐ray energy spectra is essential. Simulated CT dose index (CTDI₁₀₀) free‐in‐air is often used together with measured values to calibrate the intensity of the spectra. To simulate CTDI₁₀₀, a computational model of the ion chamber is required. Various chamber modeling methods have been reported. The purpose of this study was to investigate systematically how chamber modeling affects simulated dose response. Both the pencil chamber and the thimble chamber recently recommended by AAPM TG111 were studied.Methods:A Monte Carlo program previously validated for a clinical CT system (SOMATOM Definition Flash, Siemens Healthcare) was used. To examine the effect of chamber modeling, a highly detailed model of an actual CTDI₁₀₀ pencil chamber (model 10×5−3CT, Radcal Corporation) was first defined. Five additional models with reduced detail were then created. They differed from the first model in terms of their dimensions, component parts, material composition, and the use of quadric or voxel geometry. Two models of a TG111 thimble chamber were also created, representing a highly detailed and a rather crude rendition of an actual thimble chamber (model 10×0.6−3CT, Radcal Corporation). Single axial scans for CTDI₁₀₀ chamber models and helical scans for TG111 chamber models were simulated free‐in‐air at 70 and 120 kVp. Simulated dose responses were then compared amongst different chamber models.Results:For the six models of the CTDI₁₀₀ pencil chamber, the coefficient of variation of the simulated CTDI₁₀₀ was 0.9% at 70 kVp and 1.0% at 120 kVp. For the two models of the TG111 thimble chamber, the difference in the simulated equilibrium‐dose‐pitch product was 1.9% at 70 kVp and 2.1% at 120 kVp.Conclusion:In Monte Carlo simulations of CT ion chambers, detailed modeling of chamber geometry and material composition is not necessary.This research is supported in part by a Faculty Startup Fund from Cleveland State University.

SU‐E‐T‐480: IMRT/VMAT QA Employing a High Resolution Measurement Array: Initial Experience and Limitations

Purpose:Patient specific QA for IMRT/VMAT is time‐consuming. A high resolution ionization chamber array was employed which rotates synchronously with the gantry. Commissioning, initial experience and limitations of this measurement technique will be discussed.Methods:A high resolution ionization chamber array, Octavius1500 (PTW‐Freiburg, Germany), was employed with the Octavius4D rotating cylindrical phantom. Measured plans are reconstructed into a 3D dose distribution using measured depth dose curves (Verisoft 6.0 PTW‐Freiburg). Plans of varying complexity were selected from our patient collective treated at a TrueBeam 1.5 linac (Varian Medical Systems) with HD120 MLC (VMS). Cross‐calibration was performed using a thimble chamber (30016, PTW‐Freiburg) placed within the cylindrical phantom (SSD 840mm, SCD 1000mm). All dose distributions were recalculated using a 1.5mm dose grid (Eclipse, AAA10.0.28/Acuros10.0.28, VMS) using a virtual homogeneous cylindrical phantom provided by the array manufacturer (density 1.016 gcm‐3). All analyses were performed using Verisoft and compared to routine QA analysis (Octavius729 with octagonal phantom) results.Results:Depth dose curves from 1cm × 1cm to 26cm × 26cm were measured for 3D dose calculation commissioning in Verisoft 6.0. Excellent agreement was observed between TPS PDD and PDD derived from the Octavius1500 array measurement. For highly modulated plans the overall 3D gamma (2.0mm, 3.0%) ranged between 91.3% to 96.9%. The Octavius729 (2D) array results ranged from 97.9 to 100.0% for PTVs.Conclusion:The Octavius1500 and 4D cylindrical phantom provides an integrated QA solution. Complex VMAT plans, comprising multiple PTVs, can be verified with a single (rotating) measurement of all arcs. Hitherto two measurements (horizontal and vertical) had to be performed per PTV. Overall the results are good. However, some refinement may need to be made to the reconstruction. Further work needs to be done to determine appropriate action levels for routine analysis.

SU-E-T-283: Dose Perturbations Near Heterogeneity Junctions for Modulated-Scanning Protons

Purpose: To compare calculated and measured doses near heterogeneity junctions of tissue-substitute materials for modulated-scanning protons. Methods: Three heterogeneous phantoms were configured using slabs of various plastics to simulate lung, fat, soft-tissue (polystyrene), and bone with known relative linear stopping powers (RLSPs). Each phantom consisted of soft-tissue and a single heterogeneity of a 5 or 10 cm thickness of a non-soft-tissue material. CT images were loaded into a Syngo treatment planning system and each material contoured and assigned its RLSP. Planning target volumes (PTVs) were drawn such that a beam would partially traverse the heterogeneity and partially only soft-tissue. Lateral profiles were measured using EDR2 films at a minimum of six depths between the phantom surface and the depth corresponding to the beam range. Absolute doses were measured inside and distal to the PTV in all phantoms using either a parallel plate or thimble chamber. Additional dose measurements were made between two lung slabs. Results: Profiles measured by film generally agreed with calculations except for depths distal to lung and fat junctions. Measured lateral penumbras for depths at the distal junction of lung were found to be wider than calculated ones. Compared with calculated doses, measured doses in the PTVs were 5.19% and 2.51% lower for lung and fat respectively but for bone were 0.2% higher. Measured doses for depths distal to the PTV were up to 29.65% and 10.58% higher for lung and fat, respectively but 6.30% lower for bone. Conclusion: The low measured doses in the PTVs for lung and fat might be due to underestimation of lateral scattering of protons. The higher measured doses distal to the PTV for the lung and fat are a Result of a shortened calculated beam range whereas the higher dose distal to the bone junction is within uncertainties.

CT dose equilibration and energy absorption in polyethylene cylinders with diameters from 6 to 55 cm

ICRU Report No. 87 Committee and AAPM Task Group 200 designed a three-sectional polyethylene phantom of 30 cm in diameter and 60 cm in length for evaluating the midpoint dose DL(0) and its rise-to-the-equilibrium curve H(L) = DL(0)/D(eq) from computed tomography (CT) scanning, where D(eq) is the equilibrium dose. To aid the use of the phantom in radiation dose assessment and to gain an understanding of dose equilibration and energy absorption in polyethylene, the authors evaluated the short (20 cm) to long (60 cm) phantom dose ratio with a polyethylene diameter of 30 cm, assessed H(L) in polyethylene cylinders of 6-55 cm in diameters, and examined energy absorption in these cylinders. A GEANT4-based Monte Carlo program was used to simulate the single axial scans of polyethylene cylinders (diameters 6-55 cm and length 90 cm, as well as diameter 30 cm and lengths 20 and 60 cm) on a clinical CT scanner (Somatom Definition dual source CT, Siemens Healthcare). Axial dose distributions were computed on the phantom central and peripheral axes. An average dose over the central 23 or 100 mm region was evaluated for modeling dose measurement using a 0.6 cm(3) thimble chamber or a 10 cm long pencil ion chamber, respectively. The short (20 cm) to long (90 cm) phantom dose ratios were calculated for the 30 cm diameter polyethylene phantoms scanned at four tube voltages (80-140 kV) and a range of beam apertures (1-25 cm). H(L) was evaluated using the dose integrals computed with the 90 cm long phantoms. The resultant H(L) data were subsequently used to compute the fraction of the total energy absorbed inside or outside the scan range (E(in)/E or E(out)/E) on the phantom central and peripheral axes, where E = LD(eq) was the total energy absorbed along the z axis. The midpoint dose in the 60 cm long polyethylene phantom was equal to that in the 90 cm long polyethylene phantom. The short-to-long phantom dose ratios changed with beam aperture and phantom axis but were insensitive to tube voltage. H(L) was insensitive to tube voltage and CT scanner model. As phantom diameter increased from 6 to 55 cm, E(in)/E generally decreased but asymptotically approached constant levels on the peripheral axes of large phantoms. The curve of E(in)/E versus scan length was almost identical to that of H(L). Similarly, E(out)/E increased with scan length and asymptotically approached the equilibrium for large scan lengths. E(out)/D(eq) was much less than the equilibrium length L(eq) where H(L) = 0.98, even with scan lengths much larger than L(eq). The polyethylene phantom designed by ICRU Report No. 87 Committee and AAPM Task Group 200 is adequately long for assessing the midpoint dose and its equilibration in CT scanning. The short-to-long phantom dose ratios and the H(L) data provided in this paper allow easy evaluations of the midpoint dose, longitudinal dose distribution, and energy absorption in polyethylene phantoms. The results of dose equilibration and energy absorption presented herein may be insightful for the clinical CT scans with various subject sizes and scan lengths.