Center Of Phantom Research Articles

SU‐E‐T‐188: An Effective Quality Assurance Method for TomoTherapy Craniospinal Irradiation Patients with A 3D Semiconductor Phantom

Purpose: This study aims to develop an effective and efficient approach to perform delivery quality assurance (DQA) to any large length spine in Tomo craniospinal irradiation (CSI) with a 3D semiconductor phantom. Methods: It is challenging to use conventional method to perform DQA to an entire large length of spine because the Tomo machine limits the maximum longitudinal distance between red and green laser (=18 cm). In order to overcome this limit, three DQA plans for each CSI patient were generated for covering the entire treatment site, in which we intentionally overlapped the red and green laser in DQA calculation, then we measured the distance (d) between the centers of the phantom and lasers. When using SunNucelar QA software to compute the plannar dose, we set the center of the cylindrical surface by applying d to the IEC Z coordinate. In phantom setup, we first align the center of phantom with green laser then manually shift the phantom superiorly or inferiorly d. With this new approach, we measured five CSI patients using a commercial 3D semiconductor phantom, the ArcCHECK (SunNuclear). The measured and computed dose is compared with gamma analysis and (3%, 3 mm) agreement criteria. Results: For all CSI patients and sections measured, the acquired dose distribution covered the complete treatment length and matched the computed data very well. All the DQA passed the gamma criteria with an average passing rate of 99.1%. The total delivery time of DQA using the new method was 66.7% of the time using the conventional method. Conclusion: The measured DQA results from the five CSI patients demonstrate that this new method overcomes the distance limitation of 18 cm between the red and green laser in the conventional DQA method. The DQA delivery becomes more efficient with 33.3% reduction of beam‐on time.

SU‐E‐I‐66: Quality Assurance Program for Functional MRI

Purpose: To develop and implement an automatic algorithm for quantitatively analyzing image quality parameters in functional MRI (fMRI) studies. The image quality parameters include signal stability over time, signal‐to‐noise ratio (SNR), and ghost levels. Methods: A set of images (9 slices, 512 images per slice over a time of 8′32′) in fMRI was acquired from a spherical quality assurance (QA) phantom using a gradient‐echo echo planar imaging (EPI) sequence at 3T. For a given slice, the center of phantom was determined automatically by calculating the center of the “mass”, followed by an automatic edge‐detection algorithm to outline the phantom edge. The signal intensity in the central region of the phantom was displayed as a function of time to assess the signal stability. After a mask for the phantom was determined, the mask was shifted by 1/2 of the FOV along the phase‐encoding direction, producing a mask for the EPI ghost. The ghost mask was sub‐divided into four equal quadrants, and the average and maximum ghost intensities were evaluated in each quadrant. Ghost levels within the four quadrants were compared to infer specific root causes of the ghosts. Finally, a background noise region was automatically determined. The standard deviation and mean intensity in the noise region were used to evaluate the SNR and ghost percentage according to the ACR recommendations. Results: The above algorithm was successfully implemented in an open‐course JAVA script. The signal stability and ghost level in fMRI QA scans were automatically visualized without user intervention. Further, ghost characteristics in the four quadrants provided valuable information to reveal the causes of the ghosts, including B0‐eddy currents, linear eddy currents, and cross‐term eddy currents. Conclusion: We have developed and implemented an automated algorithm for QA analysis in fMRI studies. This algorithm is open‐source, and can be downloaded through a website.

TU‐G‐103‐09: Measurement of Planar Average Equilibrium Dose of CT

Purpose: The planar average equilibrium dose of CT is measured using the methodology suggested by AAPM report No. 111, and compared with the traditional weighted average estimate and the new proposed equal weighted average estimate. Methods: The study used a 320 slices Toshiba Aquilion ONE scanner, three CTDI body phantoms with total length of 46.5 cm (two of them are nested 3 piece CTDI phantoms with diameters of 10, 16, and 32 cm for each piece), a Radcal Accu‐Pro electrometer, and a Radcal 0.6 cc farmer chamber. In the helical mode (120 kVp, 200 mAs, nominal beam width 0.5×16 mm, pitch 0.938), we measured the cumulative dose DL(z=0) at the center of the phantoms and 12 clock position at radius of 4, 7, and 15 cm from the center, for scan length L=100, 200, 300, and 400 mm. The procedure was repeated for pitch setting of 0.641 and 1.438 and nominal beam width of 0.5×160 mm and 0.5×64 mm at L=400 mm. Results: The data obtained at four scan lengths were fit to estimate the equilibrium Dose (Deq) at radius of 0, 4, 7 and 15 cm from the phantom center. Three models (linear, quadratic, and exponential) were then used to fit DL(z=0) and Deq as a function of radius to phantom center for various scan length, pitch, and beam width combinations. We calculated the planar average cumulative (and equilibrium) dose using the above models and compared the results with the traditional weighted average estimate (Dcenter/3+2Dedge/3) and the new proposed equal weighted average estimate (Dcenter/2+Dedge/2). The planar average dose differs from the traditional weighted estimate within 2%, and equal weighted estimate within 10%. Conclusion: The traditional weighted average method provides a more accurate estimate to the CT planar average equilibrium dose than the proposed equal weighted average method.

SU‐E‐I‐49: Influence of the Phantom Length in Computed Tomography Dose Profile Dosimetry

Purpose: We measured the computed tomography (CT) dose profile with CT dose phantom for several X‐ray beam widths on 64‐slice, and 320‐slice multi detector‐row CT (MDCT) using micro ionization chamber, and assessed the association of the CTDI100 value with CT dose phantom length. Methods: The CT dose profile (CTDP) was measured as Dm using radiation monitor (Model 9015, Radcal, Monrovia, CA) with micro ionization chamber (10X5‐0.18, Radcal, Monrovia, CA) which have an active length of 19 mm and a 0.18 cm3 active area on 64‐slice MDCT (Aquilion CX, Toshiba Medical Systems, Tochigi, Japan), and 320‐slice MDCT (Aquilion One, Toshiba Medical Systems, Tochigi, Japan). The MDCT systems were used with a tube voltage of 120 kV, tube current of 100 mA, rotation time of 1 second, and nominal X‐ray beam widths of 4, 12, 32, and 160 mm (4*1 mm, 4*3 mm, 64*0.5 mm, and 320*0.5 mm axial slice acquisitions, respectively). A micro ionization chamber was placed in the center of the single CT dose phantom, and we acquired several axial scans as 5 mm increments of phantom length. Then two, and three coupled dose phantom were placed together to simulate a 300‐mm‐long, and 450‐mm‐long phantom. We acquired several axial scans in similar method. Results: The observed percent increase of the Dm with extended length from 150 mm to 300 mm of the CT dose phantom was 120 % at range from −60 to 60 mm. However, it was greatly increased toward peripheral area of the DP. With extended length from 300 mm to 450 mm of CT dose phantom, the observed percent increase of the Dm was slightly increased. There was no difference in the form of the CTDP between 300‐mm‐long phantom and 450‐mm‐long phantom. Conclusion: In upward of the 300‐mm‐long CT dose phantom, there is no difference in the form of the CTDP.

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.

SU‐E‐J‐160: A Simple Technique for Quick In‐Vivo Dose Measurements for KV CBCT and Diagnostic X‐Ray Planar Imaging Using MicroMOSFET Dosimeters

Purpose: To present a practical method for dose measurement of kV CBCT scans and diagnostic X‐ray planar imaging according to typical IGRT in radiotherapy and chest and pelvis diagnostic imaging protocols by using microMOSFET dosimeters. Once calibrated, in‐vivo measurements can be easily performed on patients on surface and/or in internal cavities. Methods: The calibration procedure uses two ADCL calibrated ion chambers for diagnostic kV X‐ray beams, one 6 cc farm chamber and one large parallel plane chamber for absolute dose references, and the microMOSFET with high sensitivity settings for calibration in air and in phantom. The calibration covered the possible dose range from 1 to 25 mGy. The calibrated dosimeters were then placed at anterior, posterior, right lateral and center of an anthromorphic phantom to measure the dose for CBCT scans using the appropriate calibration file according to kVp and dose range. Also, the dosimeters were placed inside two cadavers for internal dose measurement for typical x‐ray imaging. Results: Our measured results showed that the dose from CBCT scans were typically 16% ‐ 39% higher than the nominal dose presented by the vendor per protocol. The dose from 4D CBCT scan, 2.16 cGy, was approximately 39.4% higher than the nominal dose of 1.55 cGy. The Prostate M10 scan of 3.458 cGy was approximately 16.8% higher than the nominal dose of 2.96 cGy. The Pelvis/Chest M20 scan 2.972 cGy, was approximately 35.1% higher than the nominal dose of 2.2 cGy. Large variation was observed for planar X‐ray imaging due to use of shielding and tissue attenuations. Conclusion: The ease of implementation and using microMOSFETs for CBCT scans and X‐ray imaging allows quick and accurate in‐vivo dose measurements. Vendor specified doses are verified by this technique and should be added to increase patient quality assurance and safety.

SU-D-500-07: Evaluation of the Spatial Resolution of Proton/carbon Computed Tomography Using Modulation Transfer Function

Purpose: To characterize the modulation transfer function (MTF) of proton/carbon computed tomography using Geant4 Monte Carlo simulations. To assess the spatial resolution of proton/carbon computed tomography (pCT/cCT). Methods: A cylindrical water (1.0 g/cm3) phantom was specifically modeled with a length of 20 cm and a diameter of 20 cm. Its inserts were composed of bone and lung tissue each with two different densities: bone (1.58 g/cm3), bone (1.29 g/cm3), lung (0.6 g/cm3), and lung (0.3 g/cm3). These inserts were modeled as 10 cm × 16 mm × 8 mm boxes located in the center of the water phantom. The energy of the beams were tested at 230 MeV and 330 MeV for protons and at 430 MeV per nucleon for carbon. The contrast‐to‐noise (CNR) ratio was evaluated along a line‐of‐interest (LOI) across each water‐bone/water‐lung interface in order to ultimately characterize the spatial resolution by the 10% point of the modulation transfer function (MTF10% or MTF10) in units of line‐pairs per mm (lp/mm). Results: The MTF was generated for interfaces of water‐bone(1.58 g/cm3), water‐bone(1.29 g/cm3), water‐lung(0.6 g/cm3), and water‐lung(0.3 g/cm3). Preliminary results indicate that, at 330 MeV, pCT can distinguish the above interfaces at 2.2 lp/mm, 2.5 lp/mm, 2.3 lp/mm and 2.5 lp/mm. These values are consistent with a previous study done for proton/carbon radiography [1], in which the average resolution was 2.3 lp/mm for bone and 2.4 lp/mm for lung. Conclusion: The sub‐millimeter resolution of pCT offers clinicians a potential tool in specific tumor diagnostics (such as in lung cancer), patient‐setup for daily proton therapy, the reduction of absorbed dose relative to photon imaging, and the reduction of range uncertainties in proton/carbon therapy.

TH‐C‐144‐02: Experimental Study of Discrete Prompt Gamma Lines for In‐Vivo Proton Range Verification

Purpose: A novel method for in vivo proton beam range verification is presented, using energy‐resolvable proton‐induced prompt gamma lines. Methods: Prompt gamma‐rays emitted near the end‐of‐range of a clinical proton beam originate from transitions between the lower states of excited nuclear reaction products. We developed a prompt gamma‐ray detection system with the aim of resolving the resulting discrete gamma lines. The ability to quantify the strength of these lines can provide valuable information for in vivo range verification: known cross sections may be used as prior knowledge, and uncertainties resulting from differences in tissue composition could be reduced. A LaBr3:Ce scintillation detector was used, which provides high energy resolution. The pulses from the photomultiplier tube were acquired with analog‐to‐digital converters at high sampling rates, synchronized to the cyclotron radiofrequency to separate the quasi‐continuous neutron‐induced background from the prompt gamma signals of interest. Results: Preliminary measurements were performed with a 150 MeV proton pencil‐beam targeted on the center of a water phantom with a 15 cm diameter. A 1 cm lead slit collimator was used perpendicular to the beam direction, centered at the end‐of‐range. The acquired gamma spectra show the 6.13 MeV gamma line from oxygen‐16, and its escape peak, can be clearly resolved with an energy resolution of ∼1% FWHM. Conclusion: Our results show the feasibility of measuring discrete prompt gamma lines using a practical detection system. These gamma lines can be attributed to specific nuclear reactions with established properties, potentially enabling accurate proton range verification using a small number of measurement points. Experiments with different collimators and proton energies are currently being performed to determine an optimal configuration for range verification.

SU‐E‐I‐21: Metal Artifact Redcution in CT for Surgical Screws: Comparison Between Two Commercially Available Software Packages

Purpose: To compare the performance between two available metal artifact reduction software (MAR) packages from different CT vendors to determine applicability for surgical screws, even though such use is not supported. Methods: Two vertebral screws (Al and stainless steel) were placed in the center of a water phantom (20 cm × 15 cm), with long axis perpendicular to the scan direction. Scans were performed using a GE Discovery HD750 with Dual Energy (DE) and MARS software, and Philips iCT with iDose4 and OMAR software. Reconstruction was performed with and without MAR. Assessment was performed qualitatively and quantitatively by measuring the SD in areas surrounding the screws. For the GE scanner, mono‐energy reconstructions were performed for 70, 127, and 140 keV in addition to the MARS routine. Thickness of the screws was also measured and compared to the expected size. Results: DE reduced streak artifacts and MARS further reduced artifacts for both screws. However, a portion of the middle was distorted. SD of region next to the screw tips was reduced up to 97%; whereas SD in the region under the long axis increased in some MARS images by more than 50%. O‐MAR reduced streak artifacts but less effectively than MARS but with no loss of the middle section. With OMAR, in general most images showed reduced SD in both regions by 2% to 54%. Additionally, width was significantly reduced (20% to 50%) by MARS. This effect was not noted with O‐MAR. Conclusion: Performance of MARS and O‐MAR differed considerably. MARS effectively reduced streaks along the long axis but introduced distortions in regions near the center of the screw. With O‐MAR, streaks were reduced by a lesser degree than MARS along the screws axis, but by higher degree above and below the screws with no distortion at the center.

Shielding implications for secondary neutrons and photons produced within the patient during IMPT

Intensity modulated proton therapy (IMPT) uses a combination of computer controlled spot scanning and spot-weight optimized planning to irradiate the tumor volume uniformly. In contrast to passive scattering systems, secondary neutrons and photons produced from inelastic proton interactions within the patient represent the major source of emitted radiation during IMPT delivery. Various published studies evaluated the shielding considerations for passive scattering systems but did not directly address secondary neutron production from IMPT and the ambient dose equivalent on surrounding occupational and nonoccupational work areas. Thus, the purpose of this study was to utilize Monte Carlo simulations to evaluate the energy and angular distributions of secondary neutrons and photons following inelastic proton interactions within a tissue-equivalent phantom for incident proton spot energies between 70 and 250 MeV. Monte Carlo simulation methods were used to calculate the ambient dose equivalent of secondary neutrons and photons produced from inelastic proton interactions in a tissue-equivalent phantom. The angular distribution of emitted neutrons and photons were scored as a function of incident proton energy throughout a spherical annulus at 1, 2, 3, 4, and 5 m from the phantom center. Appropriate dose equivalent conversion factors were applied to estimate the total ambient dose equivalent from secondary neutrons and photons. A reference distance of 1 m from the center of the patient was used to evaluate the mean energy distribution of secondary neutrons and photons and the resulting ambient dose equivalent. For an incident proton spot energy of 250 MeV, the total ambient dose equivalent (3.6 × 10(-3) mSv per proton Gy) was greatest along the direction of the incident proton spot (0°-10°) with a mean secondary neutron energy of 71.3 MeV. The dose equivalent decreased by a factor of 5 in the backward direction (170°-180°) with a mean energy of 4.4 MeV. An 8 × 8 × 8 cm(3) volumetric spot distribution (5 mm FWHM spot size, 4 mm spot spacing) optimized to produce a uniform dose distribution results in an ambient dose equivalent of 4.5 × 10(-2) mSv per proton Gy in the forward direction. This work evaluated the secondary neutron and photon emission due to monoenergetic proton spots between 70 and 250 MeV, incident on a tissue equivalent phantom. Example calculations were performed to estimate concrete shield thickness based upon appropriate workload and shielding design assumptions. Although lower than traditional passive scattered proton therapy systems, the ambient dose equivalent from secondary neutrons produced by the patient during IMPT can be significant relative to occupational and nonoccupational workers in the vicinity of the treatment vault. This work demonstrates that Monte Carlo simulations are useful as an initial planning tool for studying the impact of the treatment room and maze design on surrounding occupational and nonoccupational work areas.

A study of CT dose distribution in an elliptical phantom and the influence of automatic tube current modulation in the x–y plane

Computed tomography (CT) performance assessments relating to patient dose to the body are made conventionally in 320 mm diameter cylindrical acrylic phantoms. The cross section of the human trunk is closer to an ellipse and automatic tube current modulation (ATCM) systems adjust the exposure level with orientation in the x–y plane, changing the dose distribution within the body. This study has investigated differences in the distributions of dose within a standard cylindrical body phantom and an elliptical dosimetry phantom for Toshiba, General Electric and Philips CT scanners, and recorded changes with the application of the ATCM. Single slice dose profiles have been recorded within the phantoms using Gafchromic film. CT dose indices along 100 mm lengths have been calculated and data sets combined to simulate helical scans, from which values for cumulative doses have been derived. The doses in the centre of the elliptical phantom are 70–100% larger than for the cylindrical one and in the anterior are around 20–40% larger, while the doses in the lateral positions are similar for the two phantom shapes. The differences between the anterior and lateral doses were larger for the Toshiba scanner and this is thought to be linked to the narrower profile of the beam produced by the bow-tie filter. When the ATCM mode for the Toshiba scanner is implemented, the doses in the anterior and posterior positions are reduced preferentially, bringing them closer to the doses in the lateral positions.

An investigation of image guidance dose for breast radiotherapy

Cone‐beam computed tomography (CBCT) is used for external‐beam radiation therapy setup and target localization. As with all medical applications of ionizing radiation, radiation exposure should be managed safely and optimized to achieve the necessary image quality using the lowest possible dose. The present study investigates doses from standard kilovoltage kV radiographic and CBCT imaging protocol, and proposes two novel reduced dose CBCT protocols for the setup of breast cancer patients undergoing external beam radiotherapy. The standard thorax kV and low‐dose thorax CBCT protocols available on Varian's On‐Board Imaging system was chosen as the reference technique for breast imaging. Two new CBCT protocols were created by modifying the low‐dose thorax protocol, one with a reduced gantry rotation range (“Under breast” protocol) and the other with a reduced tube current‐time product setting (“Low dose thorax 10ms” protocol). The absorbed doses to lungs, heart, breasts, and skin were measured using XRQA2 radiochromic film in an anthropomorphic female phantom. The absorbed doses to lungs, heart, and breasts were also calculated using the PCXMC Monte Carlo simulation software. The effective dose was calculated using the measured doses to the included organs and the ICRP 103 tissue weighting factors. The deviation between measured and simulated organ doses was between 3% and 24%. Reducing the protocol exposure time to half of its original value resulted in a reduction in the absorbed doses of the organs of 50%, while the reduced rotation range resulted in a dose reduction of at least 60%. Absorbed doses obtained from “Low dose thorax 10ms” protocol were higher than the doses from our departments orthogonal kV‐kV imaging protocol. Doses acquired from “Under breast” protocol were comparable to the doses measured from the orthogonal kV‐kV imaging protocol. The effective dose per fraction using the CBCT for standard low‐dose thorax protocol was 5.00±0.30 mSv; for the “Low dose thorax 10ms” protocol it was 2.44±0.21 mSv; and for the “Under breast” protocol it was 1.23±0.25 mSv when the image isocenter was positioned at the phantom center and 1.17±0.30 mSv when the image isocenter was positioned in the middle of right breast. The effective dose per fraction using the orthogonal kV‐kV protocol was 1.14±0.16 mSv. The reduction of the scan exposure time or beam rotation range of the CBCT imaging significantly reduced the dose to the organs investigated. The doses from the “Under breast” protocol and orthogonal kV‐kV imaging protocol were comparable. Simulated organ doses correlated well with measured doses. Effective doses from imaging techniques should be considered with the increase use of kV imaging protocols in order to support the use of IGRT.PACS numbers: 87.55.Qr, 87.55.ne, 87.53Bn, 87.55.kh

Measurement comparison and Monte Carlo analysis for volumetric‐modulated arc therapy (VMAT) delivery verification using the ArcCHECK dosimetry system

The objective of this study is to validate the capabilities of a cylindrical diode array system for volumetric‐modulated arc therapy (VMAT) treatment quality assurance (QA). The VMAT plans were generated by the Eclipse treatment planning system (TPS) with the analytical anisotropic algorithm (AAA) for dose calculation. An in‐house Monte Carlo (MC) code was utilized as a validation tool for the TPS calculations and the ArcCHECK measurements. The megavoltage computed tomography (MVCT) of the ArcCHECK system was adopted for the geometry reconstruction in the TPS and for MC simulations. A 10×10 cm2 open field validation was performed for both the 6 and 10 MV photon beams to validate the absolute dose calibration of the ArcCHECK system and also the TPS dose calculations for this system. The impact of the angular dependency on noncoplanar deliveries was investigated with a series of 10×10 cm2 fields delivered with couch rotation 0° to 40°. The sensitivity of detecting the translational (1 to 10 mm) and the rotational (1° to 3°) misalignments was tested with a breast VMAT case. Ten VMAT plans (six prostate, H&N, pelvis, liver, and breast) were investigated to evaluate the agreement of the target dose and the peripheral dose among ArcCHECK measurements, and TPS and MC dose calculations. A customized acrylic plug holding an ion chamber was used to measure the dose at the center of the ArcCHECK phantom. Both the entrance and the exit doses measured by the ArcCHECK system with and without the plug agreed with the MC simulation to 1.0%. The TPS dose calculation with a 2.5 mm grid overestimated the exit dose by up to 7.2% when the plug was removed. The agreement between the MC and TPS calculations for the ArcCHECK without the plug improved significantly when a 1 mm dose calculation grid was used in the TPS. The noncoplanar delivery test demonstrated that the angular dependency has limited impact on the gamma passing rate (<1.2% drop) for the 2%–3% dose and 2 mm–3 mm DTA criteria. A 1° rotational misalignment introduces 11.3% (3%/3 mm) to 21.3% (1%/1 mm) and 0.2% (3%/3 mm) to 0.8% (1%/1 mm) Gamma passing rate drop for ArcCHECK system and MatriXX system, respectively. Both systems have comparable sensitivity to the AP misalignments. However, a 2 mm RL misalignment introduces gamma passing rate drop ranging from 0.9% (3%/3 mm) to 4.0% (1%/1 mm) and 5.0% (3%/3 mm) to 12.0% (1%/1 mm) for ArcCHECK and MatriXX measurements, respectively. For VMAT plan QA, the gamma analysis passing rates ranged from 96.1% (H&N case) to 99.9% (prostate case), when using the 3%/3 mm DTA criteria for the peripheral dose validation between the TPS and ArcCHCEK measurements. The peripheral dose validation between the MC simulation and ArcCHECK measurements showed at least 97.9% gamma passing rates. The central dose validation also showed an agreement within 2.2% between TPS/MC calculations and ArcCHECK measurements. The worst discrepancy was found in the H&N case, which is the most complex VMAT case. The ArcCHECK system is suitable for VMAT QA evaluation based on the sensitivity to detecting misalignments, the clinical impact of the angular dependency, and the correlation between the dose agreements in the peripheral region and the central region. This work also demonstrated the importance of carrying out a thorough validation of both the TPS and the dosimetry system prior to utilizing it for QA, and the value of having an independent dose calculation tool, such as the MC method, in clinical practice.PACS number: 87.55.Qr

Evaluation of Absorbed Dose and Skin Dose with MDCT Using Ionization Chamber and TLD

Recently, the uses of Multi-Detector Computed Tomography (MDCT) for radiation treatment simulation and planning which is used for intensity modulated radiation therapy with high technique are increasing. Because of the increasing uses of MDCT, additional doses are also increasing. The objective of this study is to evaluate the absorbed dose of body and skin undergoing in MDCT scans. In this study, the exposed dose at the surface and the center of the cylindrical water phantom was measured using an pencil ionization chamber, 30 cc ionization chamber and TL Powder. The results of MDCT were 31.84 mGy, 33.58 mGy and 32.73 mGy respectively. The absorbed dose at the surface showed that the TL reading value was 33.92 mGy from MDCT. These results showed that the surface dose was about 3.5% from the MDCT exposure higher than a dose which is located at the center of the phantom. These results mean that the total exposed dose undergoing MDCT 4 times (diagnostic, radiation therapy planning, follow-up et al.), is about 14 cGy, and have to be considered significantly to reduce the exposed dose from CT scan. 󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏󰠏

Normal and shear strain imaging using 2D deformation tracking on beam steered linear array datasets.

Previous publications have reported on the use of one-dimensional cross-correlation analysis with beam-steered echo signals. However, this approach fails to accurately track displacements at larger depths (>4.5 cm) due to lower signal-to-noise. In this paper, the authors present the use of adaptive parallelogram shaped two-dimensional processing blocks for deformation tracking. Beam-steered datasets were acquired using a VFX 9L4 linear array transducer operated at a 6 MHz center frequency for steered angles from -15 to 15° in increments of 1°, on both uniformly elastic and single-inclusion tissue-mimicking phantoms. Echo signals were acquired to a depth of 65 mm with the focus set at 40 mm corresponding to the center of phantom. Estimated angular displacements along and perpendicular to the beam direction are used to compute axial and lateral displacement vectors using a least-squares approach. Normal and shear strain tensor component are then estimated based on these displacement vectors. Their results demonstrate that parallelogram shaped two-dimensional deformation tracking significantly improves spatial resolution (factor of 7.79 along the beam direction), signal-to-noise (5 dB improvement), and contrast-to-noise (8-14 dB improvement) associated with strain imaging using beam steering on linear array transducers. Parallelogram shaped two-dimensional deformation tracking is demonstrated in beam-steered radiofrequency data, enabling its use in the estimation of normal and shear strain components.

Development and implementation of an EPID‐based method for localizing isocenter

The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter to an accuracy of less than 1 mm using the EPID (Electronic Portal Imaging Device). The proposed solution uses a collimator setting of 10×10cm2 to acquire EPID images of a new phantom constructed from LEGO blocks. Images from a number of gantry and collimator angles are analyzed by automated analysis software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180° collimator rotation to determine if the phantom is centered in the dimension being investigated. Repeated tests show that the system is reproducibly independent of the imaging session, and calculated offsets of the phantom from radiation isocenter are a function of phantom setup only. Accuracy of the algorithm's calculated offsets were verified by imaging the LEGO phantom before and after applying the calculated offset. These measurements show that the offsets are predicted with an accuracy of approximately 0.3 mm, which is on the order of the detector's pitch. Comparison with a star‐shot analysis yielded agreement of isocenter location within 0.5 mm. Additionally, the phantom and software are completely independent of linac vendor, and this study presents results from two linac manufacturers. A Varian Optical Guidance Platform (OGP) calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment and OGP calibration on a monthly basis.PACS number: 87.55.Qr

Multi‐system repeatability and reproducibility of apparent diffusion coefficient measurement using an ice‐water phantom

To determine quantitative quality control procedures to evaluate technical variability in multi-center measurements of the diffusion coefficient of water as a prerequisite to use of the biomarker apparent diffusion coefficient (ADC) in multi-center clinical trials. A uniform data acquisition protocol was developed and shared with 18 participating test sites along with a temperature-controlled diffusion phantom delivered to each site. Usable diffusion weighted imaging data of ice water at five b-values were collected on 35 clinical MRI systems from three vendors at two field strengths (1.5 and 3 Tesla [T]) and analyzed at a central processing site. Standard deviation of bore-center ADCs measured across 35 scanners was <2%; error range: -2% to +5% from literature value. Day-to-day repeatability of the measurements was within 4.5%. Intra-exam repeatability at the phantom center was within 1%. Excluding one outlier, inter-site reproducibility of ADC at magnet isocenter was within 3%, although variability increased for off-center measurements. Significant (>10%) vendor-specific and system-specific spatial nonuniformity ADC bias was detected for the off-center measurement that was consistent with gradient nonlinearity. Standardization of DWI protocol has improved reproducibility of ADC measurements and allowed identifying spatial ADC nonuniformity as a source of error in multi-site clinical studies.

A dose point kernel database using GATE Monte Carlo simulation toolkit for nuclear medicine applications: Comparison with other Monte Carlo codes

GATE is a Monte Carlo simulation toolkit based on the Geant4 package, widely used for many medical physics applications, including SPECT and PET image simulation and more recently CT image simulation and patient dosimetry. The purpose of the current study was to calculate dose point kernels (DPKs) using GATE, compare them against reference data, and finally produce a complete dataset of the total DPKs for the most commonly used radionuclides in nuclear medicine. Patient-specific absorbed dose calculations can be carried out using Monte Carlo simulations. The latest version of GATE extends its applications to Radiotherapy and Dosimetry. Comparison of the proposed method for the generation of DPKs was performed for (a) monoenergetic electron sources, with energies ranging from 10 keV to 10 MeV, (b) beta emitting isotopes, e.g., (177)Lu, (90)Y, and (32)P, and (c) gamma emitting isotopes, e.g., (111)In, (131)I, (125)I, and (99m)Tc. Point isotropic sources were simulated at the center of a sphere phantom, and the absorbed dose was stored in concentric spherical shells around the source. Evaluation was performed with already published studies for different Monte Carlo codes namely MCNP, EGS, FLUKA, ETRAN, GEPTS, and PENELOPE. A complete dataset of total DPKs was generated for water (equivalent to soft tissue), bone, and lung. This dataset takes into account all the major components of radiation interactions for the selected isotopes, including the absorbed dose from emitted electrons, photons, and all secondary particles generated from the electromagnetic interactions. GATE comparison provided reliable results in all cases (monoenergetic electrons, beta emitting isotopes, and photon emitting isotopes). The observed differences between GATE and other codes are less than 10% and comparable to the discrepancies observed among other packages. The produced DPKs are in very good agreement with the already published data, which allowed us to produce a unique DPKs dataset using GATE. The dataset contains the total DPKs for (67)Ga, (68)Ga, (90)Y, (99m)Tc, (111)In, (123)I, (124)I, (125)I, (131)I, (153)Sm, (177)Lu (186)Re, and (188)Re generated in water, bone, and lung. In this study, the authors have checked GATE's reliability for absorbed dose calculation when transporting different kind of particles, which indicates its robustness for dosimetry applications. A novel dataset of DPKs is provided, which can be applied in patient-specific dosimetry using analytical point kernel convolution algorithms.

Poster - Thur Eve - 50: Planning and delivery accuracy of stereotactic radiosurgery with Tomotherapy as compared to linear-accelerator and robotic based radiosurgery.

This study includes planning and delivery comparison of three stereotactic radiosurgery techniques : Helical Tomotherapy (HT), circular collimator-based Linear-accelerator and robotic-radiosurgery. Plans were generated for two spherical targets of diameter 6 mm and 10 mm contoured at the center of a Lucite phantom, using similar planning constrains. Planning comparison showed that average conformality (0-1best) for Linear-accelerator, robotic-radiosurgery and HT was 1.43, 1.24, and 1.77 and gradient index (less is better) was 2.72, 4.50 and 13.56 respectively. For delivery comparison, plans were delivered to radiochromic film and measured dose was compared with the planned dose. For Linear-accelerator and robotic-radiosurgery more than 99% pixels-passing a gamma criteria of 3% dose difference and 1 mm distance to agreement where as for HT this value was as low as 40% for off-axis targets. Further investigation of the delivery accuracy as a function of the location of the target with in the bore was initiated using small volume A1SL (0.057 cm3 ) and MicroLion liquid ion chamber (0.0017 cm3 ). Point dose measurements for targets located at the center and 10 cm away from the center of the bore showed that delivered dose varied by more than 15% for targets placed away from the center of the bore as opposed to at the center. In conclusion, Linear-accelerator and the robotic-radiosurgery techniques showed preferable gradient and conformality. For HT, point dose measurements were significantly lower than predicted by the TPS when the target was positioned away from the isocenter, while they were found to be higher at isocenter.

SU-E-T-24: Development and Implementation of an Automated Algorithm to Determine Radiation Isocenter, Radiation vs. Light Field Coincidence, and Analyze Strip Tests.

The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter using the Electronic Portal Imaging Device (EPID). The phantom could then be used as a static reference point for performing other tests including: radiation vs. light field coincidence, MLC and Jaw strip tests, and Varian Optical Guidance Platform (OGP) calibration. The solution proposed uses a collimator setting of 10×10 cm to acquire EPID images of the new phantom constructed from LEGO® blocks. Images from a number of gantry and collimator angles are analyzed by the software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180 degree collimator rotation to determine if the phantom is centered in the dimension being investigated. The accuracy of the algorithm's measurements were verified by independent measurement to be approximately equal to the detector's pitch. Light versus radiation field as well as MLC and Jaw strip tests are performed using measurements based on the phantom center once located at the radiation isocenter. Reproducibility tests show that the algorithm's results were objectively repeatable. Additionally, the phantom and software are completely independent of linac vendor and this study presents results from two major linac manufacturers. An OGP calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty contained in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment, OGP calibration, and other tests on a monthly basis.