Volume 34, Issue 6, June 2007
Index of content:
- Therapy Scientific Session: Auditorium
- Clinical Measurements
MO‐D‐AUD‐01: A Comprehensive Study On the Heterogeneity Dose Calculation Accuracy in IMRT Using An Anthropomorphic Thorax Phantom34(2007); http://dx.doi.org/10.1118/1.2761232View Description Hide Description
Purpose: To provide a comprehensive study on the accuracy of many commonly used Intensity Modulated Radiation Therapy(IMRT)treatment planning systems using the Radiological Physics Center's (RPC) anthropomorphic thorax phantom. Method and Materials:Treatment planning systems (TPSs) from Corvus, Eclipse, Pinnacle, and Tomotherapy were evaluated using the RPC anthropomorphic phantom. Treatment plans were designed using the same clinical constraints and prescriptions so that 96% of the planning target volume (PTV) was covered by the prescription dose. The phantom is equipped with TLD located in the tumor,heart, and spinal cord and radiochromic film located in three anatomical planes intersecting the tumor center and extending into the lung.IMRT QA was performed to adjust the calculated dose distributions in order to isolate the effects of heterogeneity. Comparisons were made between each TPS calculation and measurement. In two instances, re‐calculations of the original correction based pencil beam (PB) plans were performed using the superposition convolution (SC) method. Results: TPSs employing superposition convolution algorithms predicted dose within 3.6% of the target TLD, while TPSs using correction based pencil beam algorithms predicted dose within 5.0% of the target TLD. Both algorithm types showed variations (2% to 38% in the cord and heart) in predicting low dose to normal structures. The dose distributions within the PTV and penumbra lung regions showed good agreement when using an SC algorithm. However, TPSs using the PB type algorithm overestimated dose in the PTV and underestimated the extent of penumbra broadening corresponding to the surrounding lung.Conclusion: This work demonstrated that superposition convolution algorithms found in widely used IMRTtreatment planning systems are able to calculate the dose accurately to the PTV and penumbra regions when low density heterogeneities are involved. Conflict of Interest: This work supported by PHS CA010953 and CA081647 awarded by NCI, DHHS.
MO‐D‐AUD‐02: Liquid Scintillation‐Based Proton Residual Range Measurement Using a Dynamic Biological Lung Phantom34(2007); http://dx.doi.org/10.1118/1.2761234View Description Hide Description
Purpose: To test and apply a scintillation‐based method to directly observe proton residual range and range variation in a dynamic biological lung phantom. Method and Materials: The dynamic biological lung phantom using a preserved swine lung has previously been compared to human lung and evaluated as an experimental platform for IGRT studies. Fiducial markers and a 5 cc artificial tumor were placed in the lung, and the phantom was imaged on a GE Lightspeed CT in 4D mode before and after seed and “tumor” implantation. The resulting 10‐phase DICOM datasets were evaluated in terms of anterior‐posterior (AP) water‐equivalent path length (WEL) variation throughout the ventilatory cycle. The preserved swine lung was irradiated in the AP direction with a 170 MeV proton beam through a 1 cm × 10 cm slit aperture. Proton residual range and range variation were directly observed using a custom‐built lucite chamber filled with scintillating fluid, monitored during irradiation by a video camera.Images were then analyzed using ImageJ to determine residual proton range. Results: WEL analysis on the imagedlung suggests a range of lung WEL values between 8 and 25 mm (average ∼16 mm). Peak calculated WEL difference at the tumor margin after implantation during ventilatory motion was approximately 5mm. Irradiation of the phantom demonstrated regional range variation across the lung on the order of 13 mm, with total WEL values equivalent to calculated values. Residual range variation due to ventilatory motion was less significant, at some points on the order of ±2.4 mm. Conclusion: The dynamic biological lung phantom in conjunction with the scintillation chamber has been shown to be a simple, inexpensive, and effective tool for the measurement of proton residual range and significant residual range variation in a complex biological system.
MO‐D‐AUD‐03: Verification of Lung Tumor Doses Calculated by the Eclipse AAA and Pinnacle CC Algorithms34(2007); http://dx.doi.org/10.1118/1.2761235View Description Hide Description
Purpose: The goal of this study was to investigate the dosimetric accuracy of the analytical anisotropic algorithm (AAA) for the treatment of lungtumors. This algorithm, recently implemented in Eclipse (Varian Inc., Palo Alto, CA), was also compared to the Pinnacle's (Philips Medical, Cleveland, OH) collapsed cone (CC) algorithm. Methods: Four phantoms were used in this study. Three lung‐, bone‐ and water‐equivalent slab phantoms were designed to validate the accuracy of the calculated doses in simple geometries. Doses were additionally measured in the CIRS (Norfolk, VA) thorax solid water phantom, which includes lung cavities and a cylindrical spine. All four phantoms were CT‐scanned with thermoluminescent dosimeters(TLD) in place. Plans were generated with the AAA algorithm using anterior‐posterior (AP), AP/PA, oblique, and intensity modulated (IMRT) beams with both 6MV and 18MV photons. The dose distributions for all plans, excluding the IMRT, were re‐computed with Pinnacle using monitor units matched to the AAA plans. Dose measurements were performed with TLDs and ion chambers at 6 different positions including three in tissue, one in spinal cord, and two in lung.Results: The AAA algorithm was found to calculate the dose in lung and tissue accurately to within 6% in all four phantoms. Ion chamber measurements showed 2–3% better agreement than the TLDs in the thorax phantom. For 6 MV, the differences between measured and calculated doses were less than 2% for both AAA and Pinnacle for all six points. However, for the 18 MV beam, the dose measured in the spinal cord was 5.9% greater than the calculated dose with both algorithms. Measured and calculated doses agreed to within the 2% for the IMRT plan. Conclusion: The AAA algorithm provides accurate dose calculations in and around heterogeneities, similar to that provided by the CC algorithm implemented in Pinnacle.
34(2007); http://dx.doi.org/10.1118/1.2761236View Description Hide Description
Purpose: To compare the measured dose distribution with the planned distribution over a PTV centrally located within the lung when heterogeneity corrections are taken into account. Method and Materials: The Radiological Physics Center's anthropomorphic thorax phantom includes a target (∼ 1 g/cm3) centrally located in the left lung (∼ 0.33 g/cm3). The phantom was sent to 25 institutions, each of which was instructed to design and deliver a stereotactic treatment plan. The plan was intended to deliver 20Gy (homogeneous calculation) to ⩾ 95% of the PTV and limit the lungdose at point 2 cm from the PTV edge to 11.7 Gy. The institutions were asked to recalculate the dose distribution with the heterogeneity correction using the monitor units determined from the homogeneous calculated plan. TLD and radiochromic films were used as dosimeters within the target region. Results: A total of 17 institutions met the phantom irradiation criteria: +/− 5% for DTLD/DInst, and +/−5mm DTA on all sides of the PTV, based on the heterogeneous calculated plan. For these irradiations, the delivered doses over the central 80% of the PTV were compared to the planned doses along 3 orthogonal profiles through the PTV. An average of 85% of the points in the profiles from the cases calculated with the superposition/convolution algorithm were within 5% of the calculation, while only 69% of the points from the plans using pencil beam and Clarkson were within the 5% of the plan. Conclusions: The superposition/convolution heterogeneity correction algorithm showed better agreement with the measured dose distribution across the PTV than the pencil beam and Clarkson algorithms because it more accurately accounted for the lack of lateral scatter.
Work supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.
MO‐D‐AUD‐05: Characterization of Dose in Heterogeneous Situations: A Comparison of Treatment Planning System and Computer Aided Second Check Dosimetry QA Software Dose Evaluations34(2007); http://dx.doi.org/10.1118/1.2761237View Description Hide Description
Purpose: To quantify and compare the dosimetric predictions calculated by a treatment planning system, second check dosimetric computer QA software, and ion chamber measurements in heterogeneous situations for 6 and 18 MV photon beams. Method and Materials: An assortment of plastic tissue equivalent materials was used to compare the calculated dose predictions between the Pinnacle3treatment planning system, the RadCalc® QA computer software, and ion chamber measurements. The dosimetric accuracy of the plastic water, lung, and bone equivalent slab materials was assessed and validated through the use of simple geometries. After planning, doses for each slab arrangement were measured on a Varian 21EX accelerator with a second check performed by the RadCalc® computer software. Percentage differences between the computed and measured doses were then compared and quantified, providing information on the accuracy of the dose predictions. Results: Evaluation and comparison between the calculated dose values from the Pinnacle3, RadCalc®, and measured data indicate that discrepancies exist, even for simple geometric setups. Looking at percentage differences, the Pinnacle3 system (−3.82% – 4.33%) more accurately calculates the dose in the heterogeneous locations than does the RadCalc® software (−8.30% – 4.15%). Examination of all measured point locations show only about 4% of the Pinnacle3 system dose calculations, and almost 18% of the RadCalc® software dose calculations, have a percentage difference greater than ±3%. Conclusion: This work explores the clinical application and accuracy of using RadCalc® for dosimetric second checks. Even with the use of heterogeneity corrections, it is still not guaranteed that an accurate dose calculation will result when heterogeneous material is present. The CIRS Inc. IMRT Thorax and Pelvic 3D phantoms will be utilized for the continuing investigation of the accuracy of dose calculations performed by Pinnacle3 and RadCalc® involving additional complex geometries in a more anatomical construct.
MO‐D‐AUD‐06: Dependence of Total Scatter Factors of Small Beams On the Radial Distribution of the Electron Beam Incident On the Target: A Multi‐Detector and Monte Carlo Study34(2007); http://dx.doi.org/10.1118/1.2761238View Description Hide Description
Purpose: To investigate dependence of total scatter factors (sc,p) of small beams used in radiosurgery on the radial distribution of the electron beam incident on the target. The study was performed in order to clarify discrepancies observed in sc,p measurements performed by means of different detectors at the Cyberknife radiosurgery system and was aimed at obtaining a means to infer the electron radial distribution of a specific Cyberknife unit. Method and Materials: PTW PinPoint31014 and Exradin A16 microchambers, PTW30012 diode and TM60003 diamond were used to measure sc,p. The same detectors were simulated by means of the Monte Carlo code BEAMnrc to calculate correction factors for sc,p. BEAMnrc was also used to calculate theoretical values of sc,p. Accuracy of Monte Carlo simulation depends on the choice of energy, divergence and radial distribution of the electron beam incident on the target. The energy was determined by comparison to experimental TMRs. Radial distribution of the electron beam (expressed as FWHM, gaussian shape) was chosen to be optimal when correction factors of the 4 detectors were such that corrected sc,p converged to the same value within +/−1%. Results: measured sc,p of the 5mm collimator averaged 0.638 −4%+11% with the 4 detectors. E=7.2MeV best matched calculated to experimental TMRs. Optimal FWHM=2.3mm gave sc,p=0.673 −0.7%+0.3%. Correction factors decreased with increasing FWHM, while Monte Carlo‐calculated sc,p increased. Measured (corrected) and calculated sc,p matched at 0.673 only for FWHM=2.3mm. Conclusion: Variations of electron beam focussing can explain significant variations of sc,p. If one of the investigated detectors is used, it is possible to infer actual FWHM values and thus appropriate correction factors by comparison to pure Monte Carlo‐calculated sc,p values as a function of FWHM. This fact could be exploited by centres who do not have access to Monte Carlo codes to simulate their own system.
MO‐D‐AUD‐07: Determination of Output Factors for Stereotactic Radiosurgery Beams by Monte Carlo and Measurements34(2007); http://dx.doi.org/10.1118/1.2761239View Description Hide Description
Purpose: The Varian Trilogy accelerator provides high dose rate (1000MU/min) photon beams for stereotactic radiosurgery(SRS). Different output factors have been reported and their use may impair observations of dose response and optimization of prescribed dose. In this work, we investigated the output factors for the Trilogy 6 MV SRS beam using Mote Carlo simulations and measurements. Method and Materials: The Trilogy SRS cone collimators are 5 mm to 30 mm in diameter. Chamber measurement of output factors is difficult for small cone sizes. In this work, the measurement was carried out using a 0.015cc pinpoint chamber and Gafchromic EBT film. The Monte Carlo simulation was performed using the MCBEAM/MCSIM codes for linac head simulation and phantom dose calculation. The Monte Carlo calculations were validated using the measurement results and compared with the Varian recommended beam data. Results and Conclusions The agreement in percent depth doses and dose profiles between the simulation results, the measurement results and the Varian data was within 2%/1mm for the 10×10 cm2reference field and all the cones at a 100 cm source to surface distance. The output factors obtained from the Monte Carlo simulations were in excellent agreement with the values from the film measurements. Similar agreement was found between the Monte Carlo results and the pinpoint chamber values except for cones with diameter less than 20 mm where the 2 mm chamber diameter has become comparable to the field size. However, the Varian recommended output factors are consistently higher than the Monte Carlo results, especially for the 5 mm cone, where the difference reaches 10.9%. Therefore, great caution must be taken with the use of the Varian recommended beam data.
MO‐D‐AUD‐08: Dose Verification at the Surface of Air Cavities During Radiation Therapy Using the TomoTherapy Hi‐Art System34(2007); http://dx.doi.org/10.1118/1.2761240View Description Hide Description
Purpose: Air cavities are a significant inhomogeneity in radiation therapy. TomoTherapy, a helical dose delivery system, is dependent on a heterogeneity based treatment planning system. Plan optimization can be affected by changing the pitch of treatment delivery along with the modulation factor for the treatments. The system allows for calculations using a fine, normal, or coarse calculation grid. In this investigation, we assess the dose delivered to the surface and superficial regions of the cavity, the influence of the above parameters on dose delivery, and the accuracy of the planning system to represent the dose in these regions. Method and Materials: Four different 3×3 cm2 air cavities configurations in solid water were investigated‐with 9 and 1cm solid water above the cavity, with the cavity open at the surface, and with no cavity to measure the surface dose. Each cavity had a reference plan, two plans changing pitch, and two plans changing modulation. We compared the predicted dose to measureddose using both the Attix chamber and TLDpowder.Results: A large variation in the percent difference between measured and expected dose depends on the location of the air cavity below the phantom surface. The accuracy of the delivered dose varied with both modulation factor and pitch dependent on the cavity location in the phantom. The Attix chamber and TLDpowder showed essentially the same response, but the Attix chamber data was more precise. Conclusions: The accuracy of the delivered to expected dose on the cavity surface was greater when the air cavity was far below the surface (0–5%). As the cavity moved closer to the surface, the deviations in measured to expected dose increased to minus 40–50% of the measureddose at the surface.
34(2007); http://dx.doi.org/10.1118/1.2761241View Description Hide Description
A method based on dose‐response function was developed in the past, which verifies the beamlet weight of intensity modulated radiation therapy(IMRT) from doseimage in electronic portal imaging device(EPID) and reconstructsdose in a patient. The establishment of a linear relationship between beamlets and dose responses in patient and in electronic portal imaging device(EPID) was the key to this methodology. The responses are to be predetermined by a full‐scope Monte Carlo calculation in patient and EPID structures. In this study, the method was validated through measurement using in‐phantom and exit film dosimetry which simulated in‐patient and EPID measurements, respectively. The in‐phantom film was inserted at 100 cm from the target and at 12.5 cm depth within a 25‐cm thick phantom and the exit film was placed at 139.5 cm from the target and at 2 cm depth within a 4‐cm thick EPID phantom. For the validation, a 6MV X‐ray beam with the size of 6 × 6 cm2 was perpendicularly exposed to the phantom. Responses to each beamlet (0.2 × 5 mm2) within a phantom and an EPID phantom were then calculated. Using the calculated responses, the exit film dose was used to inversely reconstruct the in‐phantom dose, which was then compared with the measured in‐phantom dose. In a second study, an IMRT beam intensity reconstruction was investigated computationally. The dose comparison in patient showed a difference of less than 3 %. Some propagated noise was found in the reconstructed intensity distribution, suggesting the need for noise filtration prior to reconstruction. The reconstruction took less than 10 seconds of calculation time and 10 MB of memory. The method is accurate as well as effective for the dosereconstruction of IMRT. A follow‐up study will include detailed modeling of a therapeutic beam and EPID and experiments.
- Measurement: Calibration and QA (I)
34(2007); http://dx.doi.org/10.1118/1.2761287View Description Hide Description
Purpose: The use of image‐guided patient positioning requires fast and reliable Quality Assurance (QA) methods to ensure the megavoltage (MV) treatment beam coincides with the integrated kilovoltage (kV) imaging and guidance system. This study describes an automated and comprehensive QA procedure to monitor the coincidence of the mechanical, radiation and imaging isocenters using cone‐beam CT (CBCT) and planar X‐ray imaging.Method and Materials: The On‐Board Imaging (OBI) system consists of a kV x‐ray tube and an amorphous‐silicon flat panel imaging detector which are attached to a medical linear accelerator. A Penta‐Guide phantom (Modus Medical Devices Inc. London, Ontario, Canada) with five internal markers and external markers for isocenter position and field sizes was imaged using the CBCT, kV and MV capabilities. The markers' location and size can be automatically determined using our graphical software system. The accuracy of CBCTimaging is assessed by comparison of the extracted marker positions and sizes against the phantom specifications. The coincidence of the imaging and dosimetric isocenters are tested by a similar analysis of the markers extracted from the kV and MV images. Additional tests are also performed such as isocenter stability with gantry angle, image size, collimator angle and size, and gantry angle. Results: The test was performed on all four IGRT‐enabled machines available in our institution. The coincidence between the mechanical, radiation and imaging isocenter are within 1 mm for all four accelerators. Isocenter stability with gantry angle was also within 1 mm. The acquisition of the images took ∼ 5 min, and the automated software analysis took less than 1 min. Conclusion: Our automated image analysis may be used as a daily QA procedure because it is completely automated and uses a single phantom setup.
34(2007); http://dx.doi.org/10.1118/1.2761288View Description Hide Description
Purpose: To develop a fast and accurate procedure for mechanical QA of linear accelerators that can be used during installation and routine mechanical QA. Method and Materials: A system was developed which consists of an IR‐camera and an IR‐marker attached to the gantry, treatment table or collimator.Software was written to process signals from the IR‐camera for the duration of gantry rotation and reconstruct the trace of the marker in three orthogonal planes. A circular fit of the trace in the plane of gantry rotation provides 3D coordinates of mechanical isocenter for gantry rotation. A linear fit of this trace in orthogonal planes provides information on gantry sag and rotational hysteresis. A similar procedure was developed for table mechanical QA. Orthogonality of the planes of table and gantry rotation is determined by the angle between linear fits for the gantry and table traces. Results: Mechanical QA tests were performed in two different treatment rooms equipped with IR‐camera systems. The accuracy of mechanical alignment of the axis of rotation of the gantry and couch was demonstrated to be within 1mm of the nominal isocenter defined by the room lasers. In each treatment room, the tests were repeated three times and reproducibility of the tests was better than 1mm. The tests revealed both the known effect of gantry sag and also rotational hysteresis. Orthogonality of the planes of gantry and table rotation was also determined. Conclusions: An IR‐based procedure for mechanical QA of radiation treatment accelerators was developed and tested. This procedure accurately detects the location of mechanical isocenter and provides additional information such as the extent of gantry sag and rotational hysteresis. The system provides quantitative 3D displacement of mechanical isocenter that can be effectively used in the initial installation of accelerators as well as in an ongoing quality assurance program.
MO‐E‐AUD‐03: Study of Total Positioning Accuracy of a Newly Developed Image‐Guided Radiotherapy System34(2007); http://dx.doi.org/10.1118/1.2761289View Description Hide Description
Purpose: The aim of this study was to evaluate system accuracy of our newly developed image‐guidedradiotherapy(IGRT) system. Method and Materials: The system has the following structural characteristics; (1) C‐band compact linear accelerator(LINAC), (2) O‐ring shaped gantry, (3) Gimbals mechanism at X‐ray head, and (4) Imaging subsystem. The whole X‐ray head with the LINAC and a MLC is equipped on gimbals mechanism to correct beam direction to isocenter. The imaging subsystem mounted on the O‐ring gantry consists of two sets of a kV X‐ray tube with an image detector, and can obtain either X‐ray images or cone‐beam CT(CBCT)images. By using Electronic Portal Imaging Device, the IGRT system also provides an automatic daily QA tool for total positioning accuracy, which means beam accuracy towards isocenter combined with image‐guiding accuracy. The daily QA requires only 3 minutes. Firstly in this study, to evaluate the image‐guiding accuracy, a spherical metal ball with a diameter of 10 mm was set at given positions within 20 mm from isocenter. X‐ray images or CBCTimages were obtained to measure the ball position, and the difference between true and measured position was calculated. Secondly, Mechanical accuracy of beam direction to isocenter was measured in various gantry angles with CCD camera attached to MLC frame. Finally, the combined accuracy was verified by using the daily QA tool. Results: The difference between true and measured position was 0.34 mm in SD and 0.9 mm at the maximum for X‐ray image, then 0.17mm in SD and 0.4 mm at the maximum for CBCT, respectively. Mechanical beam accuracy was within 0.1mm. The combined accuracy was better than 0.5 mm. Conclusion: The IGRT system has suitable accuracy for image‐guided stereotactic irradiation. Conflict of Interest: Research sponsored by New Energy and Industrial Technology Development Organization, Japan.
MO‐E‐AUD‐04: Automated Linear Accelerator Quality Assurance Using a Commercial Cylindrical Calibration Phantom34(2007); http://dx.doi.org/10.1118/1.2761290View Description Hide Description
Purpose: Precise mechanical operation of a linear accelerator is critical for accurate dose delivery. Available quantitative procedures for the linac mechanical quality assurance (QA) are time consuming and therefore conducted on a relatively infrequent basis. We present a method for evaluating the mechanical performance of a linac based on a series of projection portal images of a prototype cylindrical calibration phantom with embedded markers.
Method and Materials: We used non‐linear multiobjective optimization of information extracted from the images to determine a number of geometric parameters of interest. The markers detection included modeling the imager response to radiation beams where significantly non‐uniform background was expected. Results: The average results for the parameters of our geometric linacmodel were: gantry angle deviation 0.066 ± 0.085° (1 SD), gantry sag 0.026 ± 0.02°, imager in‐plane rotation 0.026 ± 0.055°, roll — 0.082 ± 0.16° and pitch −0.9 ± 0.604°, SDD 1489.7 ± 5 mm, SAD 998 .3 ± 1.7 mm, and the imager shift [− 0.66,3.9]±[0.30,1.6] mm. The results were corrected for the phantom center shift relative to the linac rotational center. The average rotational center was . The average couch height and angle variations were 0.15±0.9 mm and 0.154±0.1°, respectively. The image analysis quality was examined by comparing the detected set of marker coordinates to its simulated counterpart for three regions of the phantom image: central, near the edge and the intermediate region (relative to the central line of the cylinder). The upper limit of the mean difference was less than 0.25 mm with the cumulative mean of 0.146 mm and SD of 0.07 mm. The results of the primary optimization of directly detected marker coordinates virtually coincided with their counterparts based on the simulated coordinates for all the geometric parameters of the model.
Conclusion: This procedure is accurate and automated, which allows precise mechanic QA to be performed more frequently.
Conflict of Interest: partially supported by Varian Medical Systems.
MO‐E‐AUD‐05: Validating a Video‐Based 3‐D Surface Imaging System and Testing Its Use to Set‐Up CBCT QA Device34(2007); http://dx.doi.org/10.1118/1.2761291View Description Hide Description
Purpose: To validate resolution of a video‐based 3‐D surface imagingsystem before its clinical use and to test feasibility of using system in setting‐up a cone beam CT(CBCT) QA device to accelerate the total QA process. Method and Materials: A CBCT QA alignment tool called Ball‐Bearing device (BBD) (Elekta Inc.) is designed to check the coincidence between the MV and kV beams' isocenters. The micro‐stepping meter of the BBD allows precision adjustment to better than 0.01 mm in three directions. A foam phantom was added to the BBD in order to generate a large 3‐D surface image which can be detected by a video‐based 3‐D surface imagingsystem (AlignRT system, VisionRT Ltd., London UK). The BBD‐phantom has been imaged by the AlignRT system with changes in position (0.0 to 8.0 mm) in all three directions independently. The phantom shifts were determined by AlignRT and compared to the preset ones. After this validation test, AlignRT has been used in the initial set‐up of the CBCT device in replacement of the iterative process required to set the BBD to the MV isocenter. By carefully setting‐up the BBD using the iterative imaging method to place the ball‐bearing to MV beam isocenter, a reference image of BBD‐phantom was taken using AlignRT. Subsequently, AlignRT was used in BBD set‐up; multiple tests have been performed and testing continues. Results: AlignRT can detect 0.1 mm change in both lateral and longitudinal directions, 0.3 mm in vertical direction. AlignRT reduced set‐up time for CBCT QA device by a factor of two compared to traditional iterative method. Conclusions: AlignRT can detect fractional mm shifts in the BBD‐phantom; vertical resolution is not as sensitive as horizontal. AlignRT can be used in setting‐up the CBCT QA device (providing appropriate phantom added) and significantly reduces set‐up time.
MO‐E‐AUD‐06: Ultra‐Fast Gamma Index Calculation for Quality Assurance and Optimization in Radiotherapy34(2007); http://dx.doi.org/10.1118/1.2761292View Description Hide Description
Purpose It is clinically essential in IMRT to compare two dose distributions for dosequality assurance (DQA). The gamma index (Low et al, 1998), which combines both dose difference and distance to agreement, provides a quantitative measure of acceptability in DQA. However, its calculations can be time‐consuming and limit its applications to 2‐dimensional dose distributions. In this work, we propose an efficient calculation method. Method By embedding the k‐dimensional reference dose distribution in the (k+1)‐dimensional spatial‐dose space, we then use the Euclidean distance transform to find the distance to the reference dose distribution, regarded as a feature set, for every point in a range of the spatial‐dose space. This leads to a table of gamma indices. And the evaluation of the gamma indices for any dose distribution with respect to the reference dose distribution is simply table‐lookup. Our implementation uses a fast Euclidean distance transform, which was developed in Maurer et al, 2003 and proved to have only linear complexity. Results Using simulated 2‐D dose distributions of size 400×400, the pre‐calculation of the Gamma index table takes 26 sec and the table lookup to evaluate the Gamma index for each test dose distribution takes less than 0.1 sec in a 3GHz PC. On the other hand, it takes about 2400 sec using the exhaustive search on the same PC to evaluate the Gamma index for each test distribution. The speedup for 3D Gamma index calculation is expected to be 104∼105. Conclusion Numerical simulations demonstrate the efficiency of our proposed method. Thus, the clinical usage of 3D Gamma index becomes feasible. In addition, the Gamma index table can be used to determine the derivative of Gamma index over the dose distribution, which facilitates the inclusion of Gamma index in treatment planning and/or machine parameters optimization.
MO‐E‐AUD‐07: Calculating the Air‐Kerma Strength and Dose‐Rate Constant of 125I and 103Pd Low Dose Rate Brachytherapy Sources Using Spectra Measured With a High‐Purity Germanium Spectrometer34(2007); http://dx.doi.org/10.1118/1.2761293View Description Hide Description
Purpose: To calculate spectroscopic analogs of the air‐kerma strength and dose‐rate constant using the fully corrected energy spectra of and low dose‐rate (LDR) brachytherapy sources measured with a high‐purity germanium (HPGe) spectrometer.Methods and Materials: A high‐purity germaniumspectrometer (CANBERRA, GR2519), driven by CANBERRA Genie2000™ gamma analysis software, was used to measure spectra of Theragenics model 200 and Best Medical model 2301 sources. The thin beryllium entrance window (0.05 cm) allows high transmittance of low energy photons making it ideal for LDR applications. An iterative deconvolution algorithm was used to correct each measured spectrum for detector effects such as efficiency and germaniumfluorescence. The deconvolved spectrum represents the photonspectrum incident on the entrance window of the detector. The spectrum was corrected to vacuum to represent the spectrum emitted from the encapsulated source. The spectroscopic air‐kerma strength, SK,Spect, was calculated from the fully corrected emitted spectrum of each source. A spectroscopic dose‐rate constant, ΛSpect, was also calculated for both the models 200 and 2301 using the method developed by Chen and Nath (Med. Phys. 28, 86–96, 2001). Results: SK,Spect for model 200 and model 2301 sources were calculated. The values were compared with standard SK measurements showing an agreement of 3.2–3.4% for most sources, while one source exhibited a 12% difference that is being investigated further. ΛSpect was calculated to be 0.699 cGy h−1U−1 for the model 200 and 1.033 cGy h−1U−1 for the model 2301. These values are within the experimental uncertainty of the consensus dose‐rate constant values, which are 0.686 cGy h−1U−1 and 1.025 cGy h−1U−1, respectively. Conclusions: The spectroscopic method for calculating the air‐kerma strength and dose‐rate constant of LDR sources has proven to be a promising technique that can be extended to additional source models and new designs entering the market.
- Other: General
TU‐C‐AUD‐01: Visual Sensations During Megavoltage Radiotherapy to the Orbit Attributable to Cherenkov Radiation34(2007); http://dx.doi.org/10.1118/1.2761350View Description Hide Description
Purpose: To compute and to verify experimentally Cherenkov radiation production inside the eye from direct and indirect high energy x‐ray irradiation of the orbit and that it exceeds detectability of Cherenkov radiation in scotopic vision. We show that the Cherenkov yield for direct and indirect irradiation exceeds the detection threshold of 6.4×107 photons/m2s. Methods: In photon and electron beamradiotherapy of intraorbital and periorbital tissues, patients commonly report having light sensations during treatment. The explanation usually offered is that ‘nerve stimulation’ is occurring. The radiotherapy literature reports the effect to be the result of ‘phosphenes’. Although phosphenes may play a role, we propose instead that patients are predominantly seeing Cherenkov radiation resulting from electrons inside the eye having kinetic energy exceeding the Cherenkov radiation threshold of 0.26 MeV. We consider calculations for direct irradiation of the eye from a portal image and indirect irradiation from treatment of peri‐orbital tissues and show that it exceeds threshold detection. Results: We calculate the Cherenkov yield using analytic methods and the measurements were in good agreement with our calculations. The Cherenkov radiation from a distilled water phantom, a square plastic phantom and an anthropomorphic plastic phantom were readily visible with a high quality CCD video camera. A digital camera captures the images from the console monitor. Threshold detection of Cherenkov radiation emanating from the water phantom through the video camera was determined and compared to a measurable source. The calculations for the water phantom match reasonably well with measurements and are roughly 1 lux and 3 lux respectively. Conclusion: We show images of Cherenkov radiation emanating from routine radiotherapytreatments and demonstrate that measurements corresponding to these images match well with calculations. We show that the Cherenkov component generated inside ocular media is sufficient to be detected by the patient.
34(2007); http://dx.doi.org/10.1118/1.2761351View Description Hide Description
Purpose: Stray radiation exposures are of concern for patients receiving protonradiotherapy and vary strongly with several treatment factors such as proton energy, field size and modulation width. The purposes of this study were to conservatively estimate neutron exposures for a contemporary passive scatteringprotontreatment unit and to understand how they vary with treatment factors. Method and Materials: We simulated all 24 options (each range modulator and second scatterer combination is accounted for one option) for a passive scatteringproton therapy unit with MCNPX. Spectral neutron fluence from simulations was then converted to neutrondose equivalent using corresponding dose conversion factors. We studied the neutrondose equivalent per therapeutic absorbed dose (H/D) as a function of treatment factors including proton energy, location in the treatment room, treatmentfield size, and spread‐out Bragg peak (SOBP) width using Monte Carlo simulation.Results: The H/D value at isocenter for a 250‐MeV medium field size option was estimated to be 20 mSv Gy−1. H/D decreased to about 20% from 250 Mev to 160 MeV. H/D fell off sharply with distance from the treatment unit, approximately following a power law; H/D was about 10% higher for a large field option than a medium field option for the same energy. H//D almost doubled when SOBP width was increased from a pristine peak to 16 cm. An analytical model was developed, which predicted H/D values within 28% of those obtained in simulations; this value is within typical neutron measurement uncertainties. Conclusion: The results quantified how treatment factors influence H/D values. The in‐air method with a closed aperture presented here provides a simple and straightforward approach that could be adopted for facility inter‐comparisons. In addition, an analytical model was developed to quickly estimate H/D values.
TU‐C‐AUD‐03: Comparing the Expected Effectiveness of Helical Tomotherapy and MLC‐Based IMRT Using Biological Measures34(2007); http://dx.doi.org/10.1118/1.2761352View Description Hide Description
Purpose: Presently, the radiobiological parameters of the different tumours and normal tissues are typically not taken into account during dose prescription and optimization of a treatment plan. In this study, to investigate a more comprehensive treatment plan evaluation, the biologically effective uniform dose (D̿) is applied together with the complication‐free tumour control probability (P +).
Material and Methods: Three different cancer types at different anatomical sites were investigated: head & neck, lung and prostate cancers. For each cancer type, a linac MLC‐based step‐and‐shoot IMRT plan and a Helical Tomotherapy plan were developed. By using D̿ as the common prescription point of the treatment plans and plotting the tissue response probabilities vs. D̿ for a range of prescription doses, a number of plan trials can be compared based on radiobiological measures. Results: The applied plan evaluation method shows that in the head & neck cancer case the HT treatment gives better results than the MLC‐based IMRT (P + of 62.2% and 46.0%, D̿ to the internal target volume (ITV) of 72.3Gy and 70.7Gy, respectively). In the lungcancer and prostate cancer cases, the MLC‐based IMRT plans are better. For the lungcancer case, the HT and MLC‐based IMRT plans give a P + of 66.9% and 72.9%, D̿ to the ITV of 64.0Gy and 66.9Gy, respectively. Similarly, for the prostate cancer case, the two radiation modalities give a P + of 68.7% and 72.2%, D̿ to the ITV of 86.0Gy and 85.9Gy, respectively.
Discussion and Conclusions: Both MLC based‐IMRT and HT can encompass the often large ITV required while they minimize the volume of the organs at risk receiving high dose. There may exist clinical cases, which may look dosimetrically similar but in radiobiological terms may be quite different. In such situations, traditional dose based evaluation tools can be complemented by the use of P + − D̿ diagrams to compare treatment plans.
34(2007); http://dx.doi.org/10.1118/1.2761353View Description Hide Description
Purpose: Rapid developments in laser technology have facilitated proton (light ion) acceleration using laser‐induced plasmas. In this work, we investigate an experimental system for laser‐accelerated proton therapy.Method and Materials: Our system consists of a commercial 150 TW laser, custom‐made laser‐pulse compression and target chambers, particle selection and beam collimating devices, dosimetry monitoring systems and shielding constructions. We have performed initial laser‐proton acceleration experiments with thin aluminum foils as target materials. The maximum protonenergy was measured using CR‐39 film and a Thomson parabola ion analyzer. We have performed particle‐in‐cell simulations to investigate the optimal laser parameters and target configurations to facilitate laser‐proton acceleration and dosimetric studies. Results: The primary particles resulting from the laser‐target interaction are protons and electrons. Our particle in cell simulation predicted protons of up to 300 MeV and electrons of 20 MeV for a laser intensity of 1021 W/cm2. The maximum number was 1011 and 1012 per pulse for protons and electron, respectively. Our initial testing with a 1018 W/cm2 laser intensity (at 10 TW) produced up to 1 MeV protons with a broad energy spectrum. Conclusion: We have developed an experimental laser‐proton accelerator for radiation therapy applications. Initial experimental studies have demonstrated proton acceleration at low laser power levels. Further studies with laser intensities up to 2 × 1020 W/cm2 are being conducted with different target materials and configurations.