A Chu

SU‐E‐T‐628: Effect of Dose Rate and Leakage Correction for Dosimetric Leaf Gap Measurement

PURPOSE To study the dose rate response of Mapcheck and quantify/correct dose rate/leakage effect on IMRT QA. Evaluate the dose rate/leakage effect on dosimetric leaf gap (DLG) measurement. METHODS Varian Truebeam Linac with HD120 MLC was used for all measurement, it is capable to adjust dose rate from 600MU/min to 5MU/min. Fluke Advanced Therapy Doisemter and PTW 30013 Farmer chamber for chamber measurement; SunNuclear Mapcheck2 with 5cm total buildup for diode measurement. DLG was measured with both chamber and diode.Diode response was measured by varies dose rate, while fixed mapcheck setup and total MU. MLC Leakage was measured with both chamber and diode. Mapcheck measurement was saved as movie file (mcm file), which include measurement updated every 50mSec. The difference between intervals can be converted to dose and dose rate and leakage response correction can be applied to them. RESULTS DLG measurement results with chamber and diode were showed as follows, the DLG value is 0.36 vs. 0.24mm respectively. Diode dose rate response drops from 100% at 600MU/min to 95.5% at 5MU/min as follows. MLC Leakage measured with diode is 1.021%, which is 9% smaller than 1.112% from chamber measurement. By apply the dose rate and leakage correction, the residue error reduced 2/3. CONCLUSIONS Diode has lower response at lower dose rate, as low as 4.5% for 5MU/min; diode has lower energy response for low energy too, 5% lower for Co-60 than 6MV. It partially explains the leakage difference of 9% between chamber and diode. Lower DLG with diode is because of the lower response at narrower gap, in Eclipse however DLG need to increase to makeup lower response, which is over correction for chamber though. Correction can reduce error by 2/3, the rest 1/3 can be corrected by scatter effect, which is under study.

SU-E-T-431: Vertically-Oriented Farmer-Type Chamber for Small-Field Applications

PURPOSE To introduce a non-conventional measurement setup using Farmer-type chambers to accommodate several situations of small-field dose measurements without compromising accuracy. The validation of this technique was demonstrated for photon small-field output measurements, and electron small-field percentage depth-dose (PDD) measurements. METHODS Initial chamber alignment was performed using the conventional (horizontally-oriented) chamber setup. A PDD was acquired for a 4×4 cm2 field size using this arrangement. This PDD was used as a positional reference for the vertically-oriented chamber (VOC) configuration. Next, a PDD was acquired for a 4×4 cm2 field size with the VOC. The PDDs were superimposed to find the effective shift of the VOC. Using the shifted VOC setup, photon small-field output factors were measured and compared to stereotactic diode output factor measurements. Additionally, electron smallfield PDDs were acquired using the VOC setup and results were compared to electron Monte Carlo (eMC) predictions in the Eclipse treatment planning system (TPS). RESULTS (1) For photon small-field output factors field-sizes 2×2 cm2 and larger, the difference between the VOC setup and SFD measurements were less than 0.8%. For field sizes less than 2×2 cm2 discrepancies ranged from 4.0 to 10.6%. (2) PDDs measured by VOC setup show better than 1.6% agreement as compared to eMC for all electron energies measured down to the 80% depth on the 2×2 cm2 PDD curve. Disagreement between the VOC setup measurement and eMC calculations for depths down to the 50% depth on the PDD curve is 3.6% or less. CONCLUSION Using the VOC setup, it is possible to use a conventional farmer chamber for small field-size measurements down to 2×2 cm2 field size without sacrificing the accuracy of measurements.

SU‐E‐T‐118: Small Dynamic Field Dosimetry by Gfachromic Film (EBT3) and 2D‐Array Diode

PURPOSE Small dynamic multi-leaves collimator (MLC) dosimetry presents challenges to clinical radiological measurements. This study was to evaluate the small dynamic field dosimetry using one-scan film algorithm (FilmQA Pro/Ashland Inc.) and MapCheck 2D-array (Sun Nuclear). Commercial diode-array would provide practical needs for dynamic field QA (e.g. IMRT or VMAT). Alternatively Chromic film dosimetry serves the same purposes as diode array with much better spatial resolution, wider dose range and less energy dependency. One-scan algorithm minimizes the problems of conventional film dosimetry, which suffered time-consuming process, numerous non-radiation artifacts. METHODS Both dosimeters along with chamber measurements (TN31010/PTW, 0.125cm3) at central-axis (CAX) as absolute dose comparisons were tested by 6MeV photon with 120HD-MLC under a Varian iX LINAC using dynamic MLC (with gaps of 1, 5, 10, 20, 50, 100mm width) sweeping through a 5.0cm jaws-opening size. The experiments were arranged by a 2cm solid-water buildup above tested films, and the 2cm water-equivalent intrinsic buildup over the MapCheck diode array at 100cm SAD setup. Each film-measurement was calibrated by 4 dose-points in the same film by one-scan algorithm. RESULTS CAX absolute doses from both dosimeters showed very close results (+/-0.3% deviation) from the according chamber measurements for relatively large dynamic MLC fields (gap size > 20mm). Otherwise each dosimeter presented different problems with mixed deviations of 4∼8% from CAX chamber measurements. Energy-dependence (from scatters) and low-dose bias had been well-known for diode performance, which was enhanced by small gap-width (< 1cm). Film dosimetry suffered from systematic noises, especially in low-dose (8∼10 cGy), even with the one-scan algorithm corrections. The problem of one-scan protocol may be improved by the cares of the choice for calibration dose values to avoid over-correction in low-dose. CONCLUSION Film provided much better spatial details (0.35mm/pixel), e.g. MLC interleaved leakage, than 2D diode-array, and similar overall dose accuracy compared to diode array.

WE‐C‐116‐12: Camer‐Rao Lower Bound Analysis of 1H Magnetic Resonance Spectroscopic Imaging for Breast with Singular Value Decomposition, LCModel and QUEST

PURPOSE LCModel and QUEST supply error estimate capability, however no standard basis available for unknown lipids mixture in breast. Singular Value Decomposition (SVD) with Linear Combination method was developed to implement desired CRLB function. The purpose of this study is to evaluate SVD, QUEST and LCModel quantification for breast MRSI with Camer-Rao Lower Bound (CRLB) analysis. METHODS Spectrum for 6 metabolite were simulated with Vespa (STEAM SVS, 3.0T, TE=42ms, TM=10ms), mixture spectrum was simulate with known ratio. jMRUI 4.0 (QUEST function) and LCModel 6.3 was used in the study, metabolite basis was generated from individually simulated data. In-house SVD was developed with Matlab 7.6 to combine Hankel SVD and Linear Combination method. The estimated FID signals are constructed from the quantification parameters, residue error spectrum was calculated by FFT of the difference of original FID and estimated FID. In order to study the CRLB, known simulated spectrum without noise was analyzed with SVD, QUEST and LCModel. Then introduce random noise, quantification residue was analyzed, and estimated parameters with CRLB were compared with results without noise. Sample patient breast MRSI data was collected with Siemens TrioTim (PRESS CSI, 3.0T, TE=80ms). RESULTS For simulated data, both Quest and LCModel can reliably quantify with SNR of 5 and signal as low as 17% of maximum; while SVD method can quantify with SNR of 10.Representative breast MRSI data were analyzed, LCModel residue error is 13.3% of original maximum and Cho CRLB is 56%; while SVD can achieve 3.7% and 3.6% respectively. CONCLUSIONS For all methods, CRLB estimations increase with decrease the SNR. SVD is sensitive to noise, since it has no prior knowledge to reduce noise, if noise is larger than signal, fake peak can be introduced; while LCModel and QUEST can not quantify breast data well with limited simulated lipid basis.

SU‐E‐T‐100: The Influence Edge Electrons in Small Fields: Emphasis by the Difference of Copper‐Cerrobend Cutout

PURPOSE Electron cutout edge scatter is an important factor determining small-field electron dose outputs. The laterally scattered edge electrons presenting wide-spread energy spectrum are usually ignored in Monte Carlo calculation. This report demonstrated the different characteristics in edge electron scatter were highlighted by the copper-cerrobend pair comparisons. Backgournds: There are three possible differences caused by the replacement of copper cutout, i.e. differed by bremsstrahlung, transmission, and edge-electron scatters. Monte Carlo simulation (Ebert et al.) showed the very small amount of bremsstrahlung can get out cutout, and 1.5cm thickness of both can both effectively stop primary electrons (Das et al.), and the photon transmission is negligible (Zhe et al.)Methods: The chamber-measured comparisons for circular copper-and Cerrobend-cutouts (1.5cm thickness) with diameters (1.0, 2.0, 3.0, 5.0, 7.5, 10.0, and 12.5 cm) were made using electron beams with energies (6, 9, 12, 16, and 20 MeV) from 3 Varian accelerators. To demonstrate the edge-electron near the surface of phantom (0.5cm and 1.0cm solid-water depth), a bottle-shape cutout illustrates the aperture open from narrow width (1cm) and progressively increased to 3cm. The surface edge-electron dose profiles were measured by Gfachromic film with one-scan film dosimetry technique (FilmQAPro/Ashland Inc.) and pp chamber (Markus/PTW). RESULTS From the PDD curves collected from chamber-measurement in water tank with all circular cutouts irradiated by all electron energies over 3 LINACs, the beam-quality specifier, R50, was consistently higher by using copper cutout. The PDDs in smaller diameter circular cutout can clearly demonstrated the higher R50 differed by the cerrobend and copper cutouts was due to (edge-scattered) electron. The film profiles showed wide-spread edge-electron energy presented in the filtering effect from the surface at 0.5cm to 1.0cm depth, and different characteristics between copper and cerrobend cutouts. CONCLUSION The different edge-electron characteristics between copper-cerrobend pair may provide the clue for edge-electron modeling.

TU‐G‐217A‐08: Camer‐Rao Lower Bound Analysis of 1H Magnetic Resonance Spectroscopic Imaging for Breast With Singular Value Decomposition Method and Linear Combination Method

Purpose: To study the Singular Value Decomposition (SVD) and Linear Combination (LC) quantification with Camer‐Rao Lower Bound (CRLB) analysis.Methods: In‐house Singular Value Decomposition (SVD) and LC program was developed with Matlab version 7.6 (Mathworks.com). Hankel SVD method (singular value decomposition of the acquired FID signal arranged in a Hankel matrix) is used to compute the signal poles and amplitude, and from them the signal frequencies and damping factors. In LC method, poles are generated from SVD analysis of averaged 10 FID, the amplitudes of the signal components were estimated by the least square method for individual FID signal. The estimated FID signals are constructed from the quantification parameters, residue error spectrum was calculated by FFT of the difference of original FID and estimated FID. In order to study the CR lower bound, known simulated spectrum without noise was analyzed with SVD and LC method. Then introduce random noise, quantification residue was analyzed, and estimated parameters with CRLB were compared with results without noise. Results: CRLB estimation is verified with the standard deviation calculation, CRLB is larger than and comparable to standard deviation. For small signal as low as 1.5% of maximum, SVD can detect at SNR 10, LC can handle SNR 5. For SNR 5, SVD can reliably detect signal like 12.5% of maximum. Typical patient data from breast tumor data was analyzed with SVD and LC methods too, results are reliable and consistent. Conclusions: CRLB estimation is verified with the standard deviation calculation. Both SVD and LC method can be used for human MRSI, the SVD method is sensitive to noise, since it has no prior knowledge to reduce noise; if noise is larger than signal, fake peak can be introduced. LC method combined with SVD generated poles can be exploited in MRSI with best results.

SU‐E‐T‐356: Dose Response of a 2D Diode Array for Non‐Primary Photon Radiation from a Dynamic MLC

Purpose: To quantify the dose responses of a 2D‐diode array for ‘non‐primary’ radiations (scatter and transmission from MLC and gantry‐head) under dynamic MLC dose deliveries. The ‘non‐primary doses’ have been overlooked by the 2D‐diode dose‐calibration because the primary radiation dominates over a large calibration field (10×10cm2). If non‐primary dose responses of diode are significant, it would cause systematic errors when non‐primary radiations become important (in small field). Methods: The dose response is the diode‐dose normalized by chamber‐dose. Three setups under a 6MeV photon beam (Trilogy) were measured by detector (diode and chamber) at central‐axis (CAX): (1) the CAX diode was accurately located at places: either within MLC‐leaf collimation (2.5 mm) or between leaves. This measurement field is entirely blocked by MLC. (2) Dynamic MLC exposures are tested by 6 different MLC‐gaps (1, 5, 10, 20, 50 and 100 mm) sweeping over the CAX detector. (3) To estimate the scatter dose but without MLC involved, the CAX detector is blocked from primary beam by jaws only but received scatter doses from narrow‐slit apertures with distances from CAX. Results: (1) Results showed the diodes tends to underestimate dose of lower energy radiation, e.g. scatters. Experiment (3) confirmed this by showing its decreased dose response by scatter source away from CAX; i.e. more lower‐energy scatters in larger lateral scattered angles. As the slit is narrowed, less scatters and higher diode response were observed. (2) 3% difference in diode response to MLC‐leakage vs. non‐leakage was detected. However the impact of MLC leakage is not substantial in dynamic MLC delivery unless MLC‐gap is narrow enough (e.g. 1 mm). (3) Overall non‐primary radiations still significantly present as gap<10mm by 3% criterion. Conclusions: The diode dose responses to non‐primary radiations can cause more than 3% systematic errors for small‐aperture dynamic MLC QA unless appropriate corrections are made.

SU‐E‐T‐269: The Evaluation of Copper as an Alternative for Cerrobend Electron Shielding

PURPOSE To evaluate the replacement of Cerrobend by copper for electron beam cutouts. METHODS The dosimetric comparisons for circular copper-and Cerrobend-cutouts with diameters (1.0, 2.0, 3.0, 5.0, 7.5, 10.0, and 12.5 cm) were made using electron beams with energies (6, 9, 12, 16, and 20 MeV) from 3 Varian accelerators. A PTW Farmer chamber (0.125cc-volume) was used for larger cutouts (diameters > 2cm), and an electron-diode for the 2 smallest cutouts. Also a Markus parallel plate chamber was used. RESULTS (1) The tests showed little difference for the electron dosimetric characteristics, Eo, Eop, R50, Rp, and dmax. For larger cutout, the parameters were virtually the same for copper and Cerrobend. for smaller cutout (diameter = 3cm), small discrepancies were observed i.e. differences < 1mm for R50, Rp and dmax, =0.1MeV for Eop, and =0.3MeV for Eo. (2) The larger-cutout outputs at dmax were also virtually the same (difference = 0.6%). For smaller cutouts (diameters = 3cm), the copper outputs were 2.0%∼5.0% higher than Cerrobend. (3) For lower energy electrons (<12MeV), more larger-angle scattered electrons from higher-Z Cerrobend raise the Cerrobend percentage-depth-dose (PDD) curve at shallow-depths, and more forward scatter dose after dmax from lower-Z copper shifts the copper PDD slightly away from the one of Cerrobend. for higher energy electrons (= 12MeV), the shallow-dose difference becomes smaller for both cutouts, but even more forward-scattered dose from copper shifts coppers PDD further away from Cerrobends. (4) The higher X-ray transmission through copper is also observable; i.e. 12%, 10%, and 7% for 20MeV, 16MeV, and 12MeV, respectively, but such small transmitted amount is clinically insignificant. CONCLUSIONS Except for a higher x-ray transmission, other dosimetric differences brought in by the replacement of Cerrobend by copper cutout are negligible.

SU‐E‐I‐123: Quantification of 1H Magnetic Resonance Spectroscopic Imaging for Breast Cancer with Singular Value Decompsition (SVD) Method

Purpose: To study the feasibility of Singular Value Decomposition (SVD) method to quantify the MRSI data, and the effect of signal to noise ratio on SVD method was studied. Methods: In‐house SVD program was developed with Matlab version 7.6 (Mathworks.com), in frequency domain water signal peak around 4.7ppm is used for referencing shifting. Hankel SVD method is used to compute the signal poles and from them the signal frequencies and damping factors. Known spectrum without noise was analyzed. Then introduce random noise, Gaussian filter of various frequency was used to reduce noise effect, wavelet threshold de‐noise method was also tested with various hard and soft threshold selection methods. Human breast MRSI data were analyzed with optimized parameters.Results: SVD can easily detect the small signal in low noise GSH data at 2.54ppm, GSH signal ratio to NAA is only 0.5%. However SVD can not detect the signal after added random noise of SNR 15. By add Gaussian filter of 2.5Hz, peak could be detected by suppress noise, however if noise level is larger than the signal, fake peak could be resulted. For wavelet threshold method, hard threshold with wavelet principle of Steins Unbiased Risk Estimate (SURE) can generate best results, random noise of SNR 7 can be reliably detected. Typical patient data from breast tumor data was analyzed, even the detected cho peak is only 0.5% of water peak, it can be detected reliably, and the max residue error is 0.2% of water peak. Conclusions: SVD method can be used for human breast MRSI. the SVD method is sensitive to detect the signals as low as 0.5%. However it is suspected to noise, since it has no prior knowledge to reduce noise; improved SVD method with wavelet threshold de‐noise can exploited in MRSI with SNR of 7.

SU‐E‐T‐334: A Rapid Method for Testing the Accuracy of Heterogeneity Correction Algorithms Using a 2‐D Detector‐Array QA Tool

Purpose: To demonstrate an efficient quality assurance (QA) method using a 2‐D diode array to verify the absolute dose distribution generated by heterogeneity corrected treatment plans. Methods: The AAA, ETAR, and modified Batho heterogeneity corrected Eclipse treatment plans were compared against the measured data obtained with 6 and 18MV (Varian 2100CD) photon irradiation with the following two geometric setups: (1) Reference setup: An inhomogeneous phantom was sandwiched by a solid water slab (Gammex) and the 22×22cm2 MapCHECK detector (Sun Nuclear) under the incident photon beam that was perpendicular to the slabs. The sandwiched slab material consisted of low‐density foam occupying half of the detector area (11×22cm2), and another solid‐water slab on the other half. (2) Tested setup: two blocks of solid‐water separated by another slab of low‐density foam were erected vertically on the surface of the 2‐D array. A slightly oblique beam angle (<30o) allows primary photon field to pass through each slab of solid water and foam to project the field onto the array. Results: The measured 2‐D dose profiles from the first configuration (Reference setup) were reasonably predicted by all three algorithms, but only AAA among the three taking into account the electron transport shows the best results for the second setup. Conclusions: The geometric design of the second setup with oblique beam angle can enhance the characteristics of the complicated dose profile projected onto a 2‐D array as the transversal and longitudinal electron transports carry out non‐local dose deposit throughout inhomogeneous slabs by oblique primary photons. The combination of build‐up and build‐down profiles is usually illustrated by film dosimetry with much a more time‐consuming procedure.