M Ahmad

CLINICAL UTILITY OF THE MODIFIED SEGMENTAL BOOST TECHNIQUE FOR TREATMENT OF THE PELVIS AND INGUINAL NODES

PURPOSE Low-lying pelvic malignancies often require simultaneous radiation to pelvis and inguinal nodes. We previously reported improved homogeneity with the modified segmental boost technique (MSBT) compared to that with traditional methods, using phantom models. Here we report our institutional clinical experience with MSBT. METHODS AND MATERIALS MSBT patients from May 2001 to March 2007 were evaluated. Parameters analyzed included isocenter/multileaf collimation shifts, time per fraction (four fields), monitor units (MU)/fraction, femoral doses, maximal dose relative to body mass index, and inguinal node depth. In addition, a dosimetric comparison of the MSBT versus intensity modulated radiation therapy (IMRT) was conducted. RESULTS Of the 37 MSBT patients identified, 32 were evaluable. Port film adjustments were required in 6% of films. Median values for each analyzed parameter were as follows: MU/fraction, 298 (range, 226-348); delivery time, 4 minutes; inguinal depth, 4.5 cm; volume receiving 45 Gy (V45), 7%; V27.5, 87%; body mass index, 25 (range, 16.0-33.8). Inguinal dose was 100% in all cases; in-field inhomogeneity ranged from 111% to 118%. IMRT resulted in significantly decreased dose to normal tissue but required more time for treatment planning and a higher number of MUs (1,184 vs. 313 MU). CONCLUSIONS In our clinical experience, the mono-isocentric MSBT provides a high degree of accuracy, improved homogeneity compared with traditional techniques, ease of simulation, treatment planning, treatment delivery, and acceptable femoral doses for pelvic/inguinal radiation fields requiring 45 to 50.4 Gy. In addition, the MSBT delivers a relatively uniform dose distribution throughout the treatment volume, despite varying body habitus. Clinical scenarios for the use of MSBT vs. intensity-modulated radiation therapy are discussed. To our knowledge, this is the first study reporting the utility of MSBT in the clinical setting.

Clinical Implementation of Prostate Image Guided Radiation Therapy : A Prospective Study to Define the Optimal Field of Interest and Image Registration Technique Using Automated X-Ray Volumetric Imaging Software

Alignment of the CBCT with the reference CT is called image registration (IR). The parameters for utilizing the automated Elekta XVI IR software for IGRT of the prostate still remain to be defined. In this study, we compare several automated XVI IR parameters to manual registration to identify the optimal automated IR technique for the prostate gland. 280 prostate IRs were conducted as follows: 210 automated, and 70 manual IR were performed using 70 CBCT scans of seven patients. The three arms of the automated registrations were: (i) extended FOI/Bone + grey scale (double IR); (ii) limited FOI/GS (single IR); and (iii) extended FOI/GS (single IR). Automated IRs were compared to manual IRs; x, y, z shifts, failures, and errors recorded for off-line analysis. Based on the most successful parameters, a departmental protocol was developed and 432 automated IR were performed (on 20 patients) for analysis. Automated IR were classified as: Successful, failed, error, or unregistered. In arm 1, the rate of successful, failed, error, and unregistered IR were 52.8%, 1.5%, 8.6%, 37.1%, respectively, arm 2: 90% successful, 10% failed, arm 3: 100% successful. Using the arm 3 parameters for the 432 automated IRs, the incidence of unregistered scans was 0%, rescanning was required in 1% of treatments, and the time for performing the auto IR was < 5.5 minutes. We found that extended FOI + single (GS) IR results in shifts comparable to manual IR using automated XVI software. We experienced multiple unsuccessful registrations with the other methods. We conclude that when utilizing the Elekta XVI automated IR software, the extended FOI/single IR results in successful registrations most often. In addition, it is currently effectively used in our clinical practice.

Intermediate Megavoltage Photon Beams for Improved Lung Cancer Treatments.

The goal of this study is to evaluate the effects of intermediate megavoltage (3-MV) photon beams on SBRT lung cancer treatments. To start with, a 3-MV virtual beam was commissioned on a commercial treatment planning system based on Monte Carlo simulations. Three optimized plans (6-MV, 3-MV and dual energy of 3- and 6-MV) were generated for 31 lung cancer patients with identical beam configuration and optimization constraints for each patient. Dosimetric metrics were evaluated and compared among the three plans. Overall, planned dose conformity was comparable among three plans for all 31 patients. For 21 thin patients with average short effective path length (< 10 cm), the 3-MV plans showed better target coverage and homogeneity with dose spillage index R50% = 4.68±0.83 and homogeneity index = 1.26±0.06, as compared to 4.95±1.01 and 1.31±0.08 in the 6-MV plans (p < 0.001). Correspondingly, the average/maximum reductions of lung volumes receiving 20 Gy (V20Gy), 5 Gy (V5Gy), and mean lung dose (MLD) were 7%/20%, 9%/30% and 5%/10%, respectively in the 3-MV plans (p < 0.05). The doses to 5% volumes of the cord, esophagus, trachea and heart were reduced by 9.0%, 10.6%, 11.4% and 7.4%, respectively (p < 0.05). For 10 thick patients, dual energy plans can bring dosimetric benefits with comparable target coverage, integral dose and reduced dose to the critical structures, as compared to the 6-MV plans. In conclusion, our study indicated that 3-MV photon beams have potential dosimetric benefits in treating lung tumors in terms of improved tumor coverage and reduced doses to the adjacent critical structures, in comparison to 6-MV photon beams. Intermediate megavoltage photon beams (< 6-MV) may be considered and added into current treatment approaches to reduce the adjacent normal tissue doses while maintaining sufficient tumor dose coverage in lung cancer radiotherapy.

SU-E-T-221: Investigation of Lower Energy (< 6 MV) Photon Beams for Cancer Radiotherapy

PURPOSE To study the potential applications of the lower energy (< 6MV) photon beams in the radiotherapeutic management of pediatric cancer and lung cancer patients. METHODS Photon beams of 2, 3, 4, 5 and 6MV were first simulated with EGS4/BEAM and then used for Monte-Carlo dose calculations. For four pediatric patients with abdominal and brain lesions, six 3D-conformal radiotherapy (3DCRT) plans were generated using single photon energy (2 to 6MV) or mixed energies (3 and 6MV). Furthermore, a virtual machine of 3 and 6MV was commissioned in a treatment planning system (TPS) based on Monte-Carlo simulated data. Three IMRT plans of a lung cancer patient were generated on this virtual machine. All plans were normalized to D95% of target dose for 6MV plan and then compared in terms of integral dose and OAR sparing. RESULTS For the four pediatric patients, the integral dose for the 2, 3, 4 and 5MV plans increased by 9%, 5%, 3.5%, 1.7%, respectively as compared to 6MV. Almost all OARs in the 2MV plan received more than 10% more doses than 6MV. Mixed energy 3DCRT plans were of the same quality as 6MV plans. For the lung IMRT plans, both the 3MV plan and the mixed beam plan showed better OAR sparing in comparison to 6MV plan. Specifically, the maximum and mean doses to the spinal cord in the mixed energy plan were lower by 21% and 16%, respectively. CONCLUSION Single lower energy photon beam was found to be inferior to 6MV in the radiotherapy of pediatric patients and lung cancer patients when the integral doses and the doses to the OARs were considered. However, mixed energy plans combining low with high energy beams showed significant OAR sparing while maintaining the same PTV coverage. Investigation with more patient data is ongoing for further confirmation.

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‐543: Build‐Up Region Dosimetry of Megavoltage Photon Beams for Breast Radiotherapy

PURPOSE To evaluate the accuracy of the dosimetry in the build-up region for 6-and 10-MV photons for beam modeling in Pinnacle treatment planning system (TPS) for breast treatment plans using brass mesh bolus. METHOD AND MATERIALS High-resolution %DDs for 6-and 10-MV photons were measured from surface-to-5 cm depth for selected field sizes with a depth resolution of 1-mm using an advanced Markus chamber and the region beyond was measured using Farmer-type cylindrical chamber. Gafchromic-media dosimetry and optically-stimulated luminescence dosimetry (OSLD) were used to measure doses under variable thicknesses of superflab and brass mesh bolus placed on a breast phantom irradiated by tangential fields. The photon beams emerging from a Clinac 2100C/D were simulated in the build-up region with a depth resolution of 1 mm using a validated Monte Carlo code BEAMnrc and DOSXYZnrc. The central-axis doses computed by the TPS which uses the CCC dose model were compared with those obtained from MC simulations and measurements. RESULTS As expected, the doses at first few mm depths from surface, measured with cylindrical chamber were much higher than the MC calculated doses due to volume-averaging effect. In buildup region (surface-to-2cm), doses measured with film, OSLD and parallel-plate chamber agreed with MC predictions within 2-5%. However, there were larger discrepancies between TPS calculated values and the measured or Monte Carlo values; especially for the dosimetry for a few mm of depth under the different bolus materials used in this study. CONCLUSIONS The TPS as implemented for routine radiotherapy used in this study does not provide sufficiently accurate dosimetry in the build-up region for breast radiotherapy using different bolus materials and the calculated dose distributions for treatment setups using brass mesh bolus need to be verified by independent measurements using Gafchromic films, OSLD or thin-window parallel-plate chambers and/or Monte Carlo simulations.

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.

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.

TU‐E‐BRA‐04: Real‐Time Automatic Fiducial Marker Detection in Low Contrast Cine‐MV Images

PURPOSE Intrafraction motion tracking using beam-line MV images have gained much attention because no additional imaging dose is introduced. Since MV images have much lower contrast than kV images, a robust marker detection algorithm is a pre-requisite. In this work, we develop a novel, fast, and robust method to detect implanted markers in low-contrast cine-MV patient images. METHODS Several marker detection methods have been proposed in the recent years. These methods are all based on template matching or its derivatives. Template matching needs to match object shape that changes significantly for different implantation and projection angle. While these methods require a large number of templates to cover the different situations, they are often forced to use a smaller number of templates to reduce the computation load because their methods all require exhaustive search in the ROI. We solve this problem by synergetic use of modern but well-tested computer vision and AI techniques - detect implanted markers utilizing discriminant analysis for initialization and mean-shift feature space analysis for sequential tracking. This novel approach avoids exhaustive search by exploiting the temporal correlation between consecutive frames and makes it possible to perform more sophisticated detection at the beginning to improve the accuracy, followed by ultrafast sequential tracking after the initialization. The method was evaluated using 1149 cine-MV images from 2 prostate IMRT patients and compared with manual marker detection results from 6 researchers. The average of the manual detection results is considered as the ground truth. RESULTS The average RMS errors of the automatic tracking from the ground truth are 1.9 and 2.1 pixels for the 2 patients (0.26mm/pixel). The standard deviations of the results from the 6 researchers are 2.3 and 2.6 pixels. CONCLUSION The proposed method can achieve similar marker detection accuracy to manual detection in low-contract cine-MV images.

TH-E-220-05: Minimization of Imaging Dose for Real-Time Respiratory Motion Monitoring during Arc Therapy

Purpose: To develop and evaluate a methodology for using hybrid MV‐kV imaging technique for real‐time respiratory motion tracking during arc therapy with minimized extra imaging dose while maintaining high 3D targeting accuracy. Methods: In our method, at the beginning 4 seconds of a typical arc delivery for tumors in the thoracic and abdominal region, the implanted marker positions are measured continuously using stereoscopic MV‐kV imaging. At later times, only treatment‐beam cine‐MV images are acquired. The 3D time‐varying marker positions are estimated by combining the 2D projection data and the correlative relationship between the directional components of marker motion established from stereoscopic imaging, with the constraint that the position minimizing the distance to the first principal component line segment is the most likely position. A multivariate linear regression method is used to predict the marker positions 310 and 460 ms in the future. To evaluate the proposed method, computer simulations were performed using over 70 hours of thoracic and abdominal tumor motion data from 46 patients. A Gaussian noise with zero mean and 0.5 mm standard deviation was added to the projected marker positions on the imagers to simulate the systemcalibration and marker detection errors. Results: The average RMS error of the 3D position estimation was 0.76 mm. At a 310 ms latency, the average overall RMS positioning accuracy was 1.23 mm. At a 410 ms latency, the error increases to 1.46 mm. Because the kV imaging is only used for a short period of time, the extra patient dose from imaging in IGRT can be reduced for at least an order of magnitude. Conclusions: The proposed clinically‐ready hybrid MV‐kV imaging method incurs minimal imaging dose to the patient as compared with other stereoscopic imaging techniques because of an informed exploitation of “dose‐free” cine‐MV imaging without sacrificing targeting accuracy.