bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐277: Dosi Metric Comparison of the New Siemens 160 MLC and the Varian Millennium 120 MLC
Authors: Appenzoller L , Perera H , Sullivan P , Hand C , Hasson B
Published: August 2010

Purpose: Recent models of multileaf collimators (MLC) have smaller leaf widths and increased target conformality. The new Siemens 160 MLC and the Varian Millennium 120 MLC models both have individual leaf widths of 5 mm projected at isocenter. This study quantifies and compares the physical penumbra and transmission of the Siemens 160 MLC and the Varian 120 MLC for 6 and 10MV photon beams. Method and Materials: Penumbra was measured for 5×5 cm2 and 10×10 cm2 field sizes both on‐axis and off‐axis at depths ranging from 0.5 cm to 10 cm depth. Measurements were performed using the new Sun Nuclear Edge™ diode detector in a Scanditronix‐Wellhoffer scanning tank and confirmed with Kodak EDR2 film. The average of interleaf and intraleaf transmission and end leaf transmission were measured with Kodak EDR2 film and confirmed with a 0.6cc farmer type ion chamber. The films were scanned using a Vidar VXR 16‐bit scanner. The penumbra was determined for inplane and crossplane profiles. Results: The average central and off‐axis penumbra was 4.0 mm and 4.9 mm for the Siemens MLC and 4.3 mm and 4.5 mm for the Varian MLC, respectively. The penumbra increases with energy, field size, and depth for both MLC designs. The average of interleaf and intraleaf transmission was determined to be 0.7% for the Siemens MLC and 1.9% for the Varian MLC. End leaf transmission averaged 19.1% for the Siemens MLC and 30.3% for the Varian MLC. Conclusion: The Siemens MLC demonstrates a sharper off‐axis penumbra. while the Varian MLC demonstrates a sharper off‐axis penumbra. Penumbra for the Siemens MLC is on average 0.5 mm sharper on one side of the inplane profile. Due to the design and role of the Siemens MLC as a primary collimator, transmission was less through the Siemens MLC leaves than the Varian MLC.
SU‐GG‐T‐283: Tool for Automated Monthly Quality Assurance on Tomotherapy Using Film
Authors: Yaddanapudi S , Ramirez J Garcia , Green O Pechenaya , Wooten H , Palaniswaamy G , Rangaraj D , Mutic S , Goddu S
Published: August 2010

Purpose: To efficiently and accurately perform routine monthly quality assurance (QA) tests on a Helical Tomotherapy unit using an in‐house developed phantom and software Method and Materials: Helical Tomotherapy is an image guided intensity modulated radiation therapy delivery system. The system uses a dynamic delivery in which the gantry, treatment couch and multileaf collimator leaves are all in motion during treatment. The dynamic nature of helical tomotherapy requires QA that is different from a conventional linear accelerator. According to the AAPM Task Group report 148 (QA for Helical Tomotherapy) and previous guidelines, one needs to check longitudinal and transverse profiles along with field widths as part of the monthly QA. To perform this task we had to set up the tomotherapy water tank and use the tomotherapy electrometer measurement system (TEMS) for data collection and analysis. To simplify and expedite this process a film phantom was designed and MATLAB developed software was used for analyzing the irradiated film. One could simultaneously obtain several parameters like the longitudinal and transverse profiles, field widths, couch linearity and relative output factors for all the field widths from the film. A Kodak EDR2 film is placed in the phantom and exposed on the tomotherapy system to obtain all the field widths along with a step wedge for calibration on the same film. The irradiated film is then analyzed using the in‐house developed software and a report is printed out. Results and Conclusion: Using the phantom we could check the longitudinal and transverse profiles, field widths, couch linearity and relative output factors for all the field widths on the system. When compared to the couple of hours it would take for beam scanning, the film phantom and software have enabled us to cut down and quicken part of our monthly QA process to <30 min.
SU‐GG‐T‐284: A Method of Using Ir192 Brachytherapy System to Check the Constancy of PTW 2D‐Array Seven29 Ion Chambers
Authors: Yeh Kuan Chuan , Su Chih Chung , Chuang Ke‐Shih , Chen Rong‐Run , Huang Ying‐Chiang D
Published: August 2010

Purpose: To evaluate the feasibility of constancy check of PTW 2D‐Array seven29 ion chambers by Ir‐192 HDR Remote after loading system and to provide a method of checking 2D‐Array performance that ensures the results of IMRT routine QA is reliable. Method and Materials: We used Nucletron microSelectron HDR brachytherapy system which has an Ir192 source and well control of source position. By matching the markers on the surface of PTW 2D‐Array seven29 and the Nucletron source position checking tube, we collected each ion chamber's reading row by row. We adjusted the influence of Ir192 decay by mathematically calculation. By comparing the measurement data with calibration file from PTW Company and analyze data by EXCEL Macro Enterprise, we can evaluate the performance of each ion chamber. Results: The average of total measurement time was 162.97 mins, including setup and measurement time. And in 2 hours spans, the influence of Ir192 decay is 0.078%. The 729 ion chambers performance grouped into five types :< 1%, 1–2%, 2–2.5%, 2.5–3%, >3%. There have 8 ion chambers under1%, 692 ion chambers in 1–2%, 22 ion chambers in 2–2.5%, 4 ion chambers in 2.5–3% and 3 ion chambers over 3%. Conclusion: This study provides a method to check the constancy of PTW 2D‐Array seven29. By this method, we can check this IMRT QA tool's performance in hospital. Moreover, we can save time and money in order to send this whole package to Germany Calibration Center.
SU‐GG‐T‐285: Combined Effect of Intrinsic Energy Dependence and Intrinsic Linearity on TLD‐100 Response to Moderately Filtered 50 KVp X‐Ray Spectra Relative to 60Co
Authors: Liang Q , Davis S , DeWerd L
Published: August 2010

Purpose: To determine the combined effect of intrinsic linearity and intrinsic energy dependence on the relative response of LiF:Mg,Ti thermoluminescent dosimeters (TLD‐100) to moderately filtered 50 kVp x‐ray spectra versus 60Co at low and high doses. Method and Materials: Two sets of TLD‐100 chips were used to determine the relative response of TLD‐100 to an x‐ray beam versus 60Co at two doses. One set of TLDs was irradiated to doses of 0.1 Gy and 10 Gy in an x‐ray beam matched to the NIST M50 beam, while the other set was irradiated with 60Co to the same dose levels. The intrinsic linearity was found by taking the ratio of the average corrected TL per unit dose at 0.1 Gy and 10 Gy for each beam quality. In addition, the response of TLD‐100 to the M50 beam versus 60Co was found by taking the ratio of the average corrected TL for each beam quality at a particular dose. Results: The intrinsic linearity correction for TLD‐100 at a dose of 10 Gy was 0.729±0.012 for 60Co and 0.819±0.024 for the M50 x‐ray beam, which includes the overresponse of the photomultiplier tube. The response of TLD‐100 irradiated with the M50 x‐ray beam relative to 60Co is 1.122±0.027 at 0.1 Gy and 0.999±0.021 at 10 Gy. Conclusion: TLD‐100 exhibits greater supralinearity at 10 Gy when exposed to 60Co versus the low‐energy x‐rays. This indicates that intrinsic linearity is dependent on both dose and beam quality. In addition, there is a discrepancy of more than 10% in the response of TLD‐100 to M50 versus 60Co from 0.1 Gy to 10 Gy, due to the combination of intrinsic energy dependence and intrinsic linearity. This work concludes that both of these effects must be properly accounted for in dose measurements using TLD‐100, especially for high doses.
SU‐GG‐T‐290: Virtual SSD and Mean‐Square Angular Spread for Broad Electron Beams Measured with a 2D Ion Chamber Array
Authors: Li Texin , Gong Jason , Tong Shidong , Fountain Betty , Duffy Edward
Published: August 2010

Purpose: To explore the feasibility of measuring virtual source‐to‐skin distance and mean‐square angular spread for broad electron beams with a 2D ion chamber array. Method and Materials: The virtual SSD (Svir) and mean‐square angular spread (θvir2) for 6, 9, 12, 16, and 20 MeV electron beams from a Clinac 21EX (Varian Medical Systems) Linac were measured using a 2D ion chamber array PTW seven29 (PTW Inc.). Cone size 20 by 20 was used throughout the measurement to ensure side‐scatter equilibrium. The virtual SSD was measured using FWHM method (based on measurements of variation of field size with the nominal source‐to‐detector distance). The PTW seven29 was exposed perpendicularly to the beam axis at nominal SSD of 100, 105, 110,115, 120, and 125 cm. Using MultiCheck software (PTW Inc.), the FWHM and the penumbra of each beam profiles were obtained. A back projection of FWHM with nominal SSD giving straight line, the virtual SSD was deduced from the slope and the intercept of this line. At the same time, back projection of penumbra with air gap giving a straight line, the mean‐square angular spread was calculated from the slope of this line along with published parameter. Results: The measured virtual SSD and mean‐square angular spread for broad electron beams were listed below:(see PDF for table). Conclusion: Both the virtual SSD and mean‐square angular spread for broad electron beams were measured with PTW seven29 2D‐Array. It provided the necessary parameters to perform clinical broad beam electron dosimetry, especially in a filmless department.
SU‐GG‐T‐298: A New Method for Quality Control of Multileaf Collimators Using a 2D Ion Chamber Array
Authors: Moreno R Diaz , Venencia C , Garrigo E , Pipman Y
Published: August 2010

Purpose Verification of the accuracy of multileaf collimator (MLC) leaf positioning is a vital component in a quality control program in any Radiation Therapy Department. The decline of film‐based dosimetric systems reinforces the trend to use 2D detector arrays. This work introduces a new method, based on the principle of partial volume response of detectors that overcomes the problem of the somehow inferior resolution of a 2D ion chamber array. Method and Material The 2D array PTW‐729 was used for the verification of the 82 leaf‐MLC Optifocus of a Primus linear accelerator, Siemens. The partial volume response curve for each ion chamber was obtained by irradiating variable rectangular patterns, similar to that used in the Bayouth Test, so that each leaf covered a specific detector in a known proportion. Afterwards, it was developed an algorithm implemented into Matlab to predict the position of the leaf from the detector normalized response. Planes with intentionally introduced deviations of the leaf positions of ±1mm and ±2mm were irradiated upon the PTW‐729 and the predicted positions were compared with those included in the planification files. Result It was possible to detect, and correctly quantify, all the positioning errors into leaves positioning files, with a minimum number of false errors. The results obtained with this method compare favorably with those obtained with the Bayouth test using the same radiation patterns. Conclusion This method provides a superior substitute for film based QA method of MLC performance, with excellent spatial resolution. It detects and correctly quantified all the positioning errors intentionally introduced. It provides and effective and easy to use tool for quantitative measurement of MLC leaf positions, without compromising resolution. It can be easy applied to other 2D array as log as they exhibit a partial volume detector response.
SU‐GG‐T‐310: Dose Perturbation Caused by MOSFET Dosimeter during in Vivo Dosimetry of Photon and Electron Beam Radiotherapy ‐ a Film Dosimetry Study
Authors: Manigandan D , Ganesan S , Aruna P , Bharandharani G , Subramani V , Sharma S , Rath G
Published: August 2010

Purpose: In in vivo dosimetry, build‐up caps were used to (i) promote electronic equilibrium (ii) reduce correction factors (iii) avoid steep dose gradient of depth dose curve (iv) radiation damage per unit accumulated dose. However, it attenuates the beam both qualitatively and quantitatively. Hence, the dose attenuation MOSFET detector with and without build‐up was studied at different depths for 60Co beams, 6 and 15 MV X‐rays and electrons using film dosimetry. Method and Materials: Dose attenuation was studied using solid water phantom. For electron beams, Kodak Xomat‐V film was used and EDR‐2 films used for photon beams. In 6, 9 12 and 15 MeV, attenuation of bare MOSFET was studied. In 15 MeV, Wide Energy Hemi‐spherical build‐up cap (WEHSBC) was used as recommended by the manufacturer. For 60Co beams, 5 mm bolus was used. WEHSBC was used for 6 and 15 MV X‐rays. Dose profiles were analyzed at surface and Dmax. Doses were normalized at the central axis of the field measured without MOSFET. Results: Presence of bolus with the MOSFET increased the surface dose up to 85% and decreased the dmax dose to 5%. Field area of 1.5 × 1.5 cm2 was perturbated. Attenuation of WEHSBC at dmax is 21.1% and 4.8% for 6 and 15 MV. Total area of 1.4 × 1.4 cm2 and 1.6 × 1.6 cm2 attenuated. Bare MOSFET showed 10%, 3.9%, 2.9% and 2.8% reduction in 6, 9, 12 and 15 MeV, respectively. No attenuation was found at dmax. In 15 MeV, MOSFET+WEHSBC, showed 35.6% and 54% reduction at surface and dmax. Conclusion: WEHSBC can be used as a build‐up cap for 6 and 15 MV X‐rays with limited number of fractions. Attenuation of bare MOSFET in electron beams is negligible. WEHSBC should not be used for electron beam in vivo dose measurements.
SU‐GG‐T‐322: Dose Perturbation Due to Metallic Breast Tissue Expander in Electron Treatment of Breast Cancer
Authors: Srivastava S , Das I , Andrews J , Kumar A , Cheng C , Johnstone P
Published: August 2010

Purpose: Breast expanders are made of silicone elastomere filled with silicone gel or saline that is electively used for cosmetic purposes in conservative breast surgery. Some expanders may contain a steel port embedded for saline injection (metallic breast expander, MBE). Electron beams are used for radiation treatment due to sharp dose fall off and limited range to minimize dose to underlying structures like lung and heart. However the presence of MBE produces dose perturbation which is investigated in this study. Materials & Methods: An MBE was acquired from the manufacturer (Mentor, Santa Barbara, CA) for the electron dose distribution study. A small water phantom with the MBE mounted at a depth of 2 cm was scanned on a GE CT simulator with a slice thickness of 2.5 mm and the CT dataset was sent to an Eclipse treatment planning system (TPS) with Monte Carlo (EMC) algorithm. Electron beams of 9–22 MeV were used in this study from a Varian accelerator. Measurements were performed and compared with calculations. Results: The measured dose has pronounced backscatter (5%) at the entrance and significant dose reduction up to 70% on the exit side of the MBE respectively. On the other hand, Monte Carlo calculation did not show backscatter and the magnitude of the dose reduction was limited to only 30%. The differences between measured and TPS is energy dependent that varied between −140% to −80% for 9MeV‐22MeV beam. Conclusions: The dose perturbation in the presence of MBE is significant and cannot be predicted accurately by EMC. The differences could be due to CT‐electron density calibration and probably poor modeling of the MC for high‐Z materials. Based on these finding, it could be concluded that high‐Z materials in electron beam should be avoided or measured data should be incorporated in the TPS for actual dose.
SU‐GG‐T‐323: Verification of the Accuracy of the Proton Range Compensator Scatter Factor and Dose within Inhomogeneity Calculations Using the Eclipse Treatment Planning System
Authors: Pollard J , Poenisch F , Sahoo N , Zhu X , Gillin M
Published: August 2010

Purpose: To determine the accuracy of the proton range compensator scatter factor (CSF) and dose near inhomogeneity calculations in the Eclipse Treatment Planning System version 8.1 (TPS). Method and Materials: A head phantom was constructed that contains bone‐equivalent structures as well as several channels designed for ion chamber placement. A plan was created in Eclipse using the CT scan of the phantom that calculated dose at several selected reference points. Ion chamber measurements were made with a .125 cc cylindrical Farmer chamber at the same locations in the Eclipse plan. Measurements and calculations in water‐equivalent phantoms were made with and without compensator for clinical patient plans to determine CSF. Results: The measured doses at the selected points in a low dose gradient region were found to be within 1 to 6% of those from TPS. Dose comparisons between measured and Eclipse estimated dose for points lying closer than 1cm from the bone‐equivalent material/water interface, gave dose estimates that were 11% to 14% lower than measured values. The distance to agreement were within the range uncertainties associated with CT number stopping power calibration. In addition, in a homogeneous phantom we calculated the compensator scatter factors (CSF) by taking the ratio of the verification plan dose without the compensator at the samewater to the verification plan dose without the compensator at the same water equivalent depth for 4 clinical patients. We calculated CSF values for these patients between 0.965 and 1.062; compensator and patient scatter factors (CPSF) values from 0.983 to 1.068 and patient scatter factors (PSF) ranging from 0.97 to 1.019. Conclusions: Our measurements indicate that Eclipse reasonably estimates dose within 6% of measurements at regions near the CAX approximately 1cm or further from sharp inhomogeneities.
SU‐GG‐T‐326: Experimental Validation of a Scintillating Fiber Detector for Real‐Time Quality Control of MLC‐Driven Radiotherapy Treatment
Authors: Goulet M , Gingras L , Beaulieu L
Published: August 2010

Purpose: To validate a novel type of fluence monitoring detector based on the optical attenuation of scintillating fibers to be used for on‐line quality control of radiotherapy treatments. Method and Materials: 20 long scintillating fibers were aligned along the direction of motion of each of the 20 central pairs of leaves on a Varian Clinac iX MLC and coupled on both ends to a clear optical fiber to enable light collection. Following the theoretical model of scintillation collection based on optical attenuation previously developed, the central position (xc) and integral fluence. Φint) of various radiation fields were calculated and compared to the expected values as deduced from a radiographic film and a planar dose (PD) calculated from the treatment software. Results: The difference between the measured and calculated xc was within 2mm for more than 92% of the fields with a mean around 0.8mm. Of all these deviations, 93% can be explained by statistical variations (Poisson statistics). For Φint, the difference was less than 2% in 89% of the fields, with a mean around 0.9%. Although most of these discrepancies cannot be explained by statistical variations alone, they can be imputed on a small miss‐alignment of the fiber with the collimator leaves and small systematic errors with film processing and planar dose calculation. Nevertheless, any absolute displacement of MLC leaf pair of more than 3mm or any fluence variation over 3% can be detected by our system. Conclusion: This work validates the principle that a detector based on the optical attenuation of scintillating fibers can achieve real‐time quality control of a radiotherapy treatment with good precision. At the moment, the detection threshold for Φint is limited by the robustness of the fiber positioning system. This threshold is expected to fall below 2% with a more robust design.
SU‐GG‐T‐329: Gafgui: An Open‐Source Project for Radiochromic Film Dosimetry
Authors: Bouchard H , Lacroix F
Published: August 2010

Purpose: A code, named Gafgui, is developed to handle most features required for efficient and accurate radiochromic film dosimetry. Gafgui is distributed using the GNU General Public Licence in order to allow the medical physics community to have access to a performing analysis tool and to share algorithms. This could contribute to improving the efficiency of research projects and clinical applications, especially for centers where the access to such tool is limited. Materials: The code is developed using the Matlab® (The Mathworks, Inc., Boston, MA) platform and is designed for both clinical and research use. Gafgui handles 48‐bits tiff images format as well as RIT (Radiological Imaging Technology, CO) images. Single or dual‐channel analysis modes are available, to allow analysis of, for example, EBT and EBT2 films (International Specialty Products, Wayne, NJ). A variety of tools are included: image manipulation, an automated film characterization method, an automated method for scanner homogeneity correction as well as a complete uncertainty analysis of film response based on a recent exhaustive study of realistic uncertainties in EBT film dosimetry, suitable for clinical or research applications. Data analysis include average dose over a region of interest as well 1D and 2D‐distributions. Results: In our center, the development of Gafgui has allowed improving the workflow in radiochromic film dosimetry. It has also enabled the implementation of IMRT dose verifications as well as research in nonstandard beams reference dosimetry. The code design shows great potential for developing applications, since users are allowed to modify or add features as they wish and share it with other users. Future developments will be accessible in regularly updated versions and available to the medical physics community.
SU‐GG‐T‐383: Investigation of Water Equivalency of PRESAGE Dosimeters for Electron Beam Radiotherapy
Authors: Gorjiara T , Hill R , Kuncic Z , Baldock C
Published: August 2010

Purpose: To evaluate water equivalency of PRESAGE dosimeters for electron beam radiotherapy. Method and Materials: In this work, three different PRESAGE polymer dosimeters were evaluated: the currently available PRESAGE as well as two with a new chemical formulation. Total mass stopping powers were calculated over the energy range 10keV‐20MeV using the NIST ESTAR database for the three PRESAGE dosimeters and water. Monte Carlo modeling was used to determine the differences in depth doses between the PRESAGE dosimeters and water for a clinical 12MeV electron beam with a 10×10 cm2 field size. The Monte Carlo calculations were performed using the EGSnrc/BEAMnrc package. The BEAMnrc user code was used to calculate the phase space file for the electron beam. The phase space file generated by BEAMnrc were then used to calculate depth dose curves for each of the three PRESAGE dosimeters and water using the DOSXYZnrc user code. Results: Above 10MeV, total mass stopping power of the existing PRESAGE dosimeter has the highest differences with the value of water due to having the highest effective atomic number. For the calculated relative dose versus water equivalent depth, new PRESAGE dosimeters were in better agreement with the depth doses in water. For depths of less than 2cm water equivalent, the relative dose in water is up to 2% higher than the new PRESAGE dosimeters and 3% higher than the existing PRESAGE dosimeter. This can be attributed to higher collisional stopping power of water than the PRESAGE dosimeters due to its lower effective atomic number than the PRESAGE dosimeters. The maximum energy deposition in the three PRESAGE dosimeters is approximately in the same depth as water (2.7cm depth). Conclusion: The PRESAGE dosimeters with the new chemical formulations are more water equivalent than the existing PRESAGE dosimeter and are suitable for dosimetry of electron beams.
MO‐D‐BRA‐02: Image‐Guided Drug Delivery Developments in Cancer
Authors: Lanza G
Published: August 2010

Rapid advancement in cancer imaging is giving the phrase “seek and destroy” new meaning in the discovery and treatment of cancer. Hybridization of imaging technologies provides high‐resolution visualization of cancer needed to direct tissue‐ablating ultrasound or to guide localized catheter deposition of chemotherapeutic release matrices. The use of Nanomedicine probes, envisioned for almost two decades, have now begun to reach the clinic, promising sensitive image‐based detection, characterization and quantification of cancer in combination with delivering classic chemotherapeutics or newer DNA and RNA‐based treatments. Collectively, these novel image‐guided drug delivery approaches present a unique opportunity to personalize and focus cancer management strategies toward improved clinical outcomes with reduced morbidity. While the path from bench to patient is challenging, the vision of what can be achieved is evolving in animal models and patients alike. In a world economy in which human and economic resources are increasingly precious and pharmaceutical development risk remains high, image‐guided drug delivery presents an unprecedented opportunity for academic, government, and private sectors to collaborate and to accelerate the clinical translation pace of these high impact technologies.
MO‐D‐204B‐01: Characterizing Uncertainties in Hypoxia Imaging‐Based Dose Painting Prescriptions
Authors: Bowen S , Bentzen S , Jeraj R
Published: August 2010

Purpose: Inadequate characterization of variability from dose escalation models limits the precise definition of dose painting prescriptions. The purpose of this study was to quantify uncertainties in hypoxia imaging‐based prescriptions using an empirically‐driven and patient‐specific model. Materials and Methods: The model coupled an electrochemical formalism, which related Cu‐ATSM (hypoxia surrogate) uptake and pO2, with a modified oxygen enhancement ratio (OER) equation to yield non‐uniform dose prescriptions. This methodology was applied to eight head and neck cancer patients who underwent Cu‐ATSM PET/CT scans. Integral dose escalation was determined as a function of tumor hypoxic proportion (HP2.5 = percent tumor voxels with pO2 < 2.5 mmHg). Model parameters included intracellular acidity ‐ pH, dose boost as a function of hypoxic proportion ‐ Dboost(HP2.5), max OER ‐ OERmax, and half‐max sensitization oxygen tension ‐ Pmid. Parameters were varied within published ranges for this patient population, and uncertainties were propagated to variations in the maximum dose heterogeneity Results: Model parameters had differing impact on dose prescription uncertainties. Over the range of pH values [7.1–7.3], variation in MDH was 4 percent (maximum 9 percent). Within the 95 percent confidence intervals of the Dboost(HP2.5) functional fit, variability in MDH was 5 percent (maximum 14 percent). Between the limits of OERmax values [1.4–3.0], variation in MDH was 1 percent (maximum 2 percent). From the array of Pmid values [2–5 mmHg], variation in MDH forallparameterswas 1 percent (maximum 2 percent). The total variation in MDH for all parameters was 10 percent (maximum 17 percent). Conclusions: Dose prescription variability was on the order of 10 percent. Variations were due primarily to uncertainties in cellular acidity and dose boost functional fit, which are parameters governing calculated oxygen tension distributions. Ultimately, the derivation of a hypoxia imaging‐based prescription function will require fitting model parameters to outcome data in dose escalation clinical trials.
MO‐D‐204B‐02: Temporal Dose‐Response of Normal Lung Tissue in Patients Treated with Stereotactic Body Radiation Therapy for Lung Tumors
Authors: Diot Q , Kavanagh B , Gaspar L , Raben D , Stuhr K , Miften M
Published: August 2010

Purpose: To study the temporal changes of normal lung tissue dose‐response for patients with lung tumors who received stereotactic body radiation therapy (SBRT). Methods: Between 2003 and 2009, 63 patients received a hypofractionated treatment with a maximum of 5 fractions and a median total dose of 54 Gy (range, 30–60). RT‐induced lung density changes were evaluated after fusion of planning CT scans with a maximum of 5 post‐RT follow‐up scans corresponding to interval periods of approximately 3, 6, 12, 18, and 24 months after treatment. Patient specific dose‐response curves (DRC) were obtained by averaging CT number changes for regions receiving the same dose at 10 Gy intervals. In a secondary analysis, SBRT dose schedules were converted into 2Gy‐equivalent using a biological equivalent dose (BED) model. Results:At 6 months, transient increases of up to 100% were observed compared to the pre‐treatment lung density. After 12 months, the density changes stabilized to less than 50% of pre‐treatment levels. Different evolutions of the lung density were observed for BED levels of 0–45 Gy and 45–110 Gy. For the late response, no significant increase of lung density was visible for doses below 45 Gy. Above this threshold, the dose‐density change was linearly increasing but not as rapidly as after 6 months. Conclusion: This analysis is the first report, to our knowledge, of SBRT‐induced lung density changes showing that the lung density temporal response varied significantly with the received dose. More than 90% of the lung volume experienced an acute response that reversed to nearly no late response, indicating that the lung morphology was mostly unaffected after 12 months. Since the acute response also partially reversed for the remaining 10%, SBRT was expected to only marginally affect long term lung function.
MO‐D‐204B‐04: A Quantitative Metric Derived from DCE MRI for Assessment of Liver Response to Radiation Therapy
Authors: Wang H , Thomas A , Feng M , Pan C , Balter J , Chenevert T , Hussain H , Haken R Ten , Lawrence T , Cao Y
Published: August 2010

Purpose: To investigate a quantitative metric derived from dynamic contrast enhanced (DCE) liver MR imaging for hepatic perfusion response to radiation therapy (RT). Materials and Methods: Ten patients with intrahepatic cancers were treated by fractionated conformai RT in a dose range of 48–82 Gy, and imaged with DCE MRI before, during, 1 month and 2 month after the therapy. Voxel‐by‐voxel hepatic arterial perfusion (Fa) and portal venous perfusion (Fp) were estimated using a dual‐input single‐compartment model. To overcome some of the challenges in estimation of hepatic perfusion from DCE MRI, a portal venous perfusion ratio (PVPR) (100×Fp/(Fa+Fp)) was evaluated for liver perfusion dose‐response. Hepatic perfusion maps were co‐registered to the dose distribution via registration with the treatment planning CT. The liver voxels having PVPR between 80% and 95% before RT were considered as “normal” tissue. The relation between perfusion and dose in the “normal” liver was assessed by the venous‐perfusion‐ratio dose‐response function. Results: The PVPR in the “normal” liver regions 1 month after RT decreased compared to pre RT. The extent of the decrease was linearly correlated with the dose accumulated to the end of RT (R2=0.93). Substantial individual variations of the PVPR decreases were observed 1 month after RT. Conclusions: Our study shows dose‐dependent perfusion changes in local liver regions. There is substantial variability in the sensitivity of liver perfusion to irradiation. The PVPR may characterize the liver perfusion change in response to radiation and might be a metric for predicting radiation toxicity in the liver. The study is supported in part by R21 CA126137 and 3 P01 CA59827.
MO‐D‐204B‐07: Thermal Effusivity Changes Manifest before Skin Reactions in Locally Irriadiated Hairless Mice
Authors: Templeton A , Chu JC , Sun JG , Coon A , Yao R , Griem KL
Published: August 2010

Purpose: We aim to develop a tool to predict the patient specific risk of moist desquamation as a side‐effect of radiotherapy, enabling targeted, timely intervention (re‐planning, IMRT, radioprotectors). 3‐D thermal tomography is a translational technology, recently developed for non‐destructive, non‐invasive evaluation of various materials. Our hypothesis is that skin damage will express changes in thermal properties proportional to severity, before the degree of damage is externally observable. Method and Materials: Eleven hairless mice were irradiated to a 40 Gy skin dose in a single fraction using an Ir‐192 HDR source and a Leipzig applicator, and thermal images were acquired once per day post‐treatment. Three‐dimensional maps of thermal effusivity were reconstructed, and the thermal effusivity within equivalent regions of interest on the treated and control sides were tracked daily. Based on physical observation, mice were post hoc separated into high‐grade (n=6) and low‐grade (n=5) reaction groups. Results: Mice which developed high grade reactions showed a change in effusivity both earlier (mean 1.7 days post‐treatment) and larger (mean 14.6% over days 3–7) than the low grade group (4.4 days, 5.3%). The first visible indication of skin reaction was erythema, which occurred at days 5 and 4.3 post irradiation for the high and low grade groups, respectively. Conclusion: The presented pre‐clinical study indicates that thermal effusivity changes are induced before skin reactions develop after irradiation, particularly in subjects which would eventually develop more severe skin reactions. The differences in high grade and low grade groups were observable in the thermal effusivity data before there were any visible differences, which do not occur in the clinically useful time range. This suggests potential clinical utility for separating subjects into risk groups by measuring their individual response to their course of treatment, and proactively adjusting accordingly.
MO‐D‐204B‐09: Characterization of ADC and T2 Responses of Tumor Tissue to Radiation with Histological Interpretation
Authors: Larocque MP , Syme A , Yahya A , Wachowicz K , Allalunis‐Turner J , Lai R , Fallone BG
Published: August 2010

Purpose: To characterize the temporal, dose, and oxygenation dependence of apparent diffusion coefficient (ADC) and transverse relaxation time (T2) responses of tumor tissue after treatment with ionizing radiation, in the context of histology and observed volume changes. Methods: Human glioblastoma multiforme (GBM) tumors were grown subcutaneously in mice. Multiple groups treated with 200 kVp x‐rays and two unirradiated control groups were formed (n=6 per group). Single fraction groups received doses ranging from 50 cGy to 800 cGy. Fractionated deliveries were 800 cGy in 1, 2 or 3 fractions and fractions were separated by 24 or 72 hours. Oxygenation dependence was investigated with 800 cGy in a single fraction with a ligature in place to deplete the tumor's oxygen supply. Images were acquired on a 9.4 T MR system at multiple time points before and after treatment. Quantitative ADC and T2 maps were produced from each image set. Results: A dose dependent ADC increase was observed in irradiated tumor tissue. Maximum response was observed 7 days post‐treatment. Tumor T2 was maximum 3 days after treatment and had fallen below baseline by 14 days after treatment. Fractionation did not affect amplitude of ADC response, but delayed the time at which maximum ADC occurred and the time for T2 to fall below baseline. Hypoxic tissues showed reduced ADC response after treatment. Preliminary pathology results show ADC response correlated with reduced tumor growth rate and reduced mitotic cell count. Early increases in ADC and T2 were associated with early‐stage necrosis and edema. Later responses were associated with evolving necrosis and extracellular macromolecule accumulation. Conclusion: These results further the understanding of ADC and T2 responses and their development as biomarkers of treatment response.
MO‐EE‐A3‐05: Assessment of Volumetric Arc Therapy Plans for Constant and Variable Dose Rates
Authors: Wu Y , Dogan N , Liang X
Published: August 2010

Introduction: Recently, volumetric arc therapy (VMAT) that combines dynamic gantry motion with beam intensity modulation has become clinically available for planning and delivery of dynamic arc IMRT treatments. During VMAT optimization process, several parameters can be changed, including gantry speed and dose rate. The goal of this work was to assess the differences between the VMAT plans generated with constant and variable dose rates. Methods and Materials: Patients with H&N cancer who were treated with step‐and‐shoot IMRT (DMPO) were selected for this study. Plans were generated using Pinnacle's Smart Arc software which supports generation of both constant and variable dose rate VMAT plans. VMAT plans for 100 – 500 constant dose rates (DR) and variable dose rate were generated. Plans consisted of two 360 ° arcs with 4° arc sampling. Plans were compared using DVH parameters for target(s) and OARs and MUs. VMAT plans were also verified in a cylindrical phantom using film and ion chamber. Results: The MUs for DR=100 were 22% lower as compared to the DMPO MUs and increased with the increasing DR. The MUs for DR=400, however, is 21% higher than the DMPO MUs. The MUs for variable dose rate is the lowest (581MUs) as compared to 100 dose rate plan (832MUs). PTV coverages were almost the same up to DR=300 and degraded significantly for 400, 500 and 600. Phantom verification of all plans showed that the calculated and measured ion chamber doses at isocenter were within 4%. Conclusion: This study showed that the constant dose rate VMAT plans up to 300 produced similar plans to the ones obtained with variable dose rate and DMPO plans, in terms of target coverage and critical structure sparing. However, variable dose rate plans still the choice in terms of MU efficiency and plan quality.
MO‐EE‐A3‐06: First Clinical Results of 3D Verification of Volumetric‐Modulated Arc Therapy (VMAT) of Prostate and Lung Cancer Patients Using EPID Dosimetry
Authors: Mijnheer B , Mans A , Olaciregui‐Ruiz I , Sonke J , Stroom J , Tielenburg R , Herk M , Vijlbrief R
Published: August 2010

Purpose: To evaluate the first clinical results of 3D verification of volumetric‐modulated arc therapy (VMAT) of prostate and lung cancer patients using EPID dosimetry, both pre‐treatment and in vivo. Method and Materials: EPID images obtained during VMAT delivery using gantry‐angle resolved data acquisition are converted to 3D dose distributions inside a phantom or patient using a back‐projection model. In addition to verification of the dose at the isocentre, 3D gamma analysis is performed to compare planned and EPID‐reconstructed 3D VMAT dose distributions. Gamma evaluation parameters are mean and maximum (1%) gamma and % gamma < 1. Results: VMAT plans of 10 prostate and 10 lung cancer patients, verified by means of 3D EPID dosimetry, have been analysed. The average difference of the dose at the isocentre was for prostate cancer VMAT (−0.2±1.6)% and (+0.4±1.6)%, pre‐treatment and in vivo, respectively, and for the hypofractionated lung cancer VMAT plans (+2.0±1.2)% and (−0.7±2.3)%. 3D gamma evaluation showed almost the same results for pre‐treatment and in vivo verification of the 3D dose distribution for prostate cancer VMAT. For instance, the average of the mean gamma values of all patients is (0.42±0.12)% and (0.48±0.06)% for the pre‐treatment and in vivo results, respectively. Larger deviations have been observed for lung cancer VMAT verification; e.g. the average of the mean gamma values of all patients is (0.41±0.03)% for pre‐treatment verification, and (0.68±0.27)% for the in vivo results. Conclusion: The results of this study show that EPID dosimetry is a valuable tool for 3D verification of VMAT delivery, both pre‐treatment and in vivo. More clinical data are needed to assess acceptance criteria for 3D gamma evaluation of VMAT verification. Conflict of Interest: Research sponsored by Elekta Oncology Systems Corporation.
MO‐FF‐A3‐06: Does KV‐MV Dual‐Energy Computed Tomography Have an Advantage in Measuring Proton Stopping Power Ratio in Patients?
Authors: Yang M , Virshup G , Clayton J , Zhu X , Mohan R , Dong L
Published: August 2010

Purpose: To evaluate different combinations of x‐ray energies when using Dual‐Energy CT (DECT) method for calculating proton stopping power ratio (SPR) in the presence of random noise and beam hardening effects. Method and Materials: We selected three representing x‐ray energy pairs: 100kVp and 140kVp for KV‐KV, 100kVp and 1MV for KV‐MV, and two 1MV x‐rays with and without external filtration for MV‐MV DECT imaging. Small perturbations in CT numbers and x‐ray spectrum used for the DECT calculation were introduced to simulate effects of random noise and beam hardening. The CT dose to generate a fixed amount of random noise was determined by Monte Carlo method in simulated CT images of a water phantom. The SPRs of 34 human tissues listed in ICRP 23 and ICRU 44 were calculated using the DECT method with all three x‐ray energy combinations. We also performed error propagation to study the sensitivity to CT number uncertainties for each of the DECT x‐ray pairs. Results: The relative sensitivity of the derived SPR to random noise in CT number was 6.59, 1.27 and 3.34 for KV‐KV, KV‐MV, MV‐MV DECT techniques, respectively. Under the same condition, the root‐mean‐square errors in SPRs derived using the KV‐MV DECT technique were at least 50% smaller than those using the KV‐KV or MV‐MV combinations. Additionally, we found CT number uncertainties for the high energy beam within the dual‐energy pair had a bigger impact to SPR accuracy than the low energy beam, suggesting it is more important to emphasize the accuracy of the high energy CT imaging than the low‐energy one in DECT applications. Conclusion: The KV‐MV combination makes the DECT method more robust in calculating proton SPRs or resolving effective atomic number for tissue composition in the presence of CT number uncertainties. Conflict of Interest: Research partially sponsored by Varian Medical Systems.
TU‐A‐204B‐02: On the Potential of CBCT for Range Verification in Proton Therapy
Authors: Bentefour E , Tang S , Both S , Chen G , Lu H
Published: August 2010

Purpose: We have investigated the potential use of cone beam CT (CBCT) for beam range verifications in proton therapy treatment, in addition to its primary role in geometric targeting. Specifically, we studied the intrinsic imaging variability of a CBCT and its effect on the water equivalent path length (WEPL) calculations, in the context of daily beam range verification/correction required for a recently proposed method of treating prostate using anterior fields. The current approach uses only lateral fields due to the lack of precise range control in patient. Materials and Methods: An anthropomorphic pelvic phantom was scanned using CBCT, in eight sessions on eight different days. In each session, the phantom was scanned twice, first at a standard position as determined by the room lasers, and then with a random shift of one centimeter in lateral directions. The Xio treatment planning system was used to perform the analysis. The average Hounsfield unit (HU) numbers for the water column in the rectal balloon was used to perform a linear calibration of the stopping power ratio, independently for each scan, as supported by the planning system. A number of WEPL values vertically from the anterior skin surface to the anterior surface of the water balloon were calculated on slices covering the region of the prostate, in relevance to a prostate treatment using an anterior field. Results: The HU number in the water column varied significantly even within the same CBCT. The average value also varied from day to day for up to 20 units. However, when these average values are used to calibrate the stopping power ratio, the variations in WEPL values along the anterior beam path are mostly within 2 mm. Conclusions: In‐room CBCT can be used in proton therapy to make online verification of protons range in patients with 2mm accuracy.
TU‐A‐204B‐03: 3‐D Fiducial Localization with MV EPID Imaging during a RapidArc Therapy
Authors: Yue Y , Aristophanous M , Rottmann J , Berbeco R
Published: August 2010

Purpose: Fiducials can be used to aid organ localization during RapidArc delivery. Specifically, the 3‐D tumor position may be found through the tracking of fiducials during beam delivery. A promising technique is the detection of fiducials in 2‐D EPID images, and then reconstruction of the 3‐D position using limited views. Methods and Materials: The CIRS dynamic thorax phantom with a custom tumor model is used in the study. Two fiducials are placed on the surface of the tumor at clinically relevant distances apart from each other. Clinical RapidArc plans are delivered to the phantom based on a single 360° arc. The fiducial detection algorithms are developed based on an image block matching filter and anisotropic diffusion techniques. A wavelet‐based block matching filter is developed to suppress the noise and texture of images while preserving the edges of fiducials. Furthermore, the anisotropic diffusion technique removes redundant edges and trivial structures of the results of the previous filtering. Experiments show that the algorithms are robust to image contrast and aperture variations during the RapidArc delivery. With detected 2‐D locations, 3‐D spatial locations of fiducials are reconstructed using the inverse 3‐D rotation matrix. Results: The position accuracy in 2D EPID is 0.77+0.58 mm by comparing with the manual outlines. The 3D fiducial positions are reconstructed using the 2D EPID images. The displacement from the references in the 3D CBCT image is within one millimeter. Conclusion: Novel filtering and detection algorithms are developed to identify the fiducials in 2D EPID images. The results show that limited angles of full rotation can rigorously reconstruct the locations of each of fiducials in 3D to within one millimeter. Conflict of Interest: Varian Medical Systems, Inc.
TU‐C‐BRA‐03: Skin Toxicity in Stereotactic Body Radiation Therapy — Improving the Treatment Planning and Delivery Process
Authors: Fontenla S , Yorke E , Lovelock D , Mechalakos J , Hunt M
Published: August 2010

Purpose: Stereotactic body radiation therapy is delivered using high doses, specialized immobilization and careful treatment planning. Although more centers now offer SBRT treatments, little data regarding toxicity and doses are available. We have previously reported increased skin toxicity with hypofractionated SBRT and correlated toxicity to tumor‐skin proximity, beam arrangement and buildup dose calculation inaccuracies. The purpose of this study was to determine the accuracy of our buildup region doses through correlation with in‐vivo measurements. Method and Materials: Surface doses were measured on 30 patients receiving single fraction 18–24 Gy SBRT. Patients were supine and immobilized with an Alpha Cradle and custom SBRT cradle. IMRT planning with approximately seven posterior, 6 or 15 MV x‐rays beams was done using an in‐house planning system (TPS) and delivered on Varian accelerators. Buildup region doses were measured with thermoluminescent (TLD) or optically stimulated luminescent (OSLD) dosimeters. An 5–10 dosimeter array was positioned on the patient's surface during treatment. Dosimeter positioning was verified by KV imaging. Calculations at corresponding points at depths 1 and 2 mm were done using both our TPS (pencil beam algorithm) and Monte Carlo and compared to measurements. For six patients the TPS calculations were also done including the immobilization equipment. Results: In vivo dosimetry showed patient surface dose could be 50% higher than that calculated by the TPS. Factors contributing to higher measured doses were: tumor proximity to surface, immobilization device thickness, beam angular segmentation, and beam energy. Calculations including the immobilization device greatly improved correlation with measurements. Conclusion: To limit skin toxicity, SBRT treatments require awareness of beam placement, energy selection and immobilization devices. At these high single fraction doses it is necessary to carefully consider immobilization devices as well as to reanalyze and improve treatment planning beam data in the buildup region.