bluuebunny/crossref_metadata_2025 huggingface dataset:
MO‐D‐204B‐05: Effect of PET Image Reconstruction Parameters on Quantitative Treatment Response Assessment
Authors: Morrison C , Vanderhoek M , Perlman S , Jeraj R
Published: August 2010

Purpose: Changes in SUV are typically used for PET‐based treatment response assessment. However, the absolute value of SUV varies with different reconstruction methods. We studied the effect of different PET image reconstruction parameters on quantitative treatment response assessment. Method and Materials: Six patients (∼2 tumors/patient) were treated with molecular targeted therapy and received three whole body [18F]FLT (cellular proliferation marker) PET/CT scans at different time points of therapy. Images were reconstructed using filtered backprojection and the OSEM algorithm with varying grid size, number of iterations, and post filter. Each tumor was contoured on a reference reconstruction. This contour was applied to all reconstruction, and SUVmean, SUVmax, SUVpeak, and SUVtotal, were calculated. Treatment response was assessed by change in SUV between scans, normalized to the first scan. The effect of the reconstruction parameters on treatment response was evaluated via the percent difference from the reference reconstruction method which used the OSEM algorithm. Results: FBP yielded higher percent differences than the OSEM reconstructions for all measures. For the OSEM reconstructions, the range was 15%, 50%, 85%, and 25% for the change in SUVmean, SUVmax, SUVpeak, and SUVtotal respectively, while FBP yielded ranges of 60%, 100%, 150%, and 60%. Change in SUVmax, and SUVpeak had the greatest percent differences from the reference reconstruction. The percent difference was as high as 65% and 115% for the change in SUVmax and SUVpeak respectively when images were reconstructed with FBP compared to 25% and 65% when they were reconstructed with OSEM. Conclusion: Image reconstruction parameters affect treatment response assessment, particularly when SUVmax, or SUVpeak are used. Overall, the greatest variation was due to the image reconstruction algorithm. Consistency of reconstruction algorithm and parameters will not eliminate the inherent differences that exist with the use of SUV measures for treatment response assessment.
MO‐D‐201C‐01: Advances in MR: State of the Art Today and Future Directions — MR Instrumentation
Authors: Pelc N.J.
Published: August 2010

Even though Magnetic Resonance as a modality is now several decades old, instrumentation continues to advance at a vigorous pace. The purpose of this presentation is to review recent advances in the current state of the art and to discuss directions of continued progress. The main magnet continues to be the central platform component from the standpoint of cost and footprint. There has been steady increase in the number of clinical systems at higher field strength (e.g., 3T vs. 1.5T), and the expected increase in signal‐to‐noise ratio is now being realized. Another significant trend has been the increase in bore diameter and decrease in magnet length, both of which are appreciated by patients. A number of systems for human in‐vivo studies at even higher field strength (e.g., 7T) are now in use. These present significant technical challenges that are being addressed, especially in B0 and B1 homogeneity and in RF power deposition. The number of channels used for simultaneous signal reception also continues to increase, with related improvements in SNR and imaging speed through parallel imaging. A newer development is simultaneous RF transmission. This new field of “parallel transmission” promises to overcome problems with B1 homogeneity and perhaps power deposition. Hybrid systems that combine MR with other diagnostic or therapeutic modalities are also receiving attention. Among these hybrids are MR guided focused ultrasound, MR/PET, X‐ray/MR hybrids, and systems for MR guided radiation therapy systems

Learning Objectives:

1. Recognize recent technology trends in Magnetic Resonance

2. Understand the benefits and disadvantages of parallel excitation and reception

3. Understand the challenges in combined modality systems

4. Examine possible future directions for Magnetic Resonance

Research sponsored by GE Healthcare.
MO‐D‐201B‐01: Carbon Nanotube X‐Ray: Promises and Challenges
Authors: Zhou Otto
Published: August 2010

Purpose: To develop a carbon nanotube based field emission x‐ray technology for medical imaging and radiation therapy Method and Materials: Carbon nanotubes (CNTs) are used as the field emission “cold cathode”. The x‐ray tube current is generated by applying an external electrical field to extract the electrons from the CNTs. By varying the extraction electrical field, x‐ray radiation with programmable waveforms can be readily generated and gated with physiological signals. Spatially distributed x‐ray source arrays with one‐ and two‐ dimensionally distributed focal spots are constructed by using matrix addressable multi‐pixel CNT cathode. Switching, scanning, and regulating the x‐ray beams are achieved by specially designed gate control electronics. Results: Programmable and spatially distributed field emission x‐ray source arrays have been developed. After substantial R&D efforts these x‐ray sources are now capable of delivering the spatial resolution, the x‐ray flux, and the energy required for medical imaging applications. Utilizing the rapid programmable capability of the CNT field emission source, a prospective respiratory and cardiac gated micro‐CT scanner has been developed that is capable of high‐resolution in vivo imaging of free‐breathing small animals. A proof‐of‐concept stationary digital breast tomosynthesis scanner with a distributed multi‐beam x‐ray source array has been demonstrated which collects project images without mechanical motion and has the potential to substantially reduce the scanning time and therefore motion blurs. Utilizing the capability of distributing the x‐ray power over a large focusing track instead of small spot on the anode, a compact high dose and dose rate microbeam radiation therapy system is currently under development at UNC. Conclusion: The distributed field emission x‐ray source array technology offers unique capabilities that are attractive for tomography imaging and potentially for radiotherapy. The flexibility in source configuration opens up new possibilities in system design. As a brand new source technology, issues such as reliable and long term stability which are critical for commercial products need further investigation and improvement.

Conflict of Interest: Zhou is a board director of XinRay Systems which develops and commercializes the CNT X‐Ray source technology.
MO‐EE‐A1‐06: Tandem and Ovoids Brachytherapy for Cervical Cancer When Evaluated by High Resolution (3.0 Tesla) MRI
Authors: Kim Y , Xia J , Sun W , Bhatia SK , Jacobson GM
Published: August 2010

Purpose: To investigate tumor coverage and organs‐at‐risk (OAR) sparing of Manchester system based high dose rate (HDR) for different high risk (HR)‐CTV volumes. Materials and Methods: We studied 25 HDR plans of six patients with FIGO stage Ib1‐<emph IV< cervical cancer. A new plan was generated per fraction based upon the system but on MRI, utilizing titanium Fletcher‐Suit‐Delclos‐style tandem‐and‐ovoids applicators. The IR (intermediate)‐CTV, HR‐CTV, rectum, bladder, and sigmoid were delineated on MRI. Three subgroups were categorized according to HR‐CTV volumes; Non‐Bulky (< 20 cc), Low‐Bulky, and Bulky (≥ 40 cc). For each group, the percent values of D100 and D90 of HR‐ and IR‐CTV, normalized to the prescription dose (Rx), were quantified, while the percent D2cc of OAR, normalized to the dose limits. Results: We found 76%, 44%, and 68% of the plans resulted in over dose, respectively. The percent D2cc was recorded up to 143% (mean 112 ± 17%), 127% (94 ± 20%), and 181% (114 ± 34%), respectively. The D90 values of HR‐CTV in the Non‐Bulky group were on average 118% (± 21%) higher than Rx, while in the Bulky group it was 64% (± 17%), showing that HR‐CTV coverage was significantly changed for different tumor size group. The doses at Point A received on average 100% (± 3%) regardless of the tumor size. The ICRU rectum and bladder point doses showed down to 45% and 23% of their D2cc values underestimating their maximum doses. These underestimated cases were found in 36% of cases (9 plans out of 25) and 60% of cases (15 plans out of 25). Conclusion: More than 40% of the plans resulted in over doses in OAR while HR‐CTV received 118% of Rx in Non‐Bulky group. Volume optimization is expected to improve OAR sparing for all tumor sizes and in tumor coverage for small tumors.
MO‐EE‐A2‐03: Comparison of Four NTCP Models to Describe Dose‐Response for Radiation‐Induced Optic Neuropathy and Retinopathy
Authors: Moiseenko V , Song W , Mell L , Bhandare N
Published: August 2010

Purpose: To compare four NTCP models to predict dose‐response for the incidence of radiation‐induced optic neuropathy (RION) and retinopathy (RIRP). Method and Materials: Logistic, log‐logistic, Poisson‐based and probit models were used to describe dose‐response. Fits to four data sets were obtained: RION in 101 patients treated twice‐daily (BID) and 172 patients treated once daily (QD), RIRP in 78 BID and 108 QD patients. Doses used in dose‐response analysis were converted to isoeffective dose in 2‐Gy fractions using α/β=1.76 Gy for RION and 2.65 Gy for RIRP. Maximum‐likelihood profile method was used to obtain model parameter values, D50 and γ, and corresponding confidence intervals. Results: Within the dose range bounded by available clinical data, the model predictions were similar. For data sets spanning a broad range of incidence a reasonable consistency between model parameters was observed, however, the log‐logistic model consistently showed larger D50 and shallower normalized slope γ. Specifically, for incidence of RION among patients treated QD, D50 ranged from 94.2 to 104.7 Gy and γ from 0.88 to 1.41. Model parameters for RIRP in the QD group were D50 from 72.2 to 74.0 Gy and γ from 1.51 to 2.16; in the QD group from 72.2 to 74.0 Gy and from 0.84 to 1.20, respectively. A large variation in model parameters was observed for RION in patients treated BID who showed incidence of complications spanning the range up to 20%. D50 ranged from 96.3 to 125.2 Gy and y from 0.80 to 1.56. Conclusion: Log‐logistic model tends to lead to larger D50 and lower γ compared to other models. Statements regarding normal tissue radiosensitivity and steepness of dose‐response based on model parameters should be made carefully as the latter are not only model‐dependent but also sensitive to the range of complication incidence exhibited by clinical data.
MO‐EE‐A2‐04: A Framework to Determine the Risk of Developing Pneumonitis in Lung Cancer Patients Undergoing Radiotherapy
Authors: Chawla A , Das S
Published: August 2010

Purpose: To determine which dose and non‐dose features of patients undergoing thoracic radiotherapy best predict the incidence of possible lung pneumonitis and to estimate the corresponding risk of developing pneumonitis. Method and Materials: The study employed a database of 235 patients, out of which 34 were diagnosed with Grade 2+ pneumonitis. A total of 94 dose and non‐dose features of each patient were analyzed. To determine the most significant features likely to affect patient outcome, a two‐fold optimization approach was developed. First, the feature values were evaluated by four machine learning models for accurate prediction. The model accuracy was measured in terms of tenfold cross‐validation‐based area under ROC curve (AUC). Next, the ROCs of the four models were combined. Towards that end, a Bayesian decision fusion methodology was developed. Finally, based on the combined ROC, the fusion model isolated the set of most significant features and also computed patient‐specific risk of injury. Results: Out of 94 original features, 27 were selected by the four models. AUC values corresponding to the four models were 0.75, 0.70, 0.79, and 0.73, respectively. Most notably, when the four models were combined, the AUC improved to 0.85. Furthermore, the fusion model identified only six features as most significant. They consisted of two dose features: EUD (a=2) and V30; and four non‐dose features: gender, tumor location, histology type, and chemotherapy schedule. The corresponding probabilities for injured patients were found to be significantly higher than those for non‐injured patients, demonstrating robustness of the over all predictive model. Conclusion: A framework was developed to predict the incidence of pneumonitis in lung cancer patients undergoing radiotherapy. The predicted probabilities of injury were found to correspond well with the known injury status of the patients. The framework may potentially be used in the clinic to improve the treatment decision‐making process.
MO‐EE‐A2‐05: A Novel Method to Incorporate Spatial Location of Lung Dose into Predictive Radiation Pneumonitis Modeling
Authors: Vinogradskiy Y , Martel M , Tucker S , Liao Z
Published: August 2010

Purpose: Current methods for prediction of severe lung toxicity after thoracic radiation therapy have used basic dosimetric indices derived from dose‐volume data. Studies have found that function may vary throughout lung. Our study will develop a novel method to incorporate spatial location of lung dose into a predictive model and test its predictive power on a large clinical database of non‐small cell lung cancer patients. Methods and Materials: The 3D dose distributions of 547 patients were mapped onto one common coordinate system. The boundaries of the coordinate system were defined by the extreme points of each individual patient lung. Spatial information was incorporated into the Lyman NTCP model. The effective dose term in the Lyman model was defined as the sum of mean lung dose and a spatially weighted lung dose. The spatially weighted lung dose was calculated by scaling each dose voxel with various spatial weighting schemes. We investigated five weighting schemes that scaled the dose linearly according to the following orientations: superior‐inferior, anterior‐posterior, medial‐lateral, left‐right, and radial. Model parameters were fit to our patient cohort with the end‐point of severe radiation pneumonitis. The accuracy of the spatial dose model was compared against a conventional dose‐volume model to determine whether adding a spatial component increased the predictive power of the model. Results: The comparison of the spatial dose model and the conventional dose‐volume model did not yield a significant difference in model accuracy. A promising trend emerged with a spatial model that placed more weight on peripheral lung dose. Conclusion: The method presented is the first method to incorporate the entire 3D dose distribution and explicitly separate location and dose‐volume parameters. While statistically significant gains in model accuracy were not reached, radial weighting yielded the best results. In future work, additional spatial weighting schemes will be explored.
MO‐EE‐A3‐02: Three‐Dimensional Dose Reconstruction Using Non‐Transmission Portal Dosimetry and Monte Carlo Calculations
Authors: Holmes J , DeBlois F
Published: August 2010

Purpose: To evaluate a convolution and Monte Carlo based non‐transmission portal dosimetry method used to reconstruct a 3D dose distribution. Method and Materials: Raw images acquired by an amorphous silicon electronic portal imaging device (EPID) were deconvolved with a scatter kernel to determine a 2D primary energy fluence distribution. The scatter kernel was characterized by two components: a Monte Carlo simulated dose deposition kernel scored in the scintillation screen, and an iteratively optimized optical photon‐spreading (or “glare”) kernel initialized by the tail of a measured in‐air fluence profile. The derived primary energy fluence was converted into a phase space file and used as a source input for the Electron Gamma Shower (EGS) user code DOSXYZnrc for Monte Carlo calculation. The reconstructed 3D dose distribution was verified by comparison with a measured beam profile and PDD data. Results: The glare kernel was determined to be the dominating component of the scatter kernel, while the dose kernel correction exhibited longer‐range radial extent from the central axis of the beamlet. The derived primary energy fluence demonstrated a similar trend as the in‐air fluence measurement, and was in good agreement with the penumbra tails. The reconstructed beam profile and PDD for a 10×10 cm2 open‐field were in excellent agreement with photodiode and ionization chamber measurements. Conclusion: This study demonstrates the potential for pretreatment IMRT verification using the primary fluence extracted from an EPID to reconstruct a Monte Carlo calculated 3D dose distribution in a patient's planning CT‐scan. Future work will include dose normalization and a more comprehensive validation measurement scheme before moving to a comparison of a 3D dose volume between treatment planning system and EPID reconstruction.
MO‐E‐BRA‐01: Accounting for the Effect of a Magnetic Field from the Bending Magnet in Monte Carlo Accelerator Treatment Head Simulation
Authors: O'Shea T , Sawkey D , Foley M , Faddegon B
Published: August 2010

Purpose: To account for a fringe magnetic field from the bending magnet in Monte Carlo treatment head simulation. Method and Materials: A Tesla meter was used to map the magnetic field between the exit window and secondary scattering foil of a clinical linac for 6–21 MeV electron beams. The foil and monitor chamber were removed to access this region. Measurements extended 12.0 cm along the central axis and 1.0–3.0 cm off‐axis. The Monte Carlo code EGSnrc was adapted to incorporate the magnetic field with accuracy validated in simple geometries. Results: The magnetic field was orientated crossplane, with the deflection of the electron beam inplane. The highest intensity, measured at the exit window, increased with nominal beam energy from 0.006 T for 6 MeV to 0.020 T for 21 MeV. The field falloff, approximated by a cubic function, was 5% at the secondary scattering foil. The field deflected the electron beams by 0.82–1.24 cm at isocenter. The peak position in the calculated and measured profiles, with no scattering foils in the beam, matched to 1 mm inplane and crossplane. A preliminary comparison of calculated and measured dose profiles with the foils and chamber in the beam support our contention that the simulations can accurately account for the deflection of the electron beam by the fringe magnetic field from the bending magnet. It appears the lateral variation in the magnetic field can be neglected. Conclusion: The calculated fluence is expected to have an improved uncertainty of 1%/1 mm, without the non‐physical adjustment in the monitor chamber and secondary foil positions required when the field is left out of the simulation, having a better match to the undulations in the measured profiles with the bremsstrahlung peak in the correct position.
MO‐E‐BRA‐04: In‐Field Risk of Radiation Induced Second Cancer in Proton Therapy and IMRT for Pediatric Patients Considering Dose Inhomogeneities
Authors: Athar B , Paganetti H , Adams Judy , Yock Torunn
Published: August 2010

Purpose: In radiation therapy organs outside of the treatment volume receive medium to low doses. These doses might lead to side effects, e.g. the induction of a second cancer. In general IMRT treatment plans require more fields than proton treatment plans to reach comparable dose conformality. In addition, photon beams do not stop in the patient. The dose distributions and organ dose levels therefore differ substantially. The purpose of this analysis was to assess the risk for developing a second cancer for organs directly in the beam path. When treating pediatric patients. Method and Materials: Age and gender specific whole‐body phantoms (4‐year old female and 14‐year old male) were implement in the treatment planning system in order to determine in‐field doses. We have used two treatment sites for our study, namely, optic glioma and pelvic sarcoma. We have used a recently published second cancer risk model which takes into account the dose levels and dose inhomogeneities by using the concept of organ equivalent doses. Lifetime attributable risks (LAR) were determined. Results: In IMRT parotids receive exit dose, therefore, the second cancer risks (LARs) for treating opticglioma from IMRT is around 0.5%, whereas, in proton therapy parotids do not receive any dose. Considering brain tissue, IMRT causes LARs between 1.2% and 2.0% considering repopulation rates between 0% and 50%. In proton therapy, the values vary from 0.4% to 1.2%, respectively. LARs in the 14‐year old patient are found to be smaller compared to LARs in the 4‐year old patient. Conclusions: Integral doses to in‐field organs are in general 2 to 3 times larger from IMRT as compared to proton therapy. We find LARs from IMRT are higher compared to those from proton therapy e.g. for brain tissue 1.2% in IMRT as opposed to 0.4% in proton therapy considering zero repopulation rate.
MO‐E‐BRA‐05: Clinical Issues Associated with the Use of Monte Carlo‐Based Prospective Planning for Lung SBRT and Spine SRS Patients
Authors: Shaikh M , Wen N , Kim J , Ren L , Kumar S , Liu D , Movsas B , Ajlouni M , Chetty I
Published: August 2010

Purpose: To investigate the clinical issues associated with the use of Monte Carlo‐based prospective planning for lung SBRT and spine SRS patients. Methods: Experimental verification of the iPlan v.4.1 MC photon beam algorithm (BrainLab) was performed using film and ion‐chamber in water phantoms and solid‐water slabs containing bone and lung‐equivalent materials for a 6 MV photon beam from a Novalis linac. MC dose verification was performed for 5 spine and 7 lung patients using an anthropomorphic phantom. Treatment plans of prospectively treated patients were examined to investigate the influence of statistical uncertainties, MC‐based dose‐to‐water (Dw) and medium (Dm) and calculation speed. Results: Agreement between calculations and measurements in the water phantom verification tests was, on average, within 2%/1 mm (high dose/high gradient), and was within ±4%/2 mm in the heterogeneous slab geometries. For spine SRS, the agreement between PB and Dm calculations were within 4 % of IC measurements, however, the difference between MC_Dw and Dm was on average 9 %. For the lung SBRT tests the average difference between calculated PB doses and IC reading was 13%, and both MC calculations agreed within 1.5 % of measurements. The use of reduced uncertainty below 2% increases dose calculation time significantly but has an insignificant effect on the dose volume histograms (DVH). Conclusion: Prospective treatment planning with a well‐commissioned MC algorithm provides improved dose coverage and can be introduced for efficient calculations in the routine clinical setting. However, further investigation is warranted on issues such as dose‐to‐medium and water, the impact of statistical uncertainties on serial and parallel organs, and proper approaches for reliable dose measurements at small field sizes, under non‐equilibrium conditions. Acknowledgement: Supported in part by a grant from the NIH/NCI (R01CA106770)
MO‐E‐BRA‐07: Fast RBE‐Corrected Dose Calculation for Carbon Therapy
Authors: Veltchev I , Fourkal E , Fan J , Li J , Ma C
Published: August 2010

Purpose: To design a fast algorithm for RBE‐corrected dose calculation for carbon therapy. The interactions of heavy ions with matter produce sharp penumbra and narrow Bragg peak, which result in better dose conformity in comparison with X‐rays and electrons. Elevated RBE near the end of the heavy ion track modifies the biologically relevant dose distribution and has to be considered during treatment planning. Method and Materials: We created a database of 10000 pre‐computed (using Geant4 Monte Carlo toolkit) particle histories that are subsequently repeated in the phantom geometry of the MCSIM code. Each history is a stack of energy deposition steps and each step holds information about the particle type, step length, and energy deposited. Since particle type and LET are all part of the database, RBE corrected dose calculation can be implemented, provided the RBE of each particle is known as a function of LET. The analysis of published experimental data resulted in a model for the RBE(LET) for a variety of ion species. This model is used to modify the database accordingly. Results: We applied our algorithm for the calculation of a RBE‐corrected spread‐out Bragg peak and compare it with the corresponding physical dose distribution. Three‐dimensional dose calculation was carried out using a CT‐based patient phantom as well. It takes <4 minutes on a 2.8GHz Intel‐Pentium4 processor to achieve 2% accuracy. The code is designed to facilitate the use of arbitrary field sizes and carbon energy spectra, thus allowing for advanced therapy calculations. Conclusion: Fast RBE‐corrected dose calculations are now possible using a track repeating technique for carbon ions. In heterogeneous phantoms the algorithm is more than 100 times faster than both FLUKA2008 and Geant4 and can be used for both fast and clinically accurate dose calculations for carbon therapy with beam scanning and intensity modulation.
MO‐E‐204C‐02: Bedside SPECT Imaging with Pinhole Collimation
Authors: Cebula A , Gilland D , Studenski M
Published: August 2010

Purpose: The objective of this study is to evaluate the imaging performance of a mobile SPECT system with a pinhole collimator and compare the results with parallel hole collimators. The goal is to obtain both planar and tomographic performance measures with Tc‐99m and F‐18. This abstract presents the results using Tc‐99m. Method and Materials: The system utilizes a small field of view camera with pixilated NaI crystal and position‐sensitive photomultiplier tubes. The pinhole collimator is a tungsten knife edge with a hole diameter of 3mm, a focal length of 12.5cm, and an acceptance angle of 90 degrees. The parallel hole collimators have been previously described along with their imaging performance evaluation [1]. The following performance measures with the pinhole collimator were obtained and are presented here: count rate performance, energy resolution, flood field uniformity, system spatial resolution, system sensitivity. Results: The maximum count rate was calculated to be 1.58×105 cps corresponding to an activity of 161 μCi at 30cm. An energy spectrum from the flood acquisition demonstrated an energy resolution of 20% FWHM. Magnification corrected, system planar spatial resolution was 1.03 cm with a system sensitivity of 1.39 cps/μCi at a source to collimator distance of 10 cm. Conclusion: The imaging performance of a mobile SPECT system with a pinhole collimator has been presented. In comparison to parallel hole collimation, the pinhole collimator provides superior spatial resolution and sensitivity at distances less than 5cm. However, at further distances, pinhole sensitivity declines while parallel hole sensitivity remains relatively constant. Research sponsored by the United States Army Medical Research and Material Command under Award No. W81XWH‐04‐1‐0594
MO‐E‐204C‐07: A Scatter Theory Based Approach for MR‐Guided, Individualized PET Attenuation Correction in PET/MR Imaging
Authors: Cao Z
Published: August 2010

Purpose: In PET/MR, creation of an attenuation map from the patient's MR image is critical to quantitative PET but remains challenging because MR signals are not directly related to photon attenuation. The existing methods segment the T1‐weighted MR image and then assign average tissue attenuation coefficients to the segmented classes. Before the segmentation, coregistration between the MR image and CT, initially corrected PET and/or a template of MR images is performed. None of the methods are based on rigorous physical theory. For the first time, we developed a method from quantum scatter theory for estimating the attenuation map using the patient's proton density weighted MR image only. No coregistration is needed. Methods: Photon attenuation in human tissues at the photon energy of 511 keV is dominated by Compton scatter of which the probability depends on Z/A of the tissues. Here Z is atomic number and A atomic mass. Since Z/A is equal or close to 1/2 for the major elements in tissues, except hydrogen (Z/A = 1), the tissue mass attenuation coefficient can be calculated from the amount of hydrogen atoms in unit tissue that is obtained from a proton density weighted MR image. Initial assessment has been done by applying the method to calculating mass attenuation coefficients and then comparing with measured values. Results: The calculated attenuation coefficients agree well with the measured values. The differences between the calculated and measured mass attenuation coefficients for water, dry air, muscle and bone are 0.17%, 0.10%, 1.8% and 0.95%, respectively. The calculated number of hydrogen atoms in 1 gram of water also agrees with the exact value with a deviation of 1.8%. Conclusions: The scatter theory based method produces accurate attenuation coefficients. However, experimental evaluation with phantoms or/and patients must be performed and is currently under way.
MO‐E‐204C‐08: Characterization of Cerenkov Optical Irradiation from Radioactive Probes in Phantoms and Living Subjects
Authors: Sun C , Liu H , Pratx G , Carpenter C , Xing L , Cheng Z
Published: August 2010

Purpose: Molecular probes labeled with beta emitters within a medium radiate visible light. A series of experiments were carried out to quantify the optical properties of different radiotracers in optical phantoms and in living subjects to evaluate the potential of this new imaging modality. Method and Materials: A number of commonly used beta emitters such as 18F, 131I, 64Cu, and 90Y, were first put into small vials with known concentrations and imaged using a Xenogen IVIS 200 optical imager. The photon yields and optical spectra of these samples were measured. The radiotracers were also placed into a tissue‐mimicking phantom at a depth of 2 mm and the optical spectrum was studied with a Xenogen IVIS Lumina 2. Radionuclide PET and optical Cerenkov imaging were also done in mice with implanted tumors and the two types of images were registered and compared quantitatively. Results: Cerenkov photons were observed for all beta emitters and no optical photons were recorded for the pure gamma emitters tested. Among different emitters, electron‐emitting 90Y had the highest photon yield (∼an order of magnitude higher than that of 18F at the same concentration). Depth studies revealed that the optical signal decays rapidly with the depth of tracer placement, due to the dominant blue emission, indicating that this technique is suitable only for applications of superficial diseases. In vivo mice measurements were consistent with phantom studies and showed the potential of the new optical imaging modality for imaging in living subjects. Conclusion: The data reported here show the characteristic properties of Cerenkov light emission for radioactive probes and demonstrate its potential for molecular imaging. Combined optical / radionuclide imaging provides a new paradigm for molecular imaging and may provide additional information for in vivo biological processes.
MO‐E‐204B‐07: Dosimetric Assessment of a CT Metal Artifacts Reduction Algorithm in IMRT Delivery
Authors: Spadea M , Verburg J , Baroni G , Seco J
Published: August 2010

Purpose: To quantify the dosimetric impact of a metal artifacts reduction algorithm for CT images in the framework of Intensity Modulated Treatment dose delivery. Materials and methods: A Gammex 467 Phantom featuring 16 tissue equivalent materials was imaged with a GE Lightspeed RT16 scanner. Three scans were acquired: 1) phantom without metal inserts (Ground Truth, GT), 2) phantom filled with two 3 cm diameter cerrobend inserts (Large Metal Artifacts, LMA) and 3) phantom filled with two 1 cm diameter titanium inserts (Small Metal Artifacts, SMA). The image sinograms, affected from metal artifacts, were processed and restored by an in‐house Metal Artifact Reduction (MAR) algorithm thus obtaining other two image set: LMA_MAR and SMA_MAR. A 5 field IMRT plan was designed on GT to cover 3 cm diameter lung density PTV. Monte Carlo dose calculation was run on the 6 image set. Comparison between SMA vs. SMA_MAR and LMA vs. LMA_MAR was performed in terms of PTV and Volume of Interest (VOI) dose coverage. Results: The initial dose error at the 95% of PTV between GT and LMA and SMA was 12.59% and 0.34% respectively. After MAR correction it was reduced to 0.29% and 0.09%. Two VOIs representing breast and brain tissue respectively, were affected by dose variations caused by LMA. In this case dose underestimation up to 0.5 Gy was found. These inaccuracies were completely corrected after applying the MAR algorithm. Conclusions: This preliminary phantom study showed the importance of restoring CT images, affected by metal artifacts, not only to increase the image quality for diagnosis and contouring purposes, but mainly to avoid large dose prediction errors in radiotherapy delivery. The MAR algorithm will be evaluated for patient IMRT treatments with metal artifacts to assess the related clinical benefit.
MO‐E‐202‐01: A Primer on Proton Therapy
Authors: Palta J , Slopsema R , Engelsman M , Dong L
Published: August 2010

The use of protons for radiation therapy offers theoretical advantages. Compared to external beam photon radiotherapy, proton therapy enables lowering of the integral dose to the patient due to the finite range of protons. However, proton therapy is less tolerant than photon therapy to uncertainties in both treatment planning and treatment delivery. Therefore, it is important to understand the potential sources of dosimetric uncertainties, the impact of these uncertainties in the accuracy and conformity of dose delivered to patients, and potential strategies that translate physical advantage of proton therapy into a maximized dosimetric benefit in the patient.

One unique difference between proton therapy and photon therapy is that the dose distributions can be significantly affected by changes in anatomy and nearby tissue heterogeneities along proton beam directions. Photon‐based PTV is purely based on a geometric concept which disassociates spatial dose distribution from the movements of the patient and internal organs. The traditional PTV concept is inadequate for proton therapy, especially in the presence of setup errors, motion, and anatomical variations.

This session will provide an overview of the state‐of‐the‐art in proton therapy. We will discuss in detail the management of planning and delivery uncertainties for both passively scattered proton therapy and the newest spot‐scanning proton therapy techniques.

Learning Objectives:

1. Understand the state‐of‐the‐art on proton therapy

2. Understand the need for knowing potential sources of treatment planning and delivery uncertainties in proton therapy

3. Learn strategies to mitigate both proton therapy planning and delivery uncertainties
MO‐FF‐A1‐06: The Impact of Source Dwell Position Uncertainty on CT‐Based Brachytherapy Plan for Cervical Cancer
Authors: Lian J , Zhu J , Jones E , Varia M , Shen J , Tang X , Potter L , Chang S
Published: August 2010

Purpose: In CT based high dose rate (HDR) brachytherapy, the longitudinal catheter tip position can be easily mis‐localized because of several reasons. These include the coarse image scanning thickness, artifact of applicators, use of ovoids shield and digitizing uncertainty during planning. With the use of anatomic structures of planning CT, this project investigates the 3D dosimetric impact when source dwell positions deviate from the true positions. Method and Materials: Five cervical cancer patients treated with intracavity HDR brachytherapy using tandem and ovoids applicators were included in this study. For each patient, we acquired CT and made clinic plan based on ICRU 38 protocol. Retrospectively, we contoured the bladder and rectum wall and replaned patients considering the scenarios of source positions shifted 5mm. For each patient we compared five plans: original clinical plan, seed positions of all three applicators shifted forward or backward and seed positions of two ovoids shifted forward or backward. Seven dosimetric endpoints are used in comparisons and they are total treatment time, ICRU bladder/rectum point dose, and 1cc/5cc bladder/rectum maximal dose. Results: The DVHs of two forward plans, all shifted or only ovoids shifted, show similar trends. Similar result was observed from two backward shifted plans. Forward shifted and backward shifted plans deviate from the original plan, but in opposite way, and the direction of deviation is structure dependent. For 1cc bladder wall maximum dose, the mean of forward plans is −3.9±3.0% and backward plans is 5.7±4.6% different from the original plan. For 1cc rectum wall maximum dose, the mean of forward plans is 14.6±6.3% and backward plans is −6.3±3.2% different from the original plan. Conclusion: Source position accuracy is a critical factor for the high quality of brachytherapy treatment planning. A 5mm discrepancy could have great impact on the dosimetric endpoints of normal structures.
MO‐FF‐A2‐01: Development of a Coincidence Detection Spectrometry System to Electronically Filter the Bremsstrahlung Continuum from a 90Y Gamma Spectrum
Authors: Paxton A , Nickles R , DeWerd L
Published: August 2010

Purpose: To demonstrate the ability of a newly constructed coincidence detection spectrometry system to better resolve the 511 keV peak present in a 90Y gamma spectrum due to the internal pair production (IPP) component of 90Y decay by electronically filtering out the bremsstrahlung continuum. Method and Materials: A recently determined low‐uncertainty branching ratio for the IPP component of 90Y decay provides the potential to spectroscopically assay the activity of 90Y by measuring the resulting 511 keV peak. However, the use of a single spectrometer to accomplish this is plagued by the presence of a large bremsstrahlung continuum. In this work, a coincidence detection system was constructed that paired a HPGe detector with a large NaI detector. A series of NIM electronics were tuned to gate the energy signal of the HPGe detector by the coincidence signal. A 90Sr /90Y source was measured for 4 h. The energy spectrum gated by coincidence, the non‐gated energy spectrum, and the energy spectrum gated by random coincidence were all collected concurrently. Environmental background (EBG) (i.e. no source present) was measured for 12 h. Canberra Genie™ 2000 software was used to analyze the spectra. Results: The EBG measured with the gated method was found to be negligible with no counts in the peak area. The EBG of the non‐gated method was subtracted from the non‐gated 90Sr /90Y spectrum. Genie™ 2000 was used to determine the 511 keV peak area and uncertainty. The uncertainty was 1.7% for the gated spectrum and 7.9% for the non‐gated spectrum. Conclusion: The ability of this detection system to electronically filter the bremsstrahlung continuum from the 511 keV peak‐of‐interest has been demonstrated. This method results in a reduced uncertainty in the peak area compared to measurement with a single detector.
MO‐FF‐A2‐02: A Technique to Determine the Integral Depth Dose of Proton Pencil Beam Spots
Authors: Anand A , Sahoo N , Zhu X , Sawakuchi G , Poenisch F , Amos R , Suzuki K , Mohan R , Gillin M
Published: August 2010

Purpose: To develop and validate a technique to determine the integral depth dose (IDD) from the measured profiles and peak doses of proton pencil beam spots (PPBS). Methods and Materials: The IDDs of 72.5, 146.4 and 221.8 MeV energy PPBS were determined by area integration of their planar dose distributions at different depths. The transverse lateral profiles of the PPBS at selected depths in water were measured by using small volume ion chambers using a beam scanning system. The peak spot dose, D0, was determined by different experimental methods using ion chamber and diode. The extent of anisotropy in spot dose distribution was determined by comparing profiles from film dosimetry along different angles with the principal axes. The IDD was calculated by integrating the measured lateral profiles and the corresponding D0. The IDDs were also measured using PTW Bragg peak chamber (BPC), designed for this purpose, and were compared with the calculated values. Results: The spot dose distributions were found to be isotropic. The magnitude of difference in the calculated and BPC measured IDDs was found to be as much as 15.7% for the lower energy spots, and decreased with increase in beam energy. This difference is mainly due to the size limitation of the BPC. The contribution from the tail region of profiles increases with decrease in beam energy, and the BPC chamber size is not large enough to account for the contribution of these long tails. Correction factors for the BPC size effect on the measured IDD were determined from our calculation. Conclusion: The technique of area integration of profile is found to be useful for determining the IDD values of PPBS, which are important input parameters for beam modeling in the treatment planning system, with reasonable accuracy. Conflict of Interest Statement: Research sponsored by Varian Medical Systems.
MO‐FF‐A2‐03: A Method for Measuring X‐Ray Beam Profiles Using a Flat Panel Detector
Authors: Sun M , Star‐Lack J , Virshup G
Published: August 2010

Purpose: Many digital x‐ray imaging applications require knowledge of the spatial distribution of the incident x‐ray flux intensity (beam profile), which is often obtained through Monte Carlo simulations or dedicated measuring equipment such as point detector that is raster scanned. Direct measurement of flux intensities using an area detector is difficult due to detector gain variations. We present a method to separate detector gain variations from flux intensity variations thus providing a practical means for efficient measurement of beam profiles using area detectors. Method and Materials: A flat panel imager is used to acquire at least two unattenuated intensity images at slightly shifted positions. Signal intensity differences read from the same pixel are assumed to be due to flux intensity variations while differences measured at a same spatial location are assumed to be due to gain variations. Detector gain maps and flux intensity maps are then created by mathematically decoupling the flux and gain intensity components from the readings. The process is repeated in the other in‐plane direction to generate 2‐D maps. To test the method, a Varian Paxscan 4030CB was used to measure the beam profile produced by a Varian G242 tube. Results were compared to Monte Carlo simulations and raster‐scanned photodiode measurements. Results: The detector gain of the imager and the beam profile of the source were separated. The beam profiles measured using the propose method were independent of how prior gain calibrations were performed and matched well with Monte Carlo predictions and photodiode measurements. Conclusions: A robust method to measure the beam profile of an x‐ray field using an area detector is presented. The method also generates a detector gain map that is independent of flux intensity variations.

Employees of Varian Medical Systems.
MO‐FF‐A3‐02: What Is the Maximum Number of Beam Spots Deliverable within One Gating Window for Synchrotron Based Scanning Proton Beam Therapy of Lung Cancer?
Authors: Tsunashima Y , Vedam S , Dong L , Zhang X , Umezawa M , Balter P , Mohan R
Published: August 2010

Purpose: To analyze the maximum number of spots achievable for each energy layer in respiratory‐gated scanning beam delivery of proton radiotherapy when optimizing efficiency and minimizing the impact of cycle‐to‐cycle breathing irregularity. The ideal delivery would be to deliver all spots in each energy level within a single respiratory gate. Method and Materials: In‐house treatment delivery simulation software was developed based on manufacturer's specification to study treatment statistics using timing information from patient's respiration patterns, and scanning beam proton treatment plans. Data from 10 patients with lung tumor were analyzed. Proton scanning beam plans were developed using 3 beams/plan. The patient's breathing traces were used to simulate respiratory gated delivery with 30%, 20% and 10 % duty cycles (DC). In the simulation, we measured 1) typical respiratory gate duration, and 2) the number of spots within this duration (spill time) in each expiration phase. We also compared the above data with number of available spots without respiratory gating. Results: The average duration of each respiratory gate signal was 1161±475, 841±323 and 517±208 msec for 30%, 20% and 10 % DCs respectively. This translated to 166, 120 and 74 available spots for 30%, 20% and 10 % DC respectively. Simulation of respiratory‐gated scanning beam delivery over 10 respiratory patterns and 10 tumors indicated that, maximum number of spots required per energy level were 85, 83 and 77 for 30%, 20% and 10% DC respectively. Conclusion: Treatment plans for respiratory‐gated scanning beam proton delivery on a synchrotron accelerator should be designed such that the number of available spots energy level are less than can be delivered in the respiratory gate. Our simulation study indicated that this condition could be easily met for various DCs of respiratory gating.
MO‐FF‐A3‐03: The Impact of Monitor Unit Constraints on Plan Quality of IMPT Planning Using Single‐Field Optimization
Authors: Zhu X , Sahoo N , Zhang X , Robertson D , Li H , Choi S , Lee A , Gillin M
Published: August 2010

Purpose: To investigate the effect of monitor unit (MU) constraints on the dose distribution created by intensity modulated proton therapy (IMPT) treatment planning using single‐field optimization (SFO). Methods and Materials: Ninety‐four energies between 72.5 and 221.8 MeV are available for scanning‐beam IMPT delivery at our institution. The minimum and maximum MUs for delivering each pencil beam (spot) are 0.005 and 0.04, respectively. These MU constraints are not considered during optimization by the treatment planning system; spots are converted to deliverable MUs during post‐processing. Treatment plans for delivering uniform doses to rectangular volumes with and without MU constraints were generated for different target doses, spot spacings, spread‐out Bragg peak (SOBP) widths, and ranges in a homogenous phantom. Four prostate cancer patients were planned with and without MU constraints using different spot spacings. Rounding errors were analyzed using an in‐house software tool. Results: From the phantom study, we have found that both the number of spots that have rounding errors and the magnitude of the distortion of the dose distribution from the ideally optimized distribution increases as the field dose, spot spacing, and range decrease and as the SOBP width increases. From our study of patient plans, it is clear that as the spot spacing decreases the rounding error increases, and the dose coverage of the target volume becomes unacceptable for very small spot spacings. Conclusions: Constraints on deliverable MU for each spot could create a significant distortion from the ideally optimized dose distributions for IMPT fields using SFO. To eliminate this problem, the treatment planning system should incorporate the MU constraints in the optimization process and the delivery system should reliably delivery smaller minimum MUs.