bluuebunny/crossref_metadata_2025 huggingface dataset:
MO‐F‐213CD‐08: Characterization of the Homogeneous Breast Tissue Mixture Approximation for Breast Image Dosimetry
Authors: Feng SSJ , Bliznakova K , Qin X , Fei B , Sechopoulos I
Published: June 2012

Purpose: To characterize the suitability of the use of a homogeneous mixture of adipose and glandular tissue approximation for breast imaging dosimetry. Methods: Fifteen patient breast computed tomography images (BCT) were classified into skin, adipose, and glandular tissue. The segmented breasts underwent simulated mechanical compression to mimic breast compression during mammographic acquisition. Using Monte Carlo simulations representing BCT and mammographic acquisitions, the radiation dose to the voxels representing glandular tissue for both the uncompressed and compressed breasts was estimated. The BCT simulations used both a 49 kVp and 80 kVp tungsten target spectrum, while the mammography simulations used the spectra corresponding to the patient's screening cranio‐caudal view mammogram. The simulations were repeated by replacing the adipose and glandular voxels with voxels representing a homogeneous mixture of the two tissues in their corresponding proportions. The normalized glandular dose (DgN) of the true heterogeneous breast tissue distribution and the homogeneous approximation was compared using a paired t‐test. Results: For mammography, the approximated homogeneous DgN was on average 61% higher than the heterogeneous DgN (p<10−6). For BCT, the homogeneous DgN overestimated dose by an average of 27% (p<10−5) and 12% (p<10−4) for the 49 kVp and 80 kVp spectra, respectively. For all conditions and all breasts the homogenous DgN was higher than the heterogeneous DgN except for one breast, in which they were equal for the BCT simulations. Conclusions: The homogeneous mixture approximation commonly used for breast imaging dosimetry significantly overestimates the actual dose to the glandular tissue. Although this may be adequate for technique, modality and/or system comparisons, it is inevitable that these dose estimates are used inappropriately (e.g. for risk estimates), making this overestimation undesirable. Further study with a larger number of patients is required to obtain appropriate correction factors to better estimate breast glandular dose in the future.
MO‐F‐213CD‐10: PShield: An Algorithm for Optimization of PET/CT Shielding
Authors: Pasciak A , Jones A
Published: June 2012

Purpose: Calculation of radiation shielding requirements for high‐workload PET installations using the methods proposed by AAPM TG‐108 can be difficult. The principle challenge that makes PET shielding design more complex than other diagnostic imaging modalities, aside from the higher photon energy, is that it is a multisource problem for which no unique solution exists. The PShield algorithm incorporates three‐dimensional numerical methods to optimize PET shielding and deliver a cost‐optimized solution while making no approximations. Methods: PShield uses a sequential quadratic programming routine to optimize PET shielding by minimizing a cost function in 3‐dimensions using extrapolations of the TG‐108 formulas. PShield makes no approximations and accounts for the contribution of every radiation source to the dose rate at every location in the problem using a discrete mesh. We used two simple examples of shielding problems to compare PShield with the TG‐108 methods. Results: The benefit of applying an optimization routine to an indeterminate problem is the identification of the only solution to the problem that minimizes the desired cost function. Choosing a poorly optimized solution can Result in a shielding design that requires as much as 50% more shielding than an optimized design to reach the same dose rate at a given control point. The increased accuracy afforded by P Shield ensures that dose rates at every point in a control area never exceed the design dose, whereas a reasonable design based on the TG‐108 methods may have hot spots where the dose rate exceeds the design dose by a factor of 2 or more. Conclusions: PShield is an exact three‐dimensional numerical solution for optimal PET shielding which identifies a singular solution which is cost‐optimized. This is especially important for modern PET/CT suites, where increases in scanner capabilities have resulted in more complex shielding problems and the potential for high occupational doses.
MO‐F‐217A‐01: Establishing a 4D MRI Program for Imaging Moving Tumors
Authors: Cai J , Hu Y , Tryggestad E , Parikh P
Published: June 2012

4D‐CT has been widely used in radiotherapy for imaging moving tumors, especially in the thoracic regions. In other body sites such as the abdominal, 4D‐CT is limited due to its low soft‐tissue contrast. Additional concern for 4D‐CT includes increased radiation dose to the patient. On the other hand, MRI has no known radiation hazard and has excellent soft‐tissue contrast. It is desirable to develop 4D‐MRI techniques to address the problems associated with 4D‐CT. There are different methods of 4D‐MRI imaging. One is to use sophisticated MR pulse sequence and/or use advanced software and hardware. Another method is to retrospectively sort slice MR images acquired continuously throughout the volume based on respiratory phases. The first method typically results in a temporal resolution of ∼0.7 frame/s and a spatial resolution of 3–4 mm. The second method requires simultaneously acquired respiratory signals, either by using external surrogate or internal/image‐based surrogate. The temporal resolution is ∼ 3 frames/s, the in‐plane spatial resolution is ∼1.5–2 mm, and the slice thickness is ∼ 3–5 mm. Both methods are doable using commercially available MR sequences. 4D‐MRI can be used to improve the determination of patient specific tumor motion margin for radiation therapy, especially for the tumors in the abdominal regions where 4D‐CT has limitations. We presented this topic as a joint imaging/therapy scientific symposium in 2011 AAPM annual meeting. This proposal aims to provide a follow up education on this topic and present technical details on how to establish the 4D‐MRI program in the clinic.

Learning Objectives:

1. Discuss the need of 4D‐MRI technique for improving patient care in radiation therapy

2. Understand technical aspects of different 4D‐MRI imaging techniques

3. Discuss the procedures to establish 4D‐MRI program in a radiation oncology clinic
MO‐G‐213AB‐01: Quantification of TomoTherapy MVCT Dose
Authors: Chen M , Chao E , Lu W
Published: June 2012

Purpose: To quantify MVCT dose of TomoTherapy for three jaw settings, J4, J1, and J0.1, corresponding to beam widths of 7 mm, 4 mm, and 3 mm, respectively, at the isocenter plane, and three imaging modes, fine, normal, and coarse, corresponding to a couch speed of 4, 8, and 12 mm/rotation, respectively. Material and Methods: An MVCT dose engine was commissioned specifically for the MVCT beams, including updates to the fluence attenuation table (FAT), energy deposition kernel, cone profiles, and penumbrae. MVCT dose calculation was then applied on real and synthesized images of cylindrical water phantoms of diameters ranging from 5 cm to 40 cm, and the results were compared with film measurement. Result: For the J1 jaw and coarse imaging mode, the maximum difference between calculation and measurement was about 6% of the center dose. Calculation on synthesized phantoms showed that the center dose decreased almost linearly as the phantom diameter increased, and that the fine mode received twice the dose of the normal mode and three times that of the coarse mode. The maximal dose due to the helical ripple ranged from 100%∼200% of the center dose, with increasing ratios for larger phantoms (due to the larger radius), smaller jaws, and faster couch speed (the latter two yielding a higher helical pitch). For all jaw settings and couch speeds, the mean dose and average surface dose vary from 95%–115% of the center dose with increasing ratios for larger phantoms. Conclusion: An MVCT dose calculator was set up with validation through film measurement and subsequently used to calculate TomoTherapy MVCT dose for various phantom sizes under various imaging parameters. The results can serve as a reference for TomoTherapy MVCT dose.
MO‐G‐213AB‐04: Investigation of Photon MLC to Deliver Energy and Intensity Modulated Electron Beams for Advanced Radiotherapy
Authors: Jin L , Eldib A , Fan J , Li J , Wang L , Ma C
Published: June 2012

Purpose: To implement energy and intensity modulated electron radiotherapy (MERT) using an existing photon multileaf collimator (pMLC) in a modern medical accelerator. Methods: Both Siemens Artiste and Varian Trilogy accelerators have been investigated for MERT using the existing pMLC in the service mode. The limitations of SSD for Varian linac were examined. A Monte Carlo (MC)‐based inverse treatment planning code was used for plan optimization and dose calculation. Water scanning data including PDDs and dose profiles at various depths were acquired for various fields shaped by pMLC for 6, 9, 12 and 15MeV electron energies of both Artiste and Varian linacs and compared with MC calculations. Results: Artiste can treat patient with an SSD down to 61 cm to reduce the field penumbra caused by electron in‐air scatters. This is comparable to the Siemens Primus accelerator that has an SSD about 60cm. The electron beam characteristics are acceptable in terms of flatness and penumbra as compared with those collimated by electron applicators. Varian Trilogy has to use a longer SSD (about 85 to 90cm) due to the current restriction of the table height. Comparisons between measured PDDs and dose profiles, and MC calculations for various energies for pMLC shaped fields of 3 × 3 cm2and 10 × 10 cm2 showed agreement to within 2%/2mm. Conclusions: With pMLC shaped fields, MC dose calculations agree with measured PDDs and dose profiles at various depths, which warrant the dose accuracy for MERT delivered by pMLC using MC‐based dose calculation planning systems. With large SSD on Trilogy, dose profiles deteriorate to some extent as compared with those on Artiste with SSD = 61cm in terms of penumbral width. However the impact of this penumbra difference on the ultimate MERT plan using energy and intensity modulated electron beams is yet to be investigated.
MO‐G‐213AB‐05: Beam Simulation and Measurement for Energy‐Intensity Modulated Electron Radiotherapy (MERT) with a Computer‐Controlled Electron Multileaf Collimator Device
Authors: Jin L , Eldib A , Li J , Fan J , Wang L , Ma C
Published: June 2012

Purpose: To verify an add‐on computer‐controlled multileaf collimator (eMLC) device on a Varian linac capable of delivering accurate dose for energy‐intensity modulated electron radiotherapy (MERT). Methods: The eMLC has 27 pairs of tungsten leaves (tongue and groove design to reduce intraleaf leakage)with 0.56cm width and 2cm thickness, providing a field size as large as 15 cm × 15 cm defined at 94cm SSD. Measurements were done to determine the appropriate jaw setting for an eMLC shaped field, mainly to reduce the leaf leakage outside the eMLC shaped field. The phase space data were acquired by Monte Carlo (MC) simulations for electron beams of energies 6, 9, 12 and 15 MeV, respectively and used as an input source in MC dose calculations in a phantom. MC calculated PDDs and dose profiles were compared with measurements for large fields (e.g. 10 cm × 10 cm) and small fields (e.g. 3.4 cm × 3.4 cm). The eMLC leakage for various energies was measured both in‐air and in phantom (at dmax) as a ratio of doses with the eMLC closed and completely open. Results: With the jaw position at 0.5 cm beyond the edge of the eMLC shaped field, it was showed to best eliminate the interleaf leakage, especially for high energies, e.g. 15 MeV. The average leaf leakage ranged from 0.3% (6 MeV) to 2.3% (15 MeV), which were consistent with lower in‐phantom values than in‐air values. MC calculated PDDs and dose profiles generally agreed with measurements to within 2mm/2%. Conclusions: This eMLC device is capable of delivering energy and intensity modulated electron beams accurately with acceptable leaf leakage for advanced MERT treatment.
MO‐G‐213AB‐06: Segment Matching Optimization for Modulated Electron Radiation Therapy
Authors: Gokhroo G , Klein E
Published: June 2012

Purpose: Modulated electron radiotherapy (MERT) has been demonstrated to be an ideal treatment for lesions close to the surface in terms of dose coverage and sparing of underlying critical organs and normal tissues. The purpose of this study was to obtain the parameters and devise strategies for segment matching which are necessary for efficient MERT treatment planning. Methods: In this study we used EGS4nrc/BEAMnrc and MCSim code for Monte Carlo calculations. The Computational Environment for Radiotherapy research (CERR) tool was used for viewing and comparing plans. A water phantom, delineated with stepped target idealized for two segments with sizes from 2 to 6 cm and electron beam energies of 6, 9, 12, 16 and 20 MeV were used for the calculations. The variation in dose distribution with different energies of abutte electron segments and change in segment size were studied. In order to obtain homogenous dose at the segment match, various gaps and smoothing between segments were introduced. Finally, a salivary gland MERT plan was executed using the segment match optimization scheme. Results: As the energy difference between two consecutive segments increased, dose uniformity at the segment junction decreased. For small segments (< 4 cm) optimization can be improved by introducing a gap (1–2 mm) between segments. However, for larger segments, smoothing was found to be more beneficial than gapping the segments. The salivary gland case was successful in terms of dose coverage and homogeneity aided by the segment matching techniques described. Conclusions: We conclude that use of bigger segments, and smaller differences in energy for abutting segments, should be prioritized for MERT planning. This study provides an optimization regimen of segment matching for MERT treatment planning with gaps/smoothing, depending on segment size and energy in order to provide better dose coverage and expedite the forward planning process. Varian Master Research Agreement
MO‐G‐BRB‐02: Rosalyn S. Yalow, PhD: A Personal and Scientific Memoir
Authors: Goldsmith S
Published: June 2012

In 1950, Rosalyn S. Yalow, PhD, a medical physicist, was hired to develop The Radioisotope Service at the Bronx VA. She was joined by Sol Berson who had completed his residency in internal medicine. Their initial investigations together were in the application of radionuclides in blood volume determination and the kinetics of iodine metabolism in health and thyroid disease. They were quickly recognized as outstanding clinical investigators. They turned their attention to understanding insulin physiology by labeling insulin and studying the plasma kinetics in a variety of subjects. Radiolabeled insulin was cleared from the blood more slowly in subjects who had received insulin injections. They concluded that the slower clearance was due to insulin antibodies and they quantified the binding phenomenon. They recognized that the quantitative competition of a radiolabeled antigen and an unlabeled antigen molecule for a limited number of binding sites on an antibody provided a basis for the quantitation of an unknown amount of a similar molecule in a sample of plasma or other biological fluid. In 1960, a manuscript entitled “Immunoassay of Endogenous Plasma Insulin in Man” appeared. Using the same principal but identifying unique characteristics of other peptide hormones, the Yalow‐Berson team went on to develop assays for Human Growth Hormone, ACTH, Parathyroid Hormone, Gastrin, Glucagon and other materials of biologic interest. Radioimmunoassay could quantify these substances in less than a drop of plasma. In recognition of these contributions, the Nobel Prize in Medicine and Physiology was awarded in 1977 to Rosalyn S. Yalow, PhD. It is not possible in the limited space and time available to review the impact that the development of radioimmunoassay has had on virtually every field of medical science but the reader is encouraged to review some of the early publications on this subject and to share the excitement and awe that they created at the time of publication and continue to do so for this observer whenever there is an opportunity to review this material. This lecture will provide an overview of the life and principal scientific contributions of a Medical Physicist, Rosalyn S. Yalow, PhD who was awarded the Nobel Prize in Medicine in 1977 in recognition of the development of radioimmunoassay.

Learning Objectives:

1. Knowledge of key events in the scientific development of Rosalyn S. Yalow, MD.

2. Understand the principles of radioimmunoassay.

3. Understand the impact of radioimmunoassay methodology on medical science.
MO‐G‐BRA‐02: Investigation of a Method for Generating Synthetic CT Models from MRI Scans for Radiation Therapy
Authors: Hsu S , Cao Y , Balter J
Published: June 2012

Purpose: MRI is advantageous for treatment planning, given that MRI can support dose calculations and image‐guided radiotherapy. This study investigates initial methods to derive synthetic CT images from MRI scans that support these needs. Methods: The framework involves a) acquiring multiple MRI image volumes each presenting different tissue/material contrasts, and b) implementing a classification scheme that identifies these materials with sufficient accuracy to assign relative attenuation properties for generating CT images. Six images, consisting of two echoes from ultra‐short TE acquisition (0.07 and 3.7 ms echoes to image cortical bone), T1 GRE, T2 FSE, and fat and water images (from a two‐point Dixon technique), were tested as inputs. Signal intensities of manually‐defined tissues were measured, and used to determine the ability of various image combinations to uniquely classify tissue types. To classify tissue using these images, a fuzzy classification algorithm was used to define membership functions of material types. Assigning attenuation properties to classified tissues yielded synthetic CT image volumes. Results: While subsets of T1, T2, and water/fat image volumes demonstrated statistical significance in identifying fat, solid tissue, and water, the 0.07 ms TE data (UTE1) was necessary to significantly separate bone and air from these tissues. The classification algorithm was able to robustly extract the various material types using subsets of three of the image volumes (T1/UTE1/water). Example CT images and resulting digitally reconstructed radiographs were generated based on attenuation properties applied to the tissue segments. Conclusions: These investigations demonstrated that MRI presents sufficient differential contrasts to separate tissues of interest, and identified the minimum necessary image contrasts for synthetic CT generation using robust classification methods. Further investigations using MRI and CT on subjects undergoing radiotherapy, as well as phantoms, will validate the accuracy of this technique.
MO‐G‐BRA‐05: High Fidelity, Low Distortion Tetrahedral Diffusion‐Weighted MRI for Radiation Treatment Planning
Authors: Paulson E
Published: June 2012

Purpose: Diffusion‐weighted (DW) MR imaging provides supplemental information that may aid in target delineation during radiation treatment planning. However, geometric distortion artifacts and the inability to acquire higher b‐value DW images in tissues with short T2s have encumbered the utilization of DW MRI in radiation treatment planning. To rectify these issues, we present here a novel DW imaging approach that combines tetrahedral diffusion encoding and a multi‐shot, variable‐density spiral (MS‐VDS) acquisition. Methods: A custom MS‐VDS pulse sequence with Stejkal‐Tanner (ST) and twice refocused spin echo (TRSE) excitation schemes was implemented on a Siemens 3T scanner. For comparison to current state‐of‐the‐art, orthogonal and tetrahedral DW images were acquired in the brains of three healthy volunteers (FOV: 220mm2, matrix: 1282, interleaves: 16, TR: 3000ms, TH: 3mm, b‐values: 0/1000 s/mm2). Self‐navigated phase correction was applied during reconstruction to correct each spiral interleaf for eddy current and motion‐induced phase errors. Results: The MS‐VDS acquisition was effective at diminishing geometric distortion artifacts. The minimum echo times for the orthogonal ST, tetrahedral ST, and tetrahedral TRSE acquisitions were 79 ms, 56 ms, and 71 ms, respectively. Although the inherent self‐navigating capability of VDS was effective at diminishing eddy current and motion‐induced phase errors in the orthogonal ST images, the tetrahedral ST images were contaminated by residual eddy current‐induced blurring. However, the tetrahedral TRSE images exhibited quality similar to the orthogonal ST images, indicating that the TRSE sequence provided effective compensation of residual eddy currents. Conclusions: Tetrahedral encoding results in a reduction in minimum echo time for a given b‐value, facilitating acquisition of DW images in tissues with shorter T2s. A TRSE excitation is required to provide eddy current compensation in tetrahedral DW acquisitions. The combined MS‐VDS TRSE tetrahedral DW imaging method permits acquisition of high fidelity, low distortion DW images suitable for radiation treatment planning.
TU‐A‐211‐02: Nuts and Bolts of MRI Physics Testing
Authors: Bell R
Published: June 2012

Training of physicians performing fluoroscopy is key to patient and staff safety. In the absence of uniform regulations regarding credentialing of physicians performing fluoroscopy, various states have adopted/implemented regulations. State of Maryland requires a 4‐hour initial training in fluoroscopy operations and safety and a one‐hour refresher course on the same subject once every 2 years. This applies to all physicians performing fluoroscopy with exception to board certified radiologists. The implementation of a program to comply with such regulatory requirements in a large academic center (Johns Hopkins) and the challenges to ensure compliance of all physicians performing fluoroscopy is the focus of this presentation. Along with that, the process on how to prepare and comply for the Joint Commission's sentinel event policy pertaining to prolonged fluoroscopy will be discussed. In addition, the presentation will also discuss process set in place to comply with regulatory requirements of monitoring personal radiation monitors, other training issues and radiation dose reduction strategies in fluoroscopy.

Learning Objectives:

1. To understand the credentialing and other requirements for physicians performing fluoroscopy.

2. To become familiar with the process set to comply with regulations at a large academic medical center.

3. To be able to utilize the information to implement similar process at other institutions.
TU‐A‐BRB‐04: Fragmentation Analysis in Heavy Ion Therapy: A Novel Experimental Approach to Measure Lateral Distributions of Secondary Charged Particles
Authors: Hartmann B , Gwosch K , Granja C , Jakubek J , Jaekel O , Martisikova M
Published: June 2012

Purpose: In carbon ion beam radiotherapy, nuclear fragmentation in tissue leads to an attenuation of the primary particle flux and the build‐up of lighter projectile fragments. The radio‐biological effectiveness of these secondary particles differs from the primary ions. Therefore, fragmentation has to be considered in physical models used for therapy planning. While until now large apparatus have been used for ion spectroscopic measurements, we developed an experimental technique to measure lateral distributions of charged particles directly within or closely behind phantoms, using a small pixelated detector. Methods: Experiments were performed at the Heidelberg Ion‐Beam Therapy Center. A carbon ion pencil beam (∼5mm nominal width, E initial=271MeV/u) was directed towards a 12.7cm thick PMMA phantom. Secondary particles leaving the phantom were detected with the Timepix detector, placed perpendicular to the beam 5cm behind the phantom. The Timepix detector consists of a 300μm thick silicon sensor and a read‐out chip with 256×256 square pixels (55μm pitch). The high spatial resolution and the pixel‐wise energy calibration enable to determine the energy‐loss of individual particles in the sensitive layer. Together with pattern recognition analysis of characteristic particle traces in the detector, we are able to distinguish types of particles and discriminate the ion species. Results: Lateral distributions of events identified as being induced by hydrogen, helium and boron ions were determined. They show approximately Gaussian shaped profiles with FWHM values of 3.2mm (boron), 6.3mm (helium) and 13.1mm (hydrogen). These results are consistent with the expectation, that lighter fragments show wider distributions. Conclusions: We present a novel method to determine secondary charged particle distributions in heavy ion therapy directly within phantoms. The main advantage lies in the flexibility of the set‐up. The performance of the method is promising to enable fragment distributions measurements in a wide phase space in the future. Research carried out in frame of the Medipix Collaboration
TU‐A‐BRB‐05: A Quenching Correction Method for Volumetric Scintillation Dosimetry of Proton Beams
Authors: Robertson D , Mirkovic D , Beddar S
Published: June 2012

Purpose: Liquid scintillator dosimetry has the potential to provide fast, high‐resolution, three‐dimensional dosimetry of radiation beams. However, organic liquid scintillators exhibit a non‐linear response at the high linear energy transfer (LET) values characteristic of ion beam Bragg peaks. The purpose of this study was to develop a method to correct for this quenching effect in scintillators used for volumetric proton beam dosimetry. Methods: Scintillation light from a miniature (4 mm̂3) liquid scintillator detector was measured along the central axis of a 161.6 MeV therapeutic proton pencil beam. Three‐dimensional dose and LET distributions were calculated for 85.6, 100.9, 144.9, and 161.6 MeV beams using a validated Monte Carlo model. LET values were also calculated using an analytical formula. The empirical parameters of a quenching correction model were determined by least squares fitting of the model to the signal from the miniature detector and the Monte Carlo data for 161.6 MeV protons. The scintillation light distribution in a tank of liquid scintillator was measured with a CCD camera at all four energies. The quenching model and LET data were then used to correct the light distribution measured in the scintillator tank. The quality of agreement between the calculated dose and corrected light signal was assessed by the percent difference on the central axis and by gamma analysis. Results: The calculated and measured light signals agreed within +/−3% on the central axis of the proton pencil beam for all energies except 85.6 MeV, where the agreement was +/−4%. The gamma analysis passing rate exceeded 95% with gamma criteria of 2%, 2 mm. Conclusion: We have developed a method to correct for scintillator quenching in a large detection volume with sufficient accuracy for dosimetric purposes. This method requires prior knowledge of the LET distribution of the beam and the quenching factors for the scintillator.
TU‐A‐BRB‐06: Characterization of a Commercial Photodiode Based Plastic Scintillation Detector Prototype
Authors: Wootton L , Klein D , Bonde C , Adams E , Schmidt D , Beaulieu L , Beddar S
Published: June 2012

Purpose: The purpose of this work is to characterize a commercial plastic scintillation detector (PSD) prototype that uses a photodiode to convert scintillation light to an electronic signal that can be measured with an electrometer. We tested the reproducibility of the detector as well as its accuracy. Methods: The detector consists of 3 mm of scintillating fiber 1 mm in diameter connected to a photodiode via approximately 2 m of optical fiber. We tested the reproducibility of the detector by making daily measurements in a cobalt beam. We then verified the accuracy of the detector by measuring percent depth dose (PDD) curves on a linac for two photon and electron energies. The measurements were compared to commissioning data (measured with an ionization chamber) to quantify the accuracy of the detector. Results: Over the course often days the mean dose the detector measured varied by 0.16% (1 standard deviation). The individual measurements on specific days varied as much as 0.27% and as little as 0.09%. PDD curves obtained with the detector differed from commissioning data by less than 0.6% for both photon energies. For both electron energies, the detector results differed from commissioning data less than 3% before the steep dose gradient towards the end of the electron range. In the dose falloff the difference was as great as 8% (likely due to positioning errors). Conclusions: This new photodiode based PSD exhibited a very high level of reproducibility both in terms of individual measurements and average measurements on different days. Depth dose curves measured with the PSD in photon and electron beams demonstrated excellent agreement with machine commissioning data, validating the accuracy of this detector as well confirming that it is an energy independent detector. This work was partly supported by an NIH/NCI SBIR phase I grant (1R43CA153824‐01) in collaboration with Standard Imaging Inc.
TU‐A‐BRB‐07: Characteristics of Diode Detectors in Proton Beam Dosimetry
Authors: Coutinho L , Das I , Würfel J , Cheng C
Published: June 2012

Purpose: Small and narrow field dosimetry in proton beam therapy is difficult due to fluence disequilibrium. Diode detectors could be a suitable choice for high resolution proton dosimetry due to their relatively small size and high sensitivity. Unfortunately, semiconductor detectors are prone to radiation damage due to charged particle interactions. Characteristics of some of the commercial diodes in proton beam dosimetry are investigated in this study specially a prototype diode from PTW. Methods: The characteristics of several diodes from different vendors were evaluated in proton beams. All diodes were irradiated in a water phantom under reference condition (10cm diameter field, 16cm range, 10 cm SOBP) at various dosages to investigate the signal loss. Based on radiation response, the prototype diode was used for dose, dose rate, depth dose and profile measurements. The results were compared with Markus parallel plate chamber for proton energies in the range of 6cm to 27cm in water. Results: The initial signal loss in various diode detectors range from 0.4%/Gy to 0.05%/Gy. The PTW prototype exhibited the smallest amount of signal degradation that was investigated up to a dose of 1.4 kGy. Relative dose measurements (depth dose and profiles) are in good agreement with the measured data from Markus chamber. The magnitude of Bragg peaks differed by −2% from the results obtained with Markus chamber for all energies. Conclusions: Diode reading is a function of pre‐irradiation dose, beam energy, dose rate and total dose. Ignoring signal degradation and pre‐treatment dose, diodes can be used for relative dose measurements such as depth dose and profile. For absolute dose measurements, especially in small fields, special attention should be given to account for the loss of signal. The LET dependence is minimal for the prototype and can be used in a wide range of beam energy.
TU‐A‐BRB‐08: A Technique to Determine the Gantry Laser and the Proton Pencil Beam Coincidence Using Computer Radiography Detector for Routine Quality Checks
Authors: Anand A , Kerns J , Sahoo N , Zhu X , Gillin M
Published: June 2012

Purpose: A computed radiography (CR) detector with photostimulable phosphors can detect both direct and indirect ionization radiation, and is even sensitive to photons in the visible range. The purpose of this work is to develop and validate a technique using CR to check both proton radiation versus localization laser coincidence and spot positioning accuracy for magnetically scanned proton pencil beam spots (PPBS). Methods: CR plate is removed from its cassette and set up perpendicular to the incident beam and exposed to an array of PPBS irradiated at predefined coordinates. Upon exposure of the particle radiation, the CR detector is then exposed to the gantry's laser systems. The plate is then put through a digital scanner which then reads the photo stimulated luminescence and converts the signal into a digital image pixel map. MATLAB®‐based analysis software and ImageJ® image processing tools were used to compute the dose gradient from the laser bleaching of the proton exposure of the detector and for quantification of the results of this QA check procedure. The technique further allows quantitative determination of both spot positions, and the relationship between X ray system used for image guided patient setup and the PPBS delivery. Results: Both the coincidence between laser representing the gantry isocenter and center of PPBS along the central axis and spot positions can be checked within 1mm accuracy with the CR detector. Conclusions: We have demonstrated that our technique allows determining the coincidence between the laser and radiation beam. The technique described here is found to be suitable for periodic QA checks of relative positions of the spot center relative to the gantry isocenter and beam central axis. Improvements of this technique by considering the effect of detector's quantum yield and modulation transfer function on the data remain subject of further study.
TU‐A‐BRB‐10: Ultrasonic Imaging of Dose to Water from a 1.5 MeV Electron Beam
Authors: Tosh R , Bateman F , Chen‐Mayer H , Malyarenko E
Published: June 2012

Purpose: To use an ultrasonic sensor array to obtain thermal images produced by absorbed ‐dose distributions in a water phantom. Method and Materials: An array of 128 PVDF ultrasonic transducers arranged in a circle with radius 177.8 mm is used to image the heating of water in a phantom irradiated by a vertical, 1.5 ‐MeV electron beam capable of delivering dose rates ranging from Gy/min to kGy/min. Fan ‐beam sampling and reconstruction are managed by a double ‐multiplexer system that selects one transducer to emit a short pulse and scans the corresponding transducers across the tank to receive the propagated pulse. Time ‐of ‐flight measurements between transmitter/receiver pairs are then converted to temperature and passed to a filtered back projection algorithm to reconstruct the desired thermal distributions. Since dose from the beam is deposited mostly within the top cm layer of water, the array is located just below the water surface. A small, 4 ‐cm diameter Al mask is placed just above the water surface to aperture the beam. Results: Preliminary measurements involving therapy ‐level radiation were obscured by system noise, but higher doses, of order 1000 Gy/min, produced detectable images within seconds. Conclusion: Imaging of absorbed dose to water has been achieved for the first time, although at dose rates that are considerably above therapy level. Planned improvements in signal processing are expected to bring the detection limit into the therapy regime. Other uses of the array, e.g. triangulation of thermoacoustic disturbances caused by pulsed radiation sources, are also being investigated.
TU‐A‐BRB‐11: A New Method for the Measurement of Reference Dosimetric Quantities in Electron Beams
Authors: Muir BR , McEwen MR , Rogers DWO
Published: June 2012

Purpose: To present and confirm the accuracy of a new method to extract data for reference dosimetry in electron beams from depth‐ionization measurements. Methods: Depth‐ionization curves are measured in water using NRC reference ion chambers and several plane‐parallel chambers in 4 to 18 MeV electron beams from the Elekta Precise clinical linear accelerator. To extract useful information from depth‐ionization results, the accuracy of setting the chamber position must be established. For this purpose, differences in the depth at which the chamber's ionization reading falls to 50% of its maximum value are used to derive EPOM values relative to those of reference chambers which have been previously established (Lacroix et al., Med. Phys. 37 (2010) 4331). After validation of positioning accuracy, relative ion chamber perturbation factors are determined using the ratio of readings to those of reference ion chambers as a function of mean electron energy, which changes smoothly with depth.Results:Repeatable results for EPOM at the 0.1 mm level indicate little uncertainty from positioning. Different chambers of the same type give EPOM results that differ by less than 0.2 mm, suggesting insignificant chamber‐to‐chamber variability. Values of EPOM at I50 determined for 4 to 18 MeV beams do not exhibit significant variation with beam energy. For well‐behaved chambers, relative perturbation factors at the same mean electron energy are consistent regardless of the incident beam energy. Relative perturbation factors for the same chambers repeated using a different reference chamber after several days are unchanged within 0.3%. Relative perturbation factors for the NACP and Roos chambers agree with published data. Conclusions: These measurements show promising results for the determination of reference dosimetry data from depth‐ionization curves. Ultimately, these relative measurements will be compared against the NRC primary standard calorimeter to get absolute perturbation factors that will prove valuable for updated dosimetry protocols.
TU‐A‐BRA‐01: A Robotic Multi‐Pinhole SPECT System for Onboard and Other Region‐Of‐Interest Imaging
Authors: Bowsher J , Yan S , Roper J , Giles W , Yin F
Published: June 2012

Purpose: Radiation therapy beam guidance and modulation at treatment time could be substantially supported by onboard molecular and functional imaging capability. Currently there is very little range of technology available for this. Herein we propose and investigate a robotic multi‐pinhole SPECT system for this purpose. Methods: A 49‐pinhole system is designed with 7 pods and 7 pinholes per pod. Pod geometry is such that the full target region‐of‐interest (ROI) is within the FOV of all 7 pinholes for all pod distances exceeding a minimum distance from the ROI. Motion of the pods relative to one another can involve just 1 or 2 degrees of freedom per pod. Motion of the entire, composite 49‐pinhole system is provided by a robot. This design enables all 49 pinholes to view the full ROI throughout detector trajectories that can achieve sampling objectives, e.g. on line integrals measured, while also providing good resolution and sensitivity via close proximity to the ROI. The design is evaluated through computer simulation studies in which 7 and 10 mm diameter structures of 6‐to‐1 radiotracer uptake are added to the XCAT phantom. Results are compared to imaging with a compact parallel‐hole collimated detector, which is utilized as a reference for conventional SPECT imaging configurations. Results: For the 7 cm‐diameter target region of interest, the 49‐pinhole system shows markedly improved imaging as compared to the reference, parallel‐hole collimated detector. Most 7 mm and 10 mm diameter structures are readily visualized with 4 minute scans. Conclusions: In only a few minutes of scan time, the designed multi‐pinhole SPECT system can provide molecular and functional information about the target region as patients are in position for radiation therapy. This work is supported by NIH grant R21‐CA156390‐01A1.
TU‐A‐BRA‐04: Real‐Time Metabolic Image‐Guidance to Aid Intraoperative Radiation Therapy: Pilot Results in a Small‐Animal Model
Authors: Carpenter C , Sun C , Pratx G , Chen K , Senadheera L , Xing L
Published: June 2012

Purpose: Real‐time biological imaging is being investigated to delineate positive tumor margins which may provide feedback during intraoperative radiation therapy. We investigated the feasibility of identifying prostate cancer invasion through the capsule of the prostate intra‐operatively in a mouse model using Cerenkov optical emission from Fluorodeoxyglucose (FDG). The potential benefit of using Cerenkov emission is that it provides real‐time images of glucose uptake while avoiding signal from the bladder and surrounding structures beyond the surgical margin. We investigated the performance of a Cerenkov imaging system during surgical prostatectomy in vivo in a tumor mouse model. Methods: All Cerenkov images were collected with a commercial optical imaging system. A transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse model was used to investigate the feasibility of imaging local invasion of the cancer beyond the prostate capsule. A mouse with a ∼1cm prostate tumor with questionable invasion as determined by T2‐W MRI was injected with 1.2mCi of FDG. The injection was followed by PET/CT after 75 minutes, and subsequent sacrifice; surgery commenced 3:45 later. After prostatectomy, Cerenkov images were acquired during an attempt to clear the surgical margin. Results: In all, 8 tissue resections were performed in the surrounding tissues including the seminal vesicles, rectum, and other nearby organs. Cerenkov imaging identified tissues both before surgical resection in vivo, and confirmed their successful excision after surgical intervention ex vivo. The Cerenkov imaging confirmed uptake in the seminal vesicles, as is expected in this tumor model. In addition, several hyperintense regions that were not identified in PET/CT were visible with the interventional system. Conclusions: This proof‐of‐concept study explored the feasibility of using Cerenkov based imaging for the interventional identification of tumor tissue that had spread beyond the prostate. A more in‐depth analysis is currently underway to determine the potential benefit to intraoperative radiation therapy. DOD Breast Cancer Fellowship W81XWH‐10‐1‐0506, Center for Biomedical Imaging at Stanford (CBIS), NCI R01 CA128908, NIH ICMIC P50CA114747.
TU‐A‐BRA‐06: EPID Operation in a Bi‐Directional MRI‐Linac System: A Monte Carlo Study
Authors: Oborn BM , Metcalfe P , Crozier S , Bailey M , Keall P
Published: June 2012

Purpose: To examine via Monte Carlo simulation the dosimetric operation of an EPID inside a bi‐directional MRI‐Linac system. The proposed system is a 1 T MRI which supports both perpendicular (transverse) and inline (longitudinal) 6 MV MRIgRT. Methods: BEAMnrc Monte Carlo simulations were used to model a Varian 6 MV linac. Phase space files were produced below the MLCs and were inputs to Geant4 Monte Carlo simulations. These consist of transporting the radiation beam through a phantom and then on to a detailed model of a Varian EPID. The Geant4 simulations take place inside magnetic field maps matched to Finite Element Modeling of the MRI‐Linac system (COMSOL). Hence the EPID operation is simulated in the same conditions as a real MRI‐linac system. Various EPID SSD's were simulated in both inline and perpendicular field MRI‐linac systems. Dose to the EPID layers were taken for each field and compared with zero magnetic field. Results: The inclusion of the magnetic fringe field had little impact in the inline direction, however in the perpendicular direction the electron‐return effect (ERE) caused secondary electrons to return to the phosphor scoring level. This induced a lateral blur and overdosing of the images that was related directly to the SSD or strength of the fringe field in the transverse direction (5% at 0.25 T, 10% at 1 T). Including 1 cm of water material below the phosphor layer significantly reduced the ERE and improved the dose image quality. Conclusion: Novel Monte Carlo simulations of the operation of an EPID working inside a bidirectional field MRI‐linac have been performed. The main effect which induced an image or dose artifact was the ERE in the perpendicular field arrangement. This can however be almost eliminated by the use of an exit side bolus on the EPID.
TU‐A‐BRA‐11: Targeted Drug Delivery Technique Employing Pulsed Focused Ultrasound for Treatment of Prostate
Authors: Chen L , Rapoport N , Chen X , Cvetkovic D , Xue J , Xu Q , Tong X , Liu H , Gupta R , Ma C
Published: June 2012

Purpose: The purpose of this study is to investigate an innovative approach to prostate cancer therapy using nanodroplet‐encapsulated drugs combined with pFUS (pulsed high intensity focused ultrasound) treatment. Pulsed FUS treatment provides a localized drug release from the carrier and enhanced intracellular uptake, which ensures temporal and spatial control of local drug delivery while lowering the systemic toxicity. Methods: LNCaP cells were implanted into the prostates of nude mice. Tumor growth was monitored using MRI. The tumor‐bearing mice were randomly divided into 5 groups (n=3). Group 1, animals were treated with an IV injection of paclitaxel (PTX)‐encapsulated nanodroplets (ND‐PTX) + pFUS. Animals in Group 2 were treated with pFUS alone. Animals in Group 3 were injected (IV) with PTX‐encapsulated nanodroplets alone, Group 4 received pFUS+empty nanodroplets and Group 5 was used as a control group. After treatment, animals were allowed to survive for 4 weeks. Tumor growth delay was monitored by MRI. The formulation of the PTX‐encapsulated nanodroplets is as follows: PTX dose 20 mg/kg, Nanoemulsion composition 0.5% PTX, 1% perfluorocarbon and 2% stabilizing block copolymer. Ultrasound treatment parameters were 1MHz, 25W acoustic power, 10% duty cycle and 60 sec for each sonication. Results: Compared with the control group, significant tumor growth delay was observed in Group 1 with p=0.004, 3 weeks after treatment. There was no significant tumor growth delay observed for Group 2 (p=0.285), Group 3 (p=0.452) and Group 4 (p=0.158). Conclusions: Our preliminary results showed a great potential for prostate cancer therapy using targeted PTX+ nanodroplets, which could be activated by pFUS. More animal studies are warranted to confirm the results. The enhancement effect of pFUS on targeted drug delivery needs to be investigated quantitatively. This work is supported by Focused Ultrasound Surgery Foundation, DOD PC073127 and DOD BC102806. Technical support from InSightec is graciously acknowledged. funding supported partially by Focused Ultrasound Surgery Foundation, DOD PC073127 and DODBC102806