bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐D‐AUD C‐07: Electron Depth Dose Measurements Using a Plastic Scintillation Detector and a Parallel‐Plate Ionization Chamber
Authors: Lacroix F , Guillot M , Archambault L , Gingras L , Beddar S , Beaulieu L
Published: July 2008

Purpose: Plastic scintillation detectors (PSDs) have been shown not to require stopping power or perturbation corrections to measure absorbed dose in high energy electron beams. The purpose of this work is to compare the performance of ionization chambers and PSDs in precise measurements of electron beams. Method and Materials: Electron beam depth dose measurements were performed on a Varian Clinac iX for 6 and 18 MeV. The ionization chamber measurements were performed using a Roos (PTW) parallel plate ionization chamber. The ionization chamber measurements were converted to dose using stopping power ratios for realistic clinical beams. The scintillation dosimetry measurements were performed using a miniature water equivalent plastic scintillation detector (1.6 mm3). This detector is made out of a (1 mm diameter by 2 mm long) BCF‐12 scintillating fiber coupled to a color CCD camera. The Cerenkov radiation produced in the PSD was filtered by a chromatic discrimination technique. Results: The percentage depth dose of the Roos chamber measured at 6 and 18 MeV agree well with the scintillation detector depth dose from Dmax to the R50 depth. As the depth increases past R50, a gradual discrepancy appears between the curves provided by the 2 detectors at both energies. Past the practical range (Rp) and into the Bremsstrahlung tail, the percentage depth dose curve of the Roos does not agree with the plastic scintillation detector at both energies (20 % and 50 % difference at 6 and 18 MeV). Conclusion: The discrepancy between the detectors suggests that: 1) The stopping power ratios do not model properly the transition between electrons and photons at the Rp depth or 2) there may be an increase in Pwall with depth for the parallel plate chamber or 3) there is loss of water equivalence for the PSD at low electron energies.
TU‐D‐332‐01: Mapping of Viable Tumor Regions Using Gd‐DTPA DCE‐MRI
Authors: Huang M , Zhang M , Chang J , Le C , Zanzonico P , Humm J , Koutcher J , Ling C
Published: July 2008

Purpose: To establish a criterion for distinguishing viable and necrotic tumor cells using the initial relative slope of the Gd‐DTPA DCE‐MRI and to apply this criterion to optimize the specificity of tumor hypoxia imaging based on PET. Method and Materials: Three nude rats with Dunning R3327‐AT prostate adenocarcinoma xenografts were imaged by dynamic MRI following tail vein injection of gadopentetate dimeglumine, with imaging parameters of 1‐mm slice thickness, 0.5‐mm spacing, and 0.13mm × 0.13mm voxel size. The time‐intensity curve of each voxel was obtained at 1‐min interval to 25‐min post‐injection. Histologic (standard H&E stain and high‐power microscopy) examination was performed of 8‐μm thick slices parallel to the DCE‐MRI imaging axis and the necrotic regions were identified. The initial relative slope, where I0 and I(t) are MR image intensity before injection and at time t, was calculated for different times and compared with the pathologically defined necrotic region to determine the threshold that best distinguishes viable and necrotic cells. This criterion was then used to study the specificity of FMISO PET with the same xenografts scanned with DCE‐MRI and PET. Results: The optimal criterion for identifying viable and necrotic tumor regions was that a DCE‐MRI voxel was necrotic if at 2 minutes after contrast injection. When this criterion was applied to the PET images of three xenografts, necrotic region was found to have a wide range of image intensities, which is inconsistent with the hypothesis that high image intensity exclusively identifies hypoxic viable tumor. Among the 319 hypoxic voxels determined from the three animals' PET, only 75% corresponded to viable cells. Conclusion: A criterion was established to identify necrotic/viable tumor cells using DCE‐MRI. Using PET alone for hypoxia imaging is problematic because the specificity might be compromised by necrotic regions with high PET image intensity.
TU‐D‐332‐05: The Benefits of Non‐Uniform Gradient Direction Specification in DTI: Simulations and Phantom Data
Authors: Yanasak N , Allison J , Zhao Q , Hu T , Dhandapani K
Published: July 2008

Purpose: The goal of this project was to optimize angular precision in determining the principle diffusion eigenvector of prolate tensors in a diffusion tensor imaging (DTI) series, using prior knowledge of principle eigenvector direction and non‐uniform specification of gradient directions. Additionally, the effect of non‐uniform gradient distributions on fractional anisotropy (FA) was characterized. Method and Materials: Simulations were conducted, representing diffusive behavior in tissue as manifest in a DTI image series. Diffusion‐encoding gradient directions were constrained in elevation, for a prolate tensor oriented along the z‐axis. Noise was added to generate multiple measurements as per a DTI ROI analysis. Tensors were calculated as well as FA. Angular precision was quantified as the dispersion in the angle between the principle eigenvector of the prolate and the z‐axis. To confirm simulations, a phantom containing glass capillary arrays (FA=0.68) was imaged with DTI using a 3.0T GE HDx scanner. Gradient directions were specified to match simulation results. Eigenvalues, eigenvectors, and FA were calculated within an ROI. Results: Simulations identify the range of elevation angles θ=30°–40° as being most sensitive to the determination of principle eigenvector direction. This result is fairly consistent within the range of FA=0.2–0.8. Prescription of gradients within a band of elevation angles having a width of Δθ ∼40°–60° and centered at θ ∼30° can improve angular precision by 30–40%, given an uncertainty in prior eigenvector direction of <30°. FA precision using this scheme is similar to uniform gradient prescriptions, and accuracy is improved at low SNR values. Data from phantom measurements generally agree with simulation results. Conclusion: This work suggests that prior knowledge of principle eigenvector direction can improve its final determination, using gradients prescribed non‐uniformly. This technique may be useful for increasing sensitivity in these conditions (e.g., spinal lesions, determining tumor infiltration of white matter tracts).
TU‐D‐342‐02: What Every Medical Physicist Should Know About Breast Tomosynthesis
Authors: Lo JY
Published: July 2008

Digital tomosynthesis (or “tomo”) is revolutionizing breast imaging. Based on modified full‐field digital mammography systems, breast tomo can achieve limited‐angle cone‐beam CT imaging which produces 3D slice images of the breast. This addresses the problem of overlapping dense tissue which is the most common cause for unnecessary callbacks as well as missed cancers in mammography screening. Tomo can provide 3D images while remaining comparable to mammography in terms of speed, resolution, cost, and dose. For these reasons, tomo may be only imaging technique with the potential to completely replace the current role of mammography as the primary tool in breast cancer screening and diagnosis. FDA approval is imminent, so it is all the more important for medical physicists to understand this new modality's potential as well as limitations.

This presentation will cover both the hype and hope surrounding breast tomosynthesis. What do the initial clinical trials suggest about its performance? What are the on‐going physics issues in terms of clinical implementation, including compression, dose, and QA? What are some of the latest results from different research groups working on optimization of radiographic techniques, acquisition modes, and reconstruction algorithms? What is coming down the road in terms of advanced applications including quantitative imaging, computer aided detection, and contrast enhanced imaging? We will explore the answers to these and other questions together.

The work discussed in this presentation was supported in part by grants from NIH/NCI, US Army Breast Cancer Research Program, Susan G. Komen for the Cure, General Electric Company, Hologic, and Siemens Medical Solutions.

Educational Objectives:

1. Understand the difference between breast tomosynthesis and dedicated breast CT.

2. Appreciate the many medical physics issues involved in the development and optimization of breast tomosynthesis.

3. Understand the clinical promise and concerns of using breast tomosynthesis.
TU‐EE‐A1‐01: Beam Characteristics for Planning Carbon Ion Therapy of Ocular Melanoma
Authors: Koyama‐Ito H , Kanai T
Published: July 2008

Purpose: We recently started orthogonal two‐port carbon ion therapy for ocular melanoma with the intent to reduce incidence of radiation complications that occur with mono‐port therapy. In order to calculate dose distributions for this therapy, biophysical beam characteristics was investigated and defined as input into the treatment planning software. Method and Materials: Uniform fields of the vertical and the horizontal carbon‐ion beams from a synchrotron were shaped with the passive beam shaping systems. Apertures and range compensators (RC) were designed for individual patients. We used a commercially available treatment planning system for proton therapy, which was customized to our carbon‐ion beams. The physical depth dose profiles were measured with an ionization chamber and LET depth profiles were calculated. Variation of the survival curves with LET for HMV‐I cell were obtained experimentally. The range modulating ridge filters were designed to produce the spread‐out Bragg peaks with region of uniform HMV‐I cell killing for delivery of 70 Gray‐Equivalent in five fractions. The beam penumbra was defined based on measurements with radiographic films and calculations. The lateral penumbra was estimated to be 1/3.6 of the proton beam. Dose distributions were calculated with broad beam and pencil beam algorisms using these defined beam data. Results: The dose distributions calculated in water for typical irradiation arrangements showed the lateral penumbra width (80–20%) to be less than 0.15 cm along full penetration depth. During two years, more than twenty patients have been treated with two fractions of vertical beams and three fractions of horizontal beams. With the use of RC, the dose distributions showed high conformity to tumors with low dose to organs at risk such as I‐C body and disk. Conclusion: The biophysical beam characteristic for clinical dose calculation was described. The employed bio‐clinical model could be evaluated by clinical outcome.
TU‐EE‐A1‐02: Validation of Treatment Planning and Delivery in Conventional Linear Accelerator Without Flattening Filter
Authors: Bayouth J , Bylund K , Siochi R
Published: July 2008

Purpose: The purpose of this study was to compare planning and delivery accuracy of IMRT on a conventional linac with and without the flattening filter. Method and Materials: The 6 MV x‐ray beam of a Siemens Oncor linac was modified by removing the flattening filter, enabling dose rates of 1000 MU/min, and modeled within a commercial treatment planning system. IMRT treatment plans created with the flattening filter (F) were reoptimized with equivalent segments and computed for the beam model without the filter (NF) on 10 clinical head‐and‐neck cases. All plans (F and NF) were compared for equivalence and delivered to phantoms; treatment time and dose agreement with planning system were determined. Results: Dose distributions are similar between the two beam lines, and clinically reasonable plans were created despite the absence of the flattening filter. PTV hot spots were <0.5% hotter and cold spots were <0.5% colder for the (NF) plan. Similarly, all other ROIs investigated were within 1% for hot spots and cold spots. On average NF plans required 15% more MU, but delivered treatments were shorter (9 min vs 11 min). Dosimetric differences between computed and ion chamber measurements were similar (−1.7%+/−1.3% − F) and (1.7%+/−1.9% − NF). The number of pixels meeting gamma map criteria of 3%/3mm, 5%/5mm, and 7%/7mm were statistically equivalent (p = 0.6): [88.5%, 96.8%, and 97.8% (NF)], [87.8%, 96.6%, and 97.9% (F)]. Conclusion: The planning system was able to generate equivalent plans, and measured dose distributions in ion chamber and film show no difference between beam models with or without the flattening filter. At the higher dose rate, overall treatment times were reduced and could be applied to gating and hypofractionation. Conflict of Interest: Research supported by industrial grant from Siemens Medical Solutions.
TU‐EE‐A1‐04: Evaluation of Tumor Motion Effects On Dose Distribution for Hypofractionated Radiotherapy of Non‐Small‐Cell Lung Cancer
Authors: Kang H , Yorke E , Yang J , Chui C , Rosenzweig K , Amols H
Published: July 2008

Purpose: Hypofractionated Radiotherapy (HFRT), 1–4(12–30 Gy) fractions of intensity modulated radiation therapy (IMRT) treatments, may be advantageous in treating early‐stage non‐small‐cell lung cancer (NSCLC). But motion of dynamic multileaf collimator (DMLC) leaves relative to respiration‐induced tumor motion could cause substantial differences between doses predicted from the planning scan and delivered doses. Such effects average out for conventionally fractionated IMRT (20–40 fractions, 1.8–2 Gy/fx) but must be evaluated for HFRT. Method and Materials We performed numerical simulations to investigate effects of respiratory motion on HFRT IMRT of 20 Gy × 3 fractions. Using the clinical treatment plans and DMLC leaf motion files for 9 NSCLC patients (11 tumors), the planned Clinical Target Volume (CTV) and Gross Tumor Volume (GTV) dose distributions were retrospectively compared with the distributions calculated for simulated periodic motion for amplitudes, periods and directions typical of normal respiration (0.2–1.26 cm, 3–8 sec, superior‐inferior and anterior‐posterior). Results: For the largest amplitude (1.26 cm, excursion ∼2.5 cm), the average ± standard deviation of the ratio of simulated to planned values of mean dose, minimum dose, D95 and V95 was 0.98±0.01, 0.88±0.09, 0.94±0.05 and 0.94±0.07 respectively for the CTV and 0.99±0.004, 0.99±0.03, 0.98±0.02 and 1.0±0.008 for the GTV. For 0.9 cm amplitude, corresponding ratios were 1.0±0.004, 0.97±0.07, 0.98±0.02, and 0.99±0.02 for CTV, and 1.0±0.004, 0.99±0.05, 0.99±0.01, and 1.0±0.01 for GTV. There was minimal dependence on motion period and initial phases. Graphical dose distributions showed respiration induced broadening of the beam penumbras in the motion direction. More severe effects were seen with intensity patterns that are more highly modulated than typical for clinical NSCLC HFRT fields. Conclusion: For our IMRT lung cancer HFRT technique, motion effects on target coverage are minimal for amplitudes below 1 cm but could be significant for larger amplitudes, more highly modulated fields and smaller field margins.
TU‐EE‐A1‐06: Comparison of Intensity‐Modulated Radiation Therapy, Intensity‐Modulated Arc Therapy and Arc‐Modulated Radiation Therapy
Authors: Tang G , Earl M , Luan S , Wang C , Naqvi S , Yu C
Published: July 2008

Purpose: Arc‐modulated radiation therapy (AMRT) is a new technique that delivers comparable dose distributions to IMRT in a single gantry rotation and thereby reduces the delivery times to approximately 2 to 5 minutes. To evaluate the clinical feasibility of AMRT, plan quality and delivery times are compared to conventional IMRT and multi‐arc intensity‐modulated arc therapy (IMAT). Method and Materials: 12 IMRT cases including 3 HN, 3 brain, 3 lung and 3 prostate were selected for this study. For each case, static IMRT plans utilizing 7 to 9 fields were produced in the Pinnacle TPS. IMAT plans with 5 to 11 arcs were generated for each case using a two‐step planning approach: ideal intensity maps for 36 static beams were optimized in Pinnacle using the same optimization objectives as in the corresponding IMRT optimization. The ideal intensity maps were segmented into a deliverable IMAT sequence using the k‐link IMAT leaf sequencer. For AMRT, the same ideal intensity maps were used to translate into a deliverable single‐arc sequence using a graph algorithm. The resultant AMRT sequences were designed to have an equivalent number of segments as in the corresponding IMAT plans. Results: Multi‐arc IMAT plans outperformed AMRT and IMRT for all cases in terms of target coverage, target dose uniformity and sparing of OARs. AMRT attained better or comparable dose distributions as compared with IMRT in all cases even for the challenging HN cases. The delivery times for AMRT were approximately 20% less than IMRT and 50% less than IMAT while the number of MU is 3 times more than IMRT and 1.3 times more than IMAT in general. Conclusion: AMRT is a promising technique that may become a viable alternative to IMRT. The main benefit is the reduced treatment times. A refined planning algorithm may further improve AMRT plan quality.
TU‐EE‐A2‐02: Impact of Track Structure Calculations On Biological Treatment Planning in Carbon Ion Radiotherapy
Authors: Elsässer Th , Scholz M
Published: July 2008

Purpose: To analyze predictions of the relative biological effectiveness (RBE) for carbon ion treatment planning based on different descriptions of the radial dose distributions around ion tracks and to discuss the implications on clinically relevant depth dose profiles. Method and Materials: We investigate the impact of track structure on calculations of RBE values using the Local Effect Model (LEM). It calculates the RBE for cell lines or tissues starting from the corresponding experimental/clinical photon data and an amorphous track structure model. Different track structure models that use energy‐dependent as well as energy‐independent core radii are investigated and compared to in vitro and in vivo data. Additionally, we apply them to calculate the biologically effective dose along a spread‐out Bragg peak for typical chordoma treatments. Results: We show that the LEM is sensitive to different descriptions of the radial dose distributions. However, by changing a single model parameter of the photon reference curve, reasonable RBE values can be achieved for all dose distributions. The general improvement of predictions using a modified version of the LEM with an energy‐dependent core radius is demonstrated by comparison to in vitro data of human cell lines and to experimental data of the radiation tolerance of the rat spinal cord. Additionally, we find a larger therapeutic ratio for the modified model version relative to the original LEM for a typical treatment scenario for chordoma patients. Conclusion: The Local Effect Model is sensitive to the inner part (few nanometers) of different track structure descriptions. Since these interactions of ions with liquid water are neither well understood experimentally nor theoretically, more studies should address the energy deposition around ion tracks. This will be in particular useful for further optimization of carbon ion therapy in general and with respect to comparison with other treatment modalities like protons or IMRT.
TU‐EE‐A2‐05: Hypofractionation: What Does It Mean For Prostate Cancer Treatment?
Authors: Liao Y , Joiner M , Huang Y , Burmeister J
Published: July 2008

Purpose: Using current radiobiological models, we quantitatively examined the predicted clinical consequences of hypofractionation in prostate cancer radiotherapy while varying biological indices such as α/β, SF2, and total dose and dose per fraction. Method and Materials: Four hypofractionated treatment regimens, 22×2.94Gy, 16×3.63Gy, 12×4.3Gy, and 5×6.7Gy, were compared with standard fractionation of 39×2Gy. All regimens produce similar EQD2 to the tumor assuming α/β=2.4Gy for prostate carcinoma. Actual α/β values ranging from 1.5 to 8.5Gy for prostate carcinoma and fixed α/β values of 4Gy for both rectum and bladder were used to calculate biologically relevant parameters. The LQ model and gEUD formalism were used to transform the physical dose distribution into normalized equivalent uniform dose (gEUD2), from which TCP and NTCP were calculated. Final ranking of the radiotherapy plans was based on “complication‐free tumor control probability”.

Results: gEUD2 to rectum and bladder decreased by 28% and 23%, respectively, in the 6.7Gy/fraction regimen compared with 2Gy/fraction using “a”=5 for the gEUD calculation. A corresponding 10% decrease in NTCP was predicted for the 6.7Gy/fraction regimen compared with 2Gy/fraction. Conversely, predicted TCP for hypofractionated regimens decreases significantly with increasing SF2 and α/β. We found that for relatively responsive tumor cells (SF2=0.4–0.5), complication‐free tumor control probability was superior for nearly all hypofractionated regimens even for α/β values up to 8.5Gy. For less responsive tumor cells, (SF2=0.6), 6.7Gy/fx is predicted to be inferior to standard fractionation even when α/β=1.7Gy, and the other hypofractionation regimens remain superior to the standard fractionation for α/β values up to 3.5Gy. Conclusion: Existing radiobiological data and models predict better clinical results from hypofractionation than standard fractionation for prostate carcinoma when SF2<0.5 and <5×106 clonogenic cells, implying that it is superior for early stage disease. This is true even when tumor α/β is high.

Part funded by Varian Medical Systems.
TU‐EE‐A4‐02: Radiation Dose to the Female Breast in 64‐Slice Computed Tomography
Authors: Lemen L , Lamba M , Chen H , Alsip C , Samaratunga R , Mahoney M
Published: July 2008

Purpose: To evaluate in a phantom study the dose to adult female breast tissue using current clinical body CT protocols on 64‐slice systems. Method and Materials: An anthropomorphic phantom with breast modules (Rando‐ Alderson) was scanned on a variety of 64‐slice CT scanners (GE LightSpeed VCT; Toshiba Aquilion; Siemens Sensation64; Philips Brilliance — in progress). Standard clinical protocols which either directly expose the breast or have scatter/edge‐of‐field dose were evaluated: (1) lung screening (smoker); (2) chest‐abdomen‐pelvis (CAP ‐oncology follow‐up); (3) cardiac calcium scoring (60bpm); and (4) virtual colonoscopy (supine & prone). Protocols were similar, but not identical, between systems. Scan coverage was matched; no breast shields. Absorbed dose to the breast tissue was measured by loading 10 TLDs into each breast module. LiF TLDs were calibrated individually for 9mm Al HVL (120 kVp), with an NIST traceable ion chamber, and a correction applied for the CT HVL. Image noise was also measured. Results: Standard clinical protocols for an adult female, including adaptive mA methods for the CAP exam, were utilized on each scanner. Averages of the TLD dose to the breast ranged from: (1) 0.56–1.36 cGy lung exam; (2) 1.27–2.98 cGy CAP; (3) 1.01–2.98 cGy calcium scoring; (4) 0.67–1.35 cGy virtual colonoscopy. Conclusion: The expected broad range of breast tissue dose for various CT exams was seen, but also indicate a possible reduction in dose compared to earlier reports from 4‐slice (Mahoney et al, RSNA 2005) and 16‐slice systems (Hurwitz et al, 2006 — extrapolated estimate). Variation between manufacturers was observed, but note that the protocols tested were those
currently in clinical use. Further optimization of protocols for the given system design is/may be possible, especially given the significant interest from the entire radiological community to improve awareness of dose issues and to minimize exposures.
TU‐E‐AUD C‐01: Contemporary Concepts in Cellular and Molecular Biology for the Medical Physicist
Authors: Rosenstein B
Published: July 2008

Progress in translational biomedical research, whose goal is to translate basic research findings from the laboratory into the clinic to improve patient care, depends extensively upon the use of molecular and cellular concepts and tools. Thus, research activities often focus upon the use of cellular and molecular biology to examine research questions with potential clinical implications. As such, medical physicists may be at somewhat of a loss to comprehend these studies since they are frequently not equipped with an understanding of these applications. This restricts the role for the medical physicist and often limits their participation in conferences and seminars, which is unfortunate since these individuals may be able to provide a key contribution to ongoing research activities. A knowledge of cellular and molecular biology is also important for clinical applications of therapies and diagnostics which rely upon an understanding of the biologic basis behind their effectiveness. The goal of this educational session is to provide medical physicists with a basic working knowledge of the most common and important contemporary cellular and molecular approaches used in translational biomedical research.

Educational Objectives:

1. To provide medical physicists with an understanding of the following topics in cellular and molecular biology:

a. DNA, RNA and protein structure, synthesis and function.

b. Vectors and recombinant moleculesReporter gene assays.

c. Genetic libraries.

d. Gel electrophoresis.

e. Polymerase chain reaction

f. Gene analyses; southern, northern and western blots.

g. Microarrays.

h. Single nucleotide polymorphisms.

i. Knockout mice.

j. EMSA, ELISA and immunoprecipitation.

k. Monoclonal antibodies.

l. Gene silencing.

m. Detection and measurement of DNA damage.
TU‐E‐351‐01: Writing and Reviewing Medical Physics Papers
Authors: Hendee W , Slattery P , Karellas A , Rogers D
Published: July 2008

There is an art to writing a scientific paper so that it communicates accurately, succinctly, and comprehensively. Developing this art comes with experience, and sharing that experience with younger physicists is an obligation of senior scientists, especially those with editorial responsibilities for the journal. In this workshop, the preparation of a scientific manuscript will be dissected so participants can appreciate how each part is developed and then assembled into a complete paper. Then the review process for the paper will be discussed, including how to examine a paper and write an insightful and constructive review. Finally, we will consider the challenge of accommodating the concerns and recommendations of a reviewer in preparing a revision of the paper.

A second feature of the workshop will be a discussion of the process of electronic submission of a paper for consideration by Medical Physics. The web‐based PeerX‐Press engine for manuscript submission and management will be examined, with attention to special features such as EPAPS and line referencing. Finally, new features of Medical Physics will be explained, such as Vision 20/20 manuscripts, Physics Letters and the standardized formatting of book reviews.

Education Objectives:

1. Improve the participants' abilities to write a scientific manuscript.

2. Understand the review process for Medical Physics manuscripts and how to participate in and benefit from it.

3. Appreciate the many features of the PeerX‐Press electronic management process for Medical Physics manuscripts.

4. Develop a knowledge of new features of Medical Physics.
TU‐FF‐A2‐01: Unexpected Doserate Dependencies of Diodes for Beam Scanning
Authors: McEwen M , Ross C
Published: July 2008

Purpose: The doserate dependence in electron beams of four types of scanning diode were investigated — Scanditronix‐Wellhöfer EFD and SFD, PTW60012 and Sun Nuclear QED 1113000‐1. Method and Materials: The detectors were positioned at a fixed source‐detector distance in an uncollimated 20 MeV electron beam and the doserate varied by varying the incident beam intensity. There were therefore no changes in field size, scatter contribution etc. The dose per pulse was in the range 0–0.1 cGy, which is typical of clinical linacs. Measurements were also made in high gradient electron fields, e.g. depth‐dose curves and beam profiles, for a range of electron energies from 6–22 MeV. Results: Two of the diodes — the SFD and QED‐ showed little or no doserate dependence. The EFD showed a variation that just exceeded the manufacturer's specification and the PTW60012 showed a very large effect with a 15% change in sensitivity over the range of doserates investigated. These differences in response were also seen in the comparison of beam profiles and depth‐dose curves. The EFD and PTW diodes gave a difference in R50 of approximately 1 mm, which is equivalent to a 4% error for 6 MeV. Similar discrepancies were seen for the measurement of field size. Doserate corrections were obtained and corrected dose distributions generally gave agreement between all detectors within the measurement uncertainties. Conclusion: These results would indicate that users should not simply rely on manufacturers' specifications and should characterize any new diode for doserate dependence before using the device for beam measurements.
TU‐FF‐A2‐03: Characterization of a Small Photon Beam for Radiosurgery Using GafChromic EBT Film in a Water Phantom
Authors: García‐Garduño OA , Lárraga‐Gutiérrez JM , Rodríguez‐Villafuerte M , Martínez‐Daválos A , Moreno‐Jiménez S , Celis MA
Published: July 2008

Purpose: According to the literature radiochromic film GafChromic EBT® (ISP Corp., USA) is an excellent detector to be used for small beam dosimetry. By using the fact that these films can be immersed in water for radiation measurements, this work investigates its performance in a water phantom for small beam measurements. Method and Materials: Tissue maximum ratios (TMR), total output factors (OF) and off axis ratios (OARs) were measured in a water phantom (MP3XS, PTW‐Freiburg, Germany) using GafChromic EBT® film and a shielded solid state detector (PFD3G, IBA‐dosimetry, Germany). Circular collimators (4 to 20 mm in diameter) coupled to a dedicated 6 MV linear accelerator (Novalis®, BrainLAB, Germany) were used. Films were calibrated against an ionization chamber at dmax covering a dose range from 0.01 to 450 cGy. Analysis was performed using a commercial flat bed scanner (Microtek 9600XL) in transmission mode at 100 dpi and 48 bit color depth (RGB format). The films were carefully handled and analyzed following the recommendations of the TG55 report and those proposed by others authors. The measurements using both detectors were compared to evaluate accuracy and reproducibility. Results: The reproducibility of the EBT film is reported to be within 1–2% using a commercial flat bed scanner. The highest differences between GafChromic EBT film and diode measurements for all circular collimators were 0.87%, 2.69% and 7.61% for OAR, OF and TMR, respectively. Conclusion: The use of GafChromic EBT immersed in a water phantom allowed reliable dose measurements. Although differences are present in the TMR values, the fact that the diode is not tissue equivalent as the film may explain these differences. The use of Monte Carlo simulations may be helpful to enlighten this issue.
TU‐FF‐A2‐04: New High‐Resolution Method to Measure the 3D Dose Distribution Around Brachytherapy Seeds Using BANG3‐Pro Gel Dosimetry
Authors: Massillon G , Minniti R , Mitch M , Maryanski M , Soares C
Published: July 2008

Purpose: In this work, we propose a new method to measure absorbed dose distribution in 3D imparted by low‐energy photon sources used for cancer treatment using high‐resolution gel dosimetry. Method and Materials: BANG3‐Pro (12% Gelatin, 12% proprietary high‐viscosity‐component, 6% Methacrylic Acid and 70% Water) gel, which is a muscle, tissue and water‐equivalent polymer gel, originally known as BANG (bis, acrylamide, nitrogen and gelatin) gel has been used as a dosimeter and phantom. We have studied the properties of the gel under different irradiation conditions and measured the dose distribution around three different brachytherapy seeds: , or , each one with a particular internal geometry. For the seed irradiations, a 200 μm wall Barex tube is embedded inside the gel phantom. To calibrate the gel, a small gamma ray field of 1 × 1 cm2 has been used as a reference radiation field. The gel response was quantified using a special high‐resolution DRYOCTOPUS laser CT scanner recently developed by MGS Research Inc. with a resolution of 100 μm in all dimensions for brachytherapy seeds and 330 μm for irradiations. Results: The response of the gel to gamma rays is almost linear up to 15 Gy; for higher doses, the response decreases as a function of the time between irradiation and readout (scanning). At low doses, the gel is reproducible and stable. Based on these results and due to the high dose rate gradient at short distances from the sources, the dose distributions were measured using accumulative time exposures in order to evaluate the dose with acceptable accuracy very close to the seed. The spatial uncertainty was about 100 μm. Conclusion: To our knowledge, this study is the first high‐resolution gel measurement of the dose distribution in 3D for brachytherapy seeds at distances under 5 mm from the seeds.
TU‐FF‐A3‐02: Interactive Medullobalstoma Patient Setup with Real‐Time Surface Imaging
Authors: Riboldi M , Lu H , Baroni G , Chen G
Published: July 2008

Purpose: To explore the use of real‐time surface imaging technologies for interactive non‐invasive head setup in craniospinal irradiation. Method and Materials: A real‐time surface imaging system was installed in a treatment room at our proton therapy center. The system provides surface motion monitoring up to 10 frames per second. Real‐time deltas of patient misalignments can be displayed at the same frame rate. Phantom and patient studies were performed to investigate the feasibility of surface imaging based interactive setup, relying on the real‐time deltas graphic interface. Interactive setup on a head phantom was carried out by multiple users, with offline evaluation of residual shifts/rotations. Patient studies were focused on checking surface visibility in two adult subjects. Results: As for translational misalignments, all users were able to reposition the head phantom within tolerance. Rotations turned out to be more critical, with residual values slightly above the threshold. Pre‐alignment with lasers is recommended to achieve better performance. Translational misalignments can be handled more efficiently when relying on the treatment couch console to apply the displayed deltas. Patient studies revealed that, when the viewing angle is not obstructed, visibility in the neck and lower head region is satisfactory: blank areas in the 3D surface image occur where thick hair is in place. Average surface reproducibility, compared to the treatment planning CT scan, was found to be within 3 mm. Breathing motion in one patient was quantified in less than 1.5 mm. Conclusion: Interactive head setup relying on real‐time surface imaging is feasible. Residual alignment errors can be kept within tolerance even by naive users by relying on a real‐time display of measured misalignments. The application in medulloblastoma patient setup can potentially reduce imaging dose and the time needed for patient alignment, providing valuable tools for intra‐session monitoring of patient positioning and breathing during treatment.
TU‐FF‐A3‐03: Individual and Group Setup Margins with KV Imaging for Daily Patient Positioning
Authors: Niedermayr T , Herman MG
Published: July 2008

Purpose: Optimum treatment efficacy requires that setup uncertainties be determined for each institution and treatment site. Patient and site specific setup margins were derived and compared for laser alignment vs kV image‐based localization. Method and Materials: Setup uncertainties and margins were established for fifty‐five patients (head and neck: 7, abdomen: 12, pelvis: 20 and lung: 6) treated with external beam radiotherapy using the Varian OBI kV imager for daily target localization. At each treatment fraction two pairs of orthogonal kV images were acquired to assess 1) required shifts and 2) verify the patient position before treatment. Results: The group margins were calculated according to the Van Herk formalism with the intent to give 99% of the prescribed dose to 99% of the patient population. With laser alignment an average margin of 10.6 mm in each direction is required for all treatment sites whereas kV imaging alignment reduces this average margin to 2.6 mm. Optimal patient‐specific margins were calculated retrospectively and range from 3.2–12.7 mm, 2.8–8.8 mm, 4.8–8.7 and 3.6–8.4 mm for laser alignment and 1.2–2.8 mm, 1.2–3.2mm, 1.2–2.9 mm and 1.8–3.5 mm with kV imaging for pelvis, head and neck, abdomen and lung patients respectively. Conclusion: The amplitude of the 99%–99% margin (2.6 mm) is small enough to be incorporated into routine clinical practice for this institution. The benefits of daily kV imaging can thus be described in terms of increased target coverage for a larger fraction of patients. For patients set up with kV imaging the range of the individual setup margins is small enough that the group setup margin can be considered adequate for each individual patient. This alleviates the need to derive patient‐specific setup margins during treatment when organ motion is limited. Conflict of Interest: This research was funded in part by Varian Medical Systems.
WE‐E‐AUD B‐07: EGSnrc Benchmarking Against High‐Precision Angular Electron Scattering Data Through Thin Foils
Authors: Cojocaru C , Ross C , McEwen M , McDonald A , Faddegon B
Published: July 2008

Purpose: The EGSnrc Monte Carlo code was benchmarked against newly measured angular scattering distributions of electron beams through thin materials. Method and Materials: The BEAMnrc user code was used for the simulation of the experimental setup. The geometry, beam and material properties were implemented in BEAMnrc, while the DOSXYZnrc user code was used to score the absorbed dose up to angles of 10°. The electron beam was generated using the National Research Council of Canada (NRC) Vickers linear accelerator, which allows for the production of narrow pencil beams of electrons with well known energies. Six materials: Be, C, Al, Ti, Cu, Ta and Au, were used as scattering foils and measurements were made at two energies — 13 MeV and 20 MeV. Great care was taken to obtain high accuracy experimental data for comparison with the results of the simulations. Results: The obtained angular distributions were fitted with a Gaussian and the characteristic angle (the angle at which the absorbed dose decreases by 1/e) was used to compare the measured and simulated distributions. The discrepancy between the measured and the EGSnrc characteristic angles is on average about 1.5%. A careful error estimation was performed on the measured data, that resulted in a value of about 1%. The EGSnrc code shows an agreement with the measured data at the 2σ level. In general, the EGSnrc predicts narrower distributions. The full distributions were also compared and the shapes were found to be very similar even at large angles. Conclusion: The present study shows that the EGSnrc code predicts electron angular scattering distributions in agreement with the measured data at the 2% level. It is intended to make the experimental data available to the user community to be used for benchmarking of other Monte Carlo codes.

Partial support from NIH R01 CA104777‐01A2.
WE‐E‐AUD C‐05: Predictability of Patient Specific Prostate Margins From Real‐Time Intrafraction Motion Measurements
Authors: Litzenberg D , Balter J , Levine L , Hadley S
Published: July 2008

Purpose: To investigate the ability to predict individualized PTV margins for prostate treatment based on limited real‐time intrafraction motion data. Method and Materials: Under IRB approved protocols, 35 patients with 3 transponders implanted in the prostate were studied. Transponders were placed at the apex, right‐ and left‐base under ultrasound guidance. Isocenter was chosen relative to the centroid of the transponders and the patients were initially positioned using the electromagnetic system. The relative position of the transponders was monitored continuously during each fraction, at 10 Hz. The probability distribution of absolute displacements from isocenter was found in each direction for the first fraction. Cumulative margins, Mn, were found using van Herk's formula after each fraction, including all fractions (n=N), to determine the best retrospective PTV margin for each individual patient. Metrics from the first fraction's probability distribution were tested for correlation (Pearson's r, Pr) with the final cumulative margin. These metrics included the position in each direction which was >= 50% and 95% (R50, R95) of absolute deviations from isocenter. The percentage of patients for which was found after n fractions. Results: The correlation coefficients in the Left‐Right, Anterior‐Posterior, and Superior‐Inferior directions were Pr50 = (0.275, 0.422, 0.177), Pr95 = (0.435, 0.489, 0.168). The percentage of patients within 1 mm of their final margin after n=2, 3, 5, 10, 20 days was LR = (94.3, 97.1, 97.1, 100.0, 100.0)%, AP = (40.0, 57.1, 74.3, 91.4, 97.1)%, and SI = (34.3, 37.1, 65.7, 91.4)%. Conclusion: R50 and R95 from a single fraction of measured intrafraction displacements are poorly correlated with a given patient's final individualized margins. In addition, 15 to 20 fractions are required to estimate margins within 1 mm for 90% of patients. Conflict of Interest: Supported by NIHP01CA59827 and Calypso Medical.
WE‐E‐AUD C‐07: A Robust Approach for Estimating Tumor Volume Change During Radiotherapy of Lung Cancer
Authors: ElNaqa I , Apte A , Yang D , Noel C , Bradle J , Deasy J
Published: July 2008

Purpose: Radiotherapy treatment of lung cancer patients is complicated by changes in tumor position due to breathing and changes in size due to regression. Accurate quantification of these changes during the course of treatment would likely improve tumor response and reduce toxicity risks. We are investigating robust methods for tracking and estimating tumor volume changes between treatment fractions. Method and Materials: We have developed a registration—assisted segmentation approach based on the level‐set deformable model, in which pre‐treatment contours are propagated and adapted to fractions times at selected respiratory phases. At any time‐point during treatment, a reference respiratory phase is selected and corresponding 3D‐CT volumes are reconstructed from 4D‐CT acquisition data. Images are then globally aligned using an efficient registration algorithm. Pre‐treatment planning contours are copied to selected time‐points. In our tumor regression analysis, the GTV contour was used to initialize the level set algorithm in‐place and the PTV contour was used to narrowband the region, thus improving the algorithm's convergence. The feasibility of the proposed approach was investigated on a set of patients with repeated 4D scans acquired at three time‐intervals. Results: Our preliminary analysis indicates that the proposed approach can properly capture the boundary of the shrinking tumor region or split regions due to its topological adaptation ability. On an initial cohort of four NSCLC patients, the estimated tumor volume reductions ranged between 3–46% with a median of 8% by mid‐treatment and between 26–51% with a median of 34% by the end of treatment. Conclusion: We have demonstrated a new approach for tracking tumor regression during the course of radiotherapy treatment of NSCLC patients based on a novel level‐set segmentation algorithm. This approach provides us with a semi‐automated tool for quantifying tumor shrinkage and allows accurate estimates of ‘true’ accumulated dose to the tumor.

Supported by CA85181 grant.
WE‐E‐332‐03: Evaluating Effective Doses and Image Quality for Multislice CT Using Automatic Tube Current Modulation
Authors: Tsai SF , Tsai HY , Tyan YS
Published: July 2008

Purpose: Automatic tube current modulation (ATCM) is one of innovative techniques to lower radiation doses in computed tomography (CT) scanning. The purpose of this study is to estimate the image quality, organ doses and effective doses in clinical multi‐detector CT examinations using ATCM techniques. Method and materials: An anthropomorphic phantom was scanned on a Siemens Sensation 64 multi‐detector CT scanner with CARE Dose4D software which offered nine combinations of ATCM. The scanning regions included head, neck, chest, abdomen and pelvis. The image quality was derived from noise distributions in each image slice using MatLab. Organ does and effective doses were measured by high sensitivity thermoluminescent dosimeters (LiF: Mg, Cu, P) which were inserted into the anthropomorphic phantom at the selected organs. Results: CARE Dose 4D software tends to trigger increasing intensity modulation in the parts of shoulder (upper lung) and lower lung, but to trigger decreasing intensity modulation in the parts of head, neck, middle lung, abdomen and pelvis. Effective doses from ATCM were lower than those from fix tube current except neck examinations. The dose decreases vary from 36% to 68.5% in head examinations, 12.0% to 38.9% in chest examinations, 31.4% to 43.4% in abdomen examinations, and 33% to 55.5% in pelvis examination, but the dose increases are from 18.4% to 48% in neck examinations. The noise increase ratios, which were derived from the images using ATCM comparing with those using fix tube current, are from −1.9% to 2.5%, −15.0% to −19.9%, −13.1% to 0.7%, 7.6% to 15.7%, and 16.2% to 28.0% in head, neck, chest, abdomen and pelvis examinations, respectively. Conclusion: ATCM techniques with appropriate use could reduce effective doses but slightly increase image noises. ATCM is an efficient method to reduce patients' radiation doses during CT scanning if there is no influence on the diagnostic acceptability.
WE‐E‐332‐04: Skeletal Dosimetry in Cone Beam Computed Tomography
Authors: Kawrakow I , Walters B , Kramer R , Ding G
Published: July 2008

Purpose: In a recent publication, Ding et al [Med. Phys. 35, (2008)1135] demonstrated that the dose in patient bony anatomy during a typical cone beam computed tomography (CBCT) scan is up to 3–4 times higher than that in soft tissues. The purpose of this investigation is to determine the dose to red bone marrow (RBM) and bone surface cells (BSC), identified by the ICRP as the two organs at risk in the skeleton. Method and Materials: The FAX06/MAX06 EGSnrc‐based code provides the ability to compute whole organ doses, including BSC and RBM doses, in a voxelized representation of a female/male body including micro‐structural information for the spongiosa obtained from micro‐CT images. The code is modified to permit the computation of spatial dose distributions and to allow the use of phase space files from BEAMnrc simulations to be employed as sources. A typical head‐and‐neck CBCT scan from a Varian Trilogy linac is investigated. Results: The average RBM dose in the FAX06 phantom is found to be 4.6 cGy, i.e., about 30% lower then the average dose in soft tissues such as the brain (6 cGy) and the eye lens (6.4 cGy), although a small fraction of the bone marrow (7%) receives doses in excess of 10 cGy. Due to the close BSC proximity to trabecular bone, the average BSC dose (11.2 cGy) is about 80% higher than the dose to soft tissue. The dose in about 15% of the BSC volume exceeds 15 cGy. Conclusion: The dose delivered to BSC and a small fraction of the RBM in typical head‐and‐neck CBCT scans is significantly higher than the dose in soft tissues. Repeated CBCT scans may therefore increase the risk of bone malformation and may cause growth issues in pediatric radiotherapy patients.