bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐176: A Shielding Design for a High‐Energy Doorless Accelerator Vault
Authors: Bichay T , Meadows J , Chen C
Published: June 2005

Purpose: To design three doorless vaults capable of accommodating high‐energy accelerators of at least 16 MV. This design must meet the state of Michigan standards currently set at 250 μSv per year for an uncontrolled area, and 5,000 μSv per year for a controlled area. Furthermore, the design must occupy a footprint similar to traditional vaults with shielded doors. Method and Materials: Standard calculation protocols were used to calculate various thicknesses of shielding materials in the walls and the ceiling. We used commercial interlocking bricks in densities of 240 lb/cf and 288 lb/cf. Additional shielding materials used were steel plates, lead plates, standard polyethylene and 5% borated polyethylene. The mazes consist of high‐density brick (Ledite) and 5% borated polyethylene panels at various points. The mazes are 24 feet in length, 6 feet in width but are staggered to reduce the cross‐sectional profile. The opening is 6 feet wide and is at 90° to conventional openings. The space adjacent to the vaults, including the floor above, is occupied and uncontrolled. Two Siemens Oncor and a TomoTherapy unit were used in this study. Results: We have carried out extensive measurements of dose equivalents at various points inside the treatment rooms as well as in the mazes, the walls, and adjoining areas. All areas comply with the state Michigan current standards. Annual equivalent dose at the opening of the rooms, using a workload of 59,000 Gy per year at 100% 16 MV, ranges from 200–2,000 μSv per year. Conclusion: This design indicates the possibility of achieving acceptable dose at the opening of a high‐energy linear accelerator without a shielded door. This can be accomplished in a standard accelerator footprint. The doorless design enhances patient comfort, patient and staff safety, and is cost‐effective.
SU‐FF‐T‐180: An Experimental Comparison of Whole‐Body Photon Doses for IMRT and 3D‐CRT
Authors: George Xu X. , Wang B
Published: June 2005

Purpose: IMRT is a viable form of radiation treatment. As a new technology, however, IMRT techniques present many new challenges. The change from 3D‐CRT to IMRT, for example, may result in an increase in second malignancies due to more fields and longer exposure times. Although NCI and others have identified this potential problem, there has been limited guidance. The goal of this study is to establish a method for comparing in‐phantom measurement of whole body doses resulting from typical 3D CRT and IMRT treatment plans. Method and Materials: The measurements involved a RANDO phantom and MOSFET dosimeters. Three different treatment plans, 4‐field 3D‐CRT, 6‐field 3D‐CRT and 7‐field IMRT for the prostate, were used for this study. The steps to re‐construct organ doses using a physical phantom and MOSFET dosimeters are summarized in the supporting material. Results: The dosimeter readings show that the doses decrease as the distances increase for all treatment plans. At 40 cm from the target, the doses are reduced nearly 100%. At this location, however, the IMRT plan resulted in a dose that is a factor of 3–5 higher than the two 3D‐CRT plans. This is due to the increased scattered radiation from the extended exposure time for IMRT case. The total monitor unit (MU) is 2850 for the IMRT case, while 1308 and 1260 for 6‐field and 4‐field 3D‐CRT cases, respectively. Conclusion: A method has been developed and tested to use MOSFET dosimeters to measure and compare whole‐body doses resulting from IMRT and 3D CRT treatment plans. The case study for prostate shows that the IMRT indeed delivers higher photon doses to locations that are away from the prostate target. On‐going studies will use a whole‐body VIP‐Man model containing 80 segmented organs and Monte Carlo method to simulate various treatment plans for photons and protons.
SU‐FF‐T‐182: Analysis of Systematic Uncertainty in Vivo Dosimetry Using Diodes
Authors: Yu P , Chao T , Lee C , Wu C , Tung C
Published: June 2005

Purpose: This study analyzes systematic uncertainty of in vivo dosimetry using diodes. Several possible sources of error are investigated to eliminate or correct systematic errors. Method and Materials: Two diodes are placed on both sides of a phantom/patient to measure entrance and exit doses. The midline dose can thus be derived from these entrance and exit doses. We selected patients with tumors in their midline and with geometrically symmetric structures Several experiments are specially designed to measure response variation of diodes (1) due to temperature change, (2) with/without mask presence, (3) with different types of tapes used to fix the diodes, (4) with the space between mask and skin, (5) with the off field edge distance, and (6) with inhomogeneity of the patients. Results: The systematic uncertainty for measurement with/without the mask is 1%. The systematic uncertainty for temperature change is 3∼4%. The systematic uncertainty from different types of tapes is 1%. The systematic uncertainty due to space between the mask and the skin is less than 2%. If the diodes were placed off edge for distances greater than one‐fourth of the field size, the systematic uncertainty is less than 2%. Conclusion: Temperature change may be an important source of error for in vivo dosimetry using diodes. Other sources of error we investigated produced uncertainties less than 2 %.
SU‐FF‐T‐185: Commissioning of a Mobetron Electron Linear Accelerator
Authors: Woollard J , Gao M , Samsami N , Kanellitsas C , Gupta N
Published: June 2005

Purpose: The purpose of this paper is to describe the commissioning of our Mobetron electron linear accelerator and share the experience gained during the commissioning process. Method and Materials: The Mobetron is a relatively lightweight and mobile linear accelerator that has been developed for use in an operating room suite for delivering intra‐operative radiation therapy. This accelerator is capable of delivering a therapeutic beam of electrons with nominal energies of 4, 6, 9 or 12 MeV. The Mobetron comes with a variety of circular cones, ranging in field size from 3 to 10 cm diameter, in 0.5 cm increments, and 3 different angles, flat, 15 and 30 degrees. For each energy and cone, the percent depth dose (PDD), isodose distribution and output factor were measured. These measurements were performed in a water phantom. A dose per MU calibration was performed for each energy, using the 10 cm flat cone, according to the TG‐51 protocol. The stability of this output was monitored as a function of time. Finally, dose values were measured at various locations around the Mobetron during operation in order to determine shielding requirements and warm‐up procedures. Results: PDDs and isodose distributions for the various cone sizes, cone angles and beam energies are presented. Additionally, plots of output factors as a function of field size for the 3 different sets of cones and beam energies are provided. Lastly, the doses at different locations around the Mobetron are shown and the impacts of these doses on shielding and warm‐up procedures are discussed. Conclusions: The commissioning is complete. PDDs and isodose distributions measured for the Mobetron were similar to those measured for the linac used previously. Due to the machines' design, we found the output did vary with time, by as much as 3% over a period of about 10 days.
SU‐FF‐T‐192: Determination of Skin Dose for Hypo‐Fractionated Breast Treatment Using Mixed Photon and Electron Beams
Authors: Stathakis S , Li J , Paskalev K , Yang J , Wang L , Ma C
Published: June 2005

Purpose: Estimation of surface dose is very important for patients undergoing radiation therapy. This work is aimed at accurate determination of the dose at the skin surface and at a depth of 0.07 mm, the practical reference depth for skin as recommended by ICRP and ICRU, using ultra thin TLDs and Monte Carlo calculations. Method and Materials: Monte Carlo simulations and measurements were carried out for 5×5 cm2 and 10×10cm2 fields for 6, 10 and 18 MV photon beams as well as for electron beams of energies ranging from 6 to 21 MeV. For photon beams, the variation of the surface dose with angle of incidence and field size was investigated. Also, the exit dose was computed and measured for the same fields and angles of incidence. The dose at the ICRU reference depth was also computed for the electron beams at normal incident angles. Finally, the dose was measured and calculated for breast IMRT plans followed by dose uniformity enhancement by an additional electron beam for conventional and hypo‐fractionated treatment protocols. Results: Good agreement (±5%) was achieved between measurements and calculations. The surface dose at the entrance was increased as the angle of incidence and/or the field size increased. The exit dose was decreased as the angle of incidence increased but increased with field size. Conclusion: The dose at the surface of the patient is mostly dependent on the beam energy, modality and beam obliquity rather than the field size and field separation. By correlating the TLD measurements to Monte Carlo calculations, one can predict the dose at the skin surface with great accuracy. Knowing the dose received at the surface of the patient can lead to prediction of skin reactions helping with the design of new treatment techniques and different dose fractionation schemes.
SU‐FF‐T‐193: Dosimetric Responses at Different Gantry and Collimator Angles in Dynamic MLC Beam Delivery
Authors: Shou Zhenyu , Zheng Zhen , Komanduri Krishna , Heron Dwight E. , Huq M. Saiful , Yue Ning J.
Published: June 2005

Purpose: IMRT uses Dynamic Multi‐Leaf Collimator (DMLC). This study is to investigate the DMLC performance at different gantry and collimator angles based on dosimetric measurements. Conformity index and symmetry index were introduced to quantify the performance. Method and Materials: We developed a technique to measure the dosimetric impact of DMLC delivery at different gantry and collimator angles. Various DMLC patterns were designed to observe the discrepancies of the DMLC delivery at different gantry collimator angles. The discrepancies were quantified by acquiring dosimetric information under the corresponding radiation delivery conditions. The designed dynamic fields were delivered using 6 MV photon beams to a Solid Water TM phantom with an ion chamber at the isocenter. Phantom was carefully set up so that both the phantom and the chamber remained the same, geometrically, with respect to the beam's coordinates when the gantry angle was changed. The measurements were carried out for two Varian's 23Ex Liancs. Conformity index was defined to measure the output ratios at different gantry angles for a same DMLC pattern. Symmetry index was defined to assess the dosimetric discrepancies of the same pattern with opposite collimator setting at a same gantry angle. Results: The same measurements were performed for each machine for five consecutive days. It was observed that conformity index varied within between 98% and 100%. And the results varied on daily basis, which may imply the slight instability of DMLC performance. A consistent value less than 100% for the conformity index may indicate a gravity effect on the DMLC performance. In the meantime at a fixed gantry angle and at different collimator angle with a same DMCL pattern, the symmetry index varied randomly from 97.5% to 102.2%. Conclusion: The gravity may affect the DMLC dosimetric performance. This impact on IMRT dynamic delivery warrants further investigation.
SU‐FF‐T‐195: Evaluation of Film Calibration Procedures for Quality Assurance of Helical Tomotherapy Treatments
Authors: James J , Nelson I , Mackie T
Published: June 2005

Purpose: To evaluate and compare film calibration procedures using a conventional 6 MV static beam and an optimized helical tomotherapy plan for the patient‐specific QA of tomotherapy treatments. Method and Materials: The conventional 6 MV static beam was delivered by a Varian 2300CD Clinac to blocks of solid water. Three different doses were delivered to each of two Kodak EDR2 films, giving a total of six dose values. Cerrobend blocks were used to cover portions of the film outside each field in order to prevent contamination from one field to the others. The optimized helical tomotherapy plan was delivered by a TomoTherapy Hi‐Art™ System to a cylindrical solid water phantom. Nine cylindrical ROIs were contoured within the phantom along the horizontal and vertical axes. This procedure produces two films with a total of nine different dose values. All films were analyzed using in house software coded in LabVIEW™. Each calibration procedure was evaluated on the basis of accuracy, efficiency and ease of implementation. Results: Both the conventional 6 MV static beam and optimized helical tomotherapy plan were able to deliver the dose accurately, with the static beam producing a steeper gradient in the vertical direction but better uniformity in the coronal plane. It took approximately the same amount of time to complete each procedure. Conclusion: The conventional 6 MV static beam is able to deliver a slightly more uniform dose to the film plane. However, the optimized helical tomotherapy plan provides sufficient accuracy, is able to add more points to the calibration curve in the same amount of time and is delivered with the same beam and phantom used to deliver the patient treatments. For this reason, the optimized helical tomotherapy plan has been implemented as the film calibration procedure for patient‐specific QA of tomotherapy treatments.
SU‐FF‐T‐196: Evaluation of the Dose Within the Abutment Region Between Tangential and Supraclavicular Fields for Various Breast Irradiation Techniques
Authors: Homann K , Gates B , Salehpour M , Followill D , Kirsner S , Buchholz T , White R , Prado K
Published: June 2005

Purpose: To compare the uniformity of dose in the abutment region (matchline) between breast tangential and supraclavicular fields produced by irradiation techniques having different means of forming the match (e.g. external block, MLC, collimator jaw). Method and Materials: A film dosimetry system was developed and validated by comparing lateral and percent‐depth‐dose (PDD) profiles measured with EDR2 film in a plastic water phantom to ion chamber measurements made in water. Field sizes of 5×5, 10×10, and 20×20 cm2 were examined covering a range of depths. Once established, EDR2 film was placed in solid phantom at various depths and used for characterization of the abutment region produced by several three‐field intact‐breast irradiations. The abutment regions (+/− 2.5 mm on either side of the central matchline) of the various techniques were analyzed via a Dose Area Histogram (DAH) and compared to each other. Preliminary data are presented, in which DAHs are used to evaluate the abutment region between a field collimated with a jaw and another field collimated with the configurations mentioned above. Results: Characterization of the film dosimetry system demonstrates that, for all field sizes and depths, all lateral profiles and depth‐dose curves measured agree with ion chamber measurements to within < 3% in low dose gradient regions and < 2 mm distance to agreement (DTA) in high dose gradient regions (≥ 30% cm−1). Inspection of preliminary abutment region data indicates that the DAH clearly and sensitively demonstrates uniformity differences between the various methods used to create the match. Conclusion: We have developed a film‐based dosimetry tool that uses DAHs to accurately portray relative dose distributions within the abutment regions of smooth‐edge and MLC‐produced fields. This tool will be used, subsequently, in an anthropomorphic phantom using actual breast treatment fields. Conflict of Interest: Research supported in part by Varian Medical Systems.
SU‐FF‐T‐197: Filmless Radiation Isocentre Localisation Using An Electronic Portal Imaging Device
Authors: Heaton R , Norrlinger B , Smale J , Islam M
Published: June 2005

Purpose: The increased availability of electronic portal imaging devices (EPID) on linear accelerators provides a capability to replace film with enhance standard commissioning and validation measurements. A series of EPID based measurements have been developed to confirm the coincidence of the radiation and mechanical isocentre with gantry, collimator and table rotation, eliminating the need for traditional star‐shots films and enhancing the information gathered from such tests. Method and Materials: A precision phantom containing a set of radio‐opaque markers was positioned at the mechanical isocentre using a calibrated front pointer. The EPID was positioned at an SID of 140 cm, and a series of images for a 4×10 cm2 where acquired in IMRT mode for different angular rotations of gantry, collimator and treatment table. Images were exported to a MatLab based program which automatically identified the position of the radio‐opaque markers and field edges to determine the relative motion of the isocentre with angle. Results: Measurements were performed with several different models of aSi imager found on Varian and Elekta Precise treatment units, and compared to the results of traditional star shot films. The measurements were consistent, with the radiation isocentre confined to a 2 mm diameter sphere. In addition, the EPID based tests provided additional information on the longitudinal motion of isocentre typically not available from film star shots. Conclusion: EPID based measurements can replace the use of film in geometric verification tests typically performed during commissioning and annual quality assurance. These measurements are at the same time simpler to perform and analyze, while increasing the information extracted from the measurements and providing a reliable, user independent method of monitoring changes to the treatment unit isocentre over the lifetime of the machine.
SU‐FF‐T‐198: Head‐Scatter Off‐Axis for MLC Shaped Fields
Authors: Lazarescu G , Zhu T
Published: June 2005

Purpose: Off‐axis head‐scatter is the major contributor to doses outside x‐ray collimation. For an IMRT beam, these scattered photons also affect the dose inside the treatment field. This study determines the headscatter components for an accelerator with MLC as an attachment. Method and Materials: Head‐scatter is measured for the 6 and 18 MV photon beams from a Varian 2100C linear accelerator. The head scatter off‐axis, HOA, is defined as the scatter‐to‐primary ratio for head‐scatter with collimator setting cx □ cy at off‐axis position x, i.e., HOA(cx,cy,x) = (T(cx,cy,x) − P(cx,x))/P(cx,0), where T is the total dose measured in a graphite miniphantom for collimator setting cx □ cy at position x and P is the primary dose measured at the same location x under a collimator setting of 3 □ cx. The collimator setting along x, cx, is kept the same so that the primary remains unchanged. The results are fitted to a two‐source model [1]. Results: The widths of the two Gaussian sources are 1.4 and 16 cm, respectively, for both energies. Compared to the leakage outside beam collimation, HOA at 2 cm outside field width is 100 % of the primary for c = 5 cm to 400 % of the primary for c = 30 cm. Conclusion: Compared with the conventional collimator jaws, HOA for MLC shaped field is lower at off‐axis locations due to the difference in physical locations of the MLC and the collimator jaws. 1. Zhu TC and Bjarngard BE, “Head scatter off‐axis for megavoltage x rays,” Med Phys 30: 533 – 543 (2003).
SU‐FF‐T‐200: Image‐Guided IMRT Dose Verification with a Plastic Photosensitive Dosimeter
Authors: Pawlicki T , Luxton G , Maxim P , Mistry V , Bourland P , Boyer A
Published: June 2005

Purpose: To verify an IMRT treatment, multiple point dose measurements are time‐consuming to acquire. This investigation used a kilovoltage imaging system to localize an optical dosimeter to acquire rapidly three‐dimensional IMRT quality control measurements. Method and Materials: A plastic photosensitive dosimeter was used as a dosimetric verification tool for IMRT. The plastic was formed into a right cylinder 8cm in height and 9cm in diameter that fit precisely into a water equivalent phantom. The system was used to verify an IMRT dose distribution computed by inverse treatment planning and delivered using a medical linear accelerator equipped with a multi‐leaf collimator and a kilovoltage electronic portal imaging device (KV‐EPID). The KV‐EPID was used to determine the location of the dosimeter cylinder within the accelerator coordinate system. Small wires (1mm diameter) inserted into the cylinder at three locations were used as fiducial markers. An auto‐matching algorithm integral to the accelerator control system was used to assure that the position of the dosimeter matched the position in a phantom plan. The Root‐Mean‐Square localization error was measured to be 1.1mm. The dose distribution within the dosimeter was measured by means of an optical cone‐beam imaging system. The measured dose distribution was compared with the calculated dose distribution based on the known locations of the fiducial markers. Results: The KV‐EPID was easily able to visualize the fiducials in the dosimeter and to localize the dosimeter with a RMS error of 1.1mm. Conclusion: The dose distribution delivered by IMRT to a surrogate phantom can be aligned with a mean error of 1mm using the KV‐EPID. This allows optical reconstruction of a photosensitive plastic dosimeter to be aligned with the computed three‐dimensional dose distribution for dosimetric verification of an IMRT treatment plan.
SU‐FF‐T‐201: Implementation of EPID Based Quality Assurance Procedures for Radiation and Optical Fields
Authors: Smale J , Norrlinger B , Van Prooijen M , Rink A , Heaton R , Islam M
Published: June 2005

Purpose: Enhanced radiation and light field quality assurance (QA) procedures have been developed and implemented utilizing amorphous silicon electronic portal imaging devices (EPID). This effort was motivated by the fact that film exposure coupled with hand measurements are extremely time consuming and prone to many sources of error. Method and Materials: The EPID based QA procedure uses specially designed acrylic test templates, containing radio‐opaque led bars to register the position of the optical field. Images of these templates were acquired with the EPID on both Varian and Elekta linear accelerators. A MATLab based computer application was used to automatically assess light/radiation field congruence, radiation field sizes and optical field size and positioning. A three‐month pilot study using two Varian accelerators to assess both 6 MV and 18 MV photon energies was performed to validate the EPID QA procedure's efficacy and quantify its efficiency relative to film QA procedures. Results: The pilot study verified that the EPID QA system results are congruent with the hand measurements obtained from a detailed analysis of film QA measurements. Light/radiation congruence measurement performed using an EPID exhibited an absolute mean deviation of 0.37 ± 0.37 mm (1σ), relative to the film measurements. EPID based radiation field size assessments produced an absolute discrepancy of 0.57 ± 0.44mm (1σ), while film measurements yielded an absolute discrepancy of 0.61 ± 0.55mm (1σ), relative to measurements acquired using RIT software. EPID based measurements were found to be more efficiently performed, reducing the set‐up and analysis time by 55% compared to film. Conclusions: The enhanced EPID QA system can effectively replace film in routine quality assurance tasks for radiation and optical fields. This procedure saves Princess Margaret Hospital 4 man‐hours per week, and provides a user‐independent analysis procedure for objectively assessing routine QA results.
SU‐FF‐T‐202: In Vivo Dosimetry Using Diodes Detector for External Photon Treatments of Head and Neck Cancers
Authors: Yu P , Lee C , Chao T , Wu C , Tung C
Published: June 2005

Purpose: In vivo dosimetry is a tool to ensure the accuracy of delivery of the radiation treatments. By in vivo dosimetry, the incidence of errors could be monitored. Based on experiment results, we want to establish an in vivo dosimetry standard protocol. Method and Materials: Measurements of entrance and exit doses were performed with diodes on patients' surface. After obtained entrance and exit doses, we used transmission curve to derive midline dose. Before entrance and exit dose measurements, we need to measure the calibration factor for diode reading to dose and, correction factors for nonstandard SSDs and field sizes. Measurements were made for 6MV photon fields (Varian 2100C/D machine). The detectors were VeriDose 30‐472 (5‐11MV) diodes. The midline dose was calculated by our hand calculation system without inhomogeneous correction. Results: The total field number for patient measurements was 204. Before measuring in vivo doses, we delivered these fields to a plastic water phantom. Comparisons between phantom results and midline doses measured by the ionization chamber show the mean relative discrepancy was 0.184% with 0.5% corresponding SD. The mean relative discrepancy between midline doses measured by diodes and planning target doses from prescription for patient measurements was −1.08% and SD was 3.09%. If we averaged two opposed beams dose, the standard deviation of reletive error will be 2.35%. Further, we corrected our estimated dose by inhomogeneous correction depth which was averaged by two opposed beams measurements, the mean and standerd deviation we obtain was −0.16%(1.39%). Conclusion: After phantom and patient studies, we conclude that in vivo dosimetry is a practical way to inspect dose accuracy.
SU‐FF‐T‐204: In Vivo Force‐Torque Measurement During Prostate Brachytherapy
Authors: Podder T , Clark D , Messing E , Fuller D , O'Dell W , Rubens D , Strang J , Ng W , Sherman J , Brasacchio R , zhang Y , Okunieff P , Schell M , Liao L , Yu Y
Published: June 2005

Purpose: The main goal of this study is to measure the needle insertion force/torque (F/T), velocity/ acceleration (V/A), and tissue/organ deformation during actual brachytherapy procedures in the operating room (OR). These in vivo data will guide the design of a Robot‐Assisted Platform for Intratumoral Delivery (RAPID) system. Method and Materials: We have acquired F/T and V/A data from actual patients and a single surgeon placing brachytherapy needles in the OR using a hand‐held adapter equipped with a 6 degree‐of‐freedom (DOF) F/T sensor (Nano25™). During this in vivo measurement, the needle progression into the soft tissue was registered using ultrasound (US) imaging technique. A 6 DOF electromagnetic (EM)‐based position sensor (miniBIRD®) was attached to the hand‐held adapter to measure 3D position and orientation of the hand‐held adapter. Results: The in vivo data reveals that maximum needle insertion force is about 18N, velocity is about 72cm/s, and acceleration is about 3000cm/s2. This relatively high acceleration may have some implications on the inertia force, which may help the surgeon in needle insertion. We observed significant transverse force (about 1.7N). From in vitro data it was observed that the force and torque during robotic needle insertion in animal soft tissues were significantly smaller as compared to that for humans. Conclusion: Additional in vivo data are being collected from different patients to study the effects of patient specific criteria such as age, height, ethnicity, body mass index (BMI), prostate specific agent (PSA) value, special anatomy, previous treatment, etc. on needle insertion force/torque and tissue deformation. Since frequent in vivo data collection and experimentation are impractical, we will use these data to determine the soft animal tissues and soft materials which have close resemblance to human tissues.
SU‐FF‐T‐207: Linac QA Using a Computed Radiography System
Authors: Cai Y , Chu J , Bernard D , Hu Z
Published: June 2005

Purpose: The computed radiography (CR) is replacing film for linac quality assurance (QA) checks in our clinic. A few tools and methods were developed to facilitate this transition. Method, Materials and Results: A right‐triangular prism phantom was used for electron beam quality verification. The phantom was centered along the beam axis, and placed on top of a CR cassette with 110 cm source to image distance (SID). The image was processed to obtain a percentage exposure vs. wedge thickness curve, which was sensitive to electron energy changes. This method demonstrated good reproducibility. A method using a diamond‐shaped template for the light and radiation field congruence test had been previously developed. This method was analyzed and modified. In the modified version, the template was put right above the CR cassette to reduce the systematic error due to the penumbra effect. An IDL program was developed to compute the discrepancies between the light field and the radiation field. The jaw alignment was checked with a device similar to that of Lutz et al: a CR cassette sandwiched between two pairs of L‐shaped Cerrobend blocks. Each pair was embedded diagonally in a piece of foam. The pair above the cassette was at an angle of 90 degrees to the pair beneath the cassette. The radiation was delivered at collimator angles of 0 and 90 degrees to verify the x‐jaw and y‐jaw alignment, respectively. For each collimator angle, two gantry rotations ‐ 0 and 180 degrees ‐ were used, with equal monitor units delivered at an SID of 100 cm. The radiation field flatness, symmetry and penumbra were tested using CR combined with the RIT software solution. The results were consistent with those collected using films. Conclusion: QA checks using CR systems are more convenient and yield results comparable to those from film.
SU‐FF‐T‐209: New Polymer Gel Dosimeter with High Refractive Index for Artifact‐Free Optical CT Scanning
Authors: Maryanski M
Published: June 2005

Purpose: In order to enable optical CT scanning of gels with inserts representing tissue heterogeneities such as bone or air spaces as well as brachytherapy catheters and applicators, as well as to remove artifacts which arise when scanning across regions adjacent to container walls, we have explored the feasibility of developing a new polymer gel that would result in matching the refractive index of the gel to that of the container material. Method and Material: We have developed a new type of polymer gel dosimeter (and named it BANG4), based on cellulose acetate (gelling agent) in benzyl alcohol. The monomer we used in this study was methacrylic acid, which precipitates in benzyl alcohol upon polymerization. The gel is made under nitrogen atmosphere. These gels are optically clear and mechanically strong. Benzyl alcohol has the refractive index of 1.538 and methacrylic acid 1.429. Therefore, we can adjust the refractive index of the gel to match the material of the container (phantom) or insert, be it Lucite, PVC, PETG, Barex or glass. We have tested the dose response of the new gel using Torrex 150D X ray machine (EG&G, Long Beach, CA) and a spectrophotometer. We also scanned irradiated BANG4 gels in Barex and Lucite containers, using commercial OCTOPUS™ optical CT scanner (MGS Research, Madison, CT). Results: We found the dose response of the new gel to be linear up to 6 Gy. The optical CT images of BANG4 gels reveal that the refractive artifact was entirely removed, as can be seen from image profiles. Conclusion: We have successfully demonstrated the feasibility of a new class of polymer gels that can be OCT‐scanned without the refractive artifact. Further studies are planned to investigate radiological properties of the new dosimeter.
SU‐FF‐T‐387: Treatment Plan Evaluation and Optimization For SRS and IMRT Using Dose‐Surface Histograms
Authors: Li S
Published: June 2005

Purpose: To introduce a new method in determination of the periphery dose and dose differences between the tumor and surrounding tissues, including organs at risk (OARs), for stereotactic radiosurgery (SRS) and/or intensity modulation radiotherapy (IMRT) using dose‐surface histograms (DSHs). Method and Materials: Plan‐target volume (PTV) and its expansion into surrounding healthy tissue (TISSUE) by 3 mm (or 5 mm) for a small (or large) PTV are created. The contours of the TISSUE on CT or MRI slices are modified at the region where the Tissue intersects with an OAR. The PTV and TISSUE are then expanded or shrunk, denoted as PTV' and TISSUE', by a pixel size (PS) within a fine dose grid, respectively. DSH for the PTV (DSHPTV) can then be determined by (DVHPTV*PTV − DVH PTV'*PTV')/(PTV − PTV')/PS. Similarly, DSH for the TISSUE (DSHTISSUE) is given by (DVHTISSUE*Tissue − DVH TISSUE'*TISSUE')/(TISSUE − TISSUE')/PS. The mean and variation of DSHPTV characterizes the periphery doses and the difference between the mean doses on DSHPTV and DSHVOI quantifies the dose drop‐off from the PTV to the surrounding tissue. Results: Application of the new method on Gamma Knife plans and LINEAC‐based SRS plans clearly shows that gamma knife plan has better dose drop‐off but worse periphery doses for relative rounded target. However, the LINEAC‐based MLC‐shaped arcs plans are generally superior for irregular‐shape lesions. In comparison of plans using MLC‐shaped arcs, conformal beams, and IMRT, we found no improvement on periphery dose and dose drop‐off in IMRT for some cases due to lack of DSHPTV in the inverse planning systems. Conclusion: The accurate quantification of the periphery dose and dose drop‐off is extremely important because they are closely correlated to the treatment prescription and the clinical outcomes.
SU‐FF‐T‐388: A Method for Repositioning of Stereotactic Brain Patients with the Aid of Real‐Time CT Image Guidance
Authors: Paskalev K , Feigenberg S , Wang L , Movsas B , Laske D , Ma C
Published: June 2005

Purpose: Fractionated stereotactic radiotherapy to the brain is usually delivered using relocatable head frames. Previous studies suggest that with the GTC relocatable frame the position of the patient can be reproduced with uncertainties on the order of 1 to 2 mm. However, the reproducibility of the frame varies from patient to patient and can be poor especially in edentulous patients. The goal of this study was to develop and evaluate a method that recalculates the coordinates of the isocenter for patients undergoing stereotactic treatment to the brain with the GTC relocatable head frame, based on a pre‐treatment CT scan. Method and Materials: The pre‐treatment CT scans were acquired with the Primatom CT‐on‐rails system (Siemens) and they were fused with the planning scan using the XKnife treatment planning system (Radionics). Then the BRW coordinates of the beam isocenter were recalculated using 3D CT‐to‐CT image registration provided by the planning system, and a 3D CT‐to‐BRW coordinate transformation. The method was evaluated by comparing the initial BRW coordinates of the isocenter with the recalculated coordinates for eight single‐fraction patients (64 points). These patients had the BRW frame fixed to the outer table of the skull, and therefore the coordinates compared between the simulation and the pre‐treatment scan should be identical. Any difference between the two sets of coordinates was attributed to errors in the method. Results: The results showed that the systematic errors in the recalculated coordinates for the BRW frame were lower than 0.05 mm in all three directions, and they were not statistically significant. The random errors (one standard deviation) were from 0.35 mm (lateral) to 0.58 mm (vertical). The average value of the combined 3D difference was 0.75 mm. Conclusion: These results suggest that the proposed technique could be used to reduce spatial uncertainties associated with the GTC head frame.
SU‐FF‐T‐389: A Method to Treat GammaKnife Shots Otherwise Untreatable Due to Collison
Authors: Cernica G , Fenstermaker R , de Boer S , Plunkett R , Kuettel M , Podgorsak M
Published: June 2005

Purpose: To develop a technique enabling Gamma Knife shots deemed untreatable because of collision to be delivered using an appropriately shifted stereotactic frame placement without requiring re‐imaging of the patient. Method and Materials: A Leksell stereotactic frame with a CT fiducial box was applied to a head phantom. The assembly was imaged (1 mm slice thickness), allowing localization of its stereotactic space. Using GammaPlan, the (x,y,z) coordinates of the centers of four 0.5‐mm diameter steel spheres positioned as targets within the frontal, temporal, and occipital regions of the head phantom were recorded. A Leksell biopsy arc was then attached to the stereotactic frame and the (r,θ,φ) coordinates of 6 points marked on the phantom surface were recorded. The stereotactic frame was removed from the phantom and re‐attached in a different position. The biopsy arc was then used to determine the new (r,θ,φ) coordinates of the same surface points. A transformation algorithm using the two sets of (r,θ,φ) coordinates was applied to the (x,y,z) coordinates of the steel spheres in the original stereotactic space to determine the corresponding coordinates in the second stereotactic space. To test algorithm accuracy, a repeat CT‐scan of the phantom was done and the actual (x,y,z) coordinates of the targets in the second stereotactic space were found. Results: The mean distance between the algorithm‐predicted value and actual value of the target coordinates in the second frame placement was 1.03 (σ = 0.3) mm, demonstrating acceptable accuracy in most clinical situations. Conclusion: This technique may be used for treating patients with multiple lesions where collisions prevent treatment with a single frame application. Once all treatable shots are delivered, the frame is shifted and a transformation based on the spherical coordinates of surface points is applied to the untreatable shots which are then delivered in the new stereotactic space. Re‐imaging is not necessary.
SU‐FF‐T‐394: An Image Guided Target Localization System for Brain Radiosurgery and Fractionated Stereotactic Radiotherapy Using a Non‐Invasive Fixation
Authors: Jin J , Ryu S , Faber K , Movsas B
Published: June 2005

Purpose: To develop an image guided target localization technique to improve the patient positioning accuracy for brain radiosurgery and fractionated stereotactic radiotherapy using non‐invasive fixation. Materials and method: BrainLab stereotactic localization box system is used for the initial patient setup. The patient is immobilized using a thermal mask with a mouth bite piece. An image guided localization system (Novalis body system, BrainLab) is used to finely adjust the position after the initial setup. The system automatically fuses two X‐ray images with the corresponding DRR and gives the position offsets in 6 dimensions. The accuracy of the image guided localization system was evaluated in a rando phantom with a 2‐mm‐metal‐ball placed as the isocenter, and in 15 patients with total of 48 fractions for image fusion. Results: The phantom study showed that the positioning accuracy is within 1‐mm for various isocenter positions. The image fusion for each patient was checked carefully and could be evaluated easily due to its rigid structure and rich bony features. At least three distinguish features could be identified in each image, and a 1‐mm translational move generally induced a visible dis‐matching in these features. This suggests that the accuracy of alignment to the isocenter of the X‐ray system be better than 1‐mm. Considering that the isocenter of the X‐ray system could be different from the linac's, the overall positioning accuracy for the image guided system would be in the order of 1.4‐mm. The offsets of the fusion results was used for mutual testing between the image guided system and the stereotactic localization box system. The average offsets for the lateral, AP and longitudinal directions were −0.67±1.09, 0.44±1.09, and −0.84±1.44 mm, respectively. This is consistentwith the results of the non‐invasive immobilization techniques reported by other authors. Conclusion: The image guided system could improve the patient positioning accuracy.
SU‐FF‐T‐395: An Integrated GRID Boost Technique for Gamma Knife Radiosurgery
Authors: Ma L , Kwok Y , Chin L , Simard J , Regine W
Published: June 2005

Purpose: To develop an integrated boost approach with the GRID technique for Gamma Knife radiosurgery Method and Materials: The GRID technique was originally developed for external beam radiation therapy where a high dose being delivered via a multi‐hole collimator. The goal of the technique is to debulk large tumor with spatially fractionated dose distributions. In this study, we developed an integrated GRID boost approach for Gamma Knife radiosurgery where an array of high dose areas was placed inside the target volume using a series of 4‐mm shots. The dose grid was added on top of the existing dose distributions where a peripheral isodose line (e.g. 50%) covers the full target volume. We optimized the weights of the 4‐mm shots to equalize the integral dose at each shot location. Treatment plans were developed and evaluated based on peripheral dose fall‐off and dose to normal brain as compared with the conventional Gamma Knife treatment plans. The comparison was also performed using equivalent uniform dose (EUD), tumor control probabilities (TCP), and normal tissue complication probabilities (NTCP). Results: All parameters for the target volume (mean dose, EUD, and TCP) increased significantly (> 5–30%) for the GRID boost technique as compared with the conventional Gamma Knife plans. The peripheral isodose coverage of the target volume (as measured with conformity index) remained unchanged for the GRID boost technique. In contrast, the EUD and the NTCP for the normal brain tissue adjacent to the target decreased significantly (as much as 40% in NTCP) for the GRID boost plans. Slightly faster dose fall‐off near the peripheral target region was noted for the GRID delivery. Conclusion: We demonstrated a new GRID technique for Gamma Knife radiosurgery in escalating the dose to the target volume while improving adjacent normal brain sparing.
SU‐FF‐T‐396: Analytical Derivation of 6 MV Radiosurgery Cone Factors
Authors: Yang J , Pino R , Nizin P
Published: June 2005

Purpose: To derive cone factors analytically for 6 MV radiosurgery beams Method and Materials: Lack of lateral electronic equilibrium and finite detector size make experimental measurements of stereotactic radiosurgery cone factors a difficult task. In this work, cone factors for 6 MV radiosurgery beams of various cross sections are determined using an analytical model for absorbed dose in narrow photon beams and validated by means of experimental measurements in water. The model allows one to calculate absorbed dose under electronic disequilibrium conditions using measured broad‐beam data. For each cone ranging from 4 mm to 20 mm in diameter, cone factor is derived analytically and compared to that measured in water experimentally using a high resolution diode and a computerized data acquisition system. Results: The agreement between the analytical and measured cone factors is within 2.5 % of local value. Conclusion: Based on the results of this study for 6 MV x‐rays, the analytical method described here can be employed to determine radiosurgery cone factors. It provides a viable alternative to experimental methods where measurements in water are arduous. It can be used as an independent method or to validate experimental results.
SU‐FF‐T‐398: Commissioning a 5 Mm Circular Cone for Linac‐Based Stereotactic Radiosurgery Using MicroMOSFET and Polymer Gel
Authors: Wojcicka J , Kudynski R , Lasher D , Fortier G
Published: June 2005

Purpose: The accuracy of measured small cone parameters is important in the treatment of certain disorders like trigeminal neuralgia, where a single large dose is delivered via a small cone. The purpose of this presentation is to identify practical dosimeters for commissioning the cone accurately and efficiently in a community clinic. Method and Materials: Relative output factors for 5, 12.5, and 15 mm cones were measured using microMOSFET, Kodak EDR2 film, and TLD microcubes. TMRs for the 5 mm cone were measured using microMOSFET and BANG®polymer gel. OARs for the 5 mm cone were measured using radiographic and radiochromic films. Results: The output factor for the 5 mm cone measured with microMOSFET was 0.654 for a 6 MV beam and agreed with data published elsewhere. MicroMOSFET measurements agreed with EDR2 film and TLD microcubes measurements within 4.3% and 3.2% respectively for the 5 mm cone. All techniques were within 2.5% agreement for the 12.5 and 15 mm cones. TMR values measured with microMOSFET and polymer gel agreed within 3%. Radiographic and radiochromic film off‐axis ratio measurements showed differences not exceeding 1% above the 10% relative dose level. The measurements were verified using a MD Anderson Cancer Center phantom for a single static beam and polymer gel for a clinical set of three arcs. The doses reported by the institution and MDACC at dmax and 7.5 cm depth agreed within 4% and 3% respectively. The volumetric doses between the treatment planning system and the polymer gel were within 4%. Conclusion: The overall precision and accuracy of microMOSFET‐based measurement techniques are clinically acceptable. The microMOSFET is a feasible alternative with some advantages to TLD microcubes for dosimetric measurements of very small cones and fields. The polymer gel was found to be the only commercially available 3D‐dimensional verification dosimeter for these cones.
SU‐FF‐T‐399: Comparison of Stochastic and Analytical Algorithms in Selecting Gamma Knife Plug Patterns for Treatment Planning
Authors: Li K , Ma L
Published: June 2005

Purpose: In gamma knife SRS, plugging pattern is often used to decrease dose to critical structure. Two algorithms have been developed and analyzed in this study. One is the stochastic plug selection based on simulated annealing algorithm; the other is the analytical approach based on greedy algorithm. Clinical cases were implemented to compare the efficiency of these two algorithms. Method and Materials: With the gamma knife analytical model, the target volume and the critical structure contours were extracted from the Leksell Gamma knife Plan. We used the integral dose to the critical structure as the objective function. This objective function is calculated by adding each plug's dose contribution to the structure. For simulated annealing algorithm, the energy term is the objective function, by randomly selecting blocked pattern from a randomly generated plugging pattern pool. By controlling iteration number, simulated annealing algorithm produced an optimal blocked pattern with optimal energy term. The greedy search approach also produced the optimal objective function value based on the dose contribution order from different plugs. The values from the objective function and the DVH are compared to show the performance of these two algorithms. Results: A number of different objective function values are computed via simulated annealing algorithm. We found that the objective function value is generally smaller than the objective function value attained by greedy approach. And the DVH also shows that simulated annealing algorithm gives less effective result to satisfy the criteria to deliver smaller dose to critical structure. Conclusion: The greedy approach produces an optimal blocked pattern in the first order with the objective function, and the solution is generally better than the simulated annealing algorithm. In addition, the simulated annealing algorithm demands long computational time and the more fluctuations in the final results.