bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐99: Characterization of the Dose Response of a New Implantable MOSFET Detector
Authors: Briere T , Gillin M , Beddar S
Published: July 2007

Purpose: To investigate the response of the newest commercial implantable MOSFET detector (ASIC) for in vivo dosimetry under photon irradiation. Method and Materials: A new, smaller version of the implantable MOSFET detector (Sicel Technologies, Morrisville, NC) was investigated in this study. Measurements were performed in a solid water phantom at room temperature under 6 and 18 MV photon beams to the calibration dose of 200 cGy/fraction for 10 fractions. Additional measurements were performed under a 6 MV photon beam for doses outside the manufacturer's specified dosimeter functionality range: 100, 150, 250, 400, and 600 cGy. Separate detectors were studied for each energy and dose. Calibration and temperature correction factors were provided by the company and were applied as they would be when used in the field as intended. Results: At the calibration dose of 200 cGy/fraction, the average detector response was −0.9% ± 1.8% for the 6 MV beam, and −1.1% ± 2.5% for the 18 MV beam. The maximum error was −5.2% for the 6 MV irradiation and −4.6% for 18 MV. Smaller 6 MV doses of 100 and 150 cGy led to slightly larger positive errors of 2.2% and 3.3%, respectively, while larger doses of 400 and 600 cGy led to slightly larger negative errors of −2.2% and −4.0%, respectively. Conclusion: The response of these new implantable detectors will be presented and discussed in light of appropriate dose level tolerances for in vivo patient dosimetry that can reasonably be achieved at the present time. New developments in hardware and software design have made this system easy to use in a clinical environment. Conflict of Interest: Research sponsored by Sicel Technologies.
SU‐FF‐T‐103: Clinical Electron Beam Characteristics Investigations Using the Monte Carlo Method for Absorbed Dose Determination
Authors: Yoriyaz H , Siqueira P , Poli M , Furnari L , Rubo R , Rodrigues L , Fonseca G
Published: July 2007

Purpose: This work proposes a methodology to build electron beam models capable to accurately represent clinical beams using a simple assumption that it can be represented by a linear combination of monoenergetic beams. Simulations with beam aperture were also performed in order to study the influence of the beam direction on Percentage Depth Dose, PDD. Method and Materials: The representation of a clinical electron beam is made assigning weighting factors to each monoenergetic component. For this purpose, depth dose curves were obtained from the Monte Carlo simulations of a set of monoenergetic beams of various energies from 1 to 21 MeV. We defined a parameter so‐called as LDED, the Limit Distance for Energy Deposition, which assesses the distance from where the energy deposition drops to less than 1 % of the maximum value. The weighting factors are then estimated focusing on the behavior of each of these parameters as a function of beam energy. Results: The PDD close to the phantom surface up to dmax (depth of maximum dose) is not affected by the variation of beam aperture, μ, but from this point on its influence becomes very strong. The values of μ adopted were 0.997 and 0.995 for 9 and 15 MeV, respectively. Comparisons between calculated and measured PDD show discrepancies less than 1.3% and 2.8% respectively for 9 and 15 MeV in the build‐up region. Twenty million electron histories were simulated to achieve maximum standard deviations of 0.6% and 0.7%. Conclusion: This work demonstrates that a simple beam model based on a linear combination of monoenergetic beams and trial and error method can be used to represent clinical electron beams. PDD can be achieved using homogeneous intensity spectra but this it is not adequate for the beam profile, requiring heterogeneous intensity spectrum to reproduce measured data.
SU‐FF‐T‐106: Clinical Implementation of Varian Enhanced Dynamic Wedges Into the Pinnacle Treatment Planning System: Monte Carlo Dosimetry and Patient‐Specific Quality Assurance Techniques
Authors: Ahmad M , Deng J , Chen Z , Lund M , Kimmett J , Moran M , Nath R
Published: July 2007

Purpose: To present a commissioning study of enhanced dynamic wedges (EDWs) into the Pinnacle3 treatment planning system (TPS) and to evaluate various QA techniques for validating the treatment plans using EDWs. Method and Materials: Dose distribution modeling for EDW fields in the Pinnacle3 TPS is based on a combination of open‐field beam data and the Varian “Golden Segmented Treatment Table” (GSTT) unique to each photon beam. To validate the EDW models, dose profiles of 6− and 10−MV photon beams from a Clinac 2100C/D were measured in water for depths from near‐surface to 31.5 cm for a wide range of field sizes and wedge angles using a linear detector array of 25 energy‐compensated diodes (LDA‐25) to integrate the dose at multiple points in the EDW field for each exposure. The EDW output factors for square fields from 4 to 20 cm wide were measured in solid water using a cylindrical ionization chamber at a depth of 10 cm on central axis. The 6− and 10−MV photon beams emerging from the treatment head of the Clinac 2100C/D were fully simulated using the BEAMnrc and DOSXYZnrc was used to calculate the central‐axis percentage depth doses and dose profiles for the open and dynamically‐wedged fields in a water phantom. Film and MapCHECK data were compared with Pinnacle3 and MC predictions. Results: (1) Pinnacle collapsed‐cone convolution algorithm and MC simulations agreed with the measured EDW dose distributions to an accuracy of better than 3%. (2) Measured EDW output factors used for monitor‐unit calculation in Pinnacle3 TPS agreed with the MC predictions within 1–2%. (3) EDW‐based treatment plans were satisfactorily validated using MapCHECK and chamber measurements. Conclusions: We have successfully modeled EDWs into the Pinnacle3 TPS and demonstrated that MapCHECK and films dosimetry constitute valuable QA tools for validating planned dose distributions resulting from dynamically‐wedged beams.
SU‐FF‐T‐168: Dose Variations From Passively Scattered Proton Beams Due to Changes in Beam Size and Orientation
Authors: Polf J , Harvey M , Beddar A , Smith A
Published: July 2007

Purpose: For passively scattered proton beam delivery, day‐to‐day fluctuations in beam size extracted from the proton accelerator and changes to the orientation of an elliptical beam profile may cause variations in the delivered dose. The purpose of this study was to determine the magnitude of the variations in the dose delivered due to changes in size and orientation of the beam that is injected into the treatment nozzle. Method and Materials: A model of a clinical passive scattering treatment nozzle was developed using the MCNPX Monte Carlo code. This model was based on original manufacturer design documents. Monte Carlo calculations were used to study changes of the beam's in‐air energy distribution and spatial fluence profiles at isocenter, as well as, the central axis depth dose profiles in water resulting from changes in the incident beam size and orientation. Results: No significant changes in central axis dose, the in‐air lateral fluence profile, or the energy distribution at isocenter were observed due to changes in the orientation of the elliptical incident beam resulting from rotation of the treatment gantry. However, our results indicated that the energy profile of the beam hardens with increasing beam size. Also, for large increases in beam size, a decrease in the 90% proximal to 90% distal width of the SOBP was observed. Conclusions: Our findings indicate that the dose distribution is not sensitive to changes in the size and orientation of an incident elliptical proton beam that results from either rotation of the gantry or small day‐to‐day fluctuations of the beam size. However, for large increases (outside of clinical treatment limits) of the incident beam profile, the width of the delivered SOBP decreases.
SU‐FF‐T‐173: Dosimetric Comparison of Three Different Treatment Techniques for Extensive Scalp Lesion Irradiation
Authors: Wojcicka J , Lasher D , Bialkowski M , McAfee S , Fortier G
Published: July 2007

Purpose: Homogeneous scalp irradiation poses technical and dosimetric challenges due to the complex shape of the target. The purpose of this study is to compare conventional electron/photon therapy, photon IMRT, and HDR brachytherapy for treatment of extensive scalp lesions. Method and Materials: A 73‐year‐old man presented with extensive squamous cell carcinoma of the scalp. A custom helmet‐shaped mold was made and the target area delineated. The following three treatment methods were evaluated: a 3D plan using lateral photon/electron fields with moving junctions, an IMRT plan using seven coplanar beams, and a brachytherapy plan with catheters placed on the outer side of the mold. All plans were created for the same biological effective doses of 60 Gy. TLDs were used to verify the accuracy of the analyzed techniques. Coverage factors, dose conformity indexes, global conformity indexes, minimum, maximum and mean doses were presented for the scalp and critical structures including the brain and eyes. Results: Dosimetry parameters were normalized to 95% of prescription dose. Clinical target volume coverage factors in electron/photon, IMRT and HDR techniques were 0.976, 0.998, 0.957, and target conformity indexes were 1.98, 1.67, and 1.38, respectively. The dose gradient across the target was 59–136%, 91–129%, and 58–242% for electron/photon, IMRT, and HDR techniques, respectively. The lowest maximum dose to the eyes was achieved with external beam techniques. HDR produced the lowest maximum dose to brain, while IMRT irradiated the least amount of brain above 45 Gy. The measurements agreed within 5% of the calculated doses. Conclusion: IMRT provided better dose homogeneity and target coverage, and delivered lower doses to normal structures than the other techniques. While dose inhomogeneity in the target region was higher in HDR, this technique produced the most conformal plan. Therefore, our department selected IMRT and HDR for extensive scalp lesion treatment.
SU‐FF‐T‐183: Dosimetric Verification of a Commercial Research Irradiator
Authors: Wiesmeyer M , Schaefer M , Hile M , Stojadinovic S , Williamson J
Published: July 2007

Purpose: To present a collection of simple tests that can be used to verify the dosimetric characteristics of the MDS Nordion Gammacell 40 Extractor Low Dose‐Rate Research Irradiator. Method and Materials: The manufacturer specifies that the irradiator is provided with two 61.1 TBq Cesium 137 sources that together produce a central dose rate of 1.10 Gy/minute ± 15% in its sample tray. In the central 260 mm diameter by 100 mm height region, the specified dose uniformity is ± 7%.

Custom measurement jigs were made to fit inside the irradiator's sample trays and assemblies. Ionization chamber readings were made to determine exposure rate constancy and timer errors. Radiochromic film was used to measure dose and dose rate constancy, the attenuation of manufacturer‐provided attenuators, depth dose characteristics in Solid Water™, and uniformity of dose across the sample tray. Thermoluminescent devices (TLD's) were assessed as an alternate means of measuring dose and dose rate constancy. Results: The exposure rate was constant down to two seconds and timer error was about 1.4 seconds. Film measurements showed that dose reduction for the attenuation filters (33%, 60%, 80%) is within 3% of specifications. Film measurements in a central 10×10 cm area showed dose uniformity within about 2%. Depth doses in Solid Water™ showed a variation of about 10%. Film and TLD's yielded similarly accurate dose measurements and constancy estimates. Conclusion: Our facility's irradiator operates within the manufacturer's specifications except for dose variance with depth which may be somewhat greater than 7%. Although the manufacturer provides basic dosimetric information about each irradiator, it is advantageous to verify and expand this information. As with linacs, irradiators must certainly differ from machine to machine and change over time. It is prudent when doing research using these devices to validate their performance under their conditions of usage.
SU‐FF‐T‐186: Dosimetry in An IMRT Phantom Designed for a Remote Auditing Program
Authors: Han Y , Shin EH , Yoon MK , Lee SB , Ju SG , Lim CI , Park SH , Kang SK , Lah JE , Cho SH , Ibbott G , Ahn YC
Published: July 2007

Purpose: Accurate delivery of the most‐up‐to‐date treatment techniques, such as intensity‐modulated radiation therapy (IMRT), is essential. An anthropomorphic phantom was designed and constructed for a remote‐audit program that allows evaluation of an institution's accuracy for delivering IMRT. The dosimetric characteristics in the phantom were investigated. Method and Materials: The phantom has the shape of a cylinder with one target and three organ‐at‐risks (OARs) inside. The target and OARs were shaped analogous to those of nasopharynx cancer patients. TLD holders were inserted inside the target and the OARs for absolute dose measurements. The phantom allows measurements with ion chamber (IC) at the TLD locations, so that inter‐comparison between two dosimeters was possible. For the measurement of relative dose distribution, two film slots were orthogonally placed near the center of the phantom, which also enables inserting inhomogeneities near the target. Measurements with TLDs and ICs were done for four cases. The first was an anterior one‐port 6MV X‐ray (Primus, Siemens, USA) delivery; the second used the same beam geometrically, but with inhomogeneities inserted, and in the third case three ports of beams were distributed in equi‐gantry angle, and the fourth was an IMRT without inhomogeneities. For case 1–3, theoretical predictions were computed by using Monte Carlo code. Results: For anterior one port X‐ray delivery to a homogeneous/ inhomogeneous phantom, the deviation between the IC and TLD measurements, ranged from 1.7% to 2.3%. For three‐port beam delivery, the range was also similar. For IMRT delivery, the differences were 0.1–2.8%. The differences between the TLD measurements and MC predictions ranged from −0.14–1.8%. Conclusion: The TLD measurements in the developed phantom agreed with IC and MC results within acceptable differences. The developed phantom with TLD dosimeters are feasible to be used for remote monitoring of IMRT. Supported by KFDA Grant No 05102eui BangPum and 06112eui BangAn300.
SU‐FF‐T‐188: Dosimetry of Microbeam Radiotherapy Using Gel Dosimeters
Authors: Wong C , Patterson W , Powell C , Qiao G , Solomon D , Geso M
Published: July 2007

Purpose: Microdosimetry (measurement of radiation dose on the μm scale) suitable for micro‐beam radiotherapy is a challenging task with known dosimeters such as ionization chambers and others dosimeters. The purpose of this work was to use gel dosimeters for dose determination across micro‐beam radiotherapy fields. Method and Materials: A gel dosimeter that was highly resistant to radiation was developed by using a free radical scavenger in the gel to reduce the rate of polymerisation. The samples were irradiated using the x‐ray collimated micro‐beam from a synchrotron in order to measure the synchrotron's high‐dose. Gel scanning was performed using special type MRI scanner, Raman spectroscopy and also attempted using phase contrast imaging. Results: It was found that using current equipment and techniques, that MRI with resolution down to 20 μm is possible, but given the size of the beam (30 μm), is unable to adequately describe the dose distribution across the beam. Raman spectroscopy measurements of gel have been shown to be possible, and work in this area is involved in gaining accurate measurements in this method. Comparative measurements made using CCD camera on the beams show similar results. Conclusions: This work shows the dose measurement in gel down to the μm range is possible and could be applied in many fields such as microbeam radiotherapy.
SU‐FF‐T‐191: Dual Window Gated Beam Delivery for Stereotactic Body Radiotherapy
Authors: Yi B , Lerma F , Han‐Oh Y , Yu C
Published: July 2007

Purpose: It is crucial to have a small margin for stereotactic body radiotherapy (SBRT). Gated delivery is one of the methods to reduce intra‐treatment organ motion. Total treatment time increases 3 or 5 times due to low duty cycle (20 – 30 %) when gated therapy. We have designed and tested the dual window gated therapy (DWGT) to increase duty cycle while to keep the residual motion unchanged. Method and Materials: The DWGT turns beam on when breathing phase reaches the end of exhale (EE) or the end of inhale (EI), which will increase duty cycle up to 2 time than conventional gating. In order to make DWGT possible, a dynamic MLC file that has two static apertures to irradiate each gating position is generated. Dose rate is modulated to compensate the difference between gating MLC sequence and the breathing phase caused from breathing frequency variation during treatment. DWGT was tested with three breathing functions, the sinusoidal, the 4th order cosine and a patient's breathing signal (RPM, Varian). All the breathing signals used in this study have ± 20% breathing frequency variations during treatment. Results: Duty cycles which are to keep the residual motion less than 20% of the amplitude for the sinusoidal, the 4th order cosine and a patient's breathing signal were 20.7%, 46.8% and 38.3%, respectively for conventional gating. Duty cycles increased to 67.2%, 18.6% and 52.0%, respectively, which is 85%, 44% and 36% increasing effect from conventional gating when with DWGT. Little residual motion increase (less than 0.8 mm) for a patient's breathing signal, have been found between conventional gating and DWGT. Conclusions: DWGT increased the duty cycle to 1.36–1.85 times while to keep minimal residual motion increasing effect. Since the duty cycle increase effect is breathing pattern dependent, the DWTG is useful for the selected SBRT cases.
SU‐FF‐T‐195: Effect of Respiratory Gating On the Dose Distributions of Dynamic Wedge and IMRT Treatment Fields
Authors: Lindsay P , Briere T , Balter P , Sadagopan R , Starkschall G , Zhu X , Beddar S
Published: July 2007

Purpose: To determine the magnitude of error introduced into treatment delivery due to timing effects between respiratory‐gated delivery and dynamic beam delivery (enhanced dynamic wedge (EDW) and step‐and‐shoot intensity modulated radiation therapy (IMRT)). Method and Materials: EDW and IMRT fields were delivered on a linear accelerator (Trilogy, Varian Medical Systems). Gating of the beam was achieved using a commercial respiratory monitoring system (RPM, Varian Medical Systems), and respiratory motion was simulated using a commercial respiratory motion phantom (RPM phantom, Varian Medical Systems). All fields were delivered with no gating, a 1‐mm gating window, and a 2‐mm gating window.

Film and ion chamber measurements were made for both EDW and IMRT fields in a water‐equivalent IMRT QA phantom, which was stationary during all measurements. Film measurements used EDR‐2 film, while ion chamber measurements were made with a 0.04 cc volume pin‐point chamber. Leakage current was subtracted from the ion chamber readings.

Wedged fields of 15 and 60 degrees were delivered at a dose rate of 600 MU/min. Results were evaluated for the full IMRT plan and for one of the individual beams, at dose rates of 400 and 600 MU/min. Results: All ion chamber measurements were analyzed as percent difference between the gated (1‐mm or 2‐mm gating window) and non‐gated delivery. All differences were less than 1%. There were no significant differences between 1‐mm and 2‐mm gated delivery. Film results, as analyzed from isodose curves and the gamma metric, showed no substantial differences between gated and non‐gated beam delivery. Conclusion: The use of gated treatment delivery of EDW and step‐and‐shoot IMRT fields does not introduce clinically significant differences into the resulting dose distributions, as quantified by film and ion chamber measurements.
SU‐FF‐T‐196: Effect of Stranded and Loose Seeds in Relation to Prostate Edema in Permanent I‐125 Prostate Brachytherapy
Authors: Beiki‐Ardakani A , Crook J , Borg J , Elantholiparameswaran S , Yeung I , Landon A
Published: July 2007

Objectives: The dosimetric evaluation of permanent I‐125 seed prostate implant is often based on the assumption that prostate is static throughout the treatment time which lasts for months. However, the prostate in general becomes edematous due to the surgical procedure, followed by resolution of edema post implantation. Such changes in prostate volume would have an impact on the dosimetry. The aim of this study is to investigate the effect of use of stranded and loose seeds on prostate swelling. Methods: Thirty‐six prostate patients were treated with I‐125 seed brachytherapy. Nineteen of them were treated with stranded seeds while 17 were with loose seeds. Pretreatment prostate volume was measured with transrectal ultrasound and MRI was used to quantify the post‐implant volume at day 1, 8, and 30. Results: The average percentage swelling on day 1, 8, and 30 was the same for both cohorts. Edema half‐life was found to be 15 days for the loose seed and 10 days for the stranded seed patients respectively. When the data were stratified based on the pre‐implant volume < 35 cc and >= 35 cc percentage swelling was found to be significantly higher (44.8 vs. 26.2, p=0.013) only on day one for the loose seed group compared to the stranded seed for pre‐implant volume <35 cc. No significant difference in swelling with respect to seed type was found for large prostate (V >=35 cc). Conclusions: This study shows that for small pre‐implant prostate volume (V<35cc), stranded seeds show significant reduction in amount of swelling relative to the loose seeds on day one of implant.
SU‐FF‐T‐205: Evaluation of a Commercial OSL System for Dosimetry Measurements in Radiotherapy Beams
Authors: Marin A Viamonte , da Rosa L Ribeiro , Buckley L , Cherpak A , Cygler J
Published: July 2007

Purpose: To evaluate feasibility of using a commercial OSL reader for in‐vivo dosimetry in radiotherapy. Method and Materials: A commercial OSL system developed by Landauer Inc., the InLight microstar reader, was tested for dosimetry procedures in radiotherapy. The system uses in the form of “dots” as the radiation detector material. Experiments were carried out in , 6, 10 and 18 MV photon beams. A solid water phantom, 30×30 cm, with a special groove to accommodate the dot detector was used. The OSL dots were calibrated against an NE2571 ionization chamber. Some of the measurements were compared with data acquired using other radiation detectors, such as ionization chambers, diodes and MOSFETs. Results: The detector response to accumulated dose is linear from 20 to 400 cGy. The fading of the detector signal was less than 2% over a one month period. The detector response is independent of energy between 6 and 18 MV however is higher by about 2% in a beam. A PDD curve for was measured in a solid water phantom for 10×10 cm2 field size at SSD=80cm. The data were compared with the ionization chamber measurements in the same phantom and with diode measurements in water and found to be in very good agreement. The relative output factors were measured for field sizes ranging from 5×5 to 25×25 cm2 and are in very good agreement with the measurements performed using an ionization chamber. Conclusion: OSL results agree well with ionization chamber, diode and MOSFET measurements carried out under the same conditions, indicating suitability of the Landauer OSL system for radiotherapy dosimetry. Conflict of Interest: Project was supported by IAEA contracts CRP E2.40.14, CAN 13112 and BRA/13111. We gratefully acknowledge Landauer for providing us with OSL system and detectors for the duration of this project.
SU‐FF‐T‐206: Evaluation of a Computed Radiography System for Verification of Intensity Modulated Radiation Therapy
Authors: Guo P , Luo C , Harrington J , Murray J , Lee S
Published: July 2007

Purpose: There has been great interest recently in replacing conventional film with computed radiography (CR) systems. However, the study of its dosimetric applications has been limited. This work presents a detailed evaluation of its performance in quality assurance (QA) for intensity modulated radiation therapy (IMRT). Method and Materials: A CR system comprised of a Kodak ACR‐2000i laser scanner, an Agfa CR plate, and a PC was used for IMRT verification. Both the scanner and the CR plate were calibrated by following the Kodak‐provided instructions. A dose calibration curve for the CR plate was generated in the dose range of 0.5cGy to 400cGy. In the IMRT verification experiment, the CR plate with 6cm solid water build‐up was irradiated to a clinical 6‐field IMRT treatment plan (Varian Eclipse) of 6MV photons. Gantry angles were set to 0° for the dose delivery. The CR plate was scanned right after irradiation. An independent EDR2 film measurement of the delivered dose was also conducted. A 3‐way comparison of dose information between the Eclipse calculations, film measurements, and CR measurements was performed. Dose maps, profiles, isodose lines, and Gamma maps were utilized for analysis. Results: The CR results show excellent agreement with both Eclipse calculations and film measurements within the 3% dose difference and 3mm distance to agreement criteria except in penumbra regions or just outside field edges. CR and film measurements agree with each other better than with Eclipse calculations in the regions outside field edges, indicating possible limitations of calculation algorithms in those regions. Conclusion: The CR system can provide comparable accuracy in relative dose measurements to conventional film. The CR system, having the advantages of digital nature, environmental cleanliness, reusability, and high convenience, is a feasible and reliable tool for routine IMRT QA.
SU‐FF‐T‐208: Evaluation of a New Radiochromic Film/Flatbed Scanner System for Planar Clinical Dosimetry
Authors: Goss M , Oldham M
Published: July 2007

Purpose: To assess the accuracy and practicality of using GAFCHROMIC® EBT Film in combination with an Epson flat‐bed scanner for the verification of advanced treatment techniques in radiation therapy. The combination of EBT with flat‐bed scanner may represent a low cost and convenient alternative to currently used dosimetry systems. Methods and Materials: First the scanner temporal stability and variability of light source over the scanning area was investigated. Next the dependence on color bit‐depth was investigated by conducting scans of the same film using different bit‐depth parameters. The dose response and stability of EBT film was investigated in the range 0 to 8Gy with 6MV photon beams, and a depth‐dose comparison was made using 16× electrons to known ion chamber measurements. Intra and inter‐batch film reproducibility was investigated as well as dependence of OD on temperature at irradiation over a range of 10°C. Results: The reproducibility of the Epson scanner was found to be highly stable, to within 1.01%, over all ranges of OD studied. A slight non‐uniformity in background was observed, but this background was consistent enabling efficient correction. 48‐bit color depth was adopted for all experiments. The dose response curve of EBT film was found to be non‐linear but stable (within 1.06%) from immediately after irradiation to within one week after irradiation. The electron PDD curve showed excellent agreement with clinical PDD tables at all depths with the linac data. A maximum intra‐batch OD difference of 1.88% was observed, while a maximum OD difference of 5.76% was observed for samples from the same batch. The temperature sensitivity was found to be small, with maximum difference of 1.03% in optical density over the range of 10°C. Conclusions: The Epson® 4990 Scanner/ GAFCHROMIC® EBT Film dosimetry combination appears an accurate and convenient dosimetry system for radiation therapy.
SU‐FF‐T‐214: Evaluation of Heterogeneity Effect in Intra‐Operative HDR (IOHDR) Brachytherapy Dose Calculation Using Monte Carlo Simulation and GAFCHROMIC EBT Film Measurement
Authors: Le Y , Armour E , Wong J
Published: July 2007

Purpose: Investigate the heterogeneity effect due to air cavity and lead sheets in intro‐operative High‐Dose‐Rate (IOHDR) brachytherapy dose calculation using Monte Carlo (MC) simulation and GAFCHROMIC EBT film measurement. Method and Materials: The organs‐at‐risk (OARs) in IOHDR treatment are usually pushed away from applicators and covered by lead sheets. The dose estimations to OARs are affected by local heterogeneities like air gap and lead sheets and difficult to calculate by conventional treatment planning systems, where infinitely homogeneous water environment is assumed. This study used both MC simulation and Gafchromic film measurement to access the heterogeneity effect of dose calculations in a phantom setting. Nine 5×5cm2 sloid water slabs, each 2mm thick, were put on a 30×30×20cm3 solid water phantom. Gafchromic EBT films were place at 0, 4, 8 12 mm depth between the slabs. The Ir‐192HDR source was place 19 mm above the slabs in the air. 0–3 lead sheets, each 0.9mm thick, were placed directly on top of the slabs. MC Photo Transport code (PTRAN) was used to simulate doses at each film positions and compared with film measurements. Results: The MC simulation agrees with film measurements within 5% except data points directly under lead sheets where dose enhancement by secondary electrons was not simulated in PTRAN. The heterogeneity correction factor (HCF), defined as dose ratio of with and without lead sheets at surface of the slabs is 147%, 121% and 96%, for 1–3 lead sheets respectively. At 4–12mm depth in water slabs, the HCFs are approximately 81%, 65% and 53% for 1–3 lead sheets. Conclusion: Significant dose enhancement was found directly under lead sheets. HCF data obtained in this study can be used to estimate doses to OARs in IOHDR treatment. The MC simulation was proved to be useful to calculate doses to OARs at complex clinical settings.
SU‐FF‐T‐216: Evaluation of Mid‐Treatment Tumor Motion for Hypo‐Fractionated Lung Radiosurgery Using Hi‐Art TomoTherapy System
Authors: Jang S , Wu H , Sourivong P , Katz S , Rosen L
Published: July 2007

Purpose: To investigate the variation of the daily mid‐treatment positioning setup of the helical TomoTherapy with MV‐CT for hypofractionated lung radiosurgery cases utilizing BodyFix rigid immobilization. Methods and Materials: 54 cases treated by a hypo‐fractionated Image Guided‐Intensity Modulated Radiosurgery technique and immobilized with the BodyFix double vacuum system were analyzed retrospectively to investigate the daily variation of mid‐treatment motion, as measured daily by MV‐CT scanning. Scans were performed prior to treatment as well as at mid‐treatment. In the treated cases, a margin of 7–9mm around the CTV was utilized as a PTV. The BodyFix double vacuum system with a lung compression belt was used to minimize breathing motion and intra‐fractional tumor motion. Patients were treated daily by two fractions (e.g., 12Gy/day=6Gy/fr×2fr immediately sequentially delivered after MV‐CT guidance). Results: For all cases, multiple MV‐CT scans were performed immediately before treatment to reach optimal treatment position. After image registration and subsequent adjustment, treatment was delivered. A mid‐treatment MV‐CT scan was obtained immediately after the first‐half treatment and then table adjustments were compared. Table adjustment for mid‐treatment was longitudinal: 1.3±0.7mm and vertical: 0.9±0.5mm. The lateral recorded adjustments of 0.1±0.3mm could not be accurately determined as movements of <2mm were often omitted. It is found that slow MV‐CT scan results in a blurred target image essentially negating the effects of respiratory cycle. These results showed that the treatment margin of 7–9mm was adequate to cover the target as verified by mid‐treatment scan and that the BodyFix immobilization system provided clinically acceptable tolerance for lung radiosurgery. There was no significant difference in couch adjustment between upper and mid/lower lobe tumor locations, which may result from the robust immobilization tool. Conclusions: MV‐CT guided lung radio‐surgery using Hi‐Art TomoTherapy and BodyFix double vacuum system was clinically acceptable by minimizing the variation of patient position during the treatment.
SU‐FF‐T‐218: Evaluation of the Exradin #a19 Ion Chamber for Reference Dosimetry in Megavoltage Photon Beams
Authors: McEwen M
Published: July 2007

Purpose: The Exradin #a19 is a new design of reference‐class 0.6cc ion chamber that externally resembles the NE2571 Farmer chamber but internally is very similar to the well‐known Exradin #a12. This project involved the characterization of a single version of this chamber in high energy x‐rays. Method and Materials: The Exradin #a19 was compared against primary and reference standard ion chamber chambers at the National Research Council of Canada in a range of megavoltage photon beams (6–25 MV). Investigations looked at chamber settling, ion recombination and polarity, and determination of experimental kQ factors. Results: Chamber settling — the chamber response stabilizes very quickly (within 5 minutes), even after a large change in the polarizing voltage.

i) The polarity correction was measured for 6 MV and 25 MV beams and found to be small (within 0.15% of unity).

ii) The recombination correction showed a linear variation with the dose‐per‐pulse. The correction was similar to that of other Farmer‐type chambers.

iii) Beam quality conversion factors were found to be closer to that of the NE2571 than the #a12, which is not consistent with the guidance given in TG‐51. However, with only a single chamber under investigation one cannot draw any definitive conclusions.

Conclusion: An initial investigation of the Exradin #a19 Farmer‐type ion chamber suggests that it is fit for purpose for the calibration of megavoltage photon beams. Further work is in progress to evaluate this chamber in kilovoltage x‐ray and high‐energy electron beams.
SU‐FF‐T‐219: Evaluation of the Long‐Term Stability for the Cylindrical Ionization Chambers
Authors: Lah J , Lim C , Suh T
Published: July 2007

Purpose: To analyze the long‐term stability of Farmer‐type cylindrical ionization chambers using the calibration factor provided by the KFDA (Korea Food and Drug Administration). KFDA is the national secondary standard laboratory (SSDL) in Korea and approve as a member of the IAEA SSDL network. Materials and Methods: The cylindrical ionization chambers used in this study include PTW 30001(30006), 30013, 30002, 30004, 23333, Capintec PR06C, NE 2571, Exradin #a12 and Wellhofer FC65G(IC70). The Nk and ND,W calibration factors for the cylindrical chambers were analyzed and the measured ND,W was compared with the calculated ND,W calibration factor. Results: The long‐term stability of PTW 30013(30006), Wellhofer FC65G(IC70) and NE 2571 varied within 0.2 %, meaning that they are more stable than other ionization chambers. This result is different from previous ones showing that the long term stability of ionization chambers with graphite wall material was superior to ionization chambers made of PMMA material. It turns out that almost the same levels of the stability of calibration factor were shown between PTW 30013(30006) ionization chamber made of PMMA wall material and Wellhofer FC65G(IC70) ionization chambers made of graphite material, and thus the former is shown to be superior from the aspect of long‐term safety, and PMMA wall material is firmer, more convenient to use, and it is more effective from the financial aspect. Therefore, it may be considered to be more appropriate to use for the regular dose measurements in comparison with graphite type ionization chamber. The measured ND,W calibration factor was approximately 1.0% higher than the calculated ND,W that was determined using the Nk calibration factor. Conclusion: The long‐term stability of the cylindrical chambers was evaluated using the Nk and ND,W calibration factor. It should help to improve clinical electron dosimetry in radiotherapy.
SU‐FF‐T‐224: Experimental Evaluation of a Robust Optimization Method for IMRT of Moving Targets
Authors: Vrancic C , Trofimov A , Chan T , Sharp G , Bortfeld T
Published: July 2007

Purpose: Tumor motion during irradiation reduces target coverage and increases dose to healthy tissues. The standard clinical approaches using margins are overly conservative because they only take into account the maximum amplitude of the movement. Comparable coverage and reduced dose to healthy organs appears achievable with robust motion‐adaptive treatment planning, based on the nominal expected probability distribution of target position, and considering the uncertainty of its realization during treatment. We test a robust approach to IMRT [Chan et al. Phys Med Biol 51:2567–2583 (2006)] using patient data, to evaluate the effect of target motion variability and the MLC motion interplay on the outcome. Method and Materials: We built our robust framework using external marker motion data (traces), acquired during patient treatments. These included 251 traces from two lung, one cardiac and one liver patient. Long‐term shifts (such as exhale baseline drift) were removed from the data by band pass filtering. IMRT plans with varying grades of robustness were tested on 52 traces of two patients. A computer‐controlled motion phantom reproduced traces during plan delivery on a linac. The dose was measured with a two dimensional detector array consisting of 1020 chambers with a resolution of 7.6 mm. Results: Although tumor coverage differed for each delivery fraction due to MLC interplay and motion variability, it converged to the expected level after multiple fractions. For equivalent target coverage, 17mm peak‐to‐peak motion, the integral dose was reduced by 15% compared to the margin solution. Conclusions: Dosimetric tests largely confirmed that the robust motion‐adaptive approach to IMRT planning allows one to better manage the effect of motion, and can be extended to any uncertainty described by probability density functions. With target monitoring during the treatments, motion‐adaptive plans and the underlying robust framework could be verified and re‐adjusted if necessary.
SU‐FF‐T‐225: Experimental Observations of Convection in An Ultrasound Thermometer for Measuring Absorbed Dose to Water
Authors: Chen‐Mayer H , Tosh R , Malyarenko E
Published: July 2007

Purpose: To create, measure, and attempt to characterize convection effects experimentally in a water phantom and cell in terms of simple physical parameters, in order to obtain accurate calorimetric measurements of absorbed dose in both Domen‐type devices and especially the ultrasonic calorimeter under evaluation at NIST. Method and Materials: Experiments have been performed to investigate radiation induced convection in an open water phantom equipped with a bare thermistor and an ultrasound transducer in addition to using thermsitors in a sealed core; and to investigate convection threshold by using one thermistor as a point heating source. We employ both impulse‐response testing and continuous square‐wave excitation, and analyze the data in frequency domain in an attempt to decompose the responses due to conduction and convection. Results: Single shot, impulse response testing shows periodic ringing following after the response to the initial excitation has faded, observable for both thermistor and ultrasound sensors. The period for this ringing can be estimate via Rayleigh formula. The Fourier series expansion of the square wave excitation dictates that no even harmonics should be present; however, this is violated at elevated radiation durations, strongly suggest the presence of convection. This is experimentally observed in both thermistor and ultrasound detections, but suppressed when the thermistors are within the glass vessel. To understand the onset of convection, the point source experiment establishes a threshold for convection with increasing power. Conclusions: Using the frequency domain technique, we have observed experimentally the distinction of system response between convection and conduction. Both impulse‐response and stationary spectra indicate large‐scale oscillations of water in phantom that are likely to pose more of a problem for ultrasound than for thermistors with convection barrier. Once the behavior of convection is understood, one may be able to use spectral methods to remove its effects by data processing.
SU‐FF‐T‐227: Extra‐Focal Analytical Source Model for the Radiation Output From a Linear Accelerator
Authors: Torsti T , Tillikainen L
Published: July 2007

Purpose: To describe in detail the recently improved extra‐focal photon source component of an accurate analytical source model for the radiation output from a linear accelerator. Method and Materials: In the Anisotropic Analytical Algorithm used in the Eclipse™ Integrated Treatment Planning System (Varian Medical Systems Inc.) the radiation output from a linear accelerator is modeled using a multiple‐source model with separate sub‐sources for primary photon radiation, extra‐focal photon radiation and electron contamination. Closely related to this, analytical models for accessories such as hard wedges and dynamic wedges are included. We describe in detail the extra‐focal photon source component. A recent improvement to the extra‐focal source leading to enhanced accuracy in cases of small MLC apertures that are common during IMRT treatments is discussed. Another related improvement significantly improves the modeling of the Varian Enhanced Dynamic Wedge (EDW). Results: The new extra‐focal source model significantly improves the modeling of small MLC apertures that occur during dynamic IMRT treatments. In our test case to water phantom the maximum error (disregarding the buildup region) in depth dose curves has been reduced to 1% of dose maximum. Modeling of the Varian Enhanced Dynamic Wedge has been significantly improved also: in our example dataset the worst case deviation of local value at CAX has been reduced by more than 50%. Conclusion: The analytical source and accessory models used in the Anisotropic Analytical Algorithm in combination with a scheme to optimize the free parameters of the model has been already shown to be clinically acceptable. In this work we discuss two improvements related to the extra‐focal source model which further improve its accuracy. Conflict of Interest: The authors are employed by Varian Medical Systems Finland Oy.
SU‐FF‐T‐229: Fast Dose Calculation with Superposition Algorithm Using a Scaling Method
Authors: Fan Y , Nath R
Published: July 2007

Purpose: To speed up the scatter dose calculation procedure in a superposition algorithm by using a scaling method where the superposition of scatter contributions can be carried out through instead of all sites in the medium but randomly selected scatter sites. Method and Materials: The scatter dose in a heterogeneous medium was obtained by Monte Carlo simulated scatter dose in water multiplied by the ratio of the scatter dose in the heterogeneous medium to the scatter dose in water calculated with superposition algorithm. Because this ratio is the scaling of the dose calculation in the medium by the same calculation in water the superposition could be carried out through in stead of all scatter sites but randomly selected, sufficiently enough numbers of scatter sites representing the structure of the medium. The 3‐D dose distributions for heterogeneous structures in a spherical water phantom such as spherical lung or bone materials were calculated for a point photon source of energies of 30 keV, 100 keV and 400 keV. Results: For the phantom considered here, the reduction of about 106 scatter sites to 104 random scatter sites speeds up the dose calculation by a factor of 100 while keeping the fluctuation caused by the random selection within 5%. Conclusion: Dose calculation with a superposition algorithm can be significantly speeded up by this scaling method with randomly selected scatter sites.
SU‐FF‐T‐230: Feasability of a Treatment Nozzle Without a Range Modulator Wheel Using the Passive Scattering Technique
Authors: Harvey MC , Polf J , Mohan R
Published: July 2007

Purpose: The purpose of this work was to study the feasibility of producing a spread out Bragg peak (SOBP) without a range modulation wheel (RMW) using the passive scattering beam delivery technique. Method and Materials: Monte Carlo simulations of a passive scattering treatment nozzle were performed using the MCNPX Monte Carlo code. The model was developed using manufacturer design data and calculations were validated against measurements. In this work, the RMW was replaced with a fixed thickness, non‐rotating scatterer (RMW‐free configuration). For the RMW‐free method, SOBPs are produced by changing the relevant initial energy injected into the nozzle and weighting each individual energy Bragg peak by an original energy‐dependent weighting factor. The depth and cross‐field dose profiles in water were calculated for incident beam energies from 100–250 MeV. The RMW‐free SOBP results were successfully compared to both simulated and measured data. Results: SOBPs and cross‐field dose profiles calculated with the RMW‐free technique were found to be analogous to calculations and measurements from the standard treatment nozzle, with RMW‐free to standard‐RMW ratios of approximately unity for all calculated profiles. In addition, calculations showed that RMW‐free method produced a dose rate increase in water and also a reduction in the in‐air secondary neutron fluence at isocenter. Conclusion: Our results indicate that SOBPs of various widths can be produced without an RMW using only a thin fixed scatterer in its place. Comparable depth dose curves and cross‐field profiles were calculated with the RMW‐free method using a newly developed energy‐dependent weighting function based on the range of protons in water. Based on this preliminary evaluation, we believe that this method could result in an increase in the treatment dose rate and also a significant reduction in secondary neutron exposures to the patient.
SU‐FF‐T‐242: High‐Throughput Computing in Condor Using a Nested Partitions Framework for IMRT Beam and Dose Optimization
Authors: Zhang H , D'Souza W , Shi L , Meyer R
Published: July 2007

Purpose: To utilize a High Throughput Computing (HTC) system to provide distributed computing on a network of computer workstations for a metaheuristic, Nested Partitions (NP), for beam angle selection (BAS) in IMRT. Method and Materials: An important feature of the NP approach is that its partition/sampling processes are naturally parallelized, allowing the integration of this powerful optimization methodology with high‐performance distributed environments such as the Condor system. Condor is a specialized workload management system that can used on existing computer networks for compute‐intensive jobs, allowing the submission of many jobs at the same time, and performing these jobs by locating and utilizing idle computers in the network. Our algorithm first solves an integer programming formulation of the BAS problem that uses aggregate mean organ‐at‐risk data. Based on this initial angle set, NP uses sampling on a partition of the global solution space to generate alternative angle sets that are distributed via Condor for evaluation via dose optimization. The corresponding dose optimization problems for the beam set samples are thus solved in parallel. NP repeats this procedure iteratively to obtain high quality angle sets for IMRT. Results: Compared to evaluating each beam angle set sequentially, parallel evaluation of the NP‐selected beam angle sets 2457_2via Condor on 15 workstations yielded a factor of 14 speedup. Overall, BAS for a difficult clinical case in this setting required 15–28 minutes. Using a weighted sum of percentage violations of DVH thresholds for OARs as an overall measure of solution quality, a 78% improvement (head‐and‐neck case) and a 30% improvement (pelvic case) were achieved over equally‐spaced beams. Conclusions: High‐quality beam angle sets for IMRT can be efficiently obtained in an automated manner by utilizing an HTC system on an existing computer network to implement an NP‐based metaheuristic. Conflict of Interest: N/A.