bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐10: Experimental Measurement of Attenuation Factor of a New Type of Shielding Material for a HDR Ir‐192 Source
Authors: Narra V , Haffty B , Khan A , Gabel M , Goyal S , Yue N
Published: July 2008

Purpose: A relatively new type of material, Ledite has been introduced as radiation shielding material for both high dose rate (HDR) brachytherapy and mega voltage external beam treatment rooms. Currently, limited attenuation information is available for this type of material for radionuclides, especially for HDR source such as Ir‐192. Since many of the radiation sources, including Ir‐192 HDR source and Linac beams, are not mono‐energetic and the material is supplied in the form of block, the shielding calculations based on provided TVL value may potentially lead to errors. This study was undertaken to experimentally measure the attenuation parameters of Ledite blocks for an Ir‐192 high dose rate source to ensure adequate shielding design for a HDR treatment room. Method and Materials: Ledite blocks are supplied in dimension of 6 × 6 × 12 inches. A farmer type ionization chamber and a HDR Ir‐192 source (VariSource, Varian Medical Systems, CA) were used in the measurement. The measurements were conducted for thickness of 6 inches. In the measurement, the distance from the source to the chamber was kept the same for the situations of with and without the blocks placed in between, to avoid the inverse‐square effects. The source and chamber were placed at certain distances away from the blocks to account for, to a certain degree, the contributions from both primary and scatter photons. Results: The measurements showed that the attenuation factor for block of thickness of 6 was 0.024. These measured attenuation value was higher than those calculated based on the provided tenth value layer, indicating potential overestimation of radiation attenuation with the provided tenth value layer. Conclusion: Attenuation factor was measured for one type of Ledite Block. It was found that the provided TVL may overestimate radiation attenuation for the HDR source. More investigation is warranted.
SU‐GG‐T‐129: A High Resolution Fluence‐Convolution Model for Prediction of Amorphous Silicon EPID IMRT Images
Authors: Greer P , Cadman P , Bzdusek K
Published: July 2008

In this work an amorphous silicon electronic portal imaging device (a‐Si EPID) dose prediction model based on the fluence model of the Pinnacle treatment planning system Version 7 (Philips Medical Systems, Madison, WI, USA) is developed. A fluence matrix at very high resolution (0.5 mm) is used to incorporate multileaf collimator (MLC) leaf transmission effects in the predicted EPID images. The primary dose deposited in the EPID is calculated from the fluence using experimentally derived radially dependent EPID interaction coefficients for the open and MLC transmitted fluence components. A spatially invariant EPID dose deposition kernel that describes both radiative dose deposition and optical scatter is convolved with the primary dose. The kernel is further optimised to give accurate EPID scatter factor with changing MLC field size. Model predictions were compared to a‐Si EPID images corrected for pixel sensitivity variation, support‐arm backscatter and calibrated to dose for various static jaw defined and MLC defined fields and a step and shoot intensity modulated radiotherapy (IMRT) field. For the static fields the model predicts EPID off‐axis ratio, penumbral shape as well as inter‐leaf leakage. For the IMRT field with a Gamma criteria of 2% and 2 mm, 97.2% of points had a Gamma index less than 1. We found that incorporating the difference in EPID response to open and MLC transmission did not improve the accuracy of the prediction for the IMRT field. The developed model incorporates the effects of MLC design on the dose and therefore should improve the verification of IMRT treatments with EPIDs.
SU‐GG‐T‐135: Evaluation of PRESAGE/optical‐CT 3D Dosimetry for Commissioning a Linac for IMRT
Authors: Oldham M , Sterling D , Sakhalkar H , Molineu A , Adamovics J , Ibbott G
Published: July 2008

Purpose: PRESAGE is a novel radiochromic plastic which, in combination with optical‐CT imaging, has been shown to have outstanding potential for high‐resolution three‐dimensional (3D) dosimetry. Here, the first application of PRESAGE/optical‐CT to commissioning a new accelerator for IMRT treatment.is presented. Commissioning was achieved using a modified Radiological Physics Center (RPC) IMRT credentialing phantom that enabled 3D dosimetry. Method and Materials: The RPC phantom was CT scanned twice, first with the standard insert and second with the modified PRESAGE insert. A 9‐field IMRT plan was created in the Eclipse planning system to meet the credentialing requirements on the standard CT‐scan, and to deliver 6.6Gy to the primary PTV. This plan was then recalculated on the second CT scan incorporating the PRESAGE insert. In the latter case, the prescription dose was reduced to 4Gy to avoid over‐exposing the dosimeter. Consistent relative fluence between the plans was verified by MapCHECK and DynaLog file analysis. After irradiation, evaluation of the PRESAGE/optical‐CT system was performed by comparison with the independent film and TLD measurements made at the RPC. Profile comparisons and gamma maps were generated to compare both sets of measurements with the eclipse planning dose distribution. Results: An improved acquisition technique led to high quality low noise 3D data (<1%rms) achieved using the PRESAGE/optical‐CT system. Excellent agreement was observed (gamma criteria of 3%, 2mm) between the PRESAGE and the Eclipse distributions. Similar agreement was observed between the corresponding EBT and Eclipse distributions. Conclusion: Prior studies from the RPC have shown IMRT commissioning is non‐trivial and prone to error. This work demonstrates successful integration of PRESAGE/optical‐CT dosimetry in the RPC IMRT credentialing phantoms. The high quality of the 3D dosimetry data (noise ∼1%, accurate to within 3%) represents a significant improvement over previous work with PRESAGE, and a new tool for comprehensive verification of IMRT treatments.
SU‐GG‐T‐142: Organ Motional Effect On the Dose Distribution in Intensity Modulated Radiation Therapy
Authors: Song J , Yoon M , Nam T , Ahn S , Chung W , Nah B
Published: July 2008

Purpose: The respiratory organ motional effect on the dosimetric accuracy in IMRT was analyzed and the function of RPM gating system in reducing the organ motional effect was evaluated with the respiration simulating phantom. Method and Materials: The phantom system which can simulate respiratory organ motion was manufactured. Total 4 IMRT plans for liver cancer were evaluated in the 3 cases of situation that were a static status, a no‐gated free‐breathing motion and a gated radiotherapy with RPM system. The motional range was set 1.6 cm considering the real movement of tumor in the fluoroscopic images and respiratory movement cycle was fixed to 5 sec. The dosimetric accuracy of all the fields of IMRT plan was analyzed with MapCHECK (SunNuclear, USA) device laid on the moving table of the phantom. The comparison analysis between a dose distribution in plan and a real one in treatment was done with a coronal dose distribution in the depth of diode detector arrays in the MapCHECK. The decision of pass in each measurement points was verified by a gamma evaluation in the criteria of 3 mm distance and 3% dose. Results: The average pass rates in each motional status are 98.4% at static case, 49.9% at motional case and 97.9% at gated IMRT case. These data shows that the RPM gated radiation therapy decreased a large amount of dosimetric error due to the organ motional effect in IMRT. Conclusion: The considerable dosimetric error due to organ motion was occurred even in the motional range (1.6 cm) of normal breathing condition. This organ motional error could be reduced with a RPM gating system and the gated radiation therapy with a proper gated phase should be performed for the IMRT of abdominal lesions which are moved by a respiration.
SU‐GG‐T‐180: Measurement Instrumentation to Determine RF Noise Generated by a Medical Linac
Authors: Burke B , Lamey M , Rathee S , Fallone B , Carlone M
Published: July 2008

Purpose: The goal of image‐guided radiotherapy is to deliver the planned conformal radiation dose precisely to the target tumour volume and minimize dose to nearby normal tissue. To further this goal, integration of a linear accelerator (LINAC) with an MRI has been proposed. An obstacle to the integration of a LINAC with an MRI is the RF interference between the two devices. Our measurements indicate that LINAC induced RF occurs in the MHz range, with wavelengths on the order of 10s of meters, necessitating the RF measurements to be made in the near field. The purpose of this work is to develop and validate a method of measuring RF field patterns using standard dipole antennas both in near and far fields. Method and Materials: To validate the measurement technique, near and far field electric (E) and magnetic (H) field patterns were measured using specialized E and H field probes from two dipole radiation sources, and compared to the theoretical values. Antennas and probes were mounted in controlled geometry using a specially made wooden stand. The wave impedance (E/H) was also calculated and compared to theory. Results: For E and H fields, angular and radial field strength patterns demonstrate compliance with theory in both the near and far fields. The E field measurements oscillate around the theoretical falloff as a function radial distance, these oscillations are predominantly a result of reflections from the room structures. Our room simulations are consistent with this interpretation. Wave impedance in the near field agrees with theoretical predictions. Conclusion: A technique to measure the RF field strength in the near and far fields has been demonstrated. This technique is suitable for measuring RF field values in the vicinity of a medical LINAC enabling quantification of RF interference between a LINAC and MRI.
SU‐GG‐T‐182: Effects of Block Location On Dosimetry in the Total Body Irradiation (TBI)
Authors: Gao M , Glasgow G , Sethi A , Rusu I , Ke Y , Xu M , Roeske J
Published: July 2008

Purpose: Blocks are frequently used in TBI to protect critical organs such as lung. Among different cancer centers, the location of block varies significantly; for example, in the block tray, between the machine and patient, or directly in front of the patient. In this study, we investigate effects of block location on the dosimetry of blocked region. Method and Materials: Blocks at four source‐block distances were studied, i.e. 65cm (block tray), 120cm, 200cm and 350cm (in front of patient). All blocks were 5 HVL thick and were made based on the same film to protect an area of 12.5×12.5 cm2. Dosimetry of blocked area was measured using an ion chamber and film in a phantom at 10cm depth. All results were normalized to the open field dose. Results: Doses at the center of blocked area for source‐blockdistances of 65, 120, 200, 350 cm are 18.1%, 15.7%, 13.9% and 13.5% respectively. The penumbra of blocked area decreases from 0.9 cm to 0.4 cm as the block moves from the block tray closer to the phantom. For a desired transmission dose of 20% (as a percentage of open field), the effective size of blocked area (i.e. area whose dose is under the desired transmission) is 7.2cm, 10.1cm, 10.9cm and 11.4cm respectively for blocks at 65, 120, 200, 350 cm source‐block distances respectively. For a desired transmission of 30%, the effective size of blocked area is 11.7cm, 11.9cm, 12.1cm and 12.2cm respectively. For a 50 % level, it is 12.3cm, 12.5cm, 12.7cm and 12.7cm respectively. Conclusion: Dose under TBI blocks is significantly higher and the effective size of blocking is significantly smaller when block is located in block tray than directly in front of the patient. Hence, proper positioning of TBI blocks is critical to achieve the desired dose reduction.
SU‐GG‐T‐249: Analysis of Scatter Dose On a Contralateral Breast as a Function of Surface Dose and Treatment Depth
Authors: Rodriguez S , Esquivel C , Papanikolaou N
Published: July 2008

Purpose: To characterize scatter dose, as a function of depth and off‐axis distance, on the contralateral breast (CB), during treatment of ipsilateral breast cancer using 6 and 18MV photon beams, utilizing wedge techniques and IMRT. Method and Materials: A 30×30×30 solid water phantom was utilized to simulate dose delivery to a right ipsilateral breast. Both surface dose measurements and planar dose distributions at depths (d = 1cm and d = 2cm) were analyzed, as well as dose distributions along the transverse axis of the phantom perpendicular to the beam. Data was taken using EDR2 film, thermal‐luminescent dosimeters, and MOSFET detectors. A Varian 2100CD linear accelerator was used. Measurements were performed for the following arrangements: open field (8×18cm2), 30 and 45° physical wedge, 30 and 45° Enhanced Dynamic Wedge (EDW), and control points (CP) used in IMRT. Results: Analysis of the data shows that for the hard wedges used, a significant increase in scatter dose to the CB during treatment by 5–7% and 12–15% for the 6MV and 18MV respectively is present. At a depth of 1 cm (6MV), data indicates that for a position in the CB, scatter dose is approximately 7% of the central axis dose of the open beam. The relative increase of 2% occurred with the use of EDW. No increase in scatter dose was measured for the CP case. Conclusion: This study seems to indicate that it may be possible to quantify scatter dose through a mathematical function, based on beam energy, depth of treatment, and treatment modality. Application of such a function may be of benefit in improving treatment plans and providing guidelines with regard to dose delivery and potential shielding of CB during whole or partial breast radiotherapy.
SU‐GG‐T‐254: Evaluation of Radiochromic Film Within a Heterogeneous Phantom
Authors: Simon T , Kozelka J , Fox C , Simon W , Li J , Palta J , Liu C
Published: July 2008

Purpose: Photon dose calculation algorithms in treatment planning systems (TPS) have reduced dose modeling accuracy in and around heterogeneity interfaces. Monte Carlo (MC) based TPS offers a possible solution to this problem. However, the inherent uncertainty in MC simulations requires results to be validated through experimental measurement. Method and Materials: Slabs of Virtual Water (VW) were arranged to create a 25cm cube with a 5×5×5cm center core of various inserts: VW, urethane, aluminum, or steel. EBT Gafchromic film was placed through the center of each insert. A set of three films was used rather than a single film in an effort to decrease measurement noise. The phantom and film plane was irradiated with an orthogonal beam of 6MV photons. The films were scanned with an Epson V700 using published protocols, and a custom densitometer. The results were compared to Monte Carlo (PENELOPE code) simulations with a cutoff energy of 100eV. Results: With the VW insert, the dose to each film was the same. This was also the case for the urethane film set. However, for the aluminum and steel inserts, variations in absorbed dose within the 3 films of up to 12% were observed. These findings were validated using MC. Conclusion: We created a heterogeneous phantom of VW with various inserts. Films within the inserts of VW and urethane received consistent doses within their sets. Films within the inserts of aluminum and steel received varying doses which were also observed in MC simulations. This variation was attributed to low energy backscatter from the high density insert, and will be difficult to predict using MC simulations with reported photon and electron cutoff energies. Conflict of Interest: Supported in part by NCI Contract No.HHSN261200522014C, and by Sun Nuclear Corporation.
SU‐GG‐T‐257: Modeling of Beam Profiles Based On Three Gaussian Functions in Lung Stereotactic Body Radiotherapy for Acceptance Test of Radiotherapy Planning System
Authors: Anai S , Arimura H , Araki F , Tachibana M , Shioyama Y , Honda H , Nakamura K , Onizuka Y , Terashima H
Published: July 2008

Purpose: The purpose of this study was to estimate beam profiles in lung stereotactic body radiotherapy for acceptance test of radiotherapy planning system (RTP) system. The beam profiles measured by an ionization chamber were approximated by using three Gaussian functions, and compared with profile data calculated by two RTP systems. Method and Materials: X‐ray linear accelerator with 4,6,10MV (Varian 21EX) was used to deliver symmetric beam profile for a field size of 5×5cm2. A lung phantom consisted of a lung equivalent material (thickness : 170 mm) sandwiched by two Solid Waters, whose thickness were 30 mm and 50 mm for anterior and posterior sides, respectively. Measured beam profiles were approximated by manually determining three amplitudes and standard deviations of three Gaussian functions corresponding to three point spread functions of an x‐ray focus, x‐ray or electron scatter, and a detector, and by integrating the composed function. Finally, we evaluated our method by comparing the approximated beam profiles with those calculated by two algorithms in two RTP systems, i.e., Convolution/superposition (CS) (Philips Pinnacle) and analytical anisotropic algorithm (AAA) (Varian Eclipse). Results: Difference between the measured and approximated beam profiles were 4% at 20–80% doses, and 1.5% difference at the other doses. The fringe values (distance between the 50 and 90% levels) of beam profiles approximated by Gaussian functions and calculated by CS and AAA algorithms were 4.7, 6.1, 6.4 mm for 4MV x‐ray, 5.7, 6.9, 7.1 mm for 6MV x‐ray, and 6.8, 7.7, 8.2 mm for 10MV x‐ray, respectively. Conclusion: It was suggested the beam profile model based on the three Gaussian functions may be useful for acceptance test of a RTP system.
SU‐GG‐T‐266: Implementation of Total Skin Electron Irradiation (TSEI) Using High Dose Rate Mode at Siriraj Hospital
Authors: Liammookda P , Liammookda P
Published: July 2008

Purpose: To implement and commissioning of applied Stanford total skin electron irradiation (TSEI) technique using standard linear accelerator at our institute. Method and Materials: Patient was treated in six different upright patient positions using dual angle fields. This study was carried out on Varian 23EX linear accelerator with 6 MeV, 36×36 cm2 field size at collimator in HDTSe mode at a SSD of 400 cm behind a beam degrader. The dosimetric characteristics were verified experimentally using a parallel plate ion chamber, electron diodes, and TLD‐100 chip and rod types. The delivered dose is 36 Gy in 9 weeks. The basic dosimetric parameter consists of beam profile, depth dose curve and electron beam output in the treatment plane. The output and depth dose curve was measured by PTW23343 Markus chamber in solid phantom. Delivered dose from six dual fields technique in treatment position was verified on round phantom by TLD‐100. Results: The uniform vertical profile was obtained for gantry angulations of ± 17.5 degree above and below horizontal within 10% of the prescribed dose. The depth dose curve shows maximum dose at skin surface. The mean energy is 2.09 MeV with beam degrader 1 cm thick. The dose uniformity from six dual fields on round phantom is within 10%. The average measured skin dose in the anterior and posterior at the level of head, chest and pelvis on rando phantom is within 10% of prescribed point. Total X‐ray contamination in the rando phantom is about 1.35% of the delivered dose. Conclusion: The TSEI technique using six dual fields applied Stanford technique produced a uniformity at the whole patient's surface cannot be better than 15% of the prescribed dose because of variable skin distance, self‐shielding, and differences in curvature.
SU‐GG‐T‐267: Evaluation of the Accuracy of TomoTherapy Dose Calculations for Shallow‐Depth PTVs
Authors: Strydhorst J , Clark B , MacPherson M
Published: July 2008

Purpose: To investigate the accuracy of TomoTherapy dose calculations for shallow‐depth PTVs, and to examine the sensitivity of the resulting plans to positional variations. Method and Materials: TomoTherapy treatment plans were created and delivered for a cylindrical phantom with surface‐to‐PTV margins of 0 to 10 mm in 2 mm increments. Dose in the coronal plane was measured using EDR2 film and compared to the dose predicted by the TomoTherapy software. Treatment was also delivered with the phantom intentionally misaligned to investigate the sensitivity of each plan to imperfect patient alignment. Results: A margin of 0mm resulted in an excess dose on the order 15% to the region just below the surface of the phantom. This was consistent with the dose predicted by the planning software. Using a PTV to skin margin of 2mm or more resulted in only a negligible overdose to the phantom. With a margin of 0mm, the dose delivered was very sensitive to misalignment of the phantom. Moving the phantom by 4mm in one direction resulted in a peak dose slightly higher than predicted; moving it 4mm in the other direction substantially reduced the peak dose, nearly eliminating the overdosing altogether. With a margin of 4mm, misalignment had only negligible impact on the maximum dose. Conclusion: The results suggest that when treating breast cancer using tomotherapy, the PTV should be kept at least 2mm back from the surface of the skin to avoid severe dose dumping just below the skin and a maximum dose that changes significantly with only small misalignment of the patient.
SU‐GG‐T‐281: Quality Control in the Fabrication of Scintillating Fiber Probes for Radiotherapy Applications
Authors: Jones J , Briere TM , Archambault L , Beddar AS
Published: July 2008

Purpose: Miniature plastic scintillation detectors (PSDs) and plastic scintillation detector arrays are highly useful in many radiotherapy applications. However, PSDs can show large detector variability between apparently identical detectors. Such variability may impact on the performance of a PSD array. The purpose of this work is to develop a quality control procedure to increase the uniformity of PSDs. Method and Materials: A uniform procedure for fabricating PSDs was established. A quality control bench was then designed, which comprised a precise positioning mechanism, an ultraviolet LED for stimulating the scintillation light in plastic scintillators and a spectrometer to characterize the relative emission spectra of the PSDs as well as their total light output. The reproducibility of the bench itself was evaluated by measurements of a single detector through numerous mounting, un‐mounting cycles. A batch of 15 PSDs was then built and tested for quality. Results: The quality control bench could be used with an average reproducibility of 0.7%. Stimulation of the plastic scintillators by ultraviolet light produced the same emission spectrum as one obtained from ionizing radiation. Quality control measurements for the 15 PSDs showed no major discrepancies in the relative emission spectrum. However, large variations in total light output were observed. The standard deviation of the total light output was 16%, with a maximum variation of 28%. Of all detectors, 13% had a total light output of 0.85 or less relative to the mean. Conclusion: The precision of a plastic scintillation detector will suffer if its total light output is too low. In this work, we have developed a tight quality control procedure for PSDs that could improve the performances of PSDs and PSD arrays. This procedure can be used to prevent detectors with poor light output from being used.

Supported by the NCI (1R01CA120198‐01A2).
SU‐GG‐T‐288: Modeling the Increased Scatter From Tertiary Collimators
Authors: Alaei P , Higgins P
Published: July 2008

Purpose: The tertiary multileaf collimators in certain linear accelerators add to the amount of scatter produced. This scatter is often ignored at the time of commissioning and beam data collection, i.e. the collimator and phantom scatter factors are measured with the MLCs retracted. This increased scatter contributes to skin dose and affects the accuracy of the dosimeter readings when placed on the skin. For majority of conventional radiotherapy portals, the jaws are set to conform to MLCs as closely as possible. In the case of IMRT treatments, the jaw is often set at a fixed size, and the MLCs form the segments, or control points, of each beam. Thus the difference between MLC and jaw settings may be significant. Method and Materials: We have quantified the magnitude of this scatter for various field sizes defined by MLCs and jaws and have characterized the variation of this scatter with MLC and jaw settings. We have measured collimator scatter factors in air for MLC‐defined square fields of different sizes (2 cm2 and up to the jaw size) for jaw settings of 10, 15, 20, 25, 30, and 36 cm2. Results: Our data indicates that the larger the difference between MLC and jaw settings, the greater the difference between the scatter factors and, as the size of the MLC setting approaches that of the jaw, the scatter values converge into a single value at the point of equal MLC/jaw setting. We have also fitted curves to the MLC scatter for any given jaw setting, which can be used for estimation of the increased dose as a function of jaw and MLC sizes. Conclusion: The quantification of increased scatter due to MLCs can potentially be used to get more accurate skin dose estimates in treatment planning and to correct diode readings for IMRT treatments.
SU‐GG‐T‐298: Dose Verification of Mammosite Treatments with MOSFET Dosimeters
Authors: Sabbas A , Nedialkova L , Trichter S , Kulidzhanov F , Hayes M , Parashar B , Patel P , Nori D
Published: July 2008

Purpose: In recent years, accelerated partial breast irradiation using the MammoSite balloon has proven an effective and popular treatment. The selection criteria for patients, based on medical and physical (minimum skin distance) considerations have been well established. Our checklist addresses these criteria, confirms that the selected dwell position is at the center of the balloon and is used to assess its integrity prior to treatment. Recently, we have implemented in‐vivo dosimetry using MOSFET dosimeters to validate our brachytherapy calculations. Method and Materials: Five MOSFETs were calibrated for the HDR source by taping them on the surface of a 4 cm diameter cylinder. Our calibration geometry assured minimum dose gradient, less than 1.5%, within +/− 2 cm from their location. To calculate the dose at selected skin points, five radio‐opaque marks are placed on each patient's skin prior to CT. Before treatment, the five MOSFETs are taped on these skin points of the ipsilateral, contralateral breast and neck. For one patient, the 5th MOSFET was placed at the pacemaker situated 9 cm from the balloon center. At the completion of the treatment the readings are compared to the calculations. Results: There is generally good agreement between calculated versus measured dose for all five dosimeters. The average of the absolute discrepancy between calculation and measurement for the skin dose at the lateral aspect of the treated breast is 5.7+/−4.2 cGy for the 9 patients. Both measurement and calculation yield an average dose of 4 cGy to the contralateral breast. Conclusion: The use of MOSFETs is a convenient and accurate way to obtain skin dose measurements for mammosite cases. In addition to validating the calculations, the readings can provide dosimetry information for clinically significant areas like the thyroid or the pacemaker.
SU‐GG‐T‐300: Depth Dose and Profile Characteristics in the Superficial Buildup Region Near Air Interface for Megavoltage Photon Beams Using Radiochromic EBT Film Stacks
Authors: Chiu‐Tsao S , Chan M
Published: July 2008

Purpose: To measure the depth doses and profiles in the superficial buildup region for 6MV and 15MV photon beams using radiochromic EBT film stacks. Method and Materials: Eight radiochromic EBT film (lot♯35076) strips (3×20×0.024cm3) formed a stack (0.192cm). Each film stack was positioned above a solid phantom, with the top film layer at 100cm SSD, and surrounded by solid water slabs (0.2cm). The film stack was along the central cross plane of 10×10cm2 open field. This setup allowed dose measurement at depths from 0.012cm to 0.18cm at 0.024cm intervals. Each film stack was irradiated by 6MV or 15MV photon beam (Varian Cl‐iX) with 1000 MU. Four repetitive runs were performed for each energy modality. All films were scanned using Epson4870 flatbed scanner with 48bit color and 0.0169cm pixel resolution. The pixel values were converted to doses using an established calibration curve. For each energy and depth, (1) the profile along the central line (1mm wide) covering the primary field and peripheral region, and (2) the central axis (CAX) doses (1×1cm2) were obtained. Results: The primary beam doses were sensitive to depth, while those in the peripheral region remained insensitive. For 6MV and 15MV photon beams, (1) the CAX percent depth doses (PDD) at d = 0.012–0.18cm were 20%–60%, and 13%–40%, respectively; (2) the extrapolated CAX PDD at d=0 were 15% and 11%, respectively. Agreement with previously reported CAX PDD at surface using parallel plate thin window ion chamber and ultrathin TLD was found. The normalized primary beam dose profiles varied slightly with depths. Conclusion: The depth doses and profiles in the superficial buildup region near the air interface were quantified using radiochromic EBT film stacks. The proper incorporation of such characteristics in the treatment planning system should be considered when assessing the doses to skin and shallow tumors.
SU‐GG‐T‐305: Feasibility of Using a Programmable Respiratory Motion Phantom for QA and Assessment of Dosimetric Implications of Breathing Motion During Radiation Therapy
Authors: Publicover J , Vandermeer A , Norrlinger B , Alasti H
Published: July 2008

Purpose: Respiratory motion introduces uncertainties during CT and radiation therapy delivery. Reliable equipment and quality assurance (QA) techniques must be established to assess and overcome these uncertainties. Our goal was to validate the performance of a programmable motion phantom for QA and to demonstrate the dosimetric impact of breathing motion on treatment delivery. Method and Materials: The “Quasar” phantom (Modus Medical, London, ON) was assessed for suitability in QA procedures for radiation therapy involving respiratory motion. The phantom is equipped with a programmable unit, which introduces motion to cylindrical lung inserts. We tested the standard mode of motion and the “oscillation” mode, in which patient breathing profiles are imported and reproduced. Phantom motion reproducibility and accuracy were assessed using the Varian Real‐time Position Management (RPM) system and video for the extreme breathing periods (1 and 15seconds) and a patient representative breathing period of 4seconds. An in‐house designed cedar lung insert was built containing a target (4cm by 7cm). Film is placed in the insert to assess the dose distribution under phantom motion from static and dynamic delivery under phantom motion. The dynamic MLC treatment delivery was synchronized with target motion. Results: Using the RPM system, percent differences between the intended and actual periods for each were 0.57%, −1.70% and −0.22% respectively. When the amplitude was changed from 2cm to 1cm, the measured period did not change. Comparison of the breathing profiles in oscillation mode with profiles generated using the RPM system shows a close correspondence, with slight divergence at extreme direction or speed variation. Dose distribution for a phantom motion of 2cm peak‐to‐peak and period of 3.2sec along the moving direction indicates significant broadening of (80–20%) penumbra for static delivery (1.67cm) compared to dynamic delivery (0.80cm) Conclusion: The Quasar phantom is suitable for QA and dosimetric measurements of moving targets.
SU‐GG‐T‐308: Polymer Cross‐Linking and Sensitivity Enhancement in Normoxic Polymer Gels Used for Three‐Dimensional Dosimetry
Authors: Martin R , BenComo J , Heard M , Martin M , Kaluarachchi K , Wells N , Sadagopan R
Published: July 2008

Purpose: To discuss the relation between cross‐linking and sensitivity enhancement in normoxic polymer gels in a dose range suitable for intensity modulated radiotherapy, based on considerations related to actual sensor preparation methods and a theoretical description of the optical behavior of irradiated polymer gel when dose maps are obtained through optical CT. Method and Materials: Two set of samples of MAGIC gel (9% in weight of methacrylic acid) were prepared at 37°C and 45°C and irradiated with a and 6 MV photon beams to doses in the range from 0.1 to 5 Gy. The samples were scanned in an optical CT. Non‐irradiated samples were also scan. A theory based on the number of sites for water solvation, which takes into account polymer cross‐linking, is introduced in order to explain the observed changes in the optical density. The slope of the polymer gel response to dose is a measurement of sensitivity and it is analyzed for the actual experimental conditions as well as those for the optimum in connection to cross‐linking phenomena. Conclusion: The analysis showed a proper monotonic behavior for the polymer gel response and maximum sensitivity when cross‐linking occurs in an important fraction in the preparation process.
SU‐GG‐T‐309: Presenting TomoPen: The First Monte Carlo Treatment Planning for Helical Tomotherapy
Authors: Sterpin E , Olivera G , Vynckier S
Published: July 2008

Purpose: Use a previously validated Monte Carlo (MC) model, called TomoPen, to evaluate the accuracy of the convolution/superposition (C/S) algorithm of Tomotherapy in a heterogeneous multi‐layer phantom and in clinical cases. Method and Materials: The C/S algorithm delivered by Tomotherapy is based on the collapsed‐cone approach. In this algorithm, approximations are performed in the physics of transport of particles which may lead to significant deviations in inhomogeneous media. In the phantom study, an inhomogeneous phantom was build using two sets of slabs of low‐density material (0.3 g/cm3) separated by two thicker slabs of water‐like density material. The phantom was irradiated using three static fields with sizes of 1.25×2.5, 2.5×2.5 and 10×2.5 cm2. Dose distributions were calculated by both C/S and TomoPen and compared to measurements made with EBT gafchromic films. This experiment enables to evaluate the capability of the C/S algorithm to handle loss of lateral electronic equilibrium effects. Clinical cases were also studied for various tumor locations and dose‐volume histograms (DVH) obtained with C/S and TomoPen were compared. To ensure a “fair” comparison, all the calculations were performed using identical dose grid and DVH computation engine. Results: In the inhomogeneous phantom study, good agreement (within 3%/2mm) was achieved between C/S, TomoPen and measurements proving that the C/S algorithm accounts for loss of electronic equilibrium effects. For the clinical cases already compared in this ongoing study, a very good agreement was generally achieved between TomoPen and C/S excepted for small lung tumors where differences up to 3% could be observed for the mean dose to the target volume. Conclusion: Those results proved the ability of the C/S algorithm to compute accurately the dose in inhomogeneities but demonstrated also the potential of introducing TomoPen in clinical practice, especially for small lung tumors. Conflict of Interest: Research sponsored by Tomotherapy incorporated.
SU‐GG‐T‐312: Monte Carlo Investigation of Collimator Scatter of Proton‐Therapy Beams Produced Using the Passive Scattering Method
Authors: Titt U , Zheng Y , Vassiliev O , Newhauser W
Published: July 2008

Purpose: As a proton‐therapy beam passes through the field‐limiting aperture, some of the protons are scattered off the edges of the collimator. The edge‐scattered protons can degrade the dose distribution in a patient or phantom. Objective of this work was to quantify the dosimetric impact of edge‐scattered protons for a variety of clinical treatment beams. Method and Materials: The dosimetric impact was assessed using Monte Carlo simulations of proton beams from a contemporary treatment facility. The properties of the proton beams were varied, including the penetration range, width of the spread‐out Bragg peak, field size, and air‐gap, i.e. the distance between the collimator and the phantom. Results: The simulations revealed that the dosimetric impact of edge‐scattered protons increased strongly with increasing range (6% to 20%), decreased strongly with increasing field size (2% to 20%), increased moderately with increasing air gap (2% to 6%), and increased weakly with increasing SOBP width (< 4% change). In all cases examined, the effects were largest at shallow depths. 32 SOBPs and 79 lateral profiles have been compared so far. In three cases, maximum dose differences (in high dose regions) were found to be 3.0 % and one simulated penumbra was found to differ by 2.1 mm when compared to the measurement. Differences for all other cases are below the tolerance level. Conclusion: We conclude that the dose deposited by edge‐scattered protons can distort the dose proximal to the target with varying contributions due to proton range, treatment‐field size, collimator position and thickness, and width of the SOBP. Our findings also suggest that accurate predictions of dose per monitor‐unit calculations may require taking into account the dose from protons scattered from the edge of the patient‐specific collimator, particularly for fields of small lateral size and deep depths.
SU‐GG‐T‐315: McGill Monte Carlo Research Platform (MMCTP) for Dose Comparison Studies
Authors: Alexander A , DeBlois F , Seuntjens J
Published: July 2008

Purpose: To demonstrate recent improvements in the functionality of MMCTP, a radiotherapy research platform GUI, with integrated Monte Carlo (MC) dose calculation submission and dose analysis tools. MMCTP allows for quick MC dose calculations from standard radiotherapy formats. Clinical and MC dose distributions are analyzed under the same platform, which eliminates inherent planning system tool dependent effects. History: MMCTP has been internally distributed within the last year. External distribution, which has so far been limited to close contacts, will be open to the public. DICOM‐RT plans, which include: images, structures, dose distributions and beam information, have been successfully imported from TPS such as: Eclipse, Corvus and Tomotherapy. MC calculations use the images structures, and beam information to generate a beam and patient model. The GUI is capable of handling the following beam parameters: dynamic wedge, static/dynamic MLC, and static wedge. MC dose distributions are normalized to absolute dose with the beam monitor units and directly compared to the imported clinical dose distributions. The GUI includes a set of built in comparison tools: isodose lines and colour‐wash displays in transverse/sagittal/coronal view, DVH calculations, dose difference maps and dose profile graphs. Validation has been extensive and includes profile comparisons between MC and measured data. Results: Patient recalculations include a set of: conformal lung and breast patients, gated high dose lung patients, IMRT head and neck patients. Recalculations times vary depending on the number and complexity of beams, size and resolution of patient model. Heterogeneous recalculations reveal differences between planned and delivered dose distributions. Conclusion: MMCTP is a flexible research platform for the development of patient specific MC treatment planning for photon and electron external beam radiation therapy. The visualization, dose analysis tools offer extensive possibility for plan analysis and comparison to plans imported from commercial TPS through well‐documented protocols such as DICOM‐RT.
SU‐GG‐T‐317: Impact of Tumor Motion and Size in the Irradiation of Moving Tumors in Step‐And‐Shoot IMRT: A NCAT Based 4D Monte Carlo Simulation Study
Authors: McGurk R , Riboldi M , Seco J , Paganetti H , Wolfgang J , Chen G
Published: July 2008

Purpose: To quantify the dosimetric impact of the interplay effect in step‐and‐ shoot IMRT treatment of lung tumors of varying size and motion amplitude using a 4D Monte Carlo simulation framework and to find whether a threshold exists where implementing motion mitigation strategies might become important in such treatments. Method and Materials: The Non‐Uniform Rational B‐Spline (NURBS) Cardiac and Torso (NCAT) computational phantom was used to create 10‐phase 4D CT data sets of 12 theoretical patients with tumor sizes of 1–6 cm and magnitudes of tumor motion of 1 – 5 cm. Lung density for each model was changed as a function of breathing phase. IMRT leaf sequences were generated using the CORVUS treatment planning system for each of the 12 data sets and used as input into the Dose Planning Method (DPM) Monte Carlo code. Dose at each phase was mapped back to the exhale phase using the internal NCAT deformation maps. Dose volume histograms (DVHs) of tumor and organs at risk (OARs) were used as a measure of the dosimetric effect of tumor motion and size. Results: A population of computational patient phantoms has been created. The lung density of the NCAT phantom can vary from the default 0.30 gm/cm3 by up to 34% for the largest diaphragm motion. The NCAT phantom has been implemented within the CORVUS treatment planning environment as well as within our 4D Monte Carlo simulation framework. Various tumor locations in the lung and tumor amplitudes were investigated to assess dosimetric effects in IMRT therapy of lung tumors. Conclusion: This study shows the potential of using the most advanced computational model of human anatomy for finding class solutions for motion mitigation within a controlled environment. We conclude it is a valuable tool in order to study artifact free data sets for radiation therapy.
SU‐GG‐T‐319: Monte Carlo Study of Distal Edge Degradation in Proton Therapy Due to Heterogeneities. Proof of Principle with a Case Study
Authors: Titt U , Zhang X , Li Y , Mirkovic D , Sawakuchi G , Mohan R
Published: July 2008

Purpose: Degradation of the distal edge of quasi‐mono energetic proton beams occurs when the proton beams penetrate geometries containing complex inhomogeneities. Contemporary treatment planning systems fail to accurately predict the degradation and its potential consequences, such as dose spill over into healthy tissue or the effects on dose homogeneity in the target volume. This work aimed to build a Monte Carlo model which accurately predicts the degradation. Method and Materials: In this study we built an MCNPX Monte Carlo model to simulate this degradation in a scanning‐beam nozzle, using a 221.83 MeV proton beam impinging on a voxelized computer tomography‐based phantom of a human head. Dose distributions, energy and angular distributions of the proton beam were tallied in and at the surface of a water phantom distal to the CT‐based human head phantom. Results: It was found that the degradation of the distal edge and the energy distribution of the proton beams are strongly correlated to the complexity of the heterogeneity. The angular spread of the beam showed only a weak dependence. Conclusion: The Monte Carlo simulation model successfully produced the degradation of proton beams through a heterogeneous environment. It will be a useful tool to investigate the basic underlying physics and provide support for the development of optimization tools for proton therapy treatment planning and help to achieve escalated target doses and minimize collateral damage to normal tissue.
SU‐GG‐T‐324: Characteristics and Dosimetric Parameters of Small Radiosurgery Photon Beams
Authors: Ding G , Coffey C
Published: July 2008

Purpose: The accuracy of dosimetric parameters, such as water‐to‐air stopping power ratios, directly affects the accuracy of a beam calibration. This study investigates the variation of dosimetric parameters, such as energy spectrum, water‐to‐air stopping power ratios, energy absorption coefficient, etc., as a function of field size and depth in water for small radiosurgery photon beams. Method and Materials: The Monte Carlo simulation techniques were used in this study. The accelerator head geometries of a Varian Trilogy were simulated in detail including two designs of flattening filters: one for a conventional dose rate (100–600 MU/min) and the other for a high‐dose rate (1000 MU/min) 6 MV beam. The design of the flattening filter to produce a high‐dose rate beam limits its maximum field size to 10×10 cm2. The investigated field sizes range from 0.5 cm in diameter to 10×10 cm2 and depths range from 0–50 cm in water. Results: Although there are noticeable differences in the spectrum distributions for beams with different flattening filters, the variations in calculated water‐to‐air stopping power ratios (SPR) are within 0.2%. The values of calculated water‐to‐air SPR as a function of field size and depth in a water phantom show a maximum variation of 0.4% and 1% respectively. The mean energy of photons increases as depth increases at a rate of 22 keV/cm for small fields. Conclusion: There is an insignificant variation (<0.4%) in water‐to‐air stopping power ratios between a small narrow field and a reference 10×10 cm2 field. There is negligible difference (0.2%) in water‐to‐air SPR between beams with two very different beam flattening filters. Variations of water‐to‐air SPR are not significant between small and large field. Hence correction of water‐to‐air SPR between a reference field and a small field is negligible in the dose measurement for the small field.
SU‐GG‐T‐333: Neutron Yield and Angular Distribution From the 250 MeV Proton Interactions in Water: A Geant4 Monte Carlo Study
Authors: Chen Y , Ahmad S
Published: July 2008

Objective: To simulate using GEANT4 the interaction of 250 MeV protons in water to determine the neutron yield and angular distribution using various hadronic interaction processes. Method and Materials: A cylindrical water phantom (length=40 cm, diameter= 40 cm, density = 1 g/cm3, mean excitation energy = 75 eV) is surrounded by a spherical shell, that consists of 18 detectors, each of which covers 10 degree in theta. In Geant4, electromagnetic energy loss for hadrons and leptons are categorized as either “standard” or “low energy” processes. The low energy process is used to extend down particle interactions with energies below the standard process (for example: Protons 1 keV, electrons and photons 250 eV). The range cut for both low energy and standard process is lowered from the default value of 10−3 m to 1.5×10−5 m to improve the accuracy of simulation. In the case of HI, elastic processes for protons are calculated by G4LEpp and GHEISHA‐style G4LElastic; whereas the inelastic processes for proton and neutron are calculated by G4PreCompoundModel below 170 MeV, and G4CascadeInterface above 150 MeV; and for neutron also by G4NeutronHPorLEInelasticModel. The neutron elastic process is calculated using G4NeutronHPorLEModel for energy greater than 4 eV. When the neutron energy is lower than 4 eV, G4NeutronHPThermalScatteringData is used. The simulations are carried out using 1 million incident pencil beam proton. Results: Neutron fluences per incident proton as a function of theta are determined. Based on hadronic process being used in the calculation, the total number of neutrons per incident proton that reach all eighteen theta detectors ranges from 0.124 to 0.243. Conclusion: Our results show that the hadronic processes indeed play a critical role for GEANT4 simulations. We recommend that careful selection of physics processes for particle interaction is essential in order to interpret realistically simulated results.