Volume 35, Issue 6, June 2008
Index of content:
- Therapy Scientific Session: Auditorium B
- Brachytherapy I
MO‐D‐AUD B‐01: An Analytical Approach to Account for Shielding, Anatomical Heterogeneities and Patient Dimensions for 192Ir High Dose Rate Brachytherapy Applications35(2008); http://dx.doi.org/10.1118/1.2962342View Description Hide Description
Purpose: Based on a separate primary and scatterdose calculation technique and pre‐computed Monte Carlo (MC) data, we have developed an analytical approach to account for shielding, anatomical heterogeneities and patient dimensions for high dose rate (HDR) brachytherapy.Method and Materials: Using the PTRAN_CT MC code, primary and scatterdose kernels of an HDR source in water were generated. Separate 3D kernels for rectal treatment with a tungsten‐shielded applicator were also created. Photon attenuation and scatter in tissue heterogeneities were corrected for via ray tracing. To quantify the reduced backscatter close to the skin,MC simulations were performed with an isotropic point source placed at various distances from the center of a 30 cm diameter water sphere. Scatter correction factors were derived, which vary as a function of distances between (1) the source and the surface, (2) the point of interest and the source, and (3) the point of interest and the surface. We applied this analytical method for three clinical cases and compared the results with PTRAN_CT calculations. Results: Our technique accurately accounted for the effects of tungsten shielding and anatomical heterogeneities for a rectal patient plan. The reduced backscatter close to the skin was also calculated correctly for a base of tongue and a breast case. Around bony structures several centimeters away from the active dwell positions, there was a minor discrepancy due to softening of the spectrum. Differences in the lung due to reduced scattering in this low density region were also observed. Conclusion: Making use of pre‐computed 3D scatterdose data, our analytical technique is capable of calculating dose around metal shielding and the patient skin with high accuracy. Its validity and limitations have been studied for HDR applications. Conflict of Interest: Research sponsored by Nucletron BV.
MO‐D‐AUD B‐02: Dose Rate Constants Determined by a Photon Spectrometry Technique for 20 Different Models of Low‐Energy Brachytherapy Sources35(2008); http://dx.doi.org/10.1118/1.2962343View Description Hide Description
Purpose: To perform a systematic and independent determination of the dose rate constants (Λ) of available low‐energy interstitial brachytherapy sources using a recently developed photon spectrometry technique (PST). Method and Materials: A total of 60 low‐energy interstitial brachytherapy sources (20 different models with 3 sources per model) containing either (14 models), (5 models), or (one model) were included in this study. A recently developed photon spectrometry technique (Med. Phys. 34, 1412–1430, 2007) was used to determine the PSTΛ for each source. Source‐dependent variations in PSTΛ were analyzed systematically against the spectralcharacteristics of the emitted photons and the AAPM consensus values (CONΛ) when available. Results: The PSTΛ determined for the , , and sources had values of 0.661 to 0.678, 0.959 to 1.024, and 1.066 cGyh−1U−1, respectively. The variation in PSTΛ among the 5 source models was less than 3%; due mainly to the variations in spatial distribution of radioactivity. The variation in PSTΛ among the 14 source models was larger and the maximum difference was over 6%. These variations were caused primarily by the presence of silver in some source models and, to a lesser degree, by the variations in activity distribution. When silver was present, the PSTΛ exhibited strong dependence on the silver content with values varying from 0.959 to 1.019 cGyh−1U−1. When silver was absent, the PSTΛ was less variable and had values within 1% of 1.024 cGyh−1U−1. The PSTΛ was found within 2% (14 models) and 2.6% (one model) of CONΛ for 15 models current have such a value. Conclusion: Excellent agreement between PSTΛ and CONΛ was observed for all source models that currently have an AAPM consensus value. These results demonstrate that the PST is an accurate and robust technique for the determination of Λ for low‐energy brachytherapy sources.
MO‐D‐AUD B‐03: An Enabling Technology for Creating Sculpted Brachytherapy Dose Patterns with the Xoft Axxent™ System35(2008); http://dx.doi.org/10.1118/1.2962344View Description Hide Description
Purpose: Study a potential means of partially attenuating X‐rays from the Xoft Axxent™ system over controlled spatial areas, while minimizing changes to depth‐dose characteristics. This would be the basis of an enabling technology to sculpt brachytherapydose patterns to, for example, spare critical structures such as skin in breast brachytherapy treatments. Method and Materials: Measurements of output were made from the Xoft 50 kVp source attenuated by thin (0.001″) dot‐shaped layers of silver, with diameters of several mm. Measurements were made with azimuthal scans around the source at distances from 2 to 7 cm. The dots were attached to the source cooling catheter, approximately 2.6 mm from the source center. Attenuation calculations and Monte Carlo studies of the effect were performed using EGSnrc. Results: Partially attenuating dots of silver create shadows in measured dose that agree with attenuation calculations in proportion, and in the critical behavior of how the attenuation varies with distance. Monte Carlo studies were consistent with measured results. Materials other than silver (or elements nearby in atomic number) will harden the beam, and so the attenuation will lessen with distance. Silver has much less of a hardening effect, because the K absorption edge reduces the higher energy portion of the spectrum at a rate similar to the losses in the lower energy region. Conclusion: It is possible to create predictable, directed shadows in dose around the Xoft 50 kVp x‐ray source using practical thicknesses of silver foils. The shadows have soft edges owing to penumbral effects when the dots are placed on the cooling catheter, within a few mm of the source. Such shadows could be used in future applications to spare healthy tissue during brachytherapy. In a simulated breast treatment plan, using a simple model of the attenuation, isodose lines were shifted by several mm.
MO‐D‐AUD B‐04: Parameter Optimization for Brachytherapy Robotic Needle Insertion and Seed Deposition35(2008); http://dx.doi.org/10.1118/1.2962345View Description Hide Description
Purpose: To investigate influence of different needle insertion and seed deposition techniques for roboticbrachytherapy. To find optimal sets of low, normal and high translational and rotational velocities of the needle for decreasing insertion force, needle deflection and OR time, and increasing seed placement accuracy. Method and Materials: We have developed EUCLIDIAN — a fully automatic robotic prostate brachytherapy system. Robotic system parameters were optimized via preclinical experiments using two types of polyvinylchloride and tissue phantoms, cannula and stylet single‐axis force sensors, and six‐axis force‐torque sensor. Cannula sensor measures the force on the cannula during insertion, withdraw, and axial force exerted by tissue at rest. Stylet sensor measures the force while seed is expelled from the cartridge, during seed travel through the cannula, and at the moment when seed is deposited into tissue. Position of the needle tip and consequently deposition depth into the phantom was measured using optical encoders on the cannula and stylet motors. Cannula and stylet translational velocity range was 5–120 mm/s, and cannula rotation range was 0–30 rev/s. Force patterns were analyzed based on the experimental data. Results: According to the criteria for minimizing insertion force and OR time while maximizing seed deposition precision, it was found that best performances were achieved when cannula and stylet normal speed was 70 ± 10 mm/s and optimal high speed was 100 ± 10 mm/s. Optimal cannula rotation speed range was 15–25 rev/s. In order to avoid seed jam in the cartridge, optimal speed for pushing seed out of the cartridge was 2–5 mm/s. Conclusion: Optimal parameters were programmed in the EUCLIDIAN configuration files. Seed deposition techniques have significant influence on reduction of insertion force, needle deflection and seed deposition accuracy. Future investigation will be on adaptive parameter tuning for specific clinical encounters.
Acknowledgement: Supported by NCI‐R01‐CA091763.
35(2008); http://dx.doi.org/10.1118/1.2962346View Description Hide Description
Purpose: To develop and verify a method for determining source dwell position for implementing a MammoSite® treatment procedure with a Nucletron remote after‐loader. Method and Materials: When delivering partial breast brachytherapy using an implanted balloon device, a 2 mm error in source dwell position produces a 15% error in dose delivered to the prescription point. Therefore, the procedure for determining a Reference Length entered into the after‐loading device produced by one vendor (Nucletron Corporation, Veenendaall, The Netherlands) for the dwell position at the geometric center position of a MammoSite® balloon provided by another vendor (Hologic, Marlborough, MA) must be determined and verified.. Devices with scales and indicators, connecter between the two systems, dummy wires, transfer tube and software provided by both vendors need to be used together to plan and deliver the treatment. A planning and delivery procedure was developed and tested by means of: 1). phantom measurements using images of a dummy source to compare with film dosimetry of the dose pattern produced by the active source. These measurements established and verified the procedure, and 2). a patient planning and treatment procedure with appropriate imaging QA steps. Results: The interpretation and accurate use of the scales and indicators and dummy wires was established. A 2mm offset was confirmed for determining the source Reference Length. It was found to be important to verify the Reference Length value using fluoroscopic and radiographicimages acquired with the patient at a conventional simulator. For approximate 10% of the tested cases, adjustments on the order of 1mm were needed based on the simulation procedure. Conclusion: The method of determining the Reference Length for Mammosite® treatment planning should be established with images and a phantom. The Reference Length measured for each patient should be verified with an imaging procedure using a conventional simulator.
MO‐D‐AUD B‐06: Source Motion in Permanent Implant Prostate Brachytherapy Due to Ultrasound Probe Deformation35(2008); http://dx.doi.org/10.1118/1.2962347View Description Hide Description
Purpose: To determine the amount of seed motion in the prostate due to ultrasound probe deformation during permanent implant prostate brachytherapy.Method and Materials: A C‐arm was used to take variable angle images of clinical implants immediately after the last needle was delivered with the patient and ultrasound remaining in the treatment position, after the ultrasound probe was lowered, and after it had been removed with the patient remaining in the treatment position. Three dimensional seed coordinates were calculated and corresponding seed coordinates were compared to determine the motion induced by the ultrasound probe. A rigid body registration was performed and deformational effects were evaluated using the residual seed motion. Results: Seed positions over all patients moved, on average, 6.6 mm posterior, 1.6 mm caudal, 1.5 mm patient right and the mean total motion was 7.1 mm (range 2.1 mm – 12.3 mm). The mean for a single patient ranged from 5.3 mm (2.4 mm – 8.3 mm) to 9.7 mm (8.1 mm – 12.3 mm). The rigid body registrations showed rotation about an axis perpendicular to a sagittal plane in each patient (mean 4.2°, range 2.9° – 5.9°). The mean residual seed motion was 1.1 mm (0.2 mm – 4.4 mm) and showed non‐random deformational patterns. Conclusion: Final seed positions are significantly different from those delivered due to the ultrasound probe. Non‐random residual motion within the implant can be associated with deformation and may have dosimetric consequences.
MO‐D‐AUD B‐07: Analysis of Rectal Dose Variability Due to Inter‐Fractional Variations of Rectal Marker Positioning in Film‐Based HDR Cervical Brachytherapy35(2008); http://dx.doi.org/10.1118/1.2962348View Description Hide Description
Purpose: In film‐based intracavitary brachytherapy for cervical cancer, rectal dose is usually computed using rectal markers. Position of the markers may not accurately represent the anterior rectal wall. The study is to retrospectively analyze the variability of rectal dose due to variations of marker placement in a multi‐fractionated HDR treatment regimen. Method and Materials: A cohort of five patients, total 18 applications, treated with multiple‐fraction tandem/ovoid HDR brachytherapy was studied. To correlate the rectal points from different fractions to the same coordinate system, the cervical os point and the orientation of the applicators were manually matched. With the applicator matching, rectal points obtained from other fractions were input into the original treatment plan for each application. A rectal dose was then calculated from all the possible rectal points. The fractional rectal doses were summed as the new cumulative rectal dose for each patient, which was compared with the original cumulative rectal dose. The reproducibility of the results was also analyzed by repeating the matching procedure. Results: The maximum inter‐fractional variation of distances between rectal dose points and the closest source positions was 1.1 cm and the corresponding maximum variability of fractional rectal dose was 65.5%. The percentage difference in cumulative rectal dose estimation for each patient was 5.1%, 16.4%, 25.7%, 18.9%, 12.2%, respectively. Overall reproducibility of the results was within 1.8%. Conclusion: Our results show underestimation of the rectal dose caused by variations of rectal marker positioning relative to the anterior rectal wall, which should be taken into consideration in film‐based HDR cervical brachytherapy. By manually matching the rectal points into the same treatment plan, one may minimize the possibility of underestimating the rectal dose. We will also anticipate a more accurate approach for evaluating rectal doses in HDR intracavitary brachytherapy with the emerging 3‐D volume imaging based treatment planning.
MO‐D‐AUD B‐08: Treatment Planning for Complex Brachytherapy Dose Distributions Using High‐Z Shields and Conventional Software35(2008); http://dx.doi.org/10.1118/1.2962349View Description Hide Description
Purpose: Certain brachytherapydose distributions, like for LDR prostate implants, are readily modeled by treatment planningsoftware using the superposition principle of individual seeds to replicate the total dose distribution. However, dose distributions for brachytherapy treatments using high‐Z shields are currently not well‐modeled using conventional software.Method and Materials:Dose distributions from complex brachytherapy plaques determined using Monte Carlo methods were used as input data, and included COMS‐based eye plaques using , , and ; 4–8cm diameter AccuBoost peripheral breast brachytherapy applicators from Advanced Radiation Therapy; and the 2 and 3cm diameter Valencia skin applicators from Nucletron Corp. Radial dose functions, g(r), and 2D anisotropy functions, F(r,θ), were obtained by positioning the coordinate system origin along the dose distribution cylindrical axis of symmetry. Origin: tissue distance and effective active length, Leff, were chosen to minimize g(r) and F(r,θ) interpolation.Dosimetry parameters were entered into the Pinnacle treatment planning system, and dose distributions were subsequently calculated/compared to the original Monte Carlo‐derived dose distributions. Results: The planning technique was able to reproduce complex brachytherapydose distributions for all three plaque types. Agreement improved as distance from the coordinate system axis decreased, 1% errors on the axis were attributed to g(r) interpolation. Agreement was best for the Valencia applicator and worse at the plaque edge for COMS eye plaques and the AccuBoost applicator. Agreement between input and planned dose distributions improved as the spatial resolution of the fitted dosimetry parameters improved. For agreement on the order of 1%, dosimetry parameter spatial resolution of 1mm was required, and the F(r,θ) dataset included over 1,000 datapoints. Conclusion: A new technique was developed to simulate complex brachytherapydose distributions in tissue using conventional treatment planningsoftware. These results should be generalizable to other source types and planning systems. Conflict of Interest: Research sponsored by Advanced Radiation Therapy.
MO‐D‐AUD B‐09: The Role of the Radiotherapy Physicist in Intraoperative Partial Breast Irradiation Using a Low Energy X‐Ray Source, Based On 10 Years Clinical Experience35(2008); http://dx.doi.org/10.1118/1.2962350View Description Hide Description
Purpose: To describe the role of the radiotherapyphysicist in the clinical implementation of the Intrabeam™ system for intraoperative radiotherapy (Carl Zeiss, Germany), based on 10 years experience at University College London Hospital. Method and Materials: On delivery of the 50kVp electronic X‐Ray system, the Radiotherapy Physics Group undertook acceptance and commissioning. Half‐value layer measurements were made using a PTW 23342 0.02cc ion chamber. A dedicated water phantom was employed to measure variation of dose rate with radial distance from the X‐Ray source in 5 orthogonal directions and in one direction for each of 8 spherical applicators. These measurements were compared with the manufacturer supplied QA peripherals and with radiochromic film to assess radial isotropy and output constancy and stability. A radiation protection survey and risk assessment was performed for unshielded operating rooms (OR) prior to clinical introduction. The routine physics requirement comprises: pre‐treatment QA and calculation of applicator treatment times; in the OR: actively delivering and monitoring the radiation treatment, monitoring and enforcement of staff radiation protection in and around the OR and measurement of patient skindose by TLD.Results: UCLH physicists have commissioned 4 such X‐Ray sources for clinical use. We have treated 134 patients over a period of 10 years. To date, dose rate surveys during treatment have demonstrated the safe usage of the system under controlled conditions and no member of staff has had a recordable radiationdose.Conclusion: The Intrabeam™ device has been shown to be very stable dosimetrically and also practical within a standard clinical environment. The involvement of the radiotherapyphysicist in the commissioning and clinical implementation of this intraoperative radiotherapy system is imperative to ensure safe treatment delivery and radiation protection of staff and patients.
- Monte Carlo Methods
35(2008); http://dx.doi.org/10.1118/1.2962383View Description Hide Description
Purpose: The purposes of this work was to study the spallation product generation from a 355.91 MeV/nucleon carbon beam impinged onto a cylinder of Tissue Equivalent Plastic, and analyze its spectra as a function of energy distribution and distance using a three‐dimensional Monte Carlo particle transport code. Method and Materials: Simulations were performed using a 350 MeV/nucleon carbonion beam incident on a test phantom. The geometry for the test phantom was a cylinder with 20 cm height and 10 cm radius comprised of Tissue Equivalent Plastic. An energy of 350 MeV/nucleon was selected because it is in the range frequently used in carbon ion therapy. Monte Carlo analyses for spallation production were done using HETC‐HEDS. Results: A total of 23.74 % of the original beam was fragmented into charged particles between charges 1 and 5. Almost all charged fragments deposit their energies locally within the cylinder, and nothing escapes from the system.. Compared to neutrons and protons,charged particle fragment production is much lower as the percentages were 473.28 % and 295 % for neutrons and protons, respectively. For protons, most particles will exit the target following the direction of the primary beam. For the neutrons case, however, the 473.28 % exit the target in all directions make it very difficult to conclude that their direction is random. Conclusion: Secondary particle production is alarming especially for neutrons produced. Those secondary particles have a wide range of energy and are certainly able to cause secondary tumors. For the charged particles heavier than protons, almost 24% of the primary beam is produced as particles that range from charges 1 through 5. For the neutrons and the protons, the fluences were 473% and 293%. Those particles have high quality factors and could travel enough distance to cause DNA damage in healthy tissue.
MO‐E‐AUD B‐02: Antiproton Therapy: Monte Carlo Simulations of Normal Tissue Equivalent Dose From Annihilation Neutrons35(2008); http://dx.doi.org/10.1118/1.2962384View Description Hide Description
Purpose: Recent in vitro experiments at CERN have demonstrated a superior biological effectiveness for antiprotons relative to protons. A continued concern is the normal tissuedose resulting from annihilation neutrons. Using Monte Carlo simulations of a CT‐based anthropomorphic human phantom, we quantify the physical dose from annihilation byproducts and present the first organ specific calculations of normal tissue equivalent dose from neutrons in antiproton therapy. Method and Materials:MCNPX and FLUKA were utilized to model antiproton irradiation of the segmented whole‐body phantom of a 38 year old male representing the ICRP reference man. The fluence was tallied as a function of energy and organ type for a 75 MeV antiproton pencil beam with a Bragg peak located in the frontal lobe of the phantom's brain. Physical dose was calculated for each organ as a function of energy using fluence to kerma conversion coefficients (ICRU‐63). Finally, using energy dependent radiation weighting factors (ICRP‐60), the equivalent dose from neutrons was estimated for each organ.Results: The results indicate a neutron fluence on the order of 10−5 cm−2 per incident antiproton for the bladder and colon, and a neutron fluence on the order of 10−4 cm−2 per incident antiproton for the thyroid and esophagus. As anticipated, the physical and equivalent doses are dependent on the irradiation geometry and the proximity of the organs to the Bragg peak; of the organs tallied, bone and thyroid received the highest physical and equivalent dose for the given irradiation protocol. The estimates of organ physical and equivalent dose and their uncertainties are discussed. Conclusion: We have developed an anthropomorphic Monte Carlo model for antiproton therapy. The model provides a method for more sophisticated biological modeling of treatment response such as cost basis analysis of TCP and NTCP relative to other treatment modalities.
35(2008); http://dx.doi.org/10.1118/1.2962385View Description Hide Description
Purpose: The single‐bone tissue model commonly used in radiation therapy may be over simplified. Experiments and simulations have shown that in mega‐voltage x‐ray dose computation this model can cause errors of up to 10%. In the kilo‐voltage range a much higher error is expected because of the larger variation of the mass attenuation coefficients of different bone tissues. This error could produce significant errors in dose computation and potentially result in noticeable biological effect. A more effective bone model is therefore needed. Method and Materials: A model containing multiple bone compositions can be formulated to more accurately represent the distribution of attenuations found in vivo. Based on the observation that bone calcium and phosphorus contents are strongly correlated with the bone density, we propose a methodology in which the interpolated compositions of these 2 elements and the averaged compositions of other 10 major elements are assigned to a model bone based on its density. A series of 24 model bones was generated that covers the bone density range 1.1 to 2 gram/cm3. The error of primary photon total energy released per unit mass (terma) was used to evaluate the models. Photon‐tissue interaction cross‐sections are calculated and Monte Carlo simulation was performed to estimate the dose deposition error. Results: In the kilo‐voltage range, if a bone is assigned with the correct density but an inaccurate composition, the terma error by the single‐bone model can reach 25%. The 24‐bone model reduces this error to below 0.5%. A simulation shows that in a 120 kVp x‐ray radiation to a mouse brain, the skull dose predicted by the single‐bone model can be 2.13 times that by the 24‐bone model. Conclusion: A multiple‐bone model is proposed. Use of this model should significantly improve dose computation accuracy, especially in kilo‐voltage x‐ray spectrum range.
MO‐E‐AUD B‐04: Fast, Accurate Photon Beam Accelerator Modeling Using BEAMnrc and VMC++: A Systematic Investigation of Variance Reduction and Efficiency Enhancing Methods and Cross‐Section Data35(2008); http://dx.doi.org/10.1118/1.2962386View Description Hide Description
Purpose: To report on the accuracy of cross‐section data in BEAMnrc and on the performance of variance reduction and efficiency enhancing techniques for fast, accurate linac simulations using the BEAMnrc and VMC++ code systems. Method and Materials: BEAMnrc and VMC++ were used to simulate a 6 MV photon beam from a Siemens Primus linac. Phase space (PHSP) files were generated for a range of field sizes, from 10×10 to 40×40 cm2. BEAMnrc parameters under investigation were grouped by: i) photon and bremsstrahlung cross‐sections; ii) approximate efficiency improving techniques (AEIT); iii) variance reduction techniques (VRT); iv) VRT (bremsstrahlung splitting) with AEIT (range rejection). Efficiencies were obtained for the mean energy, fluence, angular and spectral distributions and PHSP files were subsequently used as input for DOSXYZnrc‐based phantom dose calculations; these calculations were verified against measurements. Results: Efficiencies were calculated for the various VRT/AEIT combinations in BEAMnrc, relative to simulations without VRT/AEIT, namely: (a) 935 (∼111 min. on a single 2.6 GHz CPU) and 200 for 10×10 and 40×40 resp. using directional bremsstrahlung splitting (DBS) and no electron splitting, (b) 420 and 175 for 10×10 and 40×40 resp. using DBS and electron splitting combined with augmented range rejection, a technique recently introduced in BEAMnrc. Calculations with VMC++ produced efficiencies of 1400 (∼6 min. on a single CPU) for 10×10 versus BEAMnrc (no VRT/AEIT). Noteworthy differences (±1–3%) were observed with the NIST bremsstrahlung cross‐sections compared with those of Bethe‐Heitler (default). However, MC calculated dose distributions (using all combinations of VRT/AEIT and cross‐section data) agreed within 2%/2 mm of measurements. Conclusion: VRT/AEIT related to DBS significantly improves the efficiency of BEAMnrc PHSP simulations. VMC++ can be used to perform simulations of the entire linac and phantom within minutes on a single processor. Further investigation of bremsstrahlung cross‐section data is warranted.
MO‐E‐AUD B‐05: Development of Whole‐Body Phantoms Representing An Average Adult Male and Female Using Surface‐Geometry Methods35(2008); http://dx.doi.org/10.1118/1.2962387View Description Hide Description
Purpose: To apply a series of Whole‐Body Phantoms Representing An Average Adult Male and Female Using Surface‐Geometry Methods to the study of external radiation dosimetry.Method and Materials: Boundary reprentation was used to deform the original organs automatically into two sets of standard RPI Adult Male/Female phantoms with volume/mass matched with those of the ICRP. To finally define the phantom geometries in Monte Carlo codes for dose calculations, we developed a software to convert the finished surface phantoms into the voxel phantoms at any desired voxel size. The voxelization used the parity count method together with the method of ray stabbing on polygon surface. The corresponding Monte Carlo input file was derived automatically by our program “Phantom Processor”. Average absorbed doses to organs were obtained by MCNPX.Results: The volume/mass data of the standard RPI Adult Male/Female phantoms match with those of the ICRP. After mesh voxelization, the volume/mass data of the voxel phantoms have the relative error less than 0.5%. The voxel resolutions of the Male/Female are 3.2 mm and 3.0 mm respectively. The average absorbed doses of internal organs were calculated using the 6 external neutron irradiation geometries. All results were normalized by the unit source fluence in accordance with the standard usage in radiation protection dosimetry for reporting fluence to absorbed dose conversion coefficients. Typically, 107 histories were simulated and the uncertainties were better than about 1% for most of the target organs.Conclusion: A series of RPI Adult Male/Female phantoms have been developed. Using our software we have developed additional registration and deformation algorithms that allow a mesh‐based phantom to “morph” into a different individual. This series of phantoms were voxelized and implanted into MCNPX. The results suggest that Monte Carlo calculations can be performed for various internal and external exposures to ionizing radiation.
35(2008); http://dx.doi.org/10.1118/1.2962388View Description Hide Description
Purpose: The accumulated dose accuracy depends by both the image registration accuracy and the dose addition strategy. In this study, we introduce a Monte Carlo‐based approach for cumulative dose computation based on energy mapping between various datasets. Method and Materials: EGSnrc/DOSXYZnrc Monte Carlo code is modified such that the energy (rather than the dose) is mapped from a “source” dataset onto a “target” dataset. The dose is subsequently calculated as the ratio of the warped energy deposited and the warped mass. The latter is the sum of all source voxels mass weighted by the ratio of the number of particles from “source” to “target” and the total number of particles scored in the “source” voxel. A lookup table for the source‐to‐target distribution is created for each voxel. The cumulative dose calculation requires displacement vector fields (DVFs) between “source” and “target” images. We use, for exemplification, two different DVFs, one generated by using the ITK “demons”, and the other by using the visco‐fluid model registration method. Results: The algorithm is integrated with Pinnacle planning system and is demonstrated through a 4D treatment plan. The average difference between doses reconstructed using the two DVFs is 15.6% for mean lung dose, 13.6% for heart, and 2.1% for ITV. It appears that lung doses are affected the most by the DVFs used. Conclusion: The forward 4D MC method introduced here uses both mass and energy deposition sampling, is potentially more efficient, as it does not require tracking of the deformed boundary, and is expected to provide more accurate results than interpolation‐based dose accumulation approaches, especially for the lung dose calculation. However, it does not eliminate the bias introduced by image registration errors.
MO‐E‐AUD B‐07: SAF Values for Internal Electron Emitters Calculated for the RPI‐P Pregnant‐Female Models Using Monte Carlo Methods35(2008); http://dx.doi.org/10.1118/1.2962389View Description Hide Description
Purpose: to calculate specific absorbed fraction (SAF) values for internal electron emitters based on more realistic RPI‐P serial pregnant female models. Method and Materials: The RPI‐P series pregnant‐female models developed by Xu and coworkers were used for Monte Carlo simulation. Those models are based on boundary‐representation method for organ delineation. The image sources are from clinical CTimage, VIP‐Man image, and public domain images. The pregnant woman models, RPI‐P3, RPI‐P6, and RPI‐P9, were implemented into a previously developed Monte Carlo user code, EGS4‐VLSI. In this study, internal electron emitters were considered for the following energies: 10, 15, 20, 30, 50, 100, 200, 500, 1000, 1500, 2000, and 4000 keV. SAF values to the fetus were calculated for each of these energies involving 35 source organs.Results and Discussion: SAF factors from source organs to the fetus have been calculated for all the three pregnant female models. Results show that electron SAF values follow linear relationship as equation:, where E is the electron energy, A and B are coefficients. A and B coefficients were calculated. R2 coefficient, the determination for the linear relationship, is ranging from 0.90∼1.00 except source organ=heart for RPI‐P3 model. It means the linear relationship between log(SAF) and log(E) is fitting well. Conclusion: SAF values have been derived based on a new developed RPI‐P series pregnant‐female models using Monte Carlo method. For electron emitters ranging from 10 keV to 4000 keV, the log(SAF) and log(energy) relationship can be approximated by linear function.
35(2008); http://dx.doi.org/10.1118/1.2962437View Description Hide Description
Purpose: To improve ionization chamber localization accuracy for depth‐dose measurements used for TPS dose calculation algorithm commissioning and periodic linear accelerator QA. Method and Materials:Ionization chamber depth‐dose scans are set to include points above the watersurface, which produces inflections in the depth‐dose curves. Monte Carlo simulations are performed with the EGSnrc Cavity usercode, which simulates the detailed ionization chamber and phantom geometries, and with DOSXYZnrc, which excludes the chamber geometry. The inflection point location in the Cavity simulation with respect to the chamber center quantifies the chamber's absolute location. The difference between the Cavity and DOSXYZnrc depth‐dose results quantifies the ion chamber's effective point of measurement (EPOM) variation as a function of depth. Measurements and simulations are performed for 6 and 18 MV photon beams for multiple field sizes. Measurement results are aligned to the surface position by matching the computed inflection points. Results: The dose inflection point due to the air‐water interface is clearly identifiable in both measurements and calculations. A Cavity simulation at 6 MV with a 10×10 cm2 field finds that the inflection point occurs when the central electrode is ∼1 mm beneath the watersurface. After applying the recommended EPOM shift to Cavity simulation results, the distance‐to‐agreement between the Cavity computed “surface” dose and the DOSXYZnrc dose was >2 mm. By 1.0 cm depth, the distance‐to‐agreement is negligible. 18 MV simulations yielded discrepancies in the in‐air dose, presumably due to differences in contaminant electrons. Conclusion: The proposed method of conducting depth‐dose measurements is trivial to implement and provides a way to automatically account for, and correct, shifts and/or offsets in initial chamber positioning. This allows for improved matching, not only of measured and calculated data, but also of measured data such as that acquired in periodic QA testing.
TU‐C‐AUD B‐02: Dosimetric Verification of the CCCS Algorithm for Spatially Fractionated Radiation Therapy35(2008); http://dx.doi.org/10.1118/1.2962438View Description Hide Description
Purpose: The comparison between measurements and the collapsed cone convolution superposition (CCCS) using a multileaf collimator(MLC) for grid therapy is demonstrated in this study. Method and Materials: Grids with the projected field openings of 8mm × 8 mm to 20mm × 20 mm were created using multiple MLC‐shaped fields for 6MV and 18MV photon beams. The separation between the grid openings and the open‐to‐blocked area ratio varied from 16mm to 36mm (from center to center) and 0.25 to 0.50 respectively. The deposited doses (profiles) with films at different depths and also the percent depth doses (PDD) were measured in a solid water phantom and compared against calculations using the CCCS algorithm in Pinnacle. Results: The PDDs were in good agreement with the calculated ones. The highest discrepancy was observed at the depth past 10cm and it was in the order of 2% for the smallest grid size. For the larger grid sizes the agreement was with in 1%. On the other hand, there was a higher discrepancy between the measured and calculated profiles. While there was a good agreement at the peaks, there was a difference at the location of the valleys. The difference in the lateral direction was in the order of about 2mm for all grid sizes and at the lowest point of each valley the CCCS algorithm over‐predicted the dose by about 50%. Conclusion: In summary we have demonstrated that the CCCS algorithm can correctly predicted the dose at the openings of the grid fields. The agreement is very good for all grid field sizes and independent of the open‐to‐blocked area ratio. However, the film measurement of the profiles showed that the CCCS algorithm over‐predicts the dose under the blocked area independent of the grid opening and the open‐to‐blocked area ratio.
35(2008); http://dx.doi.org/10.1118/1.2962439View Description Hide Description
Purpose: To compare the measured dose distribution to the planned distribution over a PTV centrally located within the lung when heterogeneity corrections are taken into account. Method and Materials: The Radiological Physics Center has constructed an anthropomorphic thorax phantom that includes a target (∼ 1 g/cm3) centrally located in the left lung (∼ 0.33 g/cm3). The phantom was irradiated by 33 institutions with results that meet criteria established for the RTOG 0236 protocol. TLDs and radiochromic films were used as dosimeters within the target region. Institutions were asked to design plans using 3D‐CRT and IMRT techniques. The TLDdose was compared to the dose calculated by the TPS at the center of the target. Film response was normalized to TLDdoses and a 2D‐gamma analysis comparison to the planned distribution was performed. Institutions whose irradiation of the phantom did not meet the RTOG 0236 criteria were not included. Results: A 2D‐Gamma analysis was done in the axial, sagittal and coronal planes through the PTV. 5% / 5mm criteria were applied. Due to limitations of the analysis software only the comparison over the axial plane is reported. 21 of the cases were planned with a superposition/convolution or AAA algorithm. For these cases, 92% ± 12% of the pixels in the analyzed region met the criteria. 12 of the cases were planned with a pencil beam or Clarkson algorithm. 74% ± 25% of the pixels meet the criteria for these cases. Conclusion: The superposition convolution heterogeneity correction algorithm showed better agreement with the measured dose distribution across the PTV than the pencil beam and Clarkson algorithms.
Work supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.
TU‐C‐AUD B‐04: Dose Perturbations Caused by Implanted Helical Gold Markers Used in Patients Receiving Proton Radiation Therapy for Prostate Cancer35(2008); http://dx.doi.org/10.1118/1.2962440View Description Hide Description
Purpose: Implanted gold fiducial markers are widely used in radiation therapy to improve targeting accuracy; however recent investigations have revealed that metallic fiducial markers can cause extreme perturbations in dose distributions for proton therapy, suggesting that smaller markers should be considered. This study's objective was to test the dosimetricimpact of various small helical, gold markers for tumor localization in patients receiving proton therapy.Method and Materials: Small, medium, and large helical wire markers with lengths of 10 mm and respective diameters of 0.04 mm, 0.25 mm and 0.5 mm were implanted in an anthropomorphic phantom. Radiographicvisibility was assessed for a kV x‐ray imaging system, and dosimetricimpact was characterized by Monte Carlo simulations and measurements of protondose. Acceptable dosimetric perturbation was estimated from previous studies to be 10%. Results:Radiographicvisibility was confirmed for all markers considered. Monte Carlo simulations indicated that the size of the marker dose perturbation depended on marker size, orientation, and distance from the beam's distal fall off. Simulations also revealed that dose perturbation in the lateral, opposed field treatment‐technique was 31% for large markers and 23% for medium markers in a typical orientation. Perturbation was not observed for the small marker, but it was deemed too fragile for transrectal implantation. Radiochromic film measurements confirmed the accuracy of the Monte Carlo model. Conclusion:Protondose perturbations from medium and large sized markers exceeded 10%. This suggests that great care should be exercised if these markers are implanted in patients receiving proton therapy for prostate cancer.Conflict of Interest: A similar presentation of this work will be made at the ICRS, International Conference on Radiation Shielding; the ICRS presentation will be more preliminary and delivered to a different audience.