Volume 33, Issue 6, June 2006
Index of content:
- Therapy Scientific Session: Room 230A
- Therapy Genera
TH‐C‐230A‐01: A Monte Carlo Investigation of the Temperature‐Pressure Correction Factor for Kilovoltage X‐Rays33(2006); http://dx.doi.org/10.1118/1.2241864View Description Hide Description
Purpose: To investigate the validity of the standard temperature‐pressure correction factor (PTP) for kilovoltage x‐rays incident on various ionization chambers using Monte Carlo simulations of radiation transport. Method and Materials: The EGSnrc Monte Carlocomputer code was used to calculate the response due to 20 kV, 40 kV and 60 kV beams as a function of chamber air density for thimble and spherical ionization chambers. The chambers studied had both graphite and C‐552 plastic walls to investigate the effect of the wall material in addition to the dimensions of the cavity. In principle, the PTP‐corrected response is independent of air density. Thus, a breakdown of the PTP correction factor is identified by any variation in the calculated response as the air density is varied. The air density associated with the reference temperature and pressure conditions in North America (22 °C, 101.325 kPa) is 1.205 kg/m3. Results: At an air density of 1.0 kg/m3, typical of Denver Colorado, the normalized PTP‐corrected response of a graphite‐walled thimble chamber due to the 20 kV and 40 kV spectra is as much as 1.7% and 1.2% below the expected response, respectively. For a graphite spherical chamber at the same air density, the calculated response is 3.8% below unity for 40 kV and 60 kV beam qualities. Calculated responses of chambers with C‐552 plastic walls are all within 0.5% of the expected response at air densities as low as 0.84 kg/m3. Comparisons of calculated air kerma calibration coefficients at different air densities indicate that the breakdown of the PTP correction factor should be easily detected experimentally. Conclusion: Variations in the PTP‐corrected response indicate that for low‐energy x‐rays the PTP correction factor inadequately accounts for the dependence of ion chamber response on the temperature and pressure. Additional correction factors are therefore necessary under these circumstances.
- Therapy General
33(2006); http://dx.doi.org/10.1118/1.2241865View Description Hide Description
Purpose: Our long‐term objective is to develop innovative carbon nanotube(CNT) based multi‐pixel microbeam array system for single cell irradiation at small temporal scales and under direct microscope observation. The electron microbeam has an adjustable energy (20–60 keV) and dose rate. The microbeam l array can simultaneously irradiate a large number of individually selected cells in vitro, instead of sequentially irradiating cells one at a time using a single radiation source. From its 2,500 or more individually controllable microbeam pixels, the device offers flexibility in irradiation pattern that can be spatially discrete or uniform and temporally continuous or pulsed. Together with new advances in biosensors and cellular imaging, the proposed device can play an important role in understanding critical signaling events for both short and long term radiation effects occurring in cellular level immediately after irradiation. Method and Materials: The CNT single cell irradiation systems utilizes the unique field emissionproperty of CNT, a quantum process that electrons escape from the metal surface under an external field. Each microbeam is generated by an individually controllable CNT pixel. The microbeam size is controlled by the Si3Ni4 electron/vacuum window and the dose rate is controlled by the external field.Results: We have fabricated the CNTfield emission electron pixels, Si3Ni4 electron/vacuum windows, and a prototype single pixel CNT microbeam device. We have performed Monte Carlo(MC) simulation on the microbeam dosimetry and the effect of the Si3Ni4 window on the electron microbeam. Initial dosimetry measurement of the CNT microbeam irradiation using GARCHROMIC film and compared with the MC result. The prototype CNT system delivered a microbeam size of 20 micron and dose rate of ∼5Gy/sec. Conclusion: Our preliminary data demonstrated that the CNT‐based low LET microbeam system is feasible to deliver large dose rate and single cell (10–20 micron) microbeam irradiation.
TH‐C‐230A‐03: A Complete MR‐Based Treatment Planning Procedure for Radiotherapy of Intracranial Lesions33(2006); http://dx.doi.org/10.1118/1.2241866View Description Hide Description
Purpose: The purpose of this study is to develop a complete treatment planning procedure for radiotherapy of brain lesions based solely on magnetic resonance imaging.Method and Materials: The MR‐based treatment planning procedure relies on converting the MRimages into CT‐like images by assigning electron density information to organ structures (i.e. brain, bone and scalp). First step in the process is to correct the MRimages for 3D geometrical distortions by applying a novel distortion correction procedure. The next stage is to segment the datasets into anatomical structures by using an automatic segmentation tool suited for MRbrainimages. Once the MRimages contain both the target volume and the electron density information, they are ready to be used for dose calculations. The resulting CT+MR and MR‐based plans were compared in terms of isodose distributions, DVHs and NTCP/TCPs. A plan ranking TCP‐based method for heterogeneous irradiation, which does not require the knowledge of radiobiological parameters, is used. Results: For all patients investigated, we found that MR‐based plans (1.5T and 3T) are in good agreement (within 1%) with their corresponding CT+MR‐based plans in terms of PTV dose coverage, DVHs and NTCP/TCPs. We compared tumor contours drawn on both 1.5T and 3T MRimages in terms of shape, volumes and their impact on CRT plans. For all patients the delineation of the tumor was simpler for 3T images due to higher contrast. For some patients the tumor volumes drawn on the 3T images were up to 60% higher than on 1.5T images. RT plan ranking shows that the 3T plans are significantly better than 1.5T ones. Conclusion: The proposed MR‐based treatment planning procedure was found to perform as good as the current clinical procedure based on CT+MR. Due to a higher contrast the tumor may be significantly better delineated on 3T images.
TH‐C‐230A‐04: Characterizing a Monochromatic X‐Ray Beam From a 1.3 GeV Synchrotron for Auger Electron Radiotherapy and Dosimetry Studies33(2006); http://dx.doi.org/10.1118/1.2241867View Description Hide Description
Purpose: Auger electron radiotherapy and dosimetry methods are being studied in preparation for future small animal irradiations using monochromatic x‐ray beams and IUdR. Aims of the present study are: (1) establish methods for characterizing the LSU CAMD synchrotron monochromatic x‐ray beam and (2) validate MCNP dose calculations in polymethylmethacrylate (PMMA). Method and Materials: The synchrotron's tunable (6–40 keV), monochromatic beam was set to 15 keV and collimated to ≈2.5‐mm wide. Beam energy was determined from photons Compton scattered by a 56 mg⋅cm−2aluminum target, whose energy was measured using a thin window 1‐mm thick × 2.54‐cm diameter NaI(Tl) scintillation detector. Beam cross section and divergence were measured using radiochromic film digitized with an Epson 1680 scanner. Depth‐dose measurements within a PMMA phantom were made using a 0.23 cm3 air‐ionization chamber. Ionization was converted to dose in air and PMMA at each depth and a percent depth‐dose curve generated. Results of MCNP5 Monte Carlo dose calculations simulating measured conditions were compared with measured data. Results: Measurements indicated the nominal 15 keV beam had energy of 15.5 keV, horizontal width of 3.1 cm, Gaussian distribution vertically with FWHM = 0.1 cm, and beam divergence <0.004 horizontally and vertically. A dose rate of 69 cGy⋅s−1, measured at 0.58‐cm depth in PMMA with 92 mA beam current, corresponds to 3.4×1011 photons⋅cm−2⋅s−1. Measured percent depth‐dose curve agreed with MCNP5 simulated curve, yielding a PMMA mass attenuation coefficient of 1.1 cm2 g−1, approximately equal to the NIST value. Conclusion: The LSU CAMD 15‐keV monochromatic beam has been characterized demonstrating utility of the measurement methods for future studies at energies suitable for iodine k‐edge capture (>33.2 keV). MCNP5 Monte Carlo calculations have been shown to predict depth dose in PMMA validating its use and showing its potential for future treatment planning dose calculations in small animals.
33(2006); http://dx.doi.org/10.1118/1.2241868View Description Hide Description
Scatteredneutrondose equivalent to a representative point for a fetus is evaluated in an anthropomorphic phantom of the mother undergoing protonradiotherapy. The effect on scatteredneutrondose equivalent to the fetus of changing the incident proton beam energy, aperture size, beam location, and air gap between the beam delivery snout and skin was studied for both a small field snout and a large field snout. Measurements of the fetus scatteredneutrondose equivalent were made by placing a neutron bubble detector 10 cm below the umbilicus of an anthropomorphic Rando® phantom enhanced by a wax bolus to simulate a second trimester pregnancy. The neutrondose equivalent in milliSieverts per protontreatment Gray increased with incident proton energy and decreased with aperture size, distance of the fetus representative point from the field edge, and increasing air gap. Neutrondose equivalent to the fetus varied from 0.025 to 0.450 mSv per proton Gray for the small field snout and from 0.097 to 0.871 mSv per proton Gray for the large field snout. There is likely to be no excess risk to the fetus of severe mental retardation for a typical protontreatment of 80 Gray to the mother since the scatteredneutrondose to the fetus of 69.7 mSv is well below the estimated radiation absorbed dose threshold of 600 mGy observed for the occurrence of severe mental retardation in prenatally exposed Japanese atomic bomb survivors. However based on the linear no threshold hypothesis and this same typical treatment for the mother, the excess risk to the fetus of radiation induced cancer death in the first 10 years of life is 17.4 per 10,000 children.
33(2006); http://dx.doi.org/10.1118/1.2241869View Description Hide Description
Purpose: To improve the contrast ratio of the multi‐terawatt Chirped‐Pulse Amplification (CPA) Ti:Sapphire laser to 1011 to allow Coulomb explosion regime of ion acceleration in the interaction of ultra‐short high‐intensity laser pulses with ultra‐thin ( < 1 micron) foils. Method and Materials: The cross‐polarized wave generation (XPW) technique in BaF2 crystals was implemented. This technique improves contrast by rejecting the low‐intensity amplified spontaneous emission(ASE) preceding the main laser pulse. Particle‐in‐cell (PIC) simulations were conducted under the anticipated experimental conditions: 225 TW in a 6.75 J, 30 fs laser pulse with no prepulse, focused to a spot size of 2.4 microns (FWHM) on thin foils of varying thickness. Results: Implementation of the crosspolarized wave generation technique resulted in a contrast improvement of three orders of magnitude to approximately 1011. The performed PIC simulations show that for a 0.2 μm thick hydrogen foil, protons with energy of about 200 MeV can be generated. In the case of the two‐layer aluminum‐hydrogen foil the maximum energy of accelerated protons is about 150 MeV, but the proton spectrum has a flatter distribution, which may be more advantageous for therapy applications. Conclusion: We demonstrated that pulse cleaning based on cross‐polarized wave generation (XPW) using two BaF2 crystals yields a 1011 contrast ratio for a 50 TW laser system. Such contrast may be sufficient for a preplasma‐free interaction of sub‐Petawatt laser pulses with a sub‐micron thickness foils at intensity of ∼1022 W/cm2. Modeling of this interaction with PIC simulations demonstrated protonenergies that are of interest for the radiation therapy.
This study was supported by the National Science Foundation through the Frontiers in Optical and Coherent Ultrafast Science Center at the University of Michigan and the National Institute of Health.
TH‐C‐230A‐07: Reconstruction of Spatially Varying Optical Properties of Human Prostate During Metexafin Lutetium PDT33(2006); http://dx.doi.org/10.1118/1.2241870View Description Hide Description
Purpose: The purpose of this study is to characterize the internal optical absorption and reduced scattering distribution of human prostate during the interstitial metexafin lutetiumphotodynamic therapy(PDT). These distributions could be utilized in PDTtreatment planning, which optimizes the arrangement and weighting of interstitial light sources to ensure sufficient photon are delivered to the treatment volume. Method and Materials: A continuous‐wave diffuse optical tomography system has been developed using a finite element reconstruction algorithm based on diffusion approximation of light distribution in biological tissue. Reconstructed images are presented from simulated data and clinical data acquired from prostate cancer patient being treated by PDT. Sourcedetector arrangement is identical in simulated data and patient data. The contour of prostate was obtained using ultrasound imaging. The meshes were generated by MATLab PDE Toolbox. Results: The synthetic measurement data were calculated for a rectangular phantom containing a single absorption anomaly and a single scattering anomaly. The model had a background of , . The absorption anomaly was located at (15mm,15mm), with a radius 5mm, ; the scattering anomaly was located at (35mm,10mm), with a radius 5mm, .A total number of 5 sources and 12 detectors yielded 60 measurements (12 detectors ×5 sources). The reconstruction basis uses a coarser mesh (100 nodes) to reduce the degree of freedom. The reconstruction images successfully recover the both anomalies with good localization. The clinical DOT imaging was performed on 70‐year old male subject. The reconstructed prostate μa varied between 0.025 and 0.07 mm−1 and μ′s ranged from 1.1 to 2 mm−1. These results are consistent with previous measurement using a point‐by‐point method. Conclusion: We have shown that this modeling and reconstruction algorithm can produce fast and reliable images of internal optical properties with fast computation time.
TH‐C‐230A‐08: A Prototype Rotational Immobilization System for a Proposed Static‐Gantry MicroRT Device with Tomographic Capabilities33(2006); http://dx.doi.org/10.1118/1.2241871View Description Hide Description
Purpose: Proposed small animal irradiation devices are typically isocentric gantry systems. Multi‐axis gantry systems require significant resources to develop and maintain. A static gantry system could provide the functionality of a multi‐axis gantry via subject rotation if subject motion could be minimized. In this study, we evaluated internal organ motion of mice within a prototype immobilization device during rotation. Method and Materials: Mice were anesthetized and immobilized in a prototype device designed to rigidly support animal positioning during rotation along the cranio‐caudal axis. The head and tail of the mouse were allowed to extend beyond the immobilization device as an internal control. Validation of internal and external immobilization was assessed using computed tomographyimaging at multiple rotation angles. CTimages were co‐registered (translated/rotated) using our research treatment planning system (CERR) into a common reference frame. Internal organ motion was assessed qualitatively by examination of internal anatomy in overlaid multiplane CTimages. Quantitative evaluation of organ motion was assessed by delineating organ structures in CERR and comparing relative organ volumes and centers of mass. Results:CTimaging demonstrated minimal exterior contour changes (<1mm) in the immobilized regions during rotation. Un‐immobilized regions demonstrated the expected gravitational positional changes. Internal organs demonstrated sub‐millimeter changes in organ centers of mass (heart and lung) and small (<5 mm3) changes in volume during rotation. These variations were similar to the differences when the same CT was re‐contoured multiple times by the same operator and likely represent intra‐observer contouring variations. Conclusion: A microRT device with a stationary irradiator, collimator, tomographic imaging system, and rotating subject would reduce the overall cost and complexity of the unit. This study demonstrates rotational immobilization of small animals is feasible.
This work supported in part by NIH R21 CA108677 and by a grant Varian, Inc.
33(2006); http://dx.doi.org/10.1118/1.2241872View Description Hide Description
Purpose: To test the feasibility of a system of software (MINERVA/PEREGRINE) developed by the Idaho National Laboratory (INL) and the Lawrence Livermore National Laboratory (LLNL) for supporting quality assurance (QA) review of cooperative‐group clinical trials treatment planning data within the Advanced Technology QA Consortium (ATC). Method and Materials: MINERVA is an open architecture, open source code system, designed to accommodate any computation engine through a plugin structure. MINERVA supports two types of data storage ‐ relational databases and XML files. Patient data is stored in a relational database. XML‐based import/export tools have been developed to transfer patient information between QA Centers and reviewers. Tools have been implemented as plugins to allow addition of more advanced tools. The research version of the LLNL PEREGRINE Monte Carlo code has been relocated to UC Davis Medical Center. The basis has been created for an integration of PEREGRINE with ITC, and it has been integrated with MINERVA as a calculation engine. Results: MINERVA supports submission of digital treatment planning data using RTOG format. Ability to import DICOM‐RT objects that satisfies the ATC DICOM conformance statement is needed. MINERVA provides display of DVHs and axial patient images with overlaid organ‐at‐risk/target‐volume contours, as well as user‐defined isodose curves. Users can edit contours, recalculate DVHs for these user‐defined structures, and display point doses. Test cases for several body sites have been calculated using PEREGRINE to demonstrate feasibility. We believe that the use of Monte Carlo simulation will become a key tool for credentialing and QA review of clinical trials treatment planning and verification data in the near future. Conclusion: The MINERVA/PEREGRINE software system appears to be well suited to meet the needs regarding QA of data submitted for future ATC‐supported clinical trials. Conflict of Interest: This work was supported by NIH U24 Grant CA81647.
33(2006); http://dx.doi.org/10.1118/1.2241873View Description Hide Description
Purpose: To be able to experiment with various topics in advanced radiation therapy planning using a stand‐alone Windows‐based software.Method and Materials: Product code for IMRT planning was merged with research code for adaptive radiation therapy and dose computations. An architecture and graphical user interface tailor‐made for representing adaptive treatments was designed. Code for protondose computation and treatment planning was developed and integrated into the system. Results: A software package that executes on a standard PC or advanced laptop has been developed. The system is capable of IMRTtreatment planning using dose‐volume based functions, EUD, Poisson‐LQ and LKB biological models, both as objectives and hard constraints. Pencil beam and heterogeneity corrected collapsed cone dose computations can be used. The system can optimize every relevant combination of pencil beam weights, SMLC segments, gantry, couch and collimator angles. It is possible to perform intensity modulated proton therapy planning. Models for tumor repopulation and repair are included and irregularities in the fractionation scheme can be compensated for by allowing the dose to vary between fractions during optimization. The system exhibits a comprehensive GUI and functionality for simulation and evaluation of adaptive/IGRT treatments with algorithms for deformable dose accumulation based on information from portal imaging and 3D imaging modalities such as onboard CT scanners. Errors due to patient setup and organ motion can be counteracted and compensated for by couch shift and on‐ and offline IMRT replanning. Conclusion: A software environment suitable for studying various advanced topics in modern radiation therapy has been developed. The system has proven useful in research and development in IMRT optimization, biological models, proton therapy and adaptive radiation therapy.Conflict of Interest: The authors are employees and stock owners of the submitting company.