Volume 33, Issue 6, June 2006
Index of content:
- Therapy Scientific Session: Room 224 C
- Teletherapy Planning and Delivery I
33(2006); http://dx.doi.org/10.1118/1.2241467View Description Hide Description
Purpose: To establish a quantitative method for evaluation of 3D conformal and IMRT plans based on organ specific tolerances and target coverage assessment. Method and Materials: We propose a novel evaluation criterion, which reflects both target coverage and overdoses in organs at risk (OARs). Critical Organ Scoring Index (COSI) is defined as: , where V >tol is the volume of OAR receiving more than tolerance dose and TC is the partial volume of target receiving at least prescription dose. To assess overall plan conformity we propose a 2D graphical representation of COSI vs. Conformity Index (CI). This method enables quantitative evaluation of competing plans in terms of multiple organs at risk. The COSI‐CI plots were tested for evaluation of the following treatment sites: maxillary sinus and pancreatic tumors, to compare non‐coplanar 3D and IMRT plans, and cavernous sinus meningiomas for stereotactic radiation with either dynamic arcs or IMRT.Results: For all three sites COSI‐CI plots assisted the physician in choosing the optimal plan, in terms of both target coverage and critical organ sparing. We verified each choice by analyzing individual DVHs and isodose distributions. Comparing our index to the widely used Conformation Number, we found that in all cases where there were discrepancies between CN and COSI in the choice of optimal treatment plan, the COSI‐CI graphs led to the better plan. Conclusion: We introduced a novel scoring index, COSI, which is a measure of both target coverage and critical organ overdose. Using the COSI index, we propose a two‐dimensional representation of plan quality for comparison purposes. The method was found it to be a quick and reliable tool in aiding physicians in the choice of correct plans. The main advantage of the proposed methodology is its ability to simultaneously compare multiple plans as well as multiple critical structures.
MO‐E‐224C‐02: Quantification of Respiration‐Induced Dosimetric Impact On Liver for Abdominal Radiotherapy33(2006); http://dx.doi.org/10.1118/1.2241468View Description Hide Description
Purpose: To quantify the dosimetric impact of respiratory motion on liverdose for IMRT abdominal radiotherapy based on 4D CTimagingMethod and Materials: 4D‐CT images of a patient with klatskin tumor were acquired on a GECT scanner during free‐breathing. Respiratory information was recorded using the Varian RPM system. The images were sorted retrospectively into ten respiratory phases. For each phase‐binned 4D data set, manual contours for liver were delineated. The Corvus inverse planning system (NOMOS Inc.) was used to generate optimized treatment plans. Two kinds of image registration methods, feature‐based iterative closest point (ICP) rigid image registration and normalized mutual information (NMI) non‐rigid image registration method, were implemented to transform each of the ten phases to a reference phase. The reference phase was the end of expiration (50%phase). A composite 4D‐DVH, which accounts for respiratory induced voxel motion, was subsequently calculated using equal weighting for each respiratory phase. A dose of 45Gy to the target was prescribed. The treatment plan was optimized to provide a minimum of 95% isodose coverage for PTV. Results: Variation of liverdose throughout each of the 10 calculated phases was within 5%. 4D effective DVH showed reduced volume coverage up to 18% for the same dose. In the high dose region of the liver DVH, the dose was less for the NMI algorithm as compared to the ICP algorithm. Conclusions: Two kinds of image registration methods have been implemented to derive 4D effective DVHs for an abdominal patient. In this study we found that respiratory‐induced motion does not produce a significant alteration of the liver DVH between the 10 phases. However, the 4D effective DVH method shows that the liver received less dose.
33(2006); http://dx.doi.org/10.1118/1.2241469View Description Hide Description
Purpose: We are presenting the design and implementation of a 3D‐graphical tool for the detection of potential collisions of various linac components for patient‐specific external treatment planningMethod and Materials: The graphical tool uses the Virtual Reality Modeling Language (VRML) to model the exact geometry of any treatment machine by reading its manufacturer's CAD design files. The robust system is based on VRML and Java programming that allows for accurate simulation of any linac hardware module based on the manufacturer's CAD drawings. Results: The tool predicted eminent collisions between different linac components graphically for a simulated Varian 2100EX for certain gantry and table angles. The collision angles were verified manually on the linac and found to agree with the predicted angles from the tool. Conclusions: We have developed a 3D graphical simulation tool that can be used as a stand alone application to assist in external treatment planning by visually simulating collisions between various linac hardware components. Unlike other anti‐collision methods developed so far in the literature, our tool would be able to model the details of the treatment linac and add‐on devices for patient‐specific setups. Hence, the tool will create patient‐specific realistic collision maps for any external treatment scenario. The tool can be used as a stand‐alone program and it is platform independent.
MO‐E‐224C‐04: Motorized Multi‐Leaf Collimator for Electrons: Measurements with a Prototype and Monte Carlo Simulations33(2006); http://dx.doi.org/10.1118/1.2241470View Description Hide Description
Purpose: To develop a motorized multi‐leaf collimator for electrons (eMLC) and to compare preliminary measurements to Monte Carlo simulations.Method and Materials: An eMLC has been developed. It is the first prototype with fully motorized capabilities. The eMLC is remotely controlled by the operator using home brewed and fully graphical software running on a Windows workstation. The control workstation is connected to the eMLC's custom‐built electronic controller which keeps track of the component states and executes various low‐level commands, including leaf displacement orders. There are 36 independent brass leaves on each side of the eMLC and the maximum size of the generated field is 25.2 × 19.5 cm2. The interleaf distance is less than 0.03 mm. The eMLC prototype is an add‐on device for the Elekta Precise linac (Elekta Ltd., England). The actual distance from the source to the bottom of the leaves is 95.3 cm. The eMLC has been modeled using the BEAMnrc Monte Carlo toolkit and every possible leaf positions is reproducible with our Monte Carlo model. Results: Various preliminary measurements were performed: open field, closed field, one leaf profile, interleaf leakage, leaf transmission, and comparison with conventional custom cutouts. For all measurements, comparison to Monte Carlo simulations are carried out. The transmission through the leaves is 2.4% for a 12 MeV field at the surface of a water phantom with SSD=100 cm. The interleaf leakage is negligible as no interleaf pattern is detected on a closed field profile for all the investigated energies (up to 12 MeV). Conclusion: The motorized eMLC prototype is versatile and easy to operate with a computer control from outside the treatment room. Possible applications of the eMLC go from simply replacing the conventional custom cutouts to complex usages like MERT or electron arc therapy.
Research sponsored by Elekta Ltd.
MO‐E‐224C‐05: Comparison of TomoTherapy with Conventional Electron/X‐Ray Treatment Plans for Chest Wall33(2006); http://dx.doi.org/10.1118/1.2241471View Description Hide Description
Purpose: Post‐mastectomy radiotherapy (PMRT) of the chest wall (CW) often will involve use of electron beams due to their rapid fall‐off of dose near the practical range of the electron energy. The TomoTherapy helical, fan‐beam delivery system, along with the inverse‐planning optimizer, has the potential to provide planning and delivery of 6 MV x‐rays to superficial target volumes conventionally treated with electrons. In the present study, five chest wall patients were planned for TomoTherapy and compared to their conventional plan that utilized electron beams. Analysis of breathing motion were not included. Methods and Materials: Five PMRT patients treated with electron beams and planned on the Pinnacle treatment planning system (TPS) were selected. A planning target volume (PTV) was generated to follow the isodose contour defined by 90% of the prescribed dose on the Pinnacle plan. Target dose homogeneity in the Tomotherapy TPS dose optimizer was relaxed to better achieve critical structure dose objectives when necessary. Normal tissue complication probabilities (NTCP) were calculated for the lung and the heart. A questionnaire form was provided for the radiation oncologist to evaluate each plan. Results: For all cases, the TomoTherapy plan was rated superior to the Pinnacle plan by the radiation oncologists. The TomoTherapy TPS produced a significantly more uniform dose distribution in the PTV. The average volume of ipsilateral lung receiving dose above 20 Gy was reduced from 21.5% to 17.7%. The average volume of heart receiving dose above 30 Gy was reduced from 2.2% to 1.3%. NTCP for the lung and heart were also reduced in the TomoTherapy plan. Conclusion: The results of this study demonstrated that TomoTherapy is able to deliver clinically‐acceptable dose distributions to a “static” model of PMRT patients conventionally treated with electron beams.
Supported in part by a research agreement with TomoTherapy, Inc.
MO‐E‐224C‐06: Improving Homogeneity of Abutment Dosimetry in Segmented‐Field Electron Conformal Therapy Using Variable Insert Positioning33(2006); http://dx.doi.org/10.1118/1.2241472View Description Hide Description
Purpose: To improve dose homogeneity in segmented‐field electron conformal therapy by matching penumbra of abutted fields. Method and Materials: A Varian 21EX electron applicator was modified to allow for energy‐dependent positioning of Cerrobend inserts resulting in energy‐dependent air gaps. Air gaps were chosen based on theoretical calculations to approximately match penumbra for 6, 9, 12, 16, and 20 MeV beams at 1.5‐cm depth in a water phantom at 100‐cm SSD. Treatment plans developed for four simulated target volumes using the modified applicator were compared to identical plans using the standard applicator. Improvement in dose homogeneity was assessed by comparing maximum and minimum dose, mean dose, and sigma of the dose distribution in the target volume for the two plans. Subsequently, electron blocks were cut with diverging edges using the Compu•cutter® system, and dose plans using the modified applicator were delivered to Kodak XV film in a polystyrene phantom to demonstrate feasibility. Results:Treatment planning results using the modified applicator showed improved dose homogeneity in all four simulated target volumes as compared to plans using the standard applicator. Averaged for all four PTVs; dose spread (Dmax − Dmin) decreased by 35%, σ of the dose distribution decreased by 29%, and D90–10 decreased by 31%. Dose delivered to a film phantom using the modified applicator was found to agree well with predictions (within approximately 5%) of the Pinnacle treatment planning system in the abutment region of the PTV (±2 cm from the abutment edge at depths ⩾1.5 cm). Conclusion: The results of this study suggest segmented‐field electron conformal therapy can be delivered with significant improvement in dose homogeneity as compared to the current method by using energy‐dependent positioning of electron inserts to match beam penumbra.
33(2006); http://dx.doi.org/10.1118/1.2241473View Description Hide Description
Purpose: To develop a 4D dynamic arc therapy with the capability of MLC‐shaped irradiation field tracking for moving tumor targets. Method and Materials: 4D CTimages at 10 different breathing phases were acquired and transferred to a treatment planning system. The tumor target was contoured and a 3D conformal arc therapy (3DCAT) plan was generated for each phase. For each 3DCAT plan, the selected control points (gantry angles) and the MLC‐defined conformal apertures were correlated to the patient's respiration phases. A program was developed to obtain a 4D DMLC leaf sequence with the capability of tracking the target motion based on the ten 3DCAT DMLC leaf sequence files. To evaluate the 4D plan, a deformable registration was adopted to combine dose distributions and DVHs and the results were compared with those obtained using the conventional 3D and gating plans. Five lungcancer cases and film measurements embedded in a moving phantom were used to investigate the feasibility of the proposed technique. Results: An efficient 4D dynamic arc therapy was implemented. Experimental measurements indicated that the dose distribution in the moving target delivered using the proposed technique is equivalent to that in a static target delivered using a conventional 3D arc therapy. Compared with the 3D plans, the target received more conformal doses and the sensitive structures, especially the lung, were better spared in the 4D plans for all the test cases. The treatment time for the 4D plan is comparable to the 3DCAT plan, much more efficient than the gating treatment. Conclusion: It is feasible to incorporate the intra‐fraction organ motion into a 4D dynamic arc treatment planning to track the moving targets. Compared with conventional treatment strategies, this technique has great potentials to provide more conformal dose distribution and better sensitive structure sparing with extremely high efficiency.
- Clinical Measurements I
33(2006); http://dx.doi.org/10.1118/1.2241536View Description Hide Description
Purpose: Liquid ionization chambers (LICs) have characteristics that can remedy some of the drawbacks of air‐filled ionization chamberdosimetry: large sensitive volumes (i.e. low spatial resolution), fluence perturbations, and energy dependence over the clinical range of beam qualities. However, there are significant problems in liquid chambers. High ionization density and low ion mobility lead to high ion recombination rates. In this work, extensive experimental work has been performed to investigate properties of a new liquid chamber. This includes chamber stability over time, chamber reproducibility, and establishing recombination corrections. Methods and Materials: The new chamber is called the GLIC‐03 (Guarded Liquid Ionization Chamber). The diameter of the collecting electrode is 1.5 mm and the plate separation is 0.4 mm, giving a sensitive volume of 0.7 mm3. The dielectric liquid used is isooctane. We used the 18 MV beam of a Varian Clinac 21EX linear accelerator. The lowest pulse rate setting, 100 MU/min, was used in order to avoid incomplete collection of charge from one pulse before the arrival of the next. Measurements were taken in solid water at 15 cm depth, with various field sizes and SSDs. Boag's theory for general collection efficiency for parallel‐plate gas ionization chambers, applied to isooctane in pulsed radiation, was used for recombination corrections. Results: The GLIC‐03 response varied by less than 1% over 10 hours, and was reproducible within 1.5% of the mean over different liquid fills. The collection efficiency decreased with increasing dose per pulse due to general recombination of ions from a larger number of ionizing particle tracks. Recombination corrections were within 1% for low dose rates and high electric field strengths. Conclusion: The establishment of these characteristics in the present work allows us to perform accurate relative measurements in high gradient non‐equilibrium fields as well as energy dependence investigations.
33(2006); http://dx.doi.org/10.1118/1.2241537View Description Hide Description
Purpose: At present, clinical dosimeters are limited to point or planar measurement, and hence do not provide the comprehensive 3D information ideal for verification of advanced delivery techniques. In this work we present a clinically viable 3D dosimetry system comprising a PRESAGE™ dosimeter with read out by an optical‐CT scanner.Method and Materials: A novel solid dosimeter called PRESAGE™ has been developed which is composed of polyurethane polymer and radiochromic leuco dyes. PRESAGE™ exhibits a stable color change and hence optical density (OD) change when exposed to ionizing radiation. A PRESAGE™ cylinder of 16cm diameter × 11cm height was taken through the treatment planning process and a 5‐field 6MV conformal radiation treatment was delivered by a Varian® linear accelerator. The radiation induced OD change was imaged in 3D by an optical‐CT scanner and this measured distribution was then compared with the corresponding dose distribution calculated by the treatment planning system, as well as to independent measurement by GAFCHROMIC® film. Intercomparisons between the three dose distributions were made by superimposing isodose lines and calculating gamma maps (with criteria 4% dose difference and 4mm distance to agreement). Results: Given stable temperature and protection from exposure to incandescent light, the dose response of PRESAGE™ was observed to be robust to all aspects of the lab. The 3D dose distribution measured in PRESAGE™ showed good agreement with the calculated treatment plan (Eclipse) as well as the independent film measurement at all percent doses >30% (i.e. in regions further than 1cm from the wall). Gamma comparison shows that the PRESAGE™ measurement agrees with both the calculation in treatment plan and the film measurement within 4% dose difference and 4mm distance to agreement. Conclusion: This work presents the PRESAGE™/optical‐CT combination as a practical 3D dosimetry system which can provide comprehensive quality assurance of advanced treatment techniques.
33(2006); http://dx.doi.org/10.1118/1.2241538View Description Hide Description
Purpose: To characterize the behavior of MOSFETs under radiation of various clinically relevant energies used in radiotherapy and radiology by evaluating its sensitivity or threshold voltage shift (CF) with regard to total integrated dose.Method and Materials: Seven p‐type, duals bias MOSFETs from Thomson & Nielsen were investigated. They were exposed to four radiationssources: (1) 60Co unit (〈E〉γ: 1.25 MeV), (2) 192Ir HDR unit (〈E〉γ;: 0.38 MeV), 30 kV beam (〈E〉γ: 14.8 keV) and (4) 150 kV beam (〈E〉γ: 70.1 keV). The MOSFET's sensitivity (CFw) was evaluated at various moments in time and was calculated as the ratio of the measurement Mw (mV) over the estimated dose value Dw (cGy) both in water. Results: The sensitivity of MOSFET is express by their calibration factor (CFw), and allows the user to associate the reading displayed by the device (mV) to a dose value (cGy). The CFw value diminishes with increasing threshold voltage, especially for low energy radiation. It is stable for 60Co irradiations, while it decreases of 6%, 5% and 15% for beam energies of 192Ir, 150 kV and 30 kV respectively. This behavior is explained by an alteration of the effective field applied on the MOSFET (bias), caused by the accumulation of holes at the SiO2 interface. It is strongly dependent on the radiation nature (LET) and particularly affects low x‐ray energies. Conclusions: Those results are of major interest since, following the company recommendations to calibrate the device every 7 000 mV, it could lead to a significantly underestimated dose. A calibration of the device before every use and performing more than one measurement (thus using a mean dose value) should compensate the observed behavior.
33(2006); http://dx.doi.org/10.1118/1.2241539View Description Hide Description
Purpose: To determine if a CR system that had been used for radiation therapy digital imaging since 2001 could also be used for IMRTquality assurance, and if the system could be used in helical tomotherapy quality assurance.Methods and Materials: The CR system used consisted of a desktop CR reader that utilizes storage phosphor plates, a 650 nm laser diode scanning beam source, and a high luminance light box for plate erasure. The CR plates are made of phosphor, coated with a photostimulable storage phosphor (BaFBR:Eu2+). Three types of dose‐to‐responsecalibrations were performed 1.) Static square fields; 2.) An IMRT step‐wedge; and 3.) A rotational helical tomotherapy delivery with concentric rings of known dose. All readings were taken with 6 MV beams. Like TLDs, some of the trapped charge carriers in the storage phosphor gradually decay with time. Because of the decay effect, it was important to determine the best time to wait between exposure and scanning. Five helical tomotherapy patients were selected as test cases for the CRdosimetry. Measurements were made in a cylindrical phantom with the CR plate and again with radiographic film. Calibration techniques #1, #2, and #3 were applied to the CRimages to determine which was the most appropriate. Dose differences and gamma comparisons were made between the calculated and measured doses.Results: A time and field size dependence was observed. After comparing readings from different time intervals, ranging from one to twenty minutes, it was decided that four minutes was an optimal time to wait between exposure and scanning. Also, gamma for the CRimages was significantly worse than the film images taken for the same patient. Conclusions: The field‐size dependences, inconsistencies between calibration techniques, and plate decay make the CR system used in this study non‐usable for IMRTdosimetry.
TU‐C‐224C‐05: Energy Response of LiF:Mg,Ti Thermoluminescent Dosimeters to Moderately Filtered X‐Ray Spectra in the Range of 20 to 250 KV Relative to 60Co33(2006); http://dx.doi.org/10.1118/1.2241540View Description Hide Description
Purpose: To use experimental methods to determine the response of LiF:Mg,Ti thermoluminescent dosimeters(TLDs) irradiated using moderately filtered (M‐series) x‐ray spectra in the energy range of 20 to 250 kV relative to the response to 60Co photons. Also, to determine if LiF:Mg,Ti TLDs are intrinsically linear detectors (i.e. the response is proportional to energy imparted). Method and Materials:TLDs were irradiated to a known air kerma using the NIST traceable M‐series x‐ray beams, which were located at an Accredited DosimetryCalibration Laboratory (ADCL), in the range of 20 to 250 kV. Using each x‐ray beam, several sets of TLDs were irradiated to different air kerma levels to take into account any dose non‐linearity. TLD response was then compared to that from several sets of TLDs irradiated at corresponding air kerma levels using 60Co. The Monte Carlo code MCNP5 was used to correct for scatter from the holder and to determine the predicted/expected TLD response to the experimentally used x‐ray beams. Results: The measured TLD energy response compared to the response to 60Co shows a rapid decrease toward very low photon energies. This response dropped to approximately 0.90 at the lowest effective energy of 11.5 keV. The highest response was found to be 1.37 at an effective energy of 28.5 keV. The results showed poor agreement between measured energy response and calculations using the mass‐energy absorption coefficients of pure LiF. A significant increase in measured response compared to calculated response was seen at effective energies higher then 25 keV. Conclusion: These results demonstrate that the measured energy response differs by up to 14% from Monte Carlo calculations and is highly dependent on the energy of the source. The results also suggest that LiF:Mg,Ti TLDs are not intrinsically linear with energy imparted.
TU‐C‐224C‐06: Determination of TG‐43U1 Recommended Parameters for Elongated RadioCoil™ Brachytherapy Sources 1.0 to 6.0 Cm in Length Using Experimental and Monte Carlo Simulation Techniques33(2006); http://dx.doi.org/10.1118/1.2241541View Description Hide Description
Purpose: In this project TG‐43U1 recommended dosimetriccharacteristics of newly designed elongated RadioCoil™ brachytherapy sources(1 to 6)cm in length have been determined following experimental and Monte Carlo simulation techniques. Monte Carlo simulated dose profiles have also been calculated for sources 1 to 6cm in length. Materials and Methods: TG‐43U1 recommended dosimetriccharacteristics (Λ, g(r), F(r,θ), φan) of RadioCoil™ sources have been determined using experimental(TLD) and Monte Carlo(MCNP5) simulation techniques. MCNP5 simulations were performed in spherical Solid Water™ and water phantoms 20cm in diameter for 108 histories. F(r,θ) of RadioCoil™ were determined for angles 0° to 90° for r⩾L/2(where L=active length), and angles 5°⩽θ⩽90° for r⩾L/2, with 5° increment. TLD measurements were performed in solid Water™ using same geometric arrangement as that of Monte Carlo simulations. Measured and calculated dose profiles along the longitudinal axis of these sources were utilized to validate the dose calculation with commercially available treatment planning systems. Results: Results of these investigations indicate good agreement between MCNP5 simulated and TLD determined values for RadioCoil™ sources. Upon the good agreement between these two methodologies, confirming the accuracy of our simulation geometry, dosimetric parameters of these sources have been determined in liquid water for their clinical applications. It has been found that in order to achieve a good agreement between the treatment planningdose distribution, the F(r,θ) of these sources have to be determined at radial distances ranging from 0.5 to 5.0cm with 0.5cm increment and L/2 ±0.2cm. Conclusion:Dosimetriccharacteristics of newly designed RadioCoil™ have been determined following TG‐43U1 recommendations using experimental and Monte Carlo simulation technique. In these determinations it has been found that the dose profile can be closely reproduced if the 2D anisotropy function is determined at 0.5 cm increments for radial distance ranging from 0.5 cm to 5cm, and L/2 ± 0.2cm.
33(2006); http://dx.doi.org/10.1118/1.2241542View Description Hide Description
Purpose: To propose dosimetric guidelines specifically designed for the Cyberknife radiosurgery system. Non‐availability of 10×10cm2 field and use of small circular collimators (5mm to 60mm) pose serious problems, that have been faced in this study by means of 8 different detectors and Monte Carlo simulation. This work is oriented to measurement of total scatter factors (Sc,p) and to reference dosimetry, though indications will also be given in view of a comprehensive guideline. Method and Materials: PTW PinPoint 31014, Exradin A16 and T14P microchambers, TN 502RDM micromosfet, PTW 30008 diode and TM60003 diamond, MD55 and EBT radiochromic films were used to measure Sc,p. Monte Carlo simulations (BEAMnrc) were used to produce phase space descriptions at the exit plane of each collimator, to calculate: 1) theoretical Sc,p values in water, and 2) correction factors to be applied to Sc,p as measured by 5 detectors (PinPoint, A16, T14P, diode, diamond), obtained by simulating shape and chemical composition of each detector. BEAMnrc was also used to calculate stopping power ratios and chamber correction factors for the Cyberknife linac, to decide whether values of kQ from the IAEA398 protocol could be applied without using a 10×10cm2 field. Results: Sc,p of the 5mm collimator as measured by simulated detectors averaged 0.653 − 9%+14%. Variation for larger collimators was smaller. After Monte Carlo correction, Sc,p of the 5mm collimator became 0.686 −2%+1%. Pure Monte Carlo calculation gave Sc,p=0.715 +/−1%. Calculation of correction factors showed that kQ values for the investigated chambers could be chosen when using IAEA398, introducing +/−0.2% uncertainty. Conclusion: Pure Monte Carlo calculation gave higher values of Sc,p compared to Monte Carlo‐corrected measurement. The latter is to be preferred because correction factors are less sensitive to beam parameters than pure calculation of Sc,p. For determination of Sc,p use of microchambers and Monte Carlo correction is recommended.
33(2006); http://dx.doi.org/10.1118/1.2241543View Description Hide Description
Purpose: To design a small radiation facility for the partial‐ and fully ‐ body irradiation of zebrafish embryos, cell cultures or any other small specimen used for radiobiology studies. Method and Materials: Zebrafish embryos larger than 1mm are the main animal to be irradiated in this micro‐irradiator. Radiation is provided by a 50 kV photon beam from a miniature x‐ray source, Xoft Inc., CA. Radiation field is delimited by a pinhole collimator. Diameter of the pinhole ranges from 0.5mm. A movable table and a video camera connected to a computer are used to position the specimen under the beam. Radiochromic film has been irradiated to test positional accuracy and dose distribution. Results: Coordinates of the position of the zebrafish with respect to the collimator are calculated from the image provided by the video camera and sent to the computer‐controlled movable table to position the specimen under the beam. The micro‐irradiator is totally portable and it can fit on the desktop. Positioning can be acquired with uncertainties of 50 μm in X and Y axis. Collimator design and portable shielding reduce both primary beam and scattering at safe radiation levels. Dose distribution is good enough for zebrafish irradiation and penumbra is in the order of 150μm ± 50μm. Conclusion: The designed micro‐irradiator has attractive characteristics to facilitate zebrafish irradiation. Its portability and shielding simplicity make it adequate for any radiobiology laboratory. Its uncertainty in positioning is significantly smaller than zebrafish embryo size and radiation penumbra is acceptable for specimen larger than 1mm. This novel micro‐irradiator design is appropriate for irradiation of partial‐and fully‐ body zebrafish, cell cultures and any other small specimen used in radiobiology.
33(2006); http://dx.doi.org/10.1118/1.2241544View Description Hide Description
Purpose: To study the dosimetric characteristics of a gated fiber‐optic‐coupled detector for measuring absorbed dose from a linear accelerator. The favorable properties of these dosimeters are their superior spatial resolution, real‐time readout and potential as in‐vivo dosimeters. In an application that takes advantage of its spatial resolution, small field tissue‐phantom‐ratio (TPRs) measurements from 0.6×0.6 to 2.0×2.0 cm2 are obtained and compared to those measured with a diamond and diode detector.Method and Materials: The detector is a short length of Cu1+ fused doped quartz fiber coupled to a fiber‐optic cable. It has an atomic number of about 10.8 with a density of 2.2 g/cm3 and is 1 mm long with a diameter of 0.4 mm. The background signal generated by Cerenkov radiation and native fluorescence within the optical fiber during irradiation is separated from the detector luminescence via gating the signal with the radiation pulses from the accelerator. A linear accelerator provides mega‐voltage photon and electron beams to investigate its energy response, dose rate dependence, dose linearity and reproducibility. Results: There is no measurable difference in the detector response between 6 and 18 MV photons. However, for electrons the dose response increases gradually by 7% from 6 to 20 MeV. Its dose rate response relative to a Farmer chamber exhibits a behavior similar to a diamonddetector decreasing by about 4.5% from 0.8 to 10.7 Gy/min for both 6 and 18 MV photons. The measured response is linear from 0.2 to 10 Gy and its reproducibility is better than 2%. The small field TPR measurements are in good agreement with the diamond and diode detector.Conclusion: The dosimetric properties of this detector compare favorably with other radiation detectors, and its small size and optical interface make it potentially very useful for small field and in‐vivo radiation dosimetry.
33(2006); http://dx.doi.org/10.1118/1.2241545View Description Hide Description
Purpose: To elaborate a transfer‐function approach to characterizing the response of watercalorimeters to abbreviated exposures in radiation beams. Method and Materials: Typical watercalorimeters in use today derive from the original design of Domen, inferring absorbed dose from temperature measurements at a single point in a partially irradiated, extended volume. The success of the approach in deducing locally absorbed dose depends on heat transfer being “sufficiently” slow temporally and broad spatially compared to time‐ and space‐scales of the measurement. In order to address these issues of “sufficiency” more quantitatively, we have undertaken an approach to the problem that assesses the impulse‐response of the calorimeter to spatially and temporally localized radiation. The approach involves simple analytical models of heat conduction and finite‐element methods that yield predictions of single‐point temperature waveforms obtained from thermistors in a calorimeter. Output from the models is compared with experiment over wide variations in shutter period (30s–10,800s) and duty cycle (5%–50%). Results: Our findings show that heat conduction due to typical dose inhomogeneities within watercalorimeters induces systematic variations in estimated dose that traditional data‐analysis techniques ignore. Moreover, these variations do not diminish at smaller duty cycle, suggesting that they are intrinsic to the exposure time of the experiment, and not to the use of periodic exposure techniques. Essential features of the measurements are reproduced by both finite‐element simulations and a simple analytical model. An RC‐circuit analogue derived from the latter suggests that conduction phenomena with a spatially masked beam proceed by a multiplicity of times scales, ranging from seconds to hours, which contribute significantly to systematic variations in estimated dose. Conclusions: The variations of estimated absorbed dose described here suggest that a small systematic error (<1%) may affect existing standards measurements based on watercalorimetry.
- Monte Carlo Methods
TU‐D‐224C‐01: Effects of Choice of the MLC Material On Neutron Dose Equivalent Outside of the Treatment Field in Proton Therapy33(2006); http://dx.doi.org/10.1118/1.2241559View Description Hide Description
Purpose: To determine the effects of choice of MLC material on neutron dose equivalent outside of the treatment field in proton therapy.Method and Materials: A four‐dimensional GEANT4 Monte Carlo simulation of a commercially available proton therapy nozzle has been used to calculate the ratio of neutron dose equivalent to therapeuticproton absorbed dose (HN/DP). HN/DP varies significantly with proton beam energy, materials used in the nozzle, treatment technique, and range modulation. Technical specifications of the nozzle have been provided by the vendor. Lateral beam spreading is achieved primarily by binary pre‐scatterer foils and a double contoured scatterer. Modulation is generated by a stepped propeller positioned at nozzle entrance, and pre‐collimation is done by nickel inserts. A preliminary design of a multi‐leaf collimator has been added to the simulation, providing final collimation. Four different data sets have been generated, each one corresponding to a different MLC composition: tungsten, brass, iron and stainless steel. Results: A 200 MeV proton beam is used in order to produce a large circular treatment field with a 10 cm radius, a depth of penetration of approximately 22 cm, and an SOBP width of 10 cm. Neutron dose equivalents are calculated outside the treatment field at three distances from isocenter: 15 cm, 25 cm, and 47 cm (along beam direction). The highest HN/DP ratios are calculated for tungsten, and vary from 10 mSv/Gy at a distance of 15 cm off axis, to 0.7 mSv/Gy at 47.5cm off axis. Corresponding HN/DP ratios for stainless steel are 6.93 mSv/Gy and 0.46 mSv/Gy, representing the lowest calculated values. Conclusion: A steel MLC produces, on average, 45% less secondary neutron dose deposit compared to a tungstenMLC. The result indicates potential for dose reduction in proton therapy when a steel collimator is used instead of a tungstencollimator.
33(2006); http://dx.doi.org/10.1118/1.2241560View Description Hide Description
Purpose: To improve the accuracy/efficiency of IMRT planning by combining Monte Carlo (MC) dose calculation with direct aperture optimization (DAO). Method and Materials: A 6 MV beam arrangement is applied to an IMRT phantom and patient examples. A phase space is calculated below the secondary jaws of a virtual Varian 21EX linac by MC simulation (BEAMnrc code (NRC, Canada)). The phase space is subdivided into 2.5×5.0 mm2 beamlets and the dose distribution from each beamlet is calculated to organs‐of‐interest within the patient/phantom using DOSXYZnrc. This information is input into DAO inverse planning software. The DAO includes multileaf collimator transmission and leaf motion limitations as it modifies the shape/weight of the treatment apertures. The optimized leaf sequence requires no additional leaf motion calculation step. A final forward MC dose calculation is performed. The MC doses are verified with ion chamber and film measurement. MC‐DAO is applied to a difficult phantom geometry, namely a c‐shaped target with embedded organ‐at‐risk located directly adjacent to a 5.0cm‐thick air slab. Clinical sites include nasopharynx and lung.Results: The MC optimization allows for accurate modeling of the electronic disequilibrium introduced by the air cavities. For the phantom example, MC reveals that the plan optimized with a pencil beam (PB) algorithm fails to provide adequate coverage to the PTV close to the air cavity, whereas the MC‐DAO plan demonstrates adequate coverage. For the nasopharynx, the PB plan showed errors during ion chamber/film verification, probably due to the small (∼5×4 cm2) fields whereas the MC‐DAO plan showed good agreement. The reduction in monitor units for MC‐DAO plans is 20 – 40% compared to a commercial fluence‐based (PB) treatment planning system. Conclusion:MC simulation generates accurate input data for IMRT inverse treatment planning in difficult‐to‐calculate regions. The addition of DAO results in a more efficient treatment plan delivery.
33(2006); http://dx.doi.org/10.1118/1.2241561View Description Hide Description
Purpose: Underlying brachytherapydosimetry characteristics for 137Cs, 125I, 192Ir, 103Pd, and 169Yb were examined using Monte Carlo methods.Sources were modeled as unencapsulated point or line sources in liquid water to negate source‐specific effects of materials and construction. Importance of phantom size (R), radiation transport mode, phantom material, and volume averaging were studied. Method and Materials:Radiation transport simulations were performed with MCNP5 using the most recent photon cross‐section libraries. The AAPM TG‐43U1 brachytherapydosimetry formalism was employed and extended to radionuclides with EAVG > 50 keV. Radiation spectra were taken from the National Nuclear Data Center and compared to those commonly referenced. Enough photon histories were simulated to maintain statistical uncertainties < 1%. Results and Discussion: For non‐infinite media, g(r) was found to degrade as r approached R, the phantom radius, and MCNP5 results were in agreement with those published using GEANT4. Dosimetry parameters calculated using coupled photon‐electron radiation transport simulations did not differ significantly from those using photon transport only. Low‐energy radionuclides 125I and 103Pd were sensitive to phantom material with up to a factor of 1.4 and 2, respectively, between tissue‐equivalent materials and water at r = 9 cm. In comparison, high‐energy photons from 137Cs, 192Ir, and 169Yb demonstrated ± 5% differences between water and tissue‐substitutes at r = 20 cm. Similarly, volume‐averaging effects were found to be more significant for low‐energy radionuclides. When modeling line sources with L ⩽ 0.5 cm, the 2‐D anisotropy function was largely within ± 0.5% of unity for 137Cs, 125I, and 192Ir. However, an energy and geometry effect was noted for 103Pd and 169Yb, with F(0.5,0°) = 1.05 and 0.98, respectively, for L = 0.5 cm. Additional radiation transport calculations using mono‐energetic photons showed energy‐dependent variations in F(r,θ) as a function of effective length and θ.