Zuofeng Li

DOSIMETRIC CHARACTERISTICS, AIR-KERMA STRENGTH CALIBRATION AND VERIFICATION OF MONTE CARLO SIMULATION FOR A NEW YTTERBIUM-169 BRACHYTHERAPY SOURCE

PURPOSE Ytterbium-169 (169Yb) is a promising new isotope for brachytherapy with a half life of 32 days and an average photon energy of 93 KeV. It has an Ir-192-equivalent dose distribution in water but a much smaller half-value layer in lead (0.2 mm), affording improved radiation protection and customized shielding of dose-limiting anatomic structures. The goals of this study are to: (a) experimentally validate Monte Carlo photon transport dose-rate calculations for this energy range, (b) to develop a secondary air-kerma strength standard for 169Yb, and (c) to present essential treatment planning data including the transverse-axis dose-rate distribution and dose correction factors for a number of local shielding materials. METHODS AND MATERIALS Several interstitial 169Yb sources (type 6) and an experimental high dose-rate source were made available for this study. Monte-Carlo photon-transport (MCPT) simulations, based upon validated geometric models of source structure, were used to calculate dose rates in water. To verify MCPT predictions, the transverse-axis dose distribution in homogeneous water medium was measured using a silicon-diode detector. For use in designing shielded applicators, heterogeneity correction factors (HCF) arising from small cylindrical heterogeneities of lead, aluminum, titanium, steel and air were measured in a water medium. Finally, to provide a sound experimental basis for comparing experimental and theoretical dose-rate distributions, the air-kerma strength of the sources was measured using a calibrated ion chamber. To eliminate the influence of measurement artifacts on the comparison of theory and measurement, simulated detector readings were compared directly to measured diode readings. The final data are presented in the format endorsed by the Interstitial Collaborative Working Group. RESULTS The in-air calibration revealed that the air-kerma strength per unit activity (mCi), as quoted by the vendor, varied from 1.30 to 1.57 cGy.cm2/mCi.h depending on seed design. The maximum difference between measured and MCPT-simulated absolute diode readings on the transverse axis was less than 2%, indicating that MCPT accurately predicts dose rate in medium for brachytherapy sources in this energy range. Comparison of measured and simulated HCFs for each of the 16 different cylindrical heterogeneities demonstrated 1-3% agreement. The HCFs vary by as much as 200% with respect to distance and by as much as 48% as a function of disk diameter, showing that HCF is strongly dependent on heterogeneity location and lateral dimensions as well as thickness. The dose-rate constant for water medium was found to be 1.225 cGy/h per kerma unit air-strength and 1.962 cGy/h per unit mCi as measured by the vendor. CONCLUSION Monte Carlo simulation is an accurate and powerful tool for dosimetric characterization of brachytherapy sources in this energy range. Thin lead foils produce shielding factors comparable to standard shielded applicators for 137Cs. Meaningful theoretical absolute dose calculations in brachytherapy require accurately implemented air-kerma strength standards.

Feasibility study of multileaf collimated electrons with a scattering foil based accelerator

BACKGROUND AND PURPOSE There is an ever evolving process to improve the technical aspects of electron beam delivery. Both the foil/applicator and scanning electron beam systems have gone through recent upheavals. Concomitantly, multileaf collimators are now a staple method for collimating photons. We undertook a study of multileaf collimated electron beam (MLCEB) using a dual scattering foil system. MATERIALS AND METHODS We compared MLCEB with applicator collimated electron beams (AEB) by examining the dosimetric aspects of the two systems using 70 and 80 cm SSDs for the MLCEB, the minimum practical SSDs achievable. Percent depth dose, isodose profiles, output factors, leakage, surface dose, bremstrahlung, effective SSDs, etc. were measured using film and/or ion chamber. Clinical fields, such as posterior neck node (PNN), were compared. We also investigated the use of MLCEB for arc therapy using segments. RESULTS In all cases, the MLCEB performed inferior, as judged by isodoses, uniformity index (UI) and penumbra analysis. The 80 cm SSD (minimum for PNN), low energy, small fields, was the worst case. For a 6 MeV beam, the UI/penumbra was 0.823/10 mm for the AEB, and 0.561/29 mm for the MLCEB at 80 cm SSD. The PNN multileaf fields exhibited narrow 90% and 80% isodose lines, and wide 20% and 10% lines. CONCLUSIONS We conclude that multileaf for PNN fields could not be matched to adjacent off-cord photon fields. The stair-stepping" effect associated with MLC photons was absent for electrons.

A comparative study of dosimetric properties of Plastic Water and Solid Water in brachytherapy applications

In this study the dosimetric properties of Plastic Water and Solid Water phantom materials are evaluated using Monte Carlo photon transport simulations. In particular, their water-equivalence with respect to absorption and attenuation of photons in the brachytherapy energy range are examined. For the given chemical compositions of the materials, the linear attenuation coefficients were calculated for photons of 1 keV-2 MeV. Moreover, absorbed doses to water in each phantom material were calculated at distances of 0.5-12.0 cm from point sources of 20 keV to 60Co gamma rays. These results show that at low photon energies (below 100 keV), there are substantial differences (up to a factor of 5) between the absorbed dose in Plastic Water and that in liquid water. The differences decrease as photon energy increases, and they become insignificant at 60Co gamma rays, as claimed by the manufacturer. In contrast, calculations show that the difference in absorbed dose in Solid Water from that in liquid water, over the entire range of photon energies employed in this study, is less than 25%. The results of this study demonstrate the necessity of careful dosimetric evaluation of a new phantom material, before its clinical application, particularly in energy ranges outside those referred to by the manufacturer.

Surface and buildup dose characteristics for 6, 10, and 18 MV photons from an Elekta Precise linear accelerator

Understanding head scatter characteristics of photon beams is vital to properly commission treatment planning (TP) algorithms. Simultaneously, having definitive surface and buildup region dosimetry is important to optimize bolus. The Elekta Precise linacs have unique beam flattening filter configurations for each photon beam (6, 10, and 18 MV) in terms of material and location. We performed a comprehensive set of surface and buildup dose measurements with a thin window parallel‐plate (PP) chamber to examine effects of field size (FS), source‐to‐skin distance (SSD), and attenuating media. Relative ionization data were converted to fractional depth dose (FDD) after correcting for bias effects and using the Gerbi method to account for chamber characteristics. Data were compared with a similar vintage Varian linac. At short SSDs the surface and buildup dose characteristics were similar to published data for Varian and Elekta accelerators. The FDD at surface (FDD0) for 6, 10, and 18 MV photons was 0.171, 0.159, and 0.199, respectively, for a 15×15 cm2, 100 cm SSD field. A blocking tray increased FDD0 to 0.200, 0.200, and 0.256, while the universal wedge decreased FDD0 to 0.107, 0.124, and 0.176. FDD0 increased linearly with FS (~1.16%/cm). FDD0 decreased exponentially for 10 and 18 MV with increasing SSD. However, the 6 MV FDD0 actually increased slightly with increasing SSD. This is likely due to the unique distal flattening filter for 6 MV The measured buildup curves have been used to optimize TP calculations and guide bolus decisions. Overall the FDD0 and buildup doses were very similar to published data. Of interest were the relatively low 10 MV surface doses, and the 6 MV FDD0s dependence on SSD.

Minimization of target positioning error in accelerator‐based radiosurgery

The stereotactic radiosurgery system used at the Mallinckrodt Institute of Radiology is patterned after that developed at the Joint Center for Radiation Therapy (Brigham & Womens Hospital, Boston, MA) and uses the Brown-Roberts-Wells computed tomography (CT) stereotactic system. The patients head is attached to a stand that rotates with the treatment couch. The irradiation is conducted using a set of converging arcs of irradiation. Because of mechanical limitations, no accelerator or treatment couch is capable of placing the center of the radiation beam at precisely the same point for all gantry and couch angles and a compromise must be made when locating the nominal isocenter. The stand settings are checked by placing a radiopaque QA sphere at the desired target location. The QA sphere is imaged using a series of eight films exposed at a set of couch and gantry angles that encompass the treatment angles. The distances between the QA sphere image and the center of the radiation field indicate if the correct coordinates were set on the stand and if the radiation beam converges to a sufficiently small region (< 0.1-cm diameter) for treatment. A mathematical procedure has been developed to use the film-measured position errors to determine a stand offset that will minimize the distance between the accelerator isocenter and the target. The technique is capable of reducing the average placement error, as measured by imaging the QA sphere, to 0.035 cm with a maximum deviation of 0.07 cm.

Monte Carlo calculations of dosimetry parameters of the Urocor Prostaseed 125I source

This report presents the results of Monte Carlo calculations of the dosimetric parameters of the Urocor ProstaSeed 1251 seed source. This source contains five spherical silver markers, of 0.5 mm diameter, with 1251 deposited on the spheres through ion exchange. The silver spheres are then encapsulated within a 0.8 mm in diameter and 4.5 mm long cylindrical shell of titanium, as is common to this type of sources. Physical dimensions of the source are confirmed by measurement. Four (4) geometric models of the source, based on different assumptions on the locations of the silver spheres within the seed, were used in dose rate calculations. Monte Carlo photon transport simulation was used to calculate the dosimetric parameters of dose rate constant, radial dose function, and anisotropy function using these geometric models of the source. The Monte Carlo calculated dose rate constant of the ProstaSeed source was found equal to 0.925 cGy/Uh with approximate uncertainties of 5%. Radial dose function values and anisotropy function values were derived from the relative dose distribution around the source calculated by the Monte Carlo code. The calculated values of these dosimetric parameters agree with previously published thermoluminescence-dosimeter-measured values for this source after consideration of measurement and calculation uncertainties.

An improved internal mammary irradiation technique in radiation treatment of locally advanced breast cancers

The purpose of the present study was to compare a new internal mammary irradiation technique with traditional techniques for locally advanced breast cancers in terms of sparing ipsilateral lung and heart and reducing the “cold” and “hot spots” in breast tissue. The new technique uses wide tangential fields for the first eight fractions of treatment. A medial internal mammary field (IMF) of electrons matched with narrowed tangential fields is used for the remaining fractions. Intensity‐modulated radiation therapy (IMRT) by means of segmented multileaf collimation (SMLC) is used in the narrowed tangential fields to improve the match between the electron and the photon fields. Treatment planning was performed to compare this technique to a wide‐tangential‐only technique and to a traditional oblique IMF technique for three patients with differing habitus. Film dosimetry was performed in a solid water phantom to confirm the planning results. For all three patients, the mean doses of the ipsilateral lung and the heart were significantly reduced with the new technique. The lung and the heart volumes were remarkably reduced at low‐dose levels (≤12GY) compared to the traditional IMF technique, and significantly reduced at all dose levels compared to the wide tangential technique. The new technique also reduced the “cold” and “hot spots” along the match plane between the IMF and the tangential fields compared to the traditional IMF technique. In conclusion, the new IMF technique shows dosimetric improvement compared to the traditional IMF technique in terms of the critical organ sparing and target dose uniformity. PACS number: 87.53.Tf

Thermoluminescent dosimetry of the SourceTech Medical model STM1251 125I seed.

Many new models of 125I seeds are being introduced, mainly due to the increase in prostate seed implants. We have evaluated the SourceTech Medical (STM), model STM1251, 125I seed using thermoluminescent dosimeters (TLDs) in a solid water phantom. TLD cubes, LiF TLD-100, with dimension 1 mm on each edge, were irradiated at various distances, 1, 2, 3, and 5 cm, at angles ranging from 0 degrees to 90 degrees in 10 degrees increments. Sensitivity calibration of the TLDs was achieved by irradiation to 10 cGy with 6 MV x rays from a clinical linear accelerator, Clinac 600C. Concurrent with the 125I seed exposures, several TLDs were also exposed to 10 cGy with the 600C as a control set. Dose rates per unit air kerma strength were determined based on the 1999 NIST traceable standard for the STM1251 seed. They are presented as a function of distance r and angle theta. The TG-43 parameters, including the dose rate constant, lambda, anisotropy function, F(r,theta), radial dose function, g(r), anisotropy factor, phian(r), and anisotropy constant, phi, were obtained for use in radiation treatment planning software. The value of lambda was determined as 1.07 +/- 5.5% cGy U(-1) h(-1), which is comparable to model 6702 and to the value determined using the point extrapolation method by Kirov and Williamson. We also find agreement between our TLD data and their Monte Carlo results for g(r), F(r,theta), phian(r), and phi. Additionally, agreement is found with the TLD data of Li and Williamson for lambda and g(r).

Verification of milled two-dimensional photon compensating filters using an electronic portal imaging device.

A computer-driven milling machine is being installed at the Mallinckrodt Institute of Radiology to fabricate photon compensating filters for conventional compensation and beam-intensity modulation. Commissioning and quality assurance procedures are being developed for the design, fabrication, and delivery systems prior to using the milled filters in the clinic. The portion of the quality assurance procedures governing the filter fabrication using a computer-driven milling machine includes, in part, a comparison of designed and fabricated filters. Test filters include geometrically regular filters, such as flat surfaces and steps. The verification of these shapes is accomplished using spatial measurements. However, to test the mills ability to generate complex curved surfaces, filters with more complicated surfaces are generated. These filters do not lend themselves to precise verification using physical measurement of the thickness distribution. A method has been developed to verify the thickness distributions of these complex filters by irradiating the filters with high-energy x rays and comparing the scattered and transmitted fluence to the fluence calculated using the intended filter shape. The fluence measurement is made using a calibrated commercial liquid ionization chamber electronic portal imaging device. The calculated fluence is separated into transmitted primary and scattered fluences and is determined using a convolution of a distributed radiation source kernel with an exponential filter attenuation function. The attenuation coefficient is measured for the filter material (Lipowitz metal) and fit to a second-order polynomial in filter thickness and off-axis distance. The distributed source kernel is measured using a split-field technique and fit to a sum of three two-dimensional Gaussian distributions. The scattered radiation is modeled by the Klein-Nishina cross section. The algorithm is tested by comparing calculated fluences with measured fluences for a series of machined filters: an open field, flat filters of 9.6-, 15.8-, and 31.6-mm thickness, split-field filters, and a pyramid-shaped filter. In each case, the algorithm calculates the fluence to within 3% of the measured values over the entire irradiated field size to within 1.5 cm of the collimated field edges.

An algorithm for automatic, computed‐tomography‐based source localization after prostate implant

Permanent implant of the prostate using I-125 and Pd-103 seeds is a popular choice of treatment for early-stage prostate cancer in the United States. Evaluation of the quality of the implant is best based on the calculated dose distribution from postimplant computed tomography (CT) images. This task, however, has been time-consuming and inaccurate. We have developed an algorithm for automatic source localization from postimplant CT images. The only requirement of this algorithm is knowledge of the number of seeds present in the prostate, thus minimizing the need for human intervention. The algorithm processes volumetric CT data from the patient, and pixels of higher CT numbers are categorized into classes of definite and potential source pixels. A multithresholding technique is used to further determine the number of seeds and their precise locations in the CT volume data. A graphic user interface was developed to facilitate operator review of and intervention in the calculation and the results of the algorithm. This algorithm was tested on two phantoms containing nonradioactive seeds, one with 20 seeds in discrete locations and another with 100 seeds with small distances between seeds. The tests showed that the algorithm was able to identify the seed locations to within 1 mm of their physical locations for discrete seed locations. It was further able to separate seeds at close proximity to each other while maintaining an average seed localization error of less than 2 mm, with no operator intervention required.