Monte carlo electron source model validation for an Elekta Precise linac.

PURPOSE: Electron radiation therapy is used frequently for the treatment of skin cancers and superficial tumors especially in the absence of kilovoltage treatment units. Head-and-neck treatment sites require accurate dose distribution calculation to minimize dose to critical structures, e.g., the eye, optic chiasm, nerves, and parotid gland. Monte Carlo simulations can be regarded as the dose calculation method of choice because it can simulate electron transport through any tissue and geometry. In order to use this technique, an accurate electron beam model should be used. METHODS: In this study, a two point-source electron beam model developed for an Elekta Precise linear accelerator was validated. Monte Carlo data were benchmarked against measured water tank data for a set of regular and circular fields and at 95, 100, and 110 cm source-to-skin-distance. EDR2 Film dose distribution data were also obtained for a paranasal sinus treatment case using a Rando phantom and compared with corresponding dose distribution data obtained from Monte Carlo simulations and a CMS XiO treatment planning system. A partially shielded electron field was also evaluated using a solid water phantom and EDR2 film measurements against Monte Carlo simulations using the developed source model. RESULTS: The major findings were that it could accurately replicate percentage depth dose and beam profile data for water measurements at source-to-skin-distances ranging between 95 and 110 cm over beam energies ranging from 4 to 15 MeV. This represents a stand-off between 0 and 15 cm. Most percentage depth dose and beam profile data (better than 95%) agreed within 2%/2 mm and nearly 100% of the data compared within 3%/3 mm. Calculated penumbra data were within 2 mm for the 20 x 20 cm2 field compared to water tank data at 95 cm source-to-skin-distance over the above energy range. Film data for the Rando phantom case showed gamma index map data that is similar in comparison with the treatment planning system and the Monte Carlo source model. The gamma index showed good agreement (2%/2 mm) between the Monte Carlo source model and the film data. CONCLUSIONS: Percentage depth dose and beam profile data were in most cases within a tolerance of 2%/2 mm. The biggest discrepancies were in most cases recorded in the first 6 mm of the water phantom. Circular fields showed local dose agreement within 3%/3mm. Good agreement was found between calculated dose distributions for a paranasal sinus case between Monte Carlo, film measurements and a CMS XiO treatment planning system. The electron beam model can be easily implemented in the BEAMnrc or DOSXYZnrc Monte Carlo codes enabling quick calculation of electron dose distributions in complex geometries.

Inclusion of compensator-induced scatter and beam filtration in pencil beam dose calculations.

Compensators can be used as beam intensity modulation devices for intensity-modulated radiation therapy applications. In contrast with multileaf collimators, compensators introduce scatter and beam hardening into the therapeutic x-ray beam. The degree of scatter and beam filtering depends on the compensator material and beam energy. Pencil beam dose calculation models can be used to derive the shape of the compensator. In this study a novel way of incorporating the effect of compensator-induced scatter and beam filtration is presented. The study was conducted using 6, 8, and 15 MV polyenergetic pencil beams (PBs). The compensator materials that were studied included wax, brass, copper, and lead. The perturbation effects of the compensators on the PB dose profiles were built in the PB dose profiles and tested for regular fields containing a step compensator and benchmarked against DOSXYZnrc Monte Carlo calculated dose profiles. These effects include compensator beam filtration and Compton-scattered photons generated in the compensator materials that influence the resulting PB dose profiles. These data were obtained from DOSXYZnrc simulations. A Gaussian function was used to model off-axis scatter and an exponential function was used to model beam hardening at any radius, r. Dose profiles were calculated under a step compensator using the method that can model beam hardening and off-axis scatter, as well as a conventional method where the PB profiles are not adjusted, but a single effective attenuation coefficient is used instead to best match the dose profiles. Both sets of data were compared to the DOSXYZnrc data. Depth and profile dose data for 10 x 10 cm2 and 20 x 20 cm2 fields indicated that at 2 cm depth in water the method that takes compensator scatter into account agrees more closely with the DOSXYZnrc data compared to the data using only an effective attenuation coefficient. Further, it was found that the effective attenuation method can only replicate the DOSXYZnrc data at 10 cm depth where it was chosen to do so. At shallower depths the effective attenuation method overestimates the dose and beyond 10 cm depth it causes an underestimation in the dose. The scatter and beam hardening inclusion method does not exhibit such properties. The exclusion of scatter can lead to dose errors of up to 4 percent with a copper compensator at 5 cm depth for a 10 X 10 cm2 field under a thickness of 5 cm at 6 MV. For materials such as lead this discrepancy could be as high as 7 to 8 percent at 6 MV. For larger fields (20 X 20 cm2) the effect of in-phantom scatter reduces the differences between the dose profiles calculated with the mentioned methods.

Radiological properties of a wax-gypsum compensator material.

In this paper the radiological properties of a compensator material consisting of wax and gypsum is presented. Effective attenuation coefficients (EACs) have been determined from transmission measurements with an ion chamber in a Perspex phantom. Measurements were made at 80 and 100 cm source-to-skin distance (SSD) for beam energies of 6, 8, and 15 MV, for field sizes ranging from narrow beam geometries up to 40 x 40 cm2, and at measurement depths of maximum dose build-up, 5 and 10 cm. A parametrization equation could be constructed to predict the EAC values within 4% uncertainty as a function of field size and depth of measurement. The EAC dependence on off-axis position was also quantified at each beam energy and SSD. It was found that the compensator material reduced the required thickness for compensation by 26% at 8 MV when compared to pure paraffin wax for a 10 x 10 cm2 field. Relative surface ionization (RSI) measurements have been made to quantify the effect of scattered electrons from the wax-gypsum compensator. Results indicated that for 80 cm SSD the RSI would exceed 50% for fields larger than 15 x 15 cm2. At 100 cm SSD the RSI values were below 50% for all field sizes used.

Monte Carlo calculation of effective attenuation coefficients for various compensator materials.

Effective attenuation coefficients for 6, 8, and 15 MV photon beams were derived and studied for various compensator materials for square beams with side lengths of 0.5, 1.0, 2.0, 3.0, and 5.0 cm. Calculations were based on depth dose data in water obtained from EGS4 based DOSXYZ Monte Carlo simulations. Depth dose data were calculated using different compensator materials as attenuators of variable thickness. The absorbed dose varied exponentially as a function of absorber thickness at any depth in water on the beam axis for all materials. The effective attenuation coefficient data were compared with measurements for wax, aluminum and brass with values from the literature. Theoretical narrow beam linear attenuation coefficients were calculated and compared with the Monte Carlo data. The effective attenuation coefficient data for all materials were parametrized as functions of field size and depth in water. The effective attenuation coefficient was also parametrized as a function of atomic number. It was found that the effective attenuation coefficients calculated from the DOSXYZ data using a simple source model correspond to measured data for wax, aluminum and brass and published data for lead.

Comparison of the Batho, ETAR and Monte Carlo dose calculation methods in CT based patient models.

This paper shows the contribution that Monte Carlo methods make in regard to dose distribution calculations in CT based patient models and the role it plays as a gold standard to evaluate other dose calculation algorithms. The EGS4 based BEAM code was used to construct a generic 8 MV accelerator to obtain a series of x-ray field sources. These were used in the EGS4 based DOSXYZ code to generate beam data in a mathematical water phantom to set up a beam model in a commercial treatment planning system (TPS), CADPLAN V.2.7.9. Dose distributions were calculated with the Batho and ETAR inhomogeneity correction algorithms in head/sinus, lung, and prostate patient models for 2 x 2, 5 x 5, and 10 X 10 cm2 open x-ray beams. Corresponding dose distributions were calculated with DOSXYZ that were used as a benchmark. The dose comparisons are expressed in terms of 2D isodose distributions, percentage depth dose data, and dose difference volume histograms (DDVH's). Results indicated that the Batho and ETAR methods contained inaccuracies of 20%-70% in the maxillary sinus region in the head model. Large lung inhomogeneities irradiated with small fields gave rise to absorbed dose deviations of 10%-20%. It is shown for a 10 x 10 cm2 field that DOSXYZ models lateral scatter in lung that is not present in the Batho and ETAR methods. The ETAR and Batho methods are accurate within 3% in a prostate model. We showed how the performance of these inhomogeneity correction methods can be understood in realistic patient models using validated Monte Carlo codes such as BEAM and DOSXYZ.

The indirect use of CT numbers to establish material properties needed for Monte Carlo calculation of dose distributions in patients.

A number of Monte Carlo codes are available, which can be used to calculate dose distributions n patients with high accuracy. Patient geometry can readily be derived with adequate spatial resolution from CT scans. To perform the Monte Carlo calculation with the same spatial resolution, it is necessary to enter the atomic composition and density of the tissue in each voxel of the CT image. This means entering 65,536 discrete values for a CT slice with a 256 x 256 matrix size. The need for automated methods of setting up the material data files is obvious. Because there is no direct unique relationship between CT numbers and material composition, the aim of our work was to devise a method whereby the atomic composition and density in each voxel could be assigned automatically by indirect derivation from the CT numbers. The set of all tissues types in the human body was divided into subsets that are dosimetrically equivalent, based on Monte Carlo calculated depth dose curves in homogeneous phantoms of each tissue. CT number ranges corresponding to each tissue subset were determined from the calibration curve linking electron density with CT number for the specific CT scanner. Further subdivision was found to be necessary for the lung and bone type tissues. This was done by keeping the atomic composition constant and varying the physical density. It was found that 57 distinct tissue subsets were needed to represent the 16 main tissue types in the body at a 1% dose level. Corresponding CT number intervals of 30 HU were needed in the lung and soft tissue region, whereas in the bone region the intervals could be increased to 100 HU. A computer algorithm was set up to convert automatically from CT number to corresponding equivalent material number for the Monte Carlo preprocessor code.

Chelator effect on ion diffusion in ferrous-sulfate-doped gelatin gel dosimeters as analyzed by MRI.

Ferrous-sulfate-doped gelatin gel dosimeters are useful tools for the measurement of three-dimensional absorbed radiation dose distributions. The diffusion of ferric ions through these gels causes degradation with time of the dose distribution image. It would be useful to reduce ferric ion diffusion without decreasing gel sensitivity. The amount of ferric ion diffusion is a function of the time delay after radiation, the gel temperature, and the gel concentration. These effects can be quantified by measuring the ferric ion diffusion coefficient. Determination of the diffusion coefficient by irradiating the lower section of a cylinder of gel, which was then imaged repeatedly over time with a clinical magnetic resonance imager, is described. Analysis of the edge spread function formed at each of several times after irradiation by drawing a profile over the imaged junction between the irradiated and unirradiated halves of the cylinder, gave estimates of the variance of the edge spread function. These variances were used to obtain an estimate of the ferric ion diffusion coefficient for the gel. A method of reducing ferric ion diffusion by adding a chelator and the cross linkage agent formaldehyde is suggested. The chelators investigated were 1,10 phenanthroline, xylenol orange, and bathophenanthroline disulfonic acid. These reduced diffusion to varying extents, and influenced the gel sensitivity. The diffusion coefficient in gels containing xylenol orange was found to be 0.44 mm2h-1. The gel sensitivity was 0.0093 s-1Gy-1. This compared with a diffusion coefficient of 0.82 mm2h-1 for the base line gel that did not contain formaldehyde or chelators. The sensitivity of this base line gel was 0.0129 s-1Gy-1. The addition of xylenol orange produced the most improved gel dosimeter of the gels studied. This gel had a decreased ferric ion diffusion coefficient and a decreased sensitivity. It was still sensitive enough to be useful.

Technical note: computed tomography imaging with a radiotherapy simulator.

An inexpensive system for obtaining cross-sectional information and accurate body outlines of patients destined for radiotherapy, using a radiotherapy simulator without any major modifications, has been investigated. The image intensifier of the simulator was moved laterally and a narrow fan beam of X-rays passed through the phantom onto the intensifier. Several television (TV) lines of the video signal from the TV camera were digitized by a frame grabber and stored for reconstruction. Multiple projections were acquired by rotating the gantry of the simulator. The field of view was enlarged by increasing the offset distance of the image intensifier and taking two sets of projections. Reconstruction was carried out by using the convolution and back-projection method. The gradient between pixel values in the reconstructed images was used to detect the outlines of structures in the images. The accuracy of outline detection was evaluated with images of a Rando phantom. The outlines of the images were compared with the actual outlines of the phantom. The spatial resolution of the simulator computed tomography (CT) was measured to be 4.05 mm. Large inhomogeneities could be clearly seen. The average difference between the measured and the actual outlines was 3.0 mm with a maximum difference of 10.0 mm at sharp curves in the outline. The simulator CT provides an inexpensive, alternative method of obtaining body outlines and does not require any modifications to the simulator. Data acquisition, processing and display can be performed on a personal computer with image processing facilities.

Optimization of the wedge filter parameters for a radiotherapy treatment planning system.

When a treatment planning system uses an empirical or semianalytical approach to describe the influence of a wedge filter on a photon beam, a number of experimentally determined parameters are required. These may be found from direct measurement. However, if the beam model is sensitive to the parameters, it will be necessary to optimize the parameter values to obtain better correspondence between dose profiles calculated by the model and actual measured profiles. The procedure is time consuming if optimization is done manually. We have developed an optimization scheme, using a personal computer, to find the set of wedge parameters which will result in the best fit of calculated wedge profiles (using the beam model of the treatment planning system) to measured wedge profiles. The procedure is efficient and calculated profiles were found to match measured profiles to within 2% of the central axis value.