D. W. O. Rogers

EGSnrc Monte Carlo calculated dosimetry parameters for 192Ir and 169Yb brachytherapy sources

This study presents the results of EGSnrc Monte Carlo calculations of the dose distribution surrounding a high dose rate Yb169 brachytherapy source and 14 high dose rate and pulsed dose rate Ir192 brachytherapy sources. Energy-weighted spectra of emitted photons, a full set of TG-43 dosimetry parameters, along-away dose tables, and a description of the materials and geometry used for each source are provided. In addition to this, separate tallies are made of the dose contributed from primary, single-scattered, and multiply-scattered photons. Separation of dose in this manner allows one to use convolution/superposition methods to calculate the dose surrounding a brachytherapy source accounting for a non-homogeneous medium. The effect of phantom size on TG-43 dosimetry parameters and scattered dose is also investigated for the Ir192 microSelectron v2 HDR source. This paper describes the calculation methods and presents the dose rate constants calculated for each source with the full set of dosimetry data being available online at the Carleton Laboratory for Radiotherapy Physics brachytherapy database (http://www.physics.carleton.ca/clrp/seeḏdatabase/).

Relationship between %dd(10)x and stopping-power ratios for flattening filter free accelerators : A Monte Carlo study

The relationship between the photon beam quality specifier %dd(10)x and the Spencer-Attix water water to air restricted mass collision stopping-power ratio, (L/rho))air(water), is studied using Monte Carlo simulation with realistic beams in contrast to the previously used realistic but uniform spectra from an isotropic point source. The differences between accelerators with and without flattening filters are investigated since flattening filter free accelerators appear to be useful for IMRT. Our results show that the standard relationship between %dd(10)x and (L/rho)air(water), which is used in the TG-51 protocol to calculate the quality conversion factor kQ, is acceptable for beams with or without a flattening filter with a maximum error of 0.4%, although a fit to the new data would reduce the maximum error to 0.2%. Reasons for differences between the individual values of %dd(10)x and (L/ rho)air(water) with and without a flattening filter are studied. Specifically the differences due to the softening of the beam, the change in shape of the profile, and the inclusion of radial variations in the photon energy spectra, are investigated. It is shown that if TPR10(20) is used as a beam quality specifier, there are two different relationships between TPR10(20) and (L/rho)air(water) which differ by 0.4%-1%. When using TPR10(20) as a beam quality specifier in a beam without a flattening filter, one should subtract 0.5% from the value of kQ for a given value of TPR10(20).

Monte Carlo dosimetry for and eye plaque brachytherapy

A Monte Carlo study of dosimetry for eye plaque brachytherapy is performed. BrachyDose, an EGSnrc user code which makes use of Yegins multi-geometry package, is used to fully model I125 (model 6711) and Pd103 (model 200) brachytherapy seeds and the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). Three-dimensional dose distributions in the eye region are obtained. In general, dose to water is scored; however, the implications of replacing water with eye tissues are explored. The effect of the gold alloy (Modulay) backing is investigated and the dose is found to be sensitive to the elemental composition of the backing. The presence of the silicone polymer (Silastic) seed carrier results in substantial dose decreases relative to water, particularly for Pd103. For a 20mm plaque with a Modulay backing and Silastic insert, fully loaded with 24 seeds, the dose decrease relative to water is of the order of 14% for I125 and 20% for Pd103 at a distance of 1cm from the inner sclera along the plaques central axis. For the configurations of seeds used in COMS plaques, interseed attenuation is a small effect within the eye region. The introduction of an air interface results in a dose reduction in its vicinity which depends on the plaques position within the eye and the radionuclide. Introducing bone in the eyes vicinity also causes dose reductions. The dose distributions in the eye for the two different radionuclides are compared and, for the same prescription dose, Pd103 generally offers a lower dose to critical normal structures. BrachyDose is sufficiently fast to allow full Monte Carlo dose calculations for routine clinical treatment planning.

On the accuracy of techniques for obtaining the calibration coefficient N{sub K} of {sup 192}Ir HDR brachytherapy sources

The accuracy of interpolation or averaging procedures for obtaining the calibration coefficient NK for Ir192 high-dose-rate brachytherapy sources has been investigated using the EGSnrc Monte Carlo simulation system. It is shown that the widely used two-point averaging procedure of Goetsch et al. [Med. Phys. 18, 462 (1991)] has some conceptual problems. Most importantly, they recommended, as did the IAEA, averaging AwallNK values whereas one should average 1∕NK values. In practice this and other issues are shown to have little effect except for Goetsch et al.s methods for determining Awall values. Their method of generalizing the Awall values measured in one geometry to other geometries is incorrect by up to 2%. However, these errors in Awall values cause systematic errors of only 0.3% in Ir192 calibration coefficients. It is shown that Awall values need not be included in the averaging technique at all, thereby simplifying the technique considerably. It is demonstrated that as long as ion chambers with a flat response are used and/or very heavily filtered 250kV (or higher) beams of x rays are used in the averaging, then almost all techniques can provide adequate accuracy.

Wall correction factors, P{sub wall}, for parallel-plate ionization chambers

The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, Pwall,, for parallel-plate ionization chambers in photon and electron beams. A set of Pwall values, computed at the reference depth in water, is presented for several commonly used parallel-plate chambers. These values differ from the standard assumption of unity used by dosimetry protocols by up to 1.7% for clinical electron beams. Calculations also show that Pwall is strongly dependent on the depth of measurement and can vary by as much as 6% for a 6 MeV beam in moving from a depth of dref to a depth of R50. In photon beams, where there is limited information available regarding Pwall for parallel-plate chambers, CSnrc calculations show Pwall values of up to 2.4% at the reference depth over a range of photon energies. The Pwall values for photon beams are in good agreement with previous estimates of the wall correction but have much lower statistical uncertainties and cover a wider range of photon beam energies.

Wall correction factors, P{sub wall}, for thimble ionization chambers

The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, Pwall, for thimble ionization chambers in photon and electron beams. CSnrc calculated values of Pwall give closer agreement with previous experimental results than do the values from the standard formalism used in current dosimetry protocols. A set of Pwall values, computed at the reference depth in water, is presented for several commonly used thimble chambers. These values differ from the commonly used values by up to 0.8% for megavoltage photon beams, particularly for nominal beam energies below 6 MV. The sleeve effect, which is not currently taken into account by the TG-51 dosimetry protocol, is computed to be up to 0.3% and is in some cases larger than the Pwal1 correction itself. In electron beams, where dosimetry protocols assume a wall correction of unity, CSnrc calculations show Pwall values of up to 0.6% at the reference depth, depending on the wall material. Pwall is shown to be sensitive to the depth of measurement, varying by 2.5% for a graphite-walled cylindrical Farmer-like chamber between a depth of 0.5 cm and R50 in a 6 MeV electron beam.

Sci—Sat AM(2): Brachy — 05: Fast Monte Carlo Dose Calculations for Brachytherapy with BrachyDose

A fast dose calculation algorithm called BrachyDose has been developed for brachytherapy applications. BrachyDose is based on the EGSnrc code system for simulating radiation transport. Complex geometries are modelled through the superposition of basic geometric entities (spheres, cuboids, cylinders, and cones) using Yegins multi-geometry package; the phantom geometry may be defined using a CT dataset. A database of brachytherapy sources has been developed and benchmarked, as has a database of eye plaque applicators. BrachyDose scores collision kerma, which is equivalent to absorbed dose for most situations of interest, using a tracklength estimator. The phase space of particles emitted from brachytherapy sources may be generated with BrachyDose and used in subsequent simulations to avoid the repeated simulation of particle transport within sources. A particle recycling feature has been implemented for multisource configurations in which the first source acts as a particle generator; particles emitted from this source are reinitiated at each source location. Dose calculations for prostate permanent implants achieving 2% average uncertainty in the prostate region take less than 30 seconds in (2 mm)3 voxels on a single 3.0 GHz Woodcrest core; calculation times for eye plaque therapy are on the order of three minutes in (0.5 mm)3 voxels. These calculation times are sufficiently fast for routine clinical treatment planning. A graphical user interface (GUI) for BrachyDose has been developed. Working towards clinical implementation, efforts are underway to integrate data in the DICOM-RT format with BrachyDose.

SU‐FF‐T‐113: BrachyDose: A New Fast Monte Carlo Code for Brachytherapy Calculations

Purpose: To develop a fast Monte Carlo code based on EGSnrc for accurate dose calculation around brachytherapy sources. Method and Materials: Sources and phantom geometries are modeled by using the Multi‐geometry technique which allows various predefined geometry elements (eg, sources, applicators, catheters) a phantom geometry. Sources such as an HDR Ir‐192 source and LDR I‐125 or Pd‐103 seed sources were modeled. One or more sources from a database can be duplicated many times and placed in arbitrary locations. Besides the above sources, BrachyDose can calculate dose around a miniature x‐ray‐tube source since it is based on EGSnrc. It also can use CT data in the phantom geometry. Variance reduction techniques are applied to speed up computation time. Dose is calculated by scoring the collision kerma using a tracklength estimator. There is an option to reuse every photon which escapes from a seed as if it came from every seed in the implant with same direction relative to the seed itself. Results: The speed of the BrachyDose calculation is specified by the time required to attain an average of 2\% statistics in the central region of an implant of 125 seeds spaced at 5 mm separation in a 1000 cm∧3 cubic phantom. The time required scales roughly as the inverse of the volume of the voxels. On an 2.4 MHz CPU, the computation time is 510 s for 1 mm voxels. The DVHs for 1 mm voxels are significantly different from those in 2 mm voxels. Changing phantom material from water to prostate tissue causes the dose to vary by +/−5% vs dose to water, depending on distance from the seeds. Conclusions: The Monte Carlo code developed is fast enough for routine clinical applications. Calculated dose values include inter‐seed effects and other effects from tissue inhomogeneities.

Anniversary Paper: Fifty years of AAPM involvement in radiation dosimetry

This article reviews the involvement of the AAPM in various aspects of radiation dosimetry over its 50 year history, emphasizing the especially important role that external beam dosimetry played in the early formation of the organization. Topics covered include the AAPMs involvement with external beam and x-ray dosimetry protocols, brachytherapy dosimetry, primary standards laboratories, accredited dosimetry chains, and audits for machine calibrations through the Radiological Physics Center.

Accuracy of the Burns equation for stopping-power ratio as a function of depth and R50.

The accuracy of the Burns et al. equation [Med. Phys. 23, 489-501 (1996)] for the Spencer-Attix water to air stopping-power ratio as a function of depth in a water phantom and electron beam quality in terms of R50 is investigated by comparison to the original data on which this fit was based. It is shown that using this equation provides dose estimates on the central axis in a clinical electron beam that are accurate to within 1% of dose maximum for all 24 clinical beams investigated except very close to the surface in swept beams. In contrast, the error in the dose as a percentage of the local dose is much higher for values of the depth/R50 greater than 1.2.