Transverse Bisector Research Articles

Proximal tibial anterior open-wedge oblique osteotomy: A novel technique to correct genu recurvatum

For successful acute correction of genu recurvatum, accurate correction of plateau angle, restoration of anatomical axis, rigid fixation, and preservation of patellar height are essential. However, a surgical treatment that satisfies all these conditions has not yet been established. The purpose of this study was to evaluate the novel technique of proximal tibial anterior open-wedge oblique osteotomy (PT-AOWOO) for the treatment of genu recurvatum. Five patients with genu recurvatum underwent PT-AOWOO from 2008 to 2013. The open-wedge center of rotation of angulation (CORA) was determined by the intersection of transverse bisector line and tibial posterior cortex to prevent secondary translational deformity. An osteotomy was performed from the distal margin of the tibial tuberosity to the planned CORA to maintain patellar height and provide sufficient space for fixation. The opening gap was calculated by the picture-archiving and communications system (PACS)-Photoshop method. The mean angle of recurvatum was 17° (range, 14 to 25°) preoperatively and -0.4° (range, -5° to 5°) postoperatively. The mean angle of the tibial plateau was 79.8° (range, 67 to 85°) preoperatively and 98.4° (range, 95 to 105°) postoperatively. Tibial anatomical axis was restored and the patellar height was maintained in all patients. The overall results were excellent in three patients and good in two patients. The PT-AOWOO and its unique osteotomy level corrected the tibial plateau angle accurately, restored the sagittal tibial axis without translation, maintained patellar height, and enabled rigid fixation. This technique showed satisfactory radiologic and clinical outcomes without any complications.

Impact of source‐production revision on the dose‐rate constant of interstitial brachytherapy sources

Since its introduction in 2004, the model CS-1 Rev.1 131Cs source has been used in many radiation therapy clinics for prostate brachytherapy. In 2006, this source model underwent a Rev.2 production revision. The aim of this work was to investigate the dosimetric influences of the Rev.2 production revision using high-resolution photon spectrometry. Three CS-1 Rev.1 and three CS-1 Rev.2 131Cs sources were used in this study. The relative photon energy spectrum emitted by each source in the transverse bisector of the source was measured using a high-resolution germanium detector designed for low-energy photon spectrometry. Based on the measured photon energy spectrum and the radioactivity distribution in the source, the dose-rate constant (lamda) of each source was determined. The effects of the Rev.2 production revision were quantified by comparing the emitted photon energy spectra and the lamda values determined for the sources manufactured before and after the production revision. The relative photon energy spectrum originating from the principal emissions of 131Cs was found to be nearly identical before and after the Rev.2 revision. However, the portion of the spectrum originating from the production of fluorescent x rays in niobium, a trace element present in the source construction materials, was found to differ significantly between the Rev.1 and Rev.2 sources. The peak intensity of the Nb Kalpha and Nb Kbeta fluorescent x rays from the Rev.2 source was approximately 35% of that from the Rev.1 source. Consequently, the nominal lamda value of the Rev.2 source was found to be greater than that determined for the Rev.1 source by approximately 0.7% +/- 0.5%. A significant reduction (65%) in relative niobium fluorescent x-ray yield was observed in the Rev.2 131Cs sources. The impact of this reduction on the dose-rate constant was found to be small, with a relative difference of less than 1%. This study demonstrates that photon spectrometry can be used as a sensitive and convenient tool for monitoring and for quantifying the dosimetric effects of brachytherapy source-production revisions. Because production revision can change both the geometry and the atomic composition of brachytherapy sources, its dosimetric impact should be carefully monitored and evaluated for each production revision.

SU‐GG‐T‐363: 3D Dosimetric Verification of Ir‐192 HDR Brachytherapy Source Irradiation

Purpose: Experimental verification of brachytherapy dose distributions, especially closer than 1 cm from the source, is very difficult with current dosimetry tools. PRESAGE, a 3D radio‐chromic plastic, has already shown promise in the realm of high energy clinical radiation dosimetry. We present a 3D verification of an IR‐192 HDR brachytherapy dose distribution based on the PRESAGE/Optical‐CT dosimetry system. Method and Materials: A 6cm diameter by 10cm height cylinder of PRESAGE was made with a 7cm long by 2mm diameter channel in the center. Irradiation took place using a GammaMed 12i HDR source, and one dwell position for total irradiation time of 37.8 seconds. Measurement of the dose distribution consisted of using an in‐house optical‐CT scanner with reconstructed pixel size of .6mm. The following parameters were used for image acquisition: 900 projections at .2 degree spacing, 5 exposure averages per projection, and 0.0774mm pixel size. Reconstructed resolution was 1.2mm. Comparisons were made of the measured dose distribution from PRESAGE with Eclipse treatment plan for a single dwell irradiation. Results: Eclipse data and PRESAGE data were normalized to relative dose distributions to the point 1cm from the transverse bisector. PRESAGE profiles in transverse plane agree with Eclipse profiles to ∼5% between .6 cm and 2 cm from the source position. There was a loss in PRESAGE measurement data due to edge reconstruction artifacts extending to 3mm on either side of the channel. Conclusion: We have shown that the PRESAGE/Optical‐CT system is capable of providing a 3D dose distribution Eclipse verification of a simple Ir‐192 HDR irradiation. The unique features of the system are very high spatial resolution isotropically in 3D, simplicity (lack of need for an external container) and robustness (e.g. lack of sensitivity to oxygen contamination).

MO-EE-A1-04: A Generalized Dose-Rate Constant Formalism for Brachytherapy Sources Emitting Anisotropic Photon Energy Spectrum

Purpose: The aim of this work was to develop a generalized formalism for calculating the dose‐rate constant, A, of radioactive sources emitting anisotropicphoton energy spectra.Method and Materials: Although the Λ is defined on the transverse bisector of a brachytherapy source, its magnitude depends on both the primary photons emitted along the transverse bisector and the scatteredphotons arising from those primary photons emitted in other directions. For sources emitting nearly isotropic energy spectrum, the contribution of scatteredphotons can be taken into account easily by using Berger's energy‐buildup factor. For sources with anisotropicphoton emissions, a new dose‐rate deposition kernel for unidirectional photon sources of unit intensity and energy E was introduced. The dose‐rate produced by such a source at any location, r→, in water can then be determined by superposition of the angular photon energy spectrum with the dose‐deposition kernel. Results: A generalized formalism for Λ was obtained: Λ ≡ Ḋ(r→ 0 ) S k = ∑ i n(E i ;θ 0 ,φ 0 ) · E i · (μ en (E i )/ρ) air · Λ(E i ) ∑ i n(E i ;θ 0 ,φ 0 ) · E i · (μ en (E i )/ρ) air with Λ(E) = ∫ Ω n(E;θ s ,φ s ) n(E;θ 0 ,φ 0 ) k(r→ 0 ;E,θ s φ s ) E · (μ en (E)/ ρ) air dΩ s where k(r→; E, θ s , φ s ) is the dose‐rate‐deposition kernel with r→ denoting the dose‐rate deposition site and (θs, φs) the orientation of an unidirectional source, n(E; θ s , φ s ) is the number of photons emitted in the (θs, φs) direction, and μ en (E)/ ρ) air is the mass energy absorption coefficient of air. The subscript 0 denotes the reference point of Λ. This formalism exhibits two unique features essential for accurate Λ calculation: 1) it only requires relative photon energy spectra and 2) it does not depend on the methodology used in the determination of dose‐deposition kernels or angular energy spectra.Conclusion: A generalized formalism, based on the dose deposition kernels of mono‐energetic unidirectional photon sources, has been developed for calculating the Λ of low‐energy brachytherapy sources. In addition, this formalism can also be used in the calculation of other dosimetry parameters for interstitial brachytherapy sources. Numerical testing of this formalism for some 125I, 103Pd, and 131Cs sources will be presented.

SU‐FF‐T‐18: On the Determination of the Absolute Dose for a 192Ir HDR Source Using a Fricke Dosimeter: Correction Factors for Several Radii Using MC Calculation

Purpose: To determine the absolute dose at the reference point using measurement with Fricke dosimeter at various transverse bisector distances. Method and Materials: In a previous work we showed how the absolute dose from a source can be measured using a Fricke dosimeter. Ring shaped fricke volume of 1 mm width and 3 mm height was used to determine the absolute dose. AlphaOmega HDR source was located at the center of the ring which was 1 cm away from the source, the uncertainty at this point was too high (k=7%). In another second study we demonstrated that there is a size detector dependence and source positioning uncertainty when doing the measurement. We did dose profile measurements along and away from a source using four different detectors (two ionization chambers and two diodes). We showed that the detector size effect goes from 1.3% at 1 cm to 0.2% at 2.5 cm. Therefore PENELOPE Monte Carlo simulations for several radii have been used to determine conversion factors to the reference point. Results: We show in this work that the absorbed dose at various distances away from the source does not have an inverse square law behavior. Therefore, factors have been determined to convert the dose at measured point to the dose at the reference point recommended by AAPM TG‐43. These conversion factors are 4.454, 7.609, 11.288 and 16.003 for the distances 1.5, 2.0, 2.5, and 3.0 cm respectively. Conclusion: Feasibility of determining the absorbed dose using a Fricke dosimeter at the reference point (1 cm distance) was demonstrated in a previous work, however the uncertainty was too high (7%). Measurement at distances greater than 1 cm was demonstrated to lower the uncertainty. PENELOPE MC code was used to obtain the conversion factors at various radii to the reference point.

Spectroscopic output of and low dose rate brachytherapy sources

The spectroscopic output of low dose rate (LDR) brachytherapy sources is dependent on the physical design and construction of the source. Characterization of the emitted photons from 12 and 3 LDR brachytherapy source models is presented. Photon spectra, both along the transverse bisector and at several polar angles, were measured in air with a high‐purity reverse electrode germanium (REGe) detector. Measured spectra were corrected to in vacuo conditions via Monte Carlo and analytical methods. The tabulated and plotted spectroscopic data provide a more complete understanding of each source model's output characteristics than can be obtained with other measurement techniques. The variation in fluorescence yield of the sources containing silver caused greater differences in the emitted spectra and average energies among these seed models than was observed for the sources or the sources that do not contain silver. Angular spectroscopic data further highlighted the effects of source construction unique to each model, as well as the asymmetric output of many seeds. These data demonstrate the need for the incorporation of such physically measured output characteristics in the Monte Carlo modeling process.

Determination of absorbed dose in water at the reference point for an HDR brachytherapy source using a Fricke system

A ring‐shaped Fricke device was developed to measure the absolute dose on the transverse bisector of a high dose rate (HDR) source at from its center in water, . It consists of a polymethylmethacrylate (PMMA) rod (axial axis) with a cylindrical cavity at its center to insert the radioactive source. A ring cavity around the source with thickness and height is centered at from the central axis of the source. This ring cavity is etched in a disk shaped base with diameter and thickness. The cavity has a wall around it thick. This ring is filled with Fricke solution, sealed, and the whole assembly is immersed in water during irradiations. The device takes advantage of the cylindrical geometry to measure . Irradiations were performed with a Nucletron microselectron HDR unit loaded with an Alpha Omega radioactive source. A Spectronic® 1001 spectrophotometer was used to measure the optical absorbance using a quartz cuvette with light pathlength. The PENELOPE Monte Carlo code (MC) was utilized to simulate the Fricke device and the Alpha Omega source in detail to calculate the perturbation introduced by the PMMA material. A NIST traceable calibrated well type ionization chamber was used to determine the air‐kerma strength, and a published dose‐rate constant was used to determine the dose rate at the reference point. The time to deliver to the reference point was calculated. This absorbed dose was then compared to the absorbed dose measured by the Fricke solution. Based on MC simulation, the PMMA of the Fricke device increases the by 2.0%. Applying the corresponding correction factor, the value assessed with the Fricke device agrees within 2.0% with the expected value with a total combined uncertainty of 3.43% . The Fricke device provides a promising method towards calibration of brachytherapy radiation sources in terms of and audit HDR source calibrations.

Experimental dosimetry of a 32P catheter-based endovascular brachytherapy source

The experimental dosimetry in a water phantom of a 32P linear source, 20 mm in length, used for the brachytherapy of coronary vessels is reported. The source content activity, A, was determined by means of a calibrated well ion-chamber and the value was compared with the contained activity reported in the manufacturer's certification. In this field of brachytherapy dosimetry, radiochromic film supplies a high enough spatial resolution. A highly sensitive radiochromic film, that presents only one active layer, was used in this work for the source dosimetry in a water phantom. The radiochromic film was characterized by electron beams produced by a clinical linac. A Monte Carlo calculation of beta spectra in water at different distances along the source transverse bisector axis allowed to take into account the low dependence of film response from the electron beam energy. The adopted experimental set-up, with the source in its catheter positioned on the film plane inside the water phantom, supplies accurate dosimetric information. The measured dose rate to water per unit of source activity at reference distance, Ḋ(r0, θ0)/A, in units of cGy s−1 GBq−1, was in agreement with the value reported in the manufacturer's certification within the experimental uncertainty. The radial dose function, g(r), is in good agreement with the literature data. The anisotropy function F(r, θ) is also reported. The analysis of the dose profile obtained at 2 mm from the source longitudinal axis shows that the uniformity is within 10% along 75% of the 20 mm treatment length. The adopted experimental set-up seems to be adequate for the quality control procedure of the dose homogeneity distribution in the water medium.

A standard dosimetry procedure for 192Ir sources used for endovascular brachytherapy

The experimental dosimetry of a high dose rate (HDR) 192Ir source used for the brachytherapy of peripheral vessels is reported. The direct determination of the reference air kerma rate K̇r agrees, within the experimental uncertainty, with the results obtained by a well ionization chamber calibrated at the NIST and the manufacturer's certification. A highly sensitive (HS) radiochromic film (RCF), that presents only one active layer, was used for the source dosimetry in a water phantom. The adopted experimental set-up, with the source in its catheter positioned on the RCF plane, seems to have given better accuracy of the RCF optical density measurements. The agreement between the measurement of the dose rate constant DKr(10 mm, π/2) and the literature data confirmed the coherence of the HS RCF calibration obtained by the kerma in air measurements. The RCF measurements supplied dosimetric information about the dose to water per reference air kerma rate Ḋ(r, θ)/K̇r along the source transverse bisector axis, the radial dose function g(r) and the anisotropy function F(r, θ). The value Ḋ(2 mm, π/2)/K̇r = 22.4 ± 1.2 cGy h−1/(μGy h−1) is supplied with a dose uncertainty that is essentially due to the indeterminacy of the source position in the catheter. The data of the radial and anisotropy functions have been compared with Monte Carlo determinations reported in the literature.

CT-based dosimetry calculations for 125I prostate implants

To evaluate the Monte Carlo code MCNP4B for low-energy brachytherapy calculations, including the effects of interseed attenuation and patient specific heterogeneities, on the calculated dose distribution from transperineal implantation of 125I. The Monte Carlo code MCNP4B was used to model and benchmark the absolute dose distribution from two 125I brachytherapy seeds (model 6711 and 6702). Based upon the physical source model, the total photon intensity and differential energy spectrum were evaluated as a function of angle from the transverse bisector of the source. These spectral and intensity data were reformatted to produce probability distributions for sampling from a virtual point source. The virtual source model and a modified version of MCNP4B is then used for simulating arbitrary brachytherapy source configurations within a homogeneous or heterogeneous patient specific computed tomography (CT)-based lattice geometry. Comparison with TG-43 data and the Monte Carlo calculations is excellent with MCNP4B predicting the radial dose function for the 125I 6711 and 6702 sources within 6% for all data points tested. Attenuation effects from neighboring seeds were investigated for pre- and postimplant seed distributions and found to be negligible. Preliminary dosimetry analysis of postimplant seed distributions comparing homogeneous water versus heterogeneous CT simulation geometries indicates an average decrease of approximately 5.6% for the volume of tissue irradiated to a prescription isodose line of 144 Gy.

Dose errors in the near field of an HDR brachytherapy stepping source

The dose rate at point P at 0.25 cm in water from the transverse bisector of a straight catheter with an active stepping source (Nucletron microSelectron HDR source) with a dwell length of 2 cm was calculated using Monte Carlo code MCNP 4.A. The source step sizes were 1 cm and 0.25 cm. The Monte Carlo (MC) results were used for comparison with the results calculated with the Nucletron brachytherapy planning system (BPS) formalism, first with BPS variants and then with its respective MC calculated radial dose function and anisotropy function. The dose differences at point P calculated using the BPS formalism and variants are +15.4% and +3.1% for the source step size of 1 cm and 0.25 cm respectively. This reduction in dose difference is caused by the increased importance of errors in the anisotropy function with the smaller step size, which counter the errors in the radial dose function. Using the MC calculated radial dose function and anisotropy function with the BPS formalism, 1% dose calculation accuracy can be achieved, even in the near field, with negligible extra demand on computation time.

Alanine/EPR dosimetry in brachytherapy

The paper reports the experimental procedure adopted to determine the absorbed dose rate in water per reference air kerma rate, , along the transverse bisector axis of a brachytherapy source. The dose rate measurements have been carried out at difference distances, d, from the source using alanine dosemeters in a water phantom. The reference air kerma rate, , was determined adopting a `direct procedure' that uses a spherical ionization chamber in air. The dose rate constant of the source examined was . The values of the radial dose function along the transverse axis, g( d), determined with an uncertainty of 3.4% (1), were found to be in good agreement with the results reported in the literature. The uncertainty in dose rate value has been estimated as 2.8% (1) for distances from the source up to 7 cm. has been determined with 1.2% (1) uncertainty. So values were determined with 3% (1) uncertainty.

Applications of the Italian protocol for the calibration of brachytherapy sources

The Associazione Italiana di Fisica Biomedica (AIFB; Italian Association of Biomedical Physics) has adopted the Italian protocol for the calibration of brachytherapy sources. The AIFB protocol allows measurements of the reference air kerma rate, , within 1.7% . To measure the AIFB protocol has identified a direct and an indirect procedure. The direct procedure is based on the use of spherical or cylindrical ionization chambers as local reference dosimeters positioned along the transverse bisector axis of the source. Once the source is specified by a value, this can be used to calibrate a field instrument, such as a well-type ionization chamber, for further source calibrations by means of an indirect procedure. This paper reports the results obtained by the Physics Laboratory of the Università Cattolica del S Cuore (PL-UCSC), in terms of calibration of five types of source, , and . The role of the determination for a brachytherapy source has been underlined when a new source such as the seed model X1267 has been proposed for clinical use. The values for spherical sources differed by 5% from the vendor's mean value. The five types of source calibrated in terms of were used to obtain the calibration factor, , of an HDR-1000 well-type ionization chamber.

Monte Carlo evaluation of specific dose constants in water for 125I seeds.

To clinically utilize relative three-dimensional dose distribution data for 125I seeds in water, the specific dose constant must be known. This constant is the ratio of absolute dose rate in medium, 1 cm from the source on its transverse bisector, to source strength. We have used the Monte Carlo method to calculate specific dose constants in water for three types of 125I seeds (models 6711, 6701, and 6702). Our photon transport code realistically models the complex internal structure of these interstitial sources as well as the exposure standardization experiments conducted at the National Bureau of Standards which form the basis of 125I source strength specification. In addition, relative attenuation factors for the three seed types are presented. Our results indicate that published data overestimate the specific dose constant by 10% to 14%.