Equivalent Tissue-air Ratio Research Articles

Investigation of the effects on dose calculations of correction-based algorithms in different tissue medium

The aim of this study is to determine the effects of correction-based algorithms on dose distributions in the inhomogeneous medium and dosimetric parameters depending on the algorithms preferred in breast and lung treatment plans. In the treatment planning system (TPS) in which the Pencil Beam Convolution (PBC) dose calculation algorithm is used, dose calculations were performed in different correction-based algorithms Modified Batho and Equivalent Tissue-Air Ratio (ETAR) for soft tissue, bone and air medium. Rectilinear virtual phantoms were created in TPS for soft tissue, bone and air materials, and deep dose values and dose distribution profiles at lateral depth were obtained. Intensity modulated radiotherapy (IMRT) treatment planning technique was applied to 20 patients with breast and lung cancer diagnosis on computed tomography (CT) sections, and dose calculations were performed. Different dosimetric parameters obtained within the target volume were calculated. Although the effects of correction-based algorithms on dose values depending on the depth and dose distribution profiles in lateral depth were calculated below 1% in dose calculations performed on soft tissue virtual phantom, dose profiles were obtained as approximately 20% in bone and air medium. It was concluded that correction-based algorithms in the different inhomogeneous mediums have a significant effect on the dose values calculated in TPS.

Accuracy Evaluation of EPL and ETAR Algorithms in the Treatment Planning Systems using CIRS Thorax Phantom.

Background: It is recommended for each set of radiation data and algorithm that subtle deliberation is done regarding dose calculation accuracy. Knowing the errors in dose calculation for each treatment plan will result in an accurate estimate of the actual dose achieved by the tumor.Objective: This study aims to evaluate the equivalent path length (EPL) and equivalent tissue air ratio (ETAR) algorithms in radiation dose calculation.Material and Methods: In this experimental study, the TEC-DOC 1583 guideline was used. Measurements and calculations were obtained for each algorithm at specific points in thorax CIRS phantom for 6 and 18 MVs and results were compared.Results: In the EPL, calculations were in agreement with measurements for 27 points and differences between them ranged from 0.1% to 10.4% at 6 MV. The calculations were in agreement with measurements for 21 points and differences between them ranged from 0.4% to 13% at 18 MV. In ETAR, calculations were also in consistent with measurements for 21 points, and differences between them ranged from 0.1% to 9% at 6 MV. Moreover, for 18 MV, the calculations were in agreement with measurements for 17 points and differences between them ranged from 0% to 11%. Conclusion: For the EPL algorithm, more dose points were in consistent with acceptance criteria. The errors in the ETAR were 1% to 2% less than the EPL. The greatest calculation error occurs in low-density lung tissue with inhomogeneities or in high-density bone. Errors were larger in shallow depths. The error in higher energy was more than low energy beam.

The choice of statistical methods for comparisons of dosimetric data in radiotherapy.

PurposeNovel irradiation techniques are continuously introduced in radiotherapy to optimize the accuracy, the security and the clinical outcome of treatments. These changes could raise the question of discontinuity in dosimetric presentation and the subsequent need for practice adjustments in case of significant modifications. This study proposes a comprehensive approach to compare different techniques and tests whether their respective dose calculation algorithms give rise to statistically significant differences in the treatment doses for the patient.MethodsStatistical investigation principles are presented in the framework of a clinical example based on 62 fields of radiotherapy for lung cancer. The delivered doses in monitor units were calculated using three different dose calculation methods: the reference method accounts the dose without tissues density corrections using Pencil Beam Convolution (PBC) algorithm, whereas new methods calculate the dose with tissues density correction for 1D and 3D using Modified Batho (MB) method and Equivalent Tissue air ratio (ETAR) method, respectively. The normality of the data and the homogeneity of variance between groups were tested using Shapiro-Wilks and Levene test, respectively, then non-parametric statistical tests were performed. Specifically, the dose means estimated by the different calculation methods were compared using Friedman’s test and Wilcoxon signed-rank test. In addition, the correlation between the doses calculated by the three methods was assessed using Spearman’s rank and Kendall’s rank tests.ResultsThe Friedman’s test showed a significant effect on the calculation method for the delivered dose of lung cancer patients (p <0.001). The density correction methods yielded to lower doses as compared to PBC by on average (−5 ± 4.4 SD) for MB and (−4.7 ± 5 SD) for ETAR. Post-hoc Wilcoxon signed-rank test of paired comparisons indicated that the delivered dose was significantly reduced using density-corrected methods as compared to the reference method. Spearman’s and Kendall’s rank tests indicated a positive correlation between the doses calculated with the different methods.ConclusionThis paper illustrates and justifies the use of statistical tests and graphical representations for dosimetric comparisons in radiotherapy. The statistical analysis shows the significance of dose differences resulting from two or more techniques in radiotherapy.

Effect of the modification of CT scanner calibration curves on dose using density correction methods for chest cancer

PurposeThis work sought to establish whether the choice of CT scanner calibration curve has a significant effect on dose computation using density correction methods for chest cancer. Material and methodsCIRS®062 phantom was used to calculate the Hounsfield Unit using 80, 120 and 140kV. Four CT calibration curves were implanted in the Eclipse® TPS. Forty-two irradiation fields for 4 patients with lung cancer were included and analysed. The patients were treated with 3-dimensional radiation therapy. For each patient, 3 treatment plans were generated using exactly the same beam configuration. In plan 1, the dose was calculated using the Modified Batho (MB) method. In plan 2, the dose was calculated using the Batho power law (BPL) method. In plan 3, the dose was calculated using the Equivalent Tissue Air Ratio (ETAR) method. To evaluate the treatment plans computed by the three methods, the monitor units, dose volume histograms, conformity index, homogeneity index, planning target volumes conformity index, geometrical index and 2D gamma index were compared. The statistical analysis was carried out using Wilcoxon signed rank test. ResultsThe three density correction methods in plans 1, 2 and 3 using tested curves produced a difference less than 1% for MUs and DVH. Wilcoxon test showed a statically significant difference for MUs using ETAR method with calibration curves based on 80 and 120kV. There was no significant difference for the quality indices between plan 1, 2 and 3, (P>0.05), but a significant difference for the planning target volumes conformity index between plans 1, 2 and 3 (P<0.05) was observed. The 2D gamma analysis showed that 100% of pixels had gamma≤1. ConclusionThe impact of the modification of CT calibration curves on dose is negligible for chest cancer using density correction methods. One calibration curve can be used to take into account the density correction for lung.

Comparison of CCC and ETAR dose calculation algorithms in pituitary adenoma radiation treatment planning; Monte Carlo evaluation

AbstractAimsTo verify the accuracy of two common absorbed dose calculation algorithms in comparison to Monte Carlo (MC) simulation for the planning of the pituitary adenoma radiation treatment.Materials and methodsAfter validation of Linac's head modelling by MC in water phantom, it was verified in Rando phantom as a heterogeneous medium for pituitary gland irradiation. Then, equivalent tissue-air ratio (ETAR) and collapsed cone convolution (CCC) algorithms were compared for a conventional three small non-coplanar field technique. This technique uses 30 degree physical wedge and 18 MV photon beams.ResultsDose distribution findings showed significant difference between ETAR and CCC of delivered dose in pituitary irradiation. The differences between MC and dose calculation algorithms were 6.40 ± 3.44% for CCC and 10.36 ± 4.37% for ETAR. None of the algorithms could predict actual dose in air cavity areas in comparison to the MC method.ConclusionsDifference between calculation and true dose value affects radiation treatment outcome and normal tissue complication probability. It is of prime concern to select appropriate treatment planning system according to our clinical situation. It is further emphasised that MC can be the method of choice for clinical dose calculation algorithms verification.

Commissioning and initial acceptance tests for a commercial convolution dose calculation algorithm for radiotherapy treatment planning in comparison with Monte Carlo simulation and measurement

In this study the commissioning of a dose calculation algorithm in a currently used treatment planning system was performed and the calculation accuracy of two available methods in the treatment planning system i.e., collapsed cone convolution (CCC) and equivalent tissue air ratio (ETAR) was verified in tissue heterogeneities. For this purpose an inhomogeneous phantom (IMRT thorax phantom) was used and dose curves obtained by the TPS (treatment planning system) were compared with experimental measurements and Monte Carlo (MCNP code) simulation. Dose measurements were performed by using EDR2 radiographic films within the phantom. Dose difference (DD) between experimental results and two calculation methods was obtained. Results indicate maximum difference of 12% in the lung and 3% in the bone tissue of the phantom between two methods and the CCC algorithm shows more accurate depth dose curves in tissue heterogeneities. Simulation results show the accurate dose estimation by MCNP4C in soft tissue region of the phantom and also better results than ETAR method in bone and lung tissues.

Conformal fields in prostate radiotherapy: A comparison between measurement, calculation and simulation

The objective of this study is to evaluate the accuracy of a treatment planning system (TPS) for calculating the dose distribution parameters in conformal fields (CF). Dosimetric parameters of CF's were compared between measurement, Monte Carlo simulation (MCNP4C) and TPS calculation. Field analyzer water phantom was used for obtaining percentage depth dose (PDD) curves and beam profiles (BP) of different conformal fields. MCNP4C was used to model conformal fields dose specification factors and head of linear accelerator varian model 2100C/D. Results showed that the distance to agreement (DTA) and dose difference (DD) of our findings were well within the acceptance criteria of 3 mm and 3%, respectively. According to this study it can be revealed that TPS using equivalent tissue air ratio calculation method is still convenient for dose prediction in non small conformal fields normally used in prostate radiotherapy. It was also showed that, since there is a close correlation with Monte Carlo simulation, measurements and TPS, Monte Carlo can be further confirmed for implementation and calculation dose distribution in non standard and complex conformal irradiation field for treatment planning systems.

Comparison of conventional and Monte Carlo dose calculations for prostate treatments

Monte Carlo (MC) calculations are rapidly finding their place in clinical dose assessments. We investigated conformal prostate dose distributions as calculated by MC, and compared them to several analytical dose calculations. The treatment distributions for twenty prostate cancer patients, treated with 18 MV 3D conformal radiation therapy, were retrospectively assessed. The BEAM code based on EGSnrc was used to model the beam from which phase space files were used as input into the XVMC algorithm. This was compared to conventional treatment planning system calculations (CADPLAN) with and without inhomogeneity corrections. Results indicate that the CADPLAN generalized Batho Power Law, modified Batho Power Law, and equivalent tissue-air ratio methods contain inaccuracies in calculated dose to 95 % of the prostate planning target volume of 3.5 %, 3.3 %, and 2.9 %, respectively. The greatest discrepancies in the organs at risk were seen in the bladder where the inhomogeneity correction methods all predicted that 50 % of the prescribed dose covered an average of 8.2 % more of the bladder volume than that predicted from the MC calculation. Water equivalent MC and water equivalent CADPLAN calculations revealed important discrepancies on the same order as those between heterogeneous MC and heterogeneous CADPLAN calculations. The data indicate that the effect of inhomogeneities is greater in the target volume than the organs at risk, and that accurately modeling the dose deposition process is important for each patient geometry, and may have a greater impact on the dose distribution in the prostate region than correcting an analytical algorithm for the presence of inhomogeneities.

Experimental and Monte Carlo evaluation of Eclipse treatment planning system for lung dose calculations

Summary Background In this study the accuracy of a pencil beam based treatment planning system (TPS) was evaluated for lung dose calculations by comparison with measurement and the Monte Carlo (MC) method. Aim In the current study we assessed the performance of the Eclipse treatment planning system in the thorax region by ionization chamber measurements and Monte Carlo calculations. We examined two analytic methods: modified Batho (MB) and equivalent tissue-air ratio “ETAR” methods for thorax region irradiations. For Monte Carlo calculations in the thorax phantom, we modelled a Varian Clinac 2100EX linac. After benchmarking our model with water phantom measurements we used this model for thorax phantom calculations. Materials/Methods 8 and 15 MV photon beams of Varian 21EX linac were used for irradiations. Using MANP4C Monte Carlo code, the geometry of the linac head was simulated. After commissioning “MC” beam models, lung doses were calculated by the Monte Carlo (MC) method. Irradiation cases were: (1) posterior fields of single lung with field sizes of 4×4 and 10×10cm 2 (2) lateral fields of thorax with 4×4 and 10×10cm 2 field sizes. Results TPS calculations involving ETAR and MB methods were in close agreement with Monte Carlo results and measurements for a 10×10 cm 2 field size at both energies. For a field size of 4×4cm 2 the maximum differences in local dose between TPS calculations and measurement were +33% (MB) and +28% (ETAR). Also, they ignored lung dose reduction due to lateral electronic equilibrium for small field size. Similar results would be expected for other TPSs implementing these algorithms. MC calculations were in excellent agreement with measurement, showing local differences of no more than 2% for all measured points. Conclusions Our study findings showed great differences between both analytical methods and measurements for 4×4cm 2 field sizes for points in the lung. Our study recommends using the MC method for small-field lung dose calculations.

Comparison of dose calculation algorithms with Monte Carlo methods for photon arcs.

The objective of this study is to seek an accurate and efficient method to calculate the dose distribution of a photon arc. The algorithms tested include Monte Carlo, pencil beam kernel (PK), and collapsed cone convolution (CCC). For the Monte Carlo dose calculation, EGS4/DOSXYZ was used. The SRCXYZ source code associated with the DOSXYZ was modified so that the gantry angle of a photon beam would be sampled uniformly within the arc range about an isocenter to simulate a photon arc. Specifically, photon beams (6/18 MV, 4 x 4 and 10 x 10 cm2) described by a phase space file generated by BEAM (MCPHS), or by two point sources with different photon energy spectra (MCDIV) were used. These methods were used to calculate three-dimensional (3-D) distributions in a PMMA phantom, a cylindrical water phantom, and a phantom with lung inhomogeneity. A commercial treatment planning system was also used to calculate dose distributions in these phantoms using equivalent tissue air ratio (ETAR), PK and CCC algorithms for inhomogeneity corrections. Dose distributions for a photon arc in these phantoms were measured using a RK ion chamber and radiographic films. For homogeneous phantoms, the measured results agreed well (approximately 2% error) with predictions by the Monte Carlo simulations (MCPHS and MCDIV) and the treatment planning system for the 180 degrees and 360 degrees photon arcs. For the dose distribution in the phantom with lung inhomogeneity with a 90 degrees photon arc, the Monte Carlo calculations agreed with the measurements within 2%, while the treatment planning system using ETAR, PK and CCC underestimated or overestimated the dose inside the lung inhomogeneity from 6% to 12%.

Radiotherapy dose compensation for lung patients

The purpose of the present paper is to provide a more homogeneous dose distribution in the target volume from compensated anterior and posterior fields while the healthy lung is spared by de-weighting the lateral fields. A compensation computation which used linear iterations to compute the most homogeneous dose distribution across the target volume was applied to produce optimum compensator designs. The equivalent tissue-air ratio (E-TAR) inhomogeneity correction was applied for the computations using a GE target series II planning computer. The compensators designed were tested for accuracy in a modified water/lung phantom using a scanning diode and an anthropomorphic phantom using thermoluminescent dosimeters. A comparison has been made between the compensated and uncompensated plans for the first nine patients who we have treated with this technique. The dose profiles produced by the computation agreed with the prediction of the computed isodose plans to within +/- 2% at the target depth. The thermoluminescent dosimeter (TLD)-measured results in the anthropomorphic phantom agreed with the planning computer within +/- 3%. A comparison of nine compensated plans of radiotherapy patients for large-volume targets in the lung region showed a maximum variation in the target to be 19% uncompensated versus 10% compensated. By providing compensated treatment fields from anterior and posterior treatment portals, a homogeneous dose that conforms well to the target volume is provided. As an added bonus, this enables the lateral lung fields to be significantly de-weighted and the healthy lung is spared considerable dose.

Implementation of the ETAR method for 3D inhomogeneity correction using FFT.

The equivalent tissue-air-ratio (ETAR) method employs three-dimensional (3D) CT pixel information to approximate scatter dose contribution for inhomogeneity correction. In general, the calculation provides better agreement with measurements than the one-dimensional (1D) methods typically used in commercial treatment planning systems. In its original implementation, the 3D formulation of the ETAR method is modified empirically as a 2D calculation in order to reduce computation time. The modification compromises the use of the method in several treatment geometries. An examination of the ETAR formulation shows that the calculation can be expressed as a convolution and thus can be performed in 3D using fast Fourier transform (FFT) techniques. The algorithm has been implemented as a 3D FFT convolution. Making use of the symmetric properties of the FFT, the new approach shows significant savings in computation time without excessive memory requirement. Despite its fundamental limitations when applied to regions of electronic disequilibrium, the ETAR method offers a practical solution to improving current dose calculation in 3D treatment planning, particularly when the more advanced scatter ray-tracing dose calculation algorithms remain impractical for clinical use. Recent work to extend the method to approximate electron transport is also encouraging.

リニアック 10 MV X 線における肺の線量補正

In radiation therapy of thorax region, it is necessary to apply absorbed dose correction for heterogeneity tissuse of lungs. Practically, however, it is not easy to do an accurate absorbed dose correction. We made a basic experiment for absorbed dose correction of heterogeneity tissue of lungs using thorax phantom for 10 MV X-rays, and evaluated the accuracy of depth dose calculation algorithms for its tissue which are available in conventional, commercial-based treatment-planning systems. Depth dose on the central axis in lung for small field of 5×5(cm)^2 was reduced by about 10% due to loss of lateral electronic equilibrium. Dose correction factor of heterogeneity was insensitive to the thickness of the build-up layer of this phantom if there was a longitudinal electronic equilibrium. The equivalent TAR (tissue-air ratio) method gave the greatest accuracy for dose calculation algorithms, of lungs whose error was within about 3% compared to the measured values. However, for the smaller field of 5×5(cm)^2, all our methods did not consider the effects of electron scattering, so that the equivalent TAR method and the ratio of TAR method gave errors of about 15% and the power law TAR method about 6%.

On the impact of tissue inhomogeneity corrections in clinical thoracic radiation therapy

The magnitude and impact of the corrections required for calculated absorbed doses due to the presence of real anatomic inhomogeneities were studied in 100 “standard” clinical thoracic cases. Correction factor distributions were generated with the equivalent tissue-air ratio method. The correction factors at the point of target absorbed dose, the average within the high dose area, and the average and maximum within the irradiated lung tissue were determined. Results indicate that among patients undergoing similar treatment techniques and radiation energy, correction factors can vary greatly. The diversity in anatomy, geometry, and density emphasizes the need for patient-specific dose calculations if the appropriate accuracy is to be achieved. Even for simple parallel-opposed fields encompassing the mediastinal and bilateral lung regions, the correction at the point of target absorbed dose was found to be as much as 5 to 16% for photon energies between 60Co and 25 MV. The average correction factor within the high dose region was generally comparable in value. For beams that traverse the lungs obliquely, the correction factor at the point of target absorbed dose was as large as 1.21 with 25 MV photons. These values correlated linearly with the lung pathlength preceding the point. Within lung tissues, the corrections were often more dramatic. Using the dose response curve for normal lung tolerance, the clinical consequence of ignoring the increased dose to lung tissue can be predicted. For 100 patients, the use of uncorrected dose would result in an underestimation of the risk for radiological radiation pneumonitis by 7% on average and by as much as 19% maximally. These data suggest that whenever lung tissue and/or tumor response levels are critical, as is often the case for radical therapy, multi-center trials, and dose response determination, inhomogeneity corrections should be implemented.

A 3-dimensional scatter correction algorithm for photon beams (dosimetry)

A new 3-dimensional scatter algorithm based on the principles of the equivalent tissue air ratio (ETAR) method has been developed. The algorithm models the relative amount of scattered radiation reaching a point in both homogeneous and inhomogeneous situations, and derives a correction factor which can be applied to the scatter component of the dose to the point. The algorithm has been installed on an in-house developed planning system running on a Sun Sparcstation and its implementation and performance are described. The new algorithm has been evaluated by comparison with measurement in experimental situations designed to provide a rigorous test of the model. Comparisons with ETAR are also made, and the new algorithm gives a considerable improvement in accuracy over ETAR without any significant increase in calculation time.

Accuracy of lung dose calculations for large-field irradiation with 6-MV x rays.

In large-field irradiations of the upper half-body, there is a potential for severe respiratory complications. The incidence of severe radiation damage to lung can be reduced by limiting the lung dose and the volume of lung irradiated. It is necessary to have an accurate method of calculating the dose actually delivered to lung, and this work deals with such calculations and measurements in large (half-body) fields of 6-MV x rays. The results of various lung dose calculations by the Batho, the modified Batho (Lulu and Bjärngard), the simple scaled tissue-maximum ratio and the detailed equivalent tissue-air ratio methods are compared with doses measured in phantoms representing the mediastinum and adjacent lungs.

The density scaling theorem applied to lateral electronic equilibrium.

Some theoretical considerations concerning the application of the density scaling theorem to secondary electrons from high-energy x-ray beams are discussed. It is concluded that the theorem is expected to be applicable with acceptable accuracy for 10-MV x-ray beams in low-density materials. Measurements reported by different groups of workers are discussed and shown to be in general agreement with the theorem, with one exception. It is shown that the theorem can be applied in combination with phantom scattering correction factors to calculate the dose in homogeneous bodies which differ from water in their density. The method of calculation is similar to the equivalent tissue-air ratio method, but takes account of the effects of loss of lateral electronic equilibrium.

Uncertainty of the equivalent tissue-air ratio method.

Uncertainty of the equivalent tissue-air ratio method.

A simple equivalent tissue-air ratio method for calculating absorbed dose in a heterogeneous medium.

Density correction factors for 60Co gamma rays and 8-MV x rays were measured with an ionization chamber in water-equivalent phantoms containing simulated lung or bone inhomogeneities larger or smaller than the beam (beam sizes, 5 x 5 to 30 x 30 cm). Measurements were compared with values calculated from the effective SSD method, the generalized Batho power law method, and a simplified equivalent tissue-air ratio (SETAR) method. The results indicate that the SETAR method gives the best overall agreement with experimental values, while the effective SSD and Batho algorithms are accurate only in certain situations. The SETAR method seems adequate for most clinical applications.

Half body radiotherapy: The use of computed tomography to determine the dose to lung

The complication of radiation pneumonitis is of major concern in upper half body irradiation. A CT-aided treatment planning system was utilized to study the variation in lung geometry and lung density and to determine the resultant dose variation within the corresponding treatment volumes. Twenty-three patients who were scheduled for upper half body irradiation, were studied with this CT-aided treatment planning system and a number of parameters were analyzed to determine the factors affecting the dose to lung. For healthy lungs, electron densities, relative to water, varied from 0.084 to 0.40; the average lung density was approximately 0.25. Using the equivalent tissue-air ratio method for dose calculation, the dose within lung increased from 10% to 24% in comparison with doses calculated on the basis of a homogeneous water like patient. This dose increase is dependant on patient thickness, lung thickness and lung density. The calculated doses were verified experimentally by thermoluminescent dosimetry in a human-like phantom for photon beam energies ranging from cobalt-60 to 25 MV x-rays. Agreement between the measured and calculated data was better than 3% in lung. A number of other dose computation procedures were found to be in error by more than ±10% in the determination of dose to the middle of lung. Some simplified procedures, which do not require a CT-aided treatment planning system, also were evaluated. The previously published dose complication curve for radiation pneumonitis now can be considered in terms of absolute dose to lung rather than the dose prescribed to unit density tissues.