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 Table of Contents  
ORIGINAL ARTICLE
Year : 2015  |  Volume : 11  |  Issue : 3  |  Page : 586-591

Assessment of target volume doses in radiotherapy based on the standard and measured calibration curves


1 Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran; Department of Radiology, Faculty of Paramedical, Mazandaran University of Medical Sciences, Sari, Iran
2 Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
3 Department of Biochemistry and Biophysics, Faculty of Medicine, Mazandaran University of Medical Sciences, Sari, Iran

Date of Web Publication9-Oct-2015

Correspondence Address:
Gholamreza Fallah Mohammadi
Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran; Department of Radiology, Faculty of Paramedical, Mazandaran University of Medical Sciences, Sari
Iran
Seyed Salman Zakariaee
Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.163696

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 > Abstract 

Context: In radiation treatments, estimation of the dose distribution in the target volume is one of the main components of the treatment planning procedure. To estimate the dose distribution, the information of electron densities is necessary. The standard curves determined by computed tomography (CT) scanner that may be different from that of other oncology centers. In this study, the changes of dose calculation due to the different calibration curves (HU-ρel ) were investigated.
Materials and Methods: Dose values were calculated based on the standard calibration curve that was predefined for the treatment planning system (TPS). The calibration curve was also extracted from the CT images of the phantom, and dose values were calculated based on this curve. The percentage errors of the calculated values were determined.
Statistical Analysis Used: The statistical analyses of the mean differences were performed using the Wilcoxon rank-sum test for both of the calibration curves.
Results and Discussion: The results show no significant difference for both of the measured and standard calibration curves (HU-ρel ) in 6, 15, and 18 MeV energies. In Wilcoxon ranked sum nonparametric test for independent samples with P < 0.05, the equality of monitor units for both of the curves to transfer 200 cGy doses to reference points was resulted. The percentage errors of the calculated values were lower than 2% and 1.5% in 6 and 15 MeV, respectively.
Conclusion: From the results, it could be concluded that the standard calibration curve could be used in TPS dose calculation accurately.

Keywords: Radiotherapy, the measured calibration curve, the monitor units, the standard calibration curve, treatment planning system dose distribution, treatment planning systems


How to cite this article:
Mohammadi GF, Alam NR, Rezaeejam H, Pourfallah TA, Zakariaee SS. Assessment of target volume doses in radiotherapy based on the standard and measured calibration curves. J Can Res Ther 2015;11:586-91

How to cite this URL:
Mohammadi GF, Alam NR, Rezaeejam H, Pourfallah TA, Zakariaee SS. Assessment of target volume doses in radiotherapy based on the standard and measured calibration curves. J Can Res Ther [serial online] 2015 [cited 2019 Nov 19];11:586-91. Available from: http://www.cancerjournal.net/text.asp?2015/11/3/586/163696


 > Introduction Top


In radiation treatments, estimation of the dose distribution in the target volume is one of the main components of the treatment planning procedure. There are different factors that effect on the dose distribution. One of the effective factors is patient's body heterogeneity. To estimate the dose distribution in the target volume, the information of electron densities are necessary. Electron densities could be extracted from computed tomography (CT) images. CT imaging is a conventional method to obtain the electron densities. [1] For dose calculation in treatment planning systems (TPSs), even in methods such as helical tomotherapy and proton beam therapy, CT images were used. [2],[3] Other imaging modalities such as magnetic resonance imaging and positron emission tomography prepared additional information, but CT method plays an essential role to obtain in vivo information of the tissue attenuation coefficients. The attenuation coefficients are reported in Hounsfield unit (HU).

Dose distribution could be predicted accurately based on the exact electron densities of tissues. [4] TPS converts HUs to the relative electron densities based on a calibration curve. [5],[6] The electron densities normalized to the electron density of water, and dose values were calculated based on these normalized electron densities. This conversion was suggested for the first time by Knöös et al. [7] Electron density values are dependent on X-ray spectrum; therefore, different scanners might report different electron densities. For a specific scanner based on selected kVp and filters, different electron densities could be obtained. [8],[9] In Guan et al. study, change of the energy beam from 80 to 130 keV (with a fixed calibration curve [HU-ñel ]) results in 2% error in dose calculation. [10]

Calibration curve (HU-ñel ) could be obtained using a tissue-equivalent phantom. [8] Test procedures of acceptance commissioning, quality assurance, and quality control (QC) of a TPS are suggested in Technical Report Series (International Atomic Energy Agency [IAEA]) No. 430. [11] Experimental tests of the dose calculation procedure are also suggested in TECDOC (IAEA) 1583 that introduce the drawing method for the HU-ñel curves. [12] In the calculation algorithms such as pencil beam, collapsed cone, Batho and etc., there is a heterogeneity correction factor for the path lengths. The path lengths were determined based on the mass density of the voxels. These quantitative data were obtained from the CT images. [4] The CT images could be considered as a virtual phantom for dose calculation, if the HU-ñel curve has been determined. For dose estimation, the calibration curve should be introduced to TPS. The standard curves determined using phantom or CT scanner that may be different from that of other oncology centers. Therefore, three-dimensional dose distributions will be calculated with considerable errors.

In this study, the changes of dose calculation due to the different calibration curves (HU-ñel ) were investigated. The calibration curve (HU-ñel ) was extracted from the CT images. The HU-ñel curve was extracted based on the recommended method by IAEA. Dose calculated based on this calibration curve was compared with the results of the standard calibration curve (predetermined).


 > Materials and methods Top


In this study, the changes of dose calculation due to the different calibration curves (HU-ñel ) were studied. Dose values were calculated based on the standard calibration curve (HU-ñel ) that was predefined for TPS. The calibration curve (HU-ñel ) was also extracted from the CT cross-sectional images of the phantom, and dose values were calculated based on this curve. The phantom was scanned by different CT scanners in three oncology centers. CT images were also analyzed by different TPS algorithms in these centers. TPS algorithms and CT models are listed in [Table 1].
Table 1: TPS algorithms and CT models used for data measurements


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In this study, the following steps were performed for data analyzing.

Drawing the calibration curve (HU-ñel )

Dose calculation was performed using the images obtained by the CT scanner in each center. The CT cross-sectional image of the standard heterogeneous thorax phantom (Model 002 LF) was used to draw the HU-ñel curve. [13] Heterogeneous thorax phantom and the points of dose measurements in the phantom are shown in [Figure 1]. QC tests were performed periodically on all CT scanners. In this study, accuracy, linearity, and uniformity of CT number were investigated before the study commence.
Figure 1: (a) Heterogeneous thorax phantom. (b) Computed tomography cross-sectional image of the phantom

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During the commissioning procedure for radiotherapy TPSs, the phantom irradiation was performed by the specific fields (direction, position, etc.). [12] Schematic diagrams of the standard fields are shown in [Figure 1]. In this study, the computerized imaging reference systems (CIRS) phantom was also irradiated by the standard fields in each oncology center. Regions of interests were selected at the standard points on the CT images of the phantom. The electron densities were determined in the CIRS phantom. The HUs of these standard points were extracted, and the HU-ñel curve was drawn as the measured curve. For dose calculation, this curve was introduced to TPS. [Figure 2] depicts the measured calibration curve (based on CIRS phantom) in this study.
Figure 2: Measured calibration curve based on computerized imaging reference systems phantom

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Validation of treatment planning system

For experimental dose measurement, the phantom was irradiated by 6 and 15 MeV in the first radiotherapy center. In the second and third radiotherapy centers, the phantom was irradiated by 6 and 18 MeV. Irradiation procedure was performed using the medical linear accelerator with the protocols suggested in TECDOC 1583 (IAEA). [12] At the standard points of each irradiation geometry, dose values were measured using an ion chamber (Ref. TN30013) made by PTW - Freiburg company. For dose readout, a conventional electrometer (UNIDOS webline T10023-000159) was used. The pressure and temperature correction coefficients were applied. For theoretical measurements, the calibration curve was obtained using the CIRS phantom for each oncology center. CT images of the CIRS phantom and the obtained calibration curve were introduced to TPS. [13],[14] Dose values were calculated using both of the standard and measured curves. [Figure 3] shows the irradiation fields that were used for dose calculation in the CIRS phantom. [15]
Figure 3: The standard irradiation fields suggested by International Atomic Energy Agency to evaluate the precision and accuracy of treatment planning system dose calculations. The reference points were shown in the cross-sectional image of computerized imaging reference systems phantom for each field

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Dose values were determined by the theoretical calculation and experimental measurement at the standard points. The percentage errors of the measured values were calculated by Equation 1: [12]

(Eq. 1)

where D Cal is the dose calculated by TPS, D Meas is the dose determined by the experimental measurement, and D Meas.ref is the dose value determined by the experimental measurement at the reference point. All of the dose values were compared with the value that measured and calculated at the reference point.

Dose comparison based on two calibration curves at the reference points

For accuracy assessment of TPS dose calculation, the experimental measurements were performed based on IAEA protocols and the standard calibration curve (HU-ñel ). Dose calculations were compared based on the results of the standard and measured calibration curves. The monitor units (MUs) were determined to deliver 200 cGy doses to the reference points of each irradiation field. The MUs were registered, and their differences (per fraction) were evaluated. The statistical analyses of the mean differences were performed using the Wilcoxon rank-sum test for both of the calibration curves. The Wilcoxon rank-sum test is a nonparametric statistical hypothesis test. In this statistical test, the estimation method is based on variable rank orders for two independent groups. The null hypothesis is equality of the average ranks for two groups. T value with a 95% confidence level (2.056 in our study) and n 1 + n 2 − 2 degree of freedom was compared with calculated T value for two groups. If T value for ordinal ranks was larger than 2.056, the null hypothesis is rejected.


 > Results Top


The percentage errors of the experimental measurements and the calculated values based on the measured calibration curve (HU-ñel ) for three oncology centers (C1, C2, and C3) are listed in [Table 2]. The last column of the table shows the acceptable percentage errors according to IAEA protocols. All of the irradiation systems have the low energy (6 MeV) mode. Higher energies 15, 18, and 18 MeV were utilized in C1, C2, and C3 radiotherapy centers, respectively.
Table 2: The percentage errors of the experimental measurements at the reference point of the phantom, the percentage errors of the calculated values based on the measured calibration curve (HU-ñel) and acceptable percentage errors suggested by IAEA

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The MUs per fraction (MU/Fr) calculated by TPS in 6, 15, and 18 MeV are listed in [Table 3] and [Table 4]. In this table, the values were calculated based on the measured and standard calibration curves (HU-ñel ) that were introduced to TPS.
Table 3: MU values calculated by TPS based on two calibration curves (HU-ρel) with a prescribed dose of 200 cGy to the reference points using 6 MV beam

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Table 4: MU values calculated by TPS based on two calibration curves (HU-ρel) with a prescribed dose of 200 cGy to the reference points using 15 MV and 18 MV photon beams

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The CIRS phantom was irradiated with the MU values that were calculated based on the standard calibration curve to deliver 200 cGy doses to the reference points. Exact dose values at the references points (cGy) for each irradiation field are listed in [Table 5].
Table 5: Exact dose values at the reference points (cGy) calculated by TPS based on standard calibration curve

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 > Discussion Top


The data listed in [Table 2] showed that the percentage errors of the dose calculations (TPS) were acceptable in comparison to the experimental measurements. Scanning condition of the CIRS phantom was similar to the default geometry of thorax imaging. The standard kernel filter was used in CT image reconstruction. The measured calibration curve (HU-ñel ) was drawn in 100 kV and 120 kV. The purpose of this study was not to analyze the algorithm of TPS, but some of the findings in [Table 2] were significantly important. The standard deviations of the percentage errors for the measured and calculated values in 6 MeV were lower than 15 and 18 MeV energies. These data were comparable with that of the Rutonjski et al. study. [15]

The data listed in [Table 3] and [Table 4] showed no significant difference for both of the measured and standard calibration curves (HU-ñel ) in 6, 15, and 18 MeV energies. In Wilcoxon ranked sum nonparametric test for independent samples with P < 0.05, the equality of two groups (null hypothesis) was derived. In this statistical test, T value was smaller than 2.056 (comparison criteria) for all ordinal ranks, and the equality of MUs for both of the curves to transfer 200 cGy doses to reference points was resulted. The MUs calculated based on the standard curve (HU-ñel ) were lower than the values calculated based on the measured curve (HU-ñel ) . The difference values were 2%, 1.7%, and 0.65% for 6, 15, and 18 MeV energies, respectively. For MUs calculated using both of the curves, difference values were reduced with energy increasing. The Wilcoxon ranked sum nonparametric analysis showed no significant difference for different energies. The TPS dose calculation could be performed based on the standard calibration curve, and the measured curve is not necessary. For scanning of the CIRS phantom in all centers, the standard Kernel filter was used to eliminate the effect of the reconstruction algorithm on dose calculation. The effects of Kernel and matrix changes on HU-ñel curves have been approved for MeV - cone beam CT systems. [16],[17],[18] In this study, the calibration curve (HU-ñel ) was obtained at 100 kV. It was approved that there was no difference in dose values calculated based on the calibration curves. The clinical utilizing of the standard calibration curve (HU-ñel ) was acceptable in 100 kV that was comparable with the results of the Skrzynski et al. study. [17] In Skrzynski's study, the clinical application of the standard calibration curve (HU-ñel ) was approved in energies around 120 kV. [19]

Exact dose values at the reference points [Table 5] were lower than the calculated values in 6 and 15 MeV. The percentage errors of the calculated values were lower than 2%, 1.5%, and 1.5% in 6, 15, and 18MeV, respectively. To evaluate the effects of the dose reduction on tumor control probability, more studies are recommended.


 > Conclusion Top


Calibration curves are one of the main calculation components of TPS. In this study, the standard and measured calibration curves (HU-ñel ) were used in conventional TPSs. The dose values estimated by these calibration curves (with a prescribed dose of 200 cGy to the target volume) were compared. The obtained results show that there was no significant difference between the MU values calculated by these calibration curves in 100 kV. The MUs were determined to deliver 200 cGy doses to target volume. From the results, it could be concluded that the use of standard calibration curve in TPS dose calculation is accurate in comparison with the use of measured CT-HU curves.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
 > References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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