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Comparison of fast-neutron contamination of different models of Siemens medical linacs with CR-39 film


1 Department of Physics, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran
2 Department of Biomedical Engineering and Medical Physics, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran
3 Department of Medical Physics, Reza Radiation Oncology Center, Mashhad, Iran

Correspondence Address:
Mahdi Ghorbani,
Department of Biomedical Engineering and Medical Physics, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, 198 5717443 Tehran
Iran
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.JCRT_342_18

 > Abstract 


Background: Nowadays, radiotherapy has an important role in the treatment of cancer. The use of medical linacs in radiotherapy can have risks for patients. When radiotherapy is performed with photons with energies higher than 8 MeV, due to the photonuclear reaction of photons with various components in the head of the accelerator, the neutron is produced. This imposes an unwanted neutron dose to the patient. The purpose of this study is evaluation and comparison of fast-neutron contamination with increasing of field size and depth for Siemens Primus (15 MV), Siemens Primus Plus (18 MV), and Siemens Artiste (15 MV) linacs.
Materials and Methods: Neutron dosimetry was carried out with CR-39 films, as a fast-neutron dosimeter, using chemical etching technique. Measurements were performed in depths of 0.5, 2, 3, and 4 cm and source-to-surface distance of 100 cm. Field sizes were 10 cm × 10 cm and 30 cm × 30 cm.
Results: The results of measurements showed that, with increasing depth, equivalent dose is reduced. In addition, fast-neutron equivalent dose decreases with increasing the field size.
Conclusion: Siemens Primus Plus had the highest neutron contamination in comparison with the two other linacs. Deeper tissues receive less fast-neutron doses. In radiation therapy with high-energy photon beams, neutron dose delivered to the patients should be taking into account.

Keywords: Chemical etching, CR39 film, linac, neutron contamination, Siemens



How to cite this URL:
Aftabi N, Yazdi MH, Ghorbani M, Abdollahi S. Comparison of fast-neutron contamination of different models of Siemens medical linacs with CR-39 film. J Can Res Ther [Epub ahead of print] [cited 2019 Dec 11]. Available from: http://www.cancerjournal.net/preprintarticle.asp?id=263532




 > Introduction Top


Cancer is the second cause of death in the world. Nowadays surgery, radiation therapy, hormone therapy, and chemotherapy are used for the treatment of cancer as single or combined treatments. Radiotherapy is an effective method for treatment and control of cancer. High-energy photons are used as sources in radiotherapy and have advantages to the lower energy photons. The advantages include more skin sparing, the lower delivered dose to the normal tissue adjacent to the treatment volume, and more uniformity in isodose curves.

Since high-energy photons exceed from the threshold of photonuclear reaction, photo-neutrons, also gamma rays from (n, γ) reaction, contribute in the total dose to the patient.[1],[2],[3],[4] Neutrons generated from the reaction of photons with high Z-atoms of different materials in the head of a medical linac including target, flattening filter, shielding, and collimators, are fast and depending of their energies, their relative biological effectiveness (RBE) can be high.[1]

Experimental measurement of the delivered neutron dose to the patient, for the purpose of secondary cancer risk assessment, is important. This is, especially true in the treatment of some tumors-like pediatric tumors, in which high precision is required. Many advances have been achieved in radiotherapy like intensity-modulated radiation therapy or volumetric modulated arc therapy that deliver dose with high precision to the treatment volume, and deliver lower dose to the adjacent normal tissues. However, secondary neutrons generated in the head of accelerators can easily pass through the multileaf collimators and expose an unexpectable dose to the patient.[1]

In a study performed by Hashemi et al.,[5] the photoneutron dose equivalent was examined for Saturne 20 MV linear accelerator by MCNP4C Monte Carlo simulation code. Results showed that the treatment room walls caused high-dose equivalent in different points because of the reflection of photoneutrons from the walls. The dose equivalent in the center of the X-ray field was maximum. It was noted that the dose equivalent was decreased gradually with increasing the distance from the center of the radiation field. In addition, decreasing field size resulted to decreasing the dose equivalent and the dose equivalent is lower in the maze near the door. The walls have an effective role in increasing the dose equivalent in the patient plane. However, the point that must be considered is the related uncertainty in the neutron dosimetry by this method.

The effect of field size and depth was investigated by Biltekin et al.[6] for three linear accelerators: Elekta synergy platform, Varian clinac DHX high performance, and Philips SL 25/75. Results showed that the dose equivalent increased with increasing the field size. They showed an extreme drop-off at 10 cm distance from the center of the field and a decreasing dose equivalent steadily with increasing the distance. Furthermore, they proved that the neutron dose was lower in shallow depths, unlike deep depths.

The dose equivalent and absorbed dose of secondary neutrons were examined by Fujibuchi et al.[2] with CR-39 plastic nuclear track detectors by evaluating the linear energy transfer (LET) spectrum from track recoils for the Varian 21EX medical linac. In that study, they used a well-established method to investigate absorbed dose that neutron dosimetry was done in the air. In addition, they evaluated the spatial distribution of secondary neutrons with CR-39 plates that were exposed by 10 MV photon beam, in a water phantom. The results showed that the horizontal and depth distribution of the neutron dose is quite different from the primary photon beam. The horizontal distribution of secondary neutron dose had a peak at the center of the photon beam and gradually decreased. The depth distribution of secondary neutrons had a peak near the surface of the water and decreased exponentially with depth. Furthermore, they showed that there was a high difference in the amount of absorbed dose compared to the similar studies. The reason for this difference was due to the types of linacs, irradiation set-up, and field size. High-energy neutrons exist even in deep depths, but low-energy neutrons are immediately absorbed by water near the surface of the water. Finally, they showed that the secondary neutron dose is much less than the amount of photon dose which is applied for the treatment.

There are many studies on neutron contamination measurement with different methods. Hashemi et al.[7] measured the flux of photoneutrons from two medical linacs, namely, Elekta SL 75/25 with 18 MV photon energy and Saturne 43 with 25 MV photon energy with a polycarbonate film. There are also studies performed by Bobble detectors.[8],[9] d'Errico et al.[10] evaluated equivalent dose with combined Bobble detector and CR-39 film. Králík et al.[11] measured neutron flux with Bonner spheres.

In previous studies, secondary neutron dose and dependence of depth and field size were investigated for different linacs with different methods. In these methods, Monte Carlo simulation method does not have an absolute response, in the other words, it calculates relative dose. Thermoluminescent dosimeter detectors have much percentage error. CR-39 dosimeters are suitable detectors for fast-neutron contamination measurement purposes.[12] While neutron contamination for different models of Siemens medial accelerator was examined before, but those investigations were not for Siemens artiste linac. For this reasons, measurement of neutron contamination of different models of Siemens medical accelerators and evaluation of dependence of neutron dose on depth and field size should be performed. The aim of this study is investigating and comparing of neutron contamination of different models of Siemens medical linacs and dependence of neutron contamination on depth and field size.


 > Materials and Methods Top


In this study, neutron contamination of three medical linear accelerators, Siemens primus (15 MV photon beam), Siemens primus plus (18 MV photon beam), and Siemens artiste (15 MV photon beam) was investigated. These accelerators are being used in the Reza Radiation Oncology Center in Mashhad (Iran). Neutron dosimetry was carried out by CR-39 films.

The PAD (poly-allyl-diglycol-carbonate, C12H18O7) films with trade name CR-39 in dimensions of 2.5 cm × 2.5 cm × 1.5 mm were provided from the TASL company [13] Bristol, London, United Kingdom. These films, due to their insensitivity to radiofrequency and low-LET radiations, are widely used for fast-neutron dosimetry. Neutron dose can be determined from the total tracks on the film by the application of a calibration curve obtained by a standard neutron source.[12]

Calibration of CR-39 films

These films were calibrated in the Atomic Energy Organization of Iran by a 241 Am-Be source. Doses that were applied for calibration were 1, 2, 3, 4, and 5 mSv. In each group, there were three films exposed. Three films were placed out of the treatment room for the measurement of background radiation. Totally, 18 dosimeters were used for calibration. Dosimeters were placed on a water phantom with 30 cm × 30 cm × 15 cm dimensions. For an equivalent dose, the considering quantity is Hp (10).

Neutron contamination measurement

These dosimeters were placed in a Perspex phantom on the treatment couch. The phantom was 50 cm × 50 cm × 45 cm in dimensions. There was a region embedded at the center of phantom according to depths which dosimeters placed at there. Totally, 72 dosimeters were used for 24 experimental states. Source-to-surface distance (SSD) was equal to 100 cm, and the monitor unit (MU) was 100 MUs. The effect of field size and depth was investigated for 10 cm × 10 cm and 30 cm × 30 cm fields and depths of 0.5, 2, 3, and 4 cm.

Chemical etching of CR-39 films

After exposure for calibration, chemical etching method was used to neutron tracks be appeared on the dosimeters. In this method, irradiated and unirradiated dosimeters were immersed in 6.25 M NaOH solution at 85°C water bath in a bain-marie for 3 h.[14] Unirradiated dosimeters are for background state. The etching condition and time for calibration and neutron contamination measurement were the same.

Read-out of CR-39 films

Finally, dosimeters were read and analyzed with a BX51 optical microscope [15] (Olympus Corporation, Shinjuku, Tokyo, Japan) with a magnification of ×200. [Figure 1] shows an example of microscopic neutron tracks on a CR-39 dosimeter. Normally, there are neutron tracks, alpha tracks, and noises (primarily dust on film) on CR-39 dosimeters. Alpha tracks are larger than neutron tracks and their shapes are different. Therefore, when the CR-39 dosimeters were read under the microscope, the large tracks or abrasions were not counted as neutron tracks.
Figure 1: Typical neutron tracks on a CR-39 film

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 > Results Top


There is a linear relation between neutron track density and equivalent dose. Therefore, after fitting on the calibration plot, a linear equation was obtained between the tracks density and neutron-equivalent dose. This linear calibration plot is shown in [Figure 2]. The results of the measurements are reported in [Table 1]. Variation of neutron-equivalent dose with increasing depth in phantom is shown in [Figure 3]. In addition, the effect of increasing field size on neutron equivalent dose is shown in [Figure 4]. Finally, the results are compared for the three linear accelerators in [Figure 5]. All the error bars in these figures are corresponding to standard deviations of readings from the average values.
Figure 2: Fast-neutron calibration curve for CR-39 films in the form of equivalent dose (mSv) versus track density (track/cm2)

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Table 1: Fast neutron equivalent dose (mSv)±standard deviation (mSv) in phantom for Siemens Primus, Siemens Primus Plus and Siemens Artiste linear accelerators

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Figure 3: Variation of neutron dose (mSv) versus depth (cm) in perspex phantom for: (a) 10 cm × 10 cm and (b) 30 cm × 30 cm fields

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Figure 4: Variation of equivalent dose (mSv) with increasing field size for: (a) Siemens Primus, (b) Siemens Primus Plus, and (c) Siemens Artiste

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Figure 5: Neutron contamination of three linear accelarators: (a) Siemens Primus and Siemens Primus Plus, (b) Siemens Primus and Siemens Artiste, (c) Siemens Primus and Siemens Primus Plus

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 > Discussion Top


Fast-neutron contamination of three linear accelerators of Siemens primus, Siemens primus plus, and Siemens artiste was investigated by CR-39 dosimeters. The field sizes were 10 cm × 10 cm and 30 cm × 30 cm and the depths were 0.5, 2, 3, and 4 cm in a Perspex phantom.

The neutron-equivalent dose and its variation by increasing depth in phantom were evaluated and reported in [Table 1]. According to this table, neutron-equivalent dose has a decreasing trend by increasing depth and hence that becomes zero in some depths. Fast-neutrons mostly lose their energies through an elastic scattering and become thermal. Therefore, in deeper regions, most of the neutrons are thermal and absorbed through the neutron capture reactions like 1 H (n, γ) H 2. This result is in agreement with other works.[2],[6],[9] In Fujibuchi et al. study,[2] they exposed dosimeters with 4000 MU irradiations. Therefore, the minimum neutron dose was obtained at 5 cm depth. Since the films in this study were exposed by 100 MU, it is expected that in some depths the neutron dose was read to become zero. Furthermore, it is possible that neutron dose in some states was not zero or more fast neutrons were observed in deeper depths if the films were exposed with higher MUs.

The results of the present study show that with increasing field size, neutron-equivalent dose decreases. In general, there is not an absolute response to the dependence of neutron dose on field size. In some studies, there was reported an increasing trend on neutron-equivalent dose by increasing field size.[6],[8],[16] Neutron contamination of two linear accelerators of Elekta Synergy Platform and Varian Clinac DHX High Performance was investigated by Biltekin, et al.[6] Their results showed that with increasing field size from 10 cm × 10 cm to 30 cm × 30 cm, neutron equivalent dose increases. Jahangiri, et al.[16] believed that with increasing field size, neutron flux increases, due to the head components of the accelerator which are located before the secondary collimator. This is due to the shape of filters, which with increasing the field size, much material is located in the path way of the photon beam. Some studies are in contrast with the older studies.[9],[17],[18],[19] Awotwi-Pratt and Spyrou [9] showed that the maximum neutron dose was observed for 5 cm × 5 cm field and the minimum dose was observed for 20 cm × 20 cm field. They observed that at depths deeper than 5 cm, the decrease of equivalent dose is independent of field size. Ghavami et al.[17] argued that with increasing field size, movable jaws are closer, therefore, less photonuclear reaction occurs between the photon beam and the high Z materials in jaws and the neutron flux decreases. Ma et al.[19] evaluated the contribution of different head components in producing secondary neutrons. They showed that the primary collimator has the most neutron production. The second component that has the most neutron production is Y-jaw. In addition, neutron production by X-jaws and the multileaf collimator increases with increasing field size.

There are some studies which were carried out using different methods on Siemens linear accelerator.[20],[21],[22],[23],[24] However, in these studies, neutron flux, LET spectrum, and neutron energy were measured. Since the methods and the measured quantities are different in these studies, and in the present study, their results cannot be compared herein. As an example, there is a study by Yucel et al. in which a different technique of measurement and linear accelerator was used, and the results cannot be compared with that study.[25]

By comparison of neutron contamination of the three models of Siemens linear accelerators, it is observed that Siemens primus plus has the most neutron production. Since the energy of the photon beam for this accelerator is 18 MV, this result is reasonable. It is expected that the cross-section of the photonuclear reaction increases with increasing the energy of the photon beam.[26] By comparing two linear accelerators of Siemens primus and Siemens artiste, the neutron contamination of Siemens primus linac is higher. This disparity may be because of different geometry or components of linac's head. Since these linacs have the same photon beam energies, to explain this difference the detailed structure of heads of both machines is required.

Generally, the total uncertainty consists of statistical and systematic uncertainties. The statistical uncertainty can be decreased by repeating the measurement and reading processes. In the present study, each measurement was repeated for three times, and also averaging was performed in the reading process of the films. Then standard deviations were calculated and reported. The systematic uncertainties in the etching process include the uncertainties in time, temperature, and concentration. The systematic uncertainties in the measurement process include the uncertainties in depth in the order of mm, SSD, field size in the order of mm, place of background dosimeters, and reading out the tracks on the films. In summary, the total uncertainty in the measurements in the present study was about 10%.[27]

Secondary neutrons are absorbed by the other organs, and they impose an undesirable neutron dose to patients. The amount of neutron-equivalent dose that was obtained in this study, are low. However, because of high RBE of photoneutrons, this amount is important. This means that they are effective in inducing second cancer. Besides, thermal neutrons are highly reactive. Therefore, this can be dangerous for body and investigation of neutron contamination of thermal neutrons for different models of Siemens medical linear accelerator is suggested for the further study.


 > Conclusion Top


In this study, fast-neutron contamination of different models of Siemens linear accelerators was investigated and compared by using CR-39 films. In addition, photoneutron generation dependence on field size and depth in phantom were investigated. Results showed that deeper tissues receive less fast-neutron dose, and in steeper depths, it becomes zero. By increasing the field size, neutron-equivalent dose decreases. Comparison of fast-neutron measurements shows that neutron contamination of Siemens primus plus linac is higher than the two other linacs. In general, the neutron has a relatively high RBE and therefore, increases the risk of secondary cancer induction in patients. Therefore, it is suggested that this effect, be taken into account in dose delivery to patients which are treated with high-energy photon beams.

Acknowledgments

This study was supported by the Ferdowsi University of Mashhad and Reza Radiation Oncology Center. We would like to appreciate them for financial and scientific support to perform this research.

Financial support and sponsorship

This study was supported financially by Ferdowsi University of Mashhad.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

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