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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 15  |  Issue : 5  |  Page : 1018-1023

Measurement of the contralateral breast photon and neutron dose in breast cancer radiotherapy: A Monte Carlo study


1 Radiation and Wave Research Center, Aja University of Medical Science, Tehran, Iran
2 Department of Medical Physics and Radiology, Faculty of Paramedical Sciences, Kashan University of Medical Sciences, Kashan, Iran
3 Department of Medical Physics, Iran University of Medical Sciences, Tehran, Iran
4 Nuclear Science and Technology Research Institute, Radiation Applications School, Tehran, Iran
5 Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran

Date of Web Publication4-Oct-2019

Correspondence Address:
Bagher Farhood
Department of Medical Physics and Radiology, Faculty of Paramedical Sciences, Kashan University of Medical Sciences, Kashan
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcrt.JCRT_1426_16

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 > Abstract 


Introduction: This study aimed to calculate the photon and neutron doses received to the contralateral breast (CB) during breast cancer radiotherapy for various field sizes in the presence of a physical wedge.
Materials and Methods: Varian 2100 C/D linear accelerator was simulated using a MCNP4C Monte Carlo code. Then, a phantom of real female chest was simulated and the treatment planning was carried out on tumoral breast (left breast). Finally, the received photon and neutron doses to CB (right breast) were calculated in the presence of a physical wedge for 18 MV photon beam energy. These calculations were performed for different field sizes including 11 cm × 13 cm, 11 cm × 17 cm, and 11 cm × 21 cm.
Results: The findings showed that the received doses (both of the photon and neutron) to CB in the presence of a physical wedge for 11 cm × 13 cm, 11 cm × 17 cm, and 11 cm × 21 cm field sizes were 9.87%, 12.91%, and 27.37% of the prescribed dose, respectively. In addition, the results showed that the received photon and neutron doses to CB increased with increment in the field size.
Conclusion: From the results of this study, it is concluded that the received photon and neutron doses to CB in the presence of a physical wedge is relatively more, and therefore, they should be reduced to as low as possible. Therefore, using a dynamic wedge instead of a physical wedge or field-in-field technique is suggested.

Keywords: Contralateral breast, Monte Carlo, neutron dose, physical wedge, radiotherapy


How to cite this article:
Bagheri H, Farhood B, Mahdavi SR, Shekarchi B, Manouchehri F, Esfandbod M. Measurement of the contralateral breast photon and neutron dose in breast cancer radiotherapy: A Monte Carlo study. J Can Res Ther 2019;15:1018-23

How to cite this URL:
Bagheri H, Farhood B, Mahdavi SR, Shekarchi B, Manouchehri F, Esfandbod M. Measurement of the contralateral breast photon and neutron dose in breast cancer radiotherapy: A Monte Carlo study. J Can Res Ther [serial online] 2019 [cited 2019 Nov 13];15:1018-23. Available from: http://www.cancerjournal.net/text.asp?2019/15/5/1018/231351




 > Introduction Top


Breast cancer is the most frequently diagnosed malignancy and is the leading global cause of malignancy death among females worldwide, accounting for 23% and 14% of cancer diagnoses and deaths each year, respectively.[1] One of the treatment modalities for breast cancer is radiotherapy, which is performed on almost 50% of all patients with localized breast cancer.[2],[3],[4] This treatment modality plays a significant role in the treatment of breast cancer as it reduces locoregional recurrence and increases chances of survival.[5],[6],[7],[8]

In some cases of radiotherapy, patients with breast cancer are treated with high-energy beams (for example, 18 MV beams).[9],[10] In energy beams >8 MV, interaction between these beams with different high-Z nuclei of the materials in the components of the linear accelerator (Linac) head generates unavoidable neutron fluence.[11] Therefore, the regions outside of the treatment field receive scatter radiation from the Linac head and the internal patient scatter radiation as well as unavoidable neutrons.[4] Although out-of-field doses are smaller than in-field doses, these doses have potential to induce secondary cancers with a long latency period (especially in radiosensitive organs).[12],[13] Radiation carcinogenesis is a stochastic process as the probability of radiation-induced cancer increases with dose value received and there is no threshold dose.[14] The incidence of these secondary cancers depends on the dose, delivered dose distribution, dose rate, size of the irradiated volume, and patient-specific factors.[15]

The standard radiotherapy treatment of breast cancer uses opposed tangential fields with wedges.[16] The contralateral breast (CB), as a normal tissue surrounding of the target volume, would receive scattered and leakage radiation doses during the treatment of the tumoral breast (TB); therefore, the probability of developing CB cancer would be a concern during treatment, especially among younger women. The dose received to CB is composed mainly of scatter dose from conventional radiotherapy of the breast, which is influenced by wedges, gantry angle, blocks, the use of half beams, irradiated volume, as well as the orientation of the field edges.[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29] Gao et al.[30] reported a relative risk of 1.15 and 1.32 for the second cancer induction in CB of women patients whose ages were over 55 years and below 45 years at the time of diagnosis, respectively. In this study, the Cox proportional hazards regression model was applied to estimate the relative risk of CB cancer.[31] Stovall et al.[32] showed that females below 40 years of age have a high risk for developing CB cancer when the healthy breast dose exceeds 100 cGy. Given that risk of CB cancer increases with dose, the received dose to CB has to be kept as low as possible. Hence, assessment of the received dose to CB is essential for accurate estimation of the risk of secondary tumors induced by radiation.

Several studies have measured the received photon dose to CB in the presence of a physical wedge. Weides et al.[23] used photon beam energy 6 MV for primary breast irradiation. The findings demonstrated that the dose outside the field in the presence of a physical wedge is more than that in the open field and dynamic wedge field. They concluded that when using a medial wedge, a dynamic wedge should be applied. In another study, Akram et al.[33] applied 6 MV and 15 MV photon beam energies for treatment primary breast malignancy. They indicated that the dynamic wedge generates less scattered dose compared to the physical wedge.

Although the received photon dose to CB in patients undergoing breast radiotherapy has been measured in the presence of a physical wedge, to the best of our knowledge, there is no measurement on the received neutron dose to CB for such patients in the presence of a physical wedge. Thus, the current study aimed to calculate the received photon and neutron doses to CB during breast cancer radiotherapy for various field sizes in the presence of a physical wedge.


 > Materials and Methods Top


Monte Carlo simulation of Varian 2100 C/D Linac

A MCNP4C Monte Carlo (MC) code (Los Alamos National Laboratory, Los Alamos, New Mexico) was applied in this study to simulate a Varian 2100 C/D Linac (Varian Medical Systems, Palo Alto, CA). This code can be used for development of a detailed three-dimensional (3D) model of a Linac head and dose calculation in different materials and complex geometries.[34],[35],[36],[37],[38] The 18 MV photon mode of a Varian 2100 C/D Linac was simulated based on the geometry information provided by the manufacturer. A water phantom with dimensions of 50 cm × 50 cm × 40 cm was defined and positioned under the treatment head of Linac at a source-to-surface distance (SSD) of 100 cm. A cylinder with 0.5 cm in radius for 10 cm × 10 cm field size was defined to perform the dose calculation in various depths of the water phantom, and then, percentage depth dose (PDD) was calculated. The cylinder axis was supposed to be on the beam central axis. In the next stage, the cylinder was divided into small cells with heights of 2 mm and named scoring cells. To calculate beam profile, some cubic scoring cells were used as the main axes of scoring cells were perpendicular to the beam central axis and dimensions of each cube were 0.2 cm × 1 cm × 1 cm. The cubes were positioned 5 cm deep in the water phantom, for 10 cm × 10 cm field size. In the simulations for PDD and profile calculations, the energy cutoffs for photons and electrons were set to 0.01 and 0.5 MeV, respectively. All input files were run by *F8 tally for 2 × 109 particle histories. The statistical uncertainties were <2% in all output files.

To verify our simulation data, we compared our MC results with the corresponding measured values using gamma function method.[39] In this method, if gamma value is between 0 and 1, it is considered as pass and values more than 1 are considered as fail. In the current study, in calculation for gamma function, dose difference and distance-to-agreement criteria equal to 3% and 3 mm were used, respectively.

Measurements were carried out on a Varian 2100 C/D medical Linac with 18 MV nominal photon energy. Dosimetry was performed using a Semiflex™ ionization chamber with a sensitive volume of 0.125 cm 3 (PTW, Freiburg, Germany) and the MP3-M water phantom (PTW, Freiburg, Germany). The dimensions of the water phantom were 50 cm × 50 cm × 40 cm.

Simulation of chest phantom and treatment planning

For simulation of the chest phantom, we attempted the geometry of the simulated phantom to be similar to a real female chest as possible. To accomplish this, the geometry of the chest phantom was simulated based on computed tomography images of several patients with breast cancer. The compositions used for the simulated phantom and their weight fractions were obtained by report No. 44 of the International Commission on Radiation Units and Measurements (ICRU).[40]

The left breast region of the simulated phantom was planned for irradiation (as TB) and the right breast region was selected as CB for calculating the photon and neutron doses. It is notable that choosing the right breast region as TB served no specific purpose and it was completely optional. Two tangential fields were planned, and then, a proper wedge was selected to create a uniform dose distribution in TB. The wedge angle was 15° and gantry angles were based on treatment planning. Field sizes used included 11 cm × 13 cm, 11 cm × 17 cm, and 11 cm × 21 cm. [Figure 1] shows the treatment planning and the geometry of the female chest simulated using the MCNP4C code.
Figure 1: Treatment planning and the geometry of female chest simulated by MCNP4C code. For chest irradiation, two tangential fields were planned and field sizes used were 11 cm × 13 cm, 11 cm × 17 cm, and 11 cm × 21 cm. The wedge angle was 15° and gantry angle was based on treatment planning

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Estimation of the received photon and neutron doses to CB

For each field size, the calculation of photon and neutron doses was performed in 4 points of CB of the phantom [Figure 2]. The distances of these points from the surface were 2 cm and 5 cm (i.e., 2 points in depth of 2 cm and 2 points in depth of 5 cm). It is notable that the calculated photon and neutron doses for each field sizes are the mean doses of 4 points. In addition, these doses were normalized to a point in TB (ICRU point).
Figure 2: Placements of dose calculation points for photon and neutron doses in the simulated phantom. The right and left breasts were considered as contralateral breast and tumoral breast, respectively. The calculated dose values in depths of 2 cm and 5 cm (contralateral breast) were normalized to a point in tumoral breast

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*F8 tally was used to calculate the photon dose at predefined points. Energy cutoffs for electrons and photons were set at 0.5 and 0.01 MeV, respectively. The input files used for the calculation of photon doses were run for 2 × 109 particles, and the MC uncertainties in the tally cells did not exceed 2.5%.

In addition, F4 tally was applied to calculate the neutron flux, which was then converted to neutron dose using the flux-to-dose conversion factor. It is notable that the calculated neutron doses were as the physical doses. The energy cutoff for both electrons and photons were set at 7 MeV. The input files used for calculation of photon dose were run for 5 × 1010 particles, and the maximum MC uncertainty in the tally cells was 1.4%.


 > Results Top


Validation of the Monte Carlo simulation of Varian 2100 C/D Linac

PDD and dose profile values obtained by MC simulations and measurements for 18 MV photon beams have been plotted in [Figure 3]a and [Figure 3]b, respectively. The data in this figure are related to 10 cm × 10 cm field size and SSD = 100 cm.
Figure 3: Comparison of percentage depth dose (a) and profile beam (b) values calculated by Monte Carlo simulation and measurement data for 18 MV photon beam energy at 10 cm × 10 cm field size and source-to-surface distance = 100 cm. The depth of beam profile is 5 cm

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As can be seen from [Figure 4], only a few points have gamma index >3%, which is expressed as fail or disagreement at these points. It is notable that gamma index >3% were only seen in regions of high-dose gradients (e.g., buildup regions).
Figure 4: Gamma function results for percentage depth dose (a) and dose profile (b) values for 10 cm × 10 cm. Dose difference and distance-to-agreement criteria were set as to 3% and 3 mm in the gamma calculations, respectively

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The received photon and neutron doses to the contralateral breast

The received photon and neutron doses to CB in the presence of a physical wedge are presented in [Table 1], for various field sizes. The maximum received photon and neutron doses to CB were related to the field size of 11 cm × 21 cm. The minimum received photon and neutron doses to CB were related to the field size of 11 cm × 13 cm.
Table 1: The received photon and neutron doses to the contralateral breast for different field sizes in the presence of the physical wedge. In each of the field sizes, the dose values are average of the doses calculated at 2 and 5 cm depths of contralateral breast which were normalized to a point in tumoral breast

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[Figure 5] shows variation in the received photon and neutron doses to CB by increasing the field size in the presence of the physical wedge.
Figure 5: The received photon and neutron doses to the contralateral breast in various field sizes for the physical wedge. In each of the field sizes, the dose values are average of the doses calculated at 2 and 5 cm depths of contralateral breast which were normalized to a point in tumoral breast

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 > Discussion Top


As is evident from the results [Figure 3] and [Figure 4], there is a good agreement between our PDD and dose profile values obtained by MC simulations and the measured values by ionization chamber. However, there are only a few points that had a gamma index >1 as these points were in regions of high-dose gradient (e.g., buildup regions). Therefore, our MC simulation of the Linac head is validated.

Given that the dose received to the normal tissues surrounding the target volume has side effects, it should be reduced to as low as possible. In this regard, techniques to minimize the radiation dose to CB are routinely used in breast cancer radiotherapy.[18],[28],[41],[42],[43],[44] The use of wedge filters in tangential fields is an effective factor in the received dose value for CB; therefore, several studies have suggested that a dynamic wedge should be used instead of a physical wedge.[19],[22],[23],[26] The reason is that the physical wedge is an absorbing block made of metallic materials and inserted manually in the beam path. As a result, it gives more scattering photons because of primary photon beams interacting with its materials. The dynamic wedge gives a less scattered dose compared to physical wedge because of less scattering resulting from the interactions of primary incident photons with physical wedge metallic materials. Recently, it was reported that using field-in-field technique compared to conventional wedged fields in breast cancer radiotherapy, in addition to improvement of dose homogeneity, received dose to surrounding tissue (such as CB) reduces.[45],[46] In a study by Borghero et al.,[45] it was concluded that received dose to CB is remarkably reduced with field-in-field forward-planned intensity-modulated radiation treatment compared to wedge-compensated techniques. In another study by Ohashi et al.,[46] it concluded that using 3D conformal radiotherapy with a field-in-field technique decreases doses to the lungs, heart, and the other normal tissues, compared with modified tangential irradiation technique.

In some cases, to obtain an appropriate dose distribution in patients with breast cancer, high-energy beams (for example, 18 MV beam) were applied in tangential fields.[10] Interaction of these beams with the components of the Linac head, which have high-Z nuclei, generates unavoidable neutrons.[11] The average energy of these unavoidable neutrons is estimated to be approximately 1 MeV.[47],[48] This energy range has the highest quality factor for neutrons.[49] Therefore, the neutrons generated should not be neglected. In radiotherapy with high-energy photons (more than 8 MV), it is significant to know the neutron dose equivalent per unit therapeutic dose in the treatment room, to design proper shielding of the bunker as well as to evaluate the equivalent dose received by patients.[50]

In the present study, the received doses (both of the photon and neutron) to CB in the presence of a physical wedge for 11 cm × 13 cm, 11 cm × 17 cm, and 11 cm × 21 cm field sizes were 9.87%, 12.91%, and 27.37% of the prescribed dose, respectively. Our results related to the received photon doses to CB in the presence of a physical wedge were consistent with those of other studies. These studies concluded that the received dose to CB is 9%–10% of the prescribed dose.[19],[28],[51] However, several studies have reported lower received photon doses to CB compared to that in our study. Alzoubi et al.[16] measured the CB dose from chest wall and breast irradiation in two- and three-field techniques. In this study, they used 6 MV photon beam energy. The finding demonstrated that the CB dose is 2.1%–4.1% of the prescribed dose. Weides et al.[23] compared the dose outside field from primary breast irradiation (using 6 MV photon beam energy) in the presence of an open-field, a regular wedge, and a dynamic wedge technique. They showed that the received photon dose to CB in the presence of a regular wedge is approximately 2%–6% of the dose at isocenter, whereas for a dynamic wedge, it is approximately l–5% with an average dose of approximately 2.5%. It should be noted that the difference between our results and the mentioned studies can be due to different used photon beam energy, filed size, technique treatment, etc.

The findings showed [Figure 5] that the received photon and neutron doses to CB increased with increment of the field size. This can be due to the increasing of the scattered photon with increment of the field size and consequently the interaction of these photons with various materials having high-Z nuclei in the direction of the beams. In addition, for all field sizes, the received photon dose to CB was a little more than the neutron dose. Although the received neutron dose to CB is less than the photon dose, the quality factor for neutron beams is much more than that for photon beams (as it depends on neutron energy), and consequently, radiobiological effects of these neutron beams will be more than that of photon beams.

For future research, it is suggested that dose measurement of photon and neutron doses should be performed using different dosimetric methods, such as thermoluminescent dosimetry.


 > Conclusion Top


Given the growing concern of the risk of secondary cancers induced by radiation, the received dose to CB is a vital issue in the radiation therapy field. Therefore, measuring the received dose to the normal tissue outside the treatment field is essential because it can be used for a more accurate estimation of the risk of secondary tumors induced by radiation.

Given that the received photon and neutron doses to CB in the presence of a physical wedge were relatively more (for example, 27.37% of the prescribed dose in 11 cm × 21 cm field size), they should be reduced to as low as possible to reduce the risk of secondary cancer. To achieve this, using a dynamic wedge instead of a physical wedge or field-in-field technique is suggested.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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