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Year : 2016  |  Volume : 12  |  Issue : 2  |  Page : 1060-1064

Study on the measurement of photo-neutron for15 MV photon beam from medical linear accelerator under different irradiation geometries using passive detectors

1 Department of Physics, Noorul Islam Centre for Higher Education, Noorul Islam University, Kanyakumari, Tamil Nadu; Department of Medical Physics, Dr. Balabhai Nanavati Hospital, Mumbai, Maharashtra, India
2 Department of Physics, Noorul Islam Centre for Higher Education, Noorul Islam University, Kanyakumari, Tamil Nadu, India
3 Department of Physics, University of Calicut, Malappuram, Kerala, India
4 Bhabha Atomic Research Center, Radiological Physics and Advisory Division, Mumbai, Maharashtra, India
5 Department of Medical Physics, Dr. Balabhai Nanavati Hospital, Mumbai, Maharashtra, India

Date of Web Publication25-Jul-2016

Correspondence Address:
Siji Cyriac Thekkedath
Department of Radiation Therapy, Dr. Balabhai Nanavati Hospital, Vile Parle (West), Mumbai - 400 056, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-1482.183187

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 > Abstract 

Aim of Study: The photo-neutron dose equivalents of 15 MV Elekta precise accelerators were measured for different depths in phantom, for various field sizes, at different distances from the isocenter in the patient plane and for various wedged fields.
Materials and Methods: Fast and thermal neutrons are measured using passive detectors such as Columbia Resin-39 and pair of thermoluminescent dosimetry (TLD) 600 and TLD 700 detector from Elekta medical linear accelerator.
Results: It is found that fast photo-neutron dose rate decreases as the depth increases, with a maximum of 0.57 ± 0.08 mSv/Gy photon dose at surface and minimum of 0.09 ± 0.02 mSv/Gy photon dose at 15 cm depth of water equivalent phantom with 10 cm backscatter. Photo neutrons decreases from 1.28 ± 0.03 mSv/Gy to 0.063 ± 0.032 when measured at isocenter and at 100 cm far from the field edge along the longitudinal direction in the patient plane. Fast and thermal neutron doses increases from 0.65 ± 0.05 mSv/Gy to 1.08 ± 0.07 mSv/Gy as the field size increases; from 5 cm × 5 cm to 30 cm × 30 cm for fast neutrons. With increase in wedge field angle from 0° to 60°, it is observed that the fast neutron dose increases from 0.42 ± 0.03 mSv/Gy to 0.95 ± 0.05 mSv/Gy.s
Conclusions: Measurements indicate the photo-neutrons at few field sizes are slightly higher than the International Electrotechnical Commission standard specifications. Photo-neutrons from Omni wedged fields are studied in details. These studies of the photo-neutron energy response will enlighten the neutron dose to radiation therapy patients and are expected to further improve radiation protection guidelines.

Keywords: Columbia Resin-39 detector, medical linear accelerator, photo-neutrons tracks, thermoluminescent dosimetry 600 and 700 detectors

How to cite this article:
Thekkedath SC, Raman R G, Musthafa M M, Bakshi A K, Pal R, Dawn S, Kummali AH, Huilgol NG, Selvam T P, Datta D. Study on the measurement of photo-neutron for15 MV photon beam from medical linear accelerator under different irradiation geometries using passive detectors. J Can Res Ther 2016;12:1060-4

How to cite this URL:
Thekkedath SC, Raman R G, Musthafa M M, Bakshi A K, Pal R, Dawn S, Kummali AH, Huilgol NG, Selvam T P, Datta D. Study on the measurement of photo-neutron for15 MV photon beam from medical linear accelerator under different irradiation geometries using passive detectors. J Can Res Ther [serial online] 2016 [cited 2020 Mar 28];12:1060-4. Available from: http://www.cancerjournal.net/text.asp?2016/12/2/1060/183187

 > Introduction Top

Today, the medical linear accelerator (LINAC) is considered as the primary tool in external beam radiotherapy. While using photon beams with energies higher than 10 MV, photo-neutron production through (γ, n) reaction occurs due to continuous impinging of photons on high Z materials along the path of photon beam,[1],[2] in the accelerator head. In modern radiotherapy techniques, higher beam on time is required and the possibility of neutron production as such increases.[3],[4] At higher photon energy, the production of unwanted fast neutrons can contaminate the therapeutic beam and also give a nonnegligible contribution to the patient dose. The biological effectiveness of neutrons are substantially higher than that of photons,[5] and hence a small neutron dose will increase the risk for secondary cancer. Therefore, it is necessary to minimize the contribution of neutron dose in the patient plane for all clinical cases.

Using the Monte Carlo (MC) methods, Pena et al.[6] and Zanin et al.[7] calculated the contribution to the production of photo-neutrons by different components of the accelerator body of Siemens Primus model operated at 15 MV photon beam. It is reported that the contribution of photo-neutron are 52%, 21%, 6.6%, 5% and 0.41% from primary collimator, secondary collimator, jaws, target, multi-leaf collimator (MLC) shielding and flattening filter respectively. The fast neutron decreases as the depth increases in the water equivalent phantom, which is described by Al-Ghamdi et al.[8] Neutron yield is highest at the middle size of the irradiation field size of 20 cm × 20 cm. The attenuation of neutron and the production of neutrons from primary and secondary collimator find a compromise in the midsize field as mentioned by Kim et al.[9] Exhaustive review of photo-neutron production in terms of source strength Q (n/Gy) in the medical LINACs manufactured by Siemens, Varian, Elekta, and GE has been reported by Naseri and Mesbahi.[10] It is clear from the above literature that photo-neutron source strength does vary depending on the photon energy, head design, etc., even if it is manufactured by the same manufacturer. Moreover, study on the photo-neutron source strength of Elekta LINAC is reported by Followill et al.,[11] for 17, 18, 22, and 25 MV photon beams for a specific field size (20 cm × 20 cm). To the best of our knowledge, no report is available in the literature, neither on the photo-neutron production in terms of n/Gy nor the measurement of dose equivalent in Elekta Precise LINAC for 15 MV photon beam. In view of the above, and also, as this machine is in use for radiotherapy in many hospitals including our hospital in our country, it was felt necessary to carry out an exhaustive measurement of photo-neutron dose equivalent for different irradiation geometries simulating the patient treatment conditions for 15 MV photon beam of Elekta Precise medical LINAC.

As explained in the ICRU report 26,[12] the use of a suitable pair of dosimeters, in which one is sensitive to neutrons and the other one is sensitive to photon, is needed to discriminate the contributions of gamma photons and neutrons in the mixed field. The use of solid state neutron track detectors, such as Columbia Resin-39 (CR-39), is therefore considered to be appropriate for fast neutron measurements with insensitivity to gamma photons,[13] in a medical LINAC field, which is used in our study.

The objectives of the present study were to estimate the neutron dose equivalent (contribution of thermal and fast neutrons) for Elekta Precise LINAC operating at 15 MV photon modes (i) for various depths along the central axis in tissue phantom, (ii) for various field sizes opening in presence of a tissue phantom, (iii) at the patient plane (surface of couch) along the longitudinal and lateral directions up to a distance of 100 cm, and (iv) for the various wedge fields for a given field size.

 > Materials and Methods Top

All the irradiations were performed using 15 MV photon beams from ELEKTA Precise medical LINAC (Elekta AB, Stockholm, Sweden) equipped with 80-leaf MLC.

Columbia Resin-39 detector

Fast neutron (100 keV to 7 MeV) measurements were performed using CR-39 chemical name polyallyl diglycol carbonate track etch detector procured from M/s Track Analysis System Ltd., (UK) of size 3 cm × 3 cm and 0.625 mm thickness. CR-39 detectors were processed through electrochemical etching consists of 1 h preetching, 3 h etching at 100 Hz and 50 min at 3.5 kHz in presence of 7N KOH solution maintained at 60°C in an incubator. The preetching step improves the response of the dosimeter because this treatment removes superficial alpha particles tracks and scratches, if any.[13]

Considering that the energy of the photo-neutron at the patient plane is of the order of 1 MeV,[14] CR-39 detectors were calibrated using 252 Cf bare neutron source at 75 cm in air. The total ambient dose equivalent H*(10) delivered was 2.6 mSv. The calibration factor calculated based on this irradiation was 7.9 µSv/tracks/cm 2. After etching, the detectors were thoroughly washed in running water and dried overnight. Counting of tracks was carried out on a PC-based automatic image analysis system developed indigenously at Bhabha Atomic Research Centre. The images and track density data can be stored against the detector number in a file in the present system. The basic parameters which are computed for the tracks in the binary image area, perimeter, and roundness factor (the parameters are set in the software such that only the tracks that fall within the set range of area and the roundness factor) are counted.

Thermoluminescent detector

For the measurement of thermal neutrons, thermoluminescent dosimetry (TLD) 600 and TLD 700 detector pair of size 3.2 mm × 3.2 mm × 0.89 mm procured from Thermofisher Scientific (I) Pvt Ltd., (81 Wyman Street, Waltham, MA USA 02451) were used in the study. Before using these detectors in the actual experiment, theses detectors (50 nos) were tested by irradiating them to 20 mSv (H*[10]) of 137 Cs gamma rays in air. Although the variation in sensitivity of the detectors was in the range of ±5% (1σ), we have used individual sensitivity factors of these TLDs for the calculation of the response during analysis of the experimental results. Calibration of TLD 600 was carried out in a thermalized neutron facility (241 Am-Be source thermalized by graphite stag) to a dose equivalent (H*[10]) of 1 mSv whereas calibration of TLD 700 was carried out by delivering gamma dose of 1 Gy using a 60 Co-based teletherapy machine under charged particle equilibrium. Calibration factors derived based on the above irradiations are 1.26 µSv/nC for TLD 600 and 0.08 Gy/µC for TLD 700. All the TLDs were read on a Harshaw reader model 3500 in the temperature range 50–250°C with a heating rate of 5°C/s after a gap of 1 week of irradiation.

Irradiation at linear accelerator

The irradiation of CR-39 and TLDs together were carried out for various field sizes such as 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm, and 40 cm × 40 cm open fields keeping the source to detector distance of 100 cm. The LINAC was set to deliver 400 cGy (4 Gy) at the point of maximum depth (dmax) dose for each field size and this was done at a maximum dose rate of ~600 MU/min. All measurements were performed along the central axis and considered for the dmax depth of 3 cm for 15 MV on the RW3 (Polystyrene [C8H8]n with an admixture of 2.1% ± 0.2% TiO2) solid water phantom (PTW, Freiburg, Germany).

For the study of the variation of photo-neutron along the depth of phantom, irradiation for various depths such as 0 cm (surface), 3 cm, 6 cm, 10 cm, and 15 cm of phantom with minimum of 10 cm backscatter was carried out. All the irradiation were carried out with 30 cm × 30 cm fields size defined using MLC [Figure 1]a by keeping the dosimeter sets at different depths of solid water phantom. For studying the variation of photo-neutron dose rate at different distance from the isocenter, detector sets were irradiated along the longitudinal, lateral directions on the surface of couch for a field size of 30 cm × 30 cm. The dosimeter sets were kept at isocenter, 10 cm, 20 cm, 50 cm, and 100 cm away from field edges [Figure 1]b.
Figure 1: Irradiation setup (a) film arrangements along the depth, (b) film arrangements along the couch direction (horizontal direction)

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Elekta medical LINACs use Omni wedges [15] to create the wedge profiles in the treatment. Different (0, 15, 30, 45, and 60°) wedge fields are created for 30 cm × 30 cm field size and the photo-neutron productions are studied. CR-39 films and TLD chips are placed at 3 cm depth from the surface of 100 cm SSD for all the measurements. A photon dose of 4 Gy is delivered at dmax and the normalized neutron dose with respect to photon dose is measured using the above detectors. All the irradiations were performed with a backscatter of 10 cm water equivalent plastic phantom. To ensure the photon dose delivery, the irradiations were associated with online 0.6 cc cylindrical ionization chamber measurement; this was further cross checked with the planned dose from the treatment planning system.

 > Results Top

[Table 1] shows the variation of the fast and thermal photo-neutrons dose equivalents along the depth of PTW RW3 tissue equivalent water phantom.
Table 1: Variation of photo-neutron dose along the depth of phantom

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[Figure 2] and [Figure 3] show the variation of gamma and neutron dose as a function of distance along the longitudinal and lateral direction in the patient plane (on the couch) with respect to the Gantry-Target direction from the isocenter for 30 cm × 30 cm MLC defined treatment fields.
Figure 2: Variation of gamma and neutron dose along longitudinal direction in the patient plane

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Figure 3: Variation of gamma and neutron dose along lateral direction in the patient plane

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The variation of fast and thermal photo-neutrons as a function of field size opening is shown in [Table 2].
Table 2: Variation of photo-neutron dose as a function of field size

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[Table 3] shows the variation of fast and thermal photo-neutrons as a function of wedge angles produced using Omni wedges used in Elekta medical LINACs.
Table 3: Variation of fast and thermal neutron dose with wedge angle

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[Table 4] shows the comparison of the photon dose measurements using ionization chamber and TLD 700 for different wedge angles.
Table 4: Photon dose comparison using ionization chamber and the thermoluminescent dosimeters 700

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 > Discussion Top

It is noted that the neutron dose equivalents reported in this study are always with respect to the photon dose at the dose maximum position (Dmax = 3 cm) at the isocenter of the beam. As shown in [Table 1], the fast neutron component decreases as the depth of phantom increases. However, the thermal neutrons increase to certain depth and then start decreasing. This is because the fast neutrons are moderated due to the interaction of fast neutrons with the hydrogen and other elements present in water equivalence phantom materials through elastic scattering. Beyond certain depth, these thermal neutrons will be absorbed by hydrogen and hence the thermal neutrons fluence/dose decreases. As described by Awotwi-Pratt and Spyrou,[16] the lower energy neutrons reach thermal equilibrium and are captured at shallower depths, while the higher energy neutrons travel longer distances before being captured. According to d'Errico et al.,[17] the fast neutrons located within the primary beam contribute significantly to the total dose equivalent and their relative contribution increases with depth because of the attenuation of the low energy neutron fluence at a shallow depth. Similar studies are conducted for thermal neutrons but the fast neutron studies are not found in literature for depth dose equivalence of neutron production.[11] Our study highlights the combined information of fast and thermal neutron contributions as a function of depth in phantom to mimic the actual spectra inside the patient body during treatment.

The early studies of photo-neutrons at different locations in the treatment rooms are mainly carried out using MC simulation and bubble detectors for thermal neutrons,[6],[7],[18] for 10 MV, 15 MV Siemens PRIMUS machine. Very few studies for fast neutrons are available in the literature.[8],[9],[19] Our study highlights the photo-neutrons equivalence on the patient plane for different directions of the couch. The decrease of photo-neutron dose from 1.288 ± 0.03 mSv/Gy to 0.062 ± 0.032 mSv/Gy along the longitudinal direction and from 0.62 ± 0.08 mSv/Gy to 0.068 ± 0.019 mSv/Gy as the distance from the isocenter increase from 0 cm to 100 cm from the field edge of 30 cm × 30 cm field size as shown in [Figure 2] and [Figure 3].

[Table 2] shows the variation of photo-neutron dose as a function of field sizes for thermal and fast neutrons. Our study shows that the fast neutrons increases as the field size increases for 15 MV Elekta Precise machines, which correlate with the studies published by Awotwi-Pratt et al.,[16] for Siemens LINAC (PRIMUS). The logical reasoning of higher fast neutron production at larger field opening was discussed in literatures.[9],[20],[21],[22] The values of neutron dose equivalent estimated are consistent with the results of other measurements reported in the previous literature and fall within the allowed limit by the International Electrotechnical Commission (IEC),[23] for field size up to 20 cm × 20 cm. The ratio of neutron dose equivalent to central axis photon are slightly higher, 1.04 ± 0.04 mSv/Gy and 1.08 ± 0.07 mSv/Gy for 20 cm × 20 cm and 30 cm × 30 cm field size, respectively, and are exceeding IEC limit of <1 mSv/Gy inside and 0.5 mSv/Gy outside of photon field. Our study shows the similar results shown by Khaled et al.,[24] for 15 MV Elekta Precise machine using CR39 detector for 15 cm × 15 cm field size.

As shown in [Table 3], by increasing the wedge angle, the fast and thermal neutron dose increases. Fast neutron increases from 0.42 ± 0.03 mSv/Gy for no wedge (0o) to 0.95 ± 0.05 mSv/Gy for 60° wedge due to the interaction of photons with the wedge through (γ, n) reaction. At the same time, the thermal neutrons decreases from 0.88 ± 0.04 mSv/Gy for 0° wedge angle to 0.30 ± 0.005 mSv/Gy for 60° because of the following reasons: (i) Degradation of energy of the fast neutron through (n × n) reaction at higher wedge angle and (ii) absorption of thermal neutrons by the phantom material of thickness 3 cm. A sudden increase in thermal neutron dose at 45o wedge angle could possibly be related to the resonance effect of TLD 600 at 200 keV. All the measurements were taken from a TPS plan for 4 Gy at 3 cm depth with 10 cm backscatter. TLD 700 readings were compared with calibrated 0.6 cc ionization chamber to ensure the almost equal photon dose is delivered for all wedge angles.

No literature measurements are available to compare the results for universal wedges (Omni wedge) from Elekta. It is seen that the photon dose is almost constant with wedge angle, which is obtained through the variation of MU [Table 4]. Doses estimated by TLD 700 are comparable with ionization chamber-based results. Slight difference in the dose estimation between TLD 700 and ionization chamber could be attributed to the sensitivity variation (±3%) among TLD 700 chips and the positional difference between the ionization chamber and the TLD 700.

 > Conclusions Top

This study shows the complete interpretations for photo-neutrons for various field sizes, depth dose in phantom equivalent to a patient body, and the wedge beam modifier of Elekta LINAC. The neutron dose to the patient is dependent on several parameters, such as the type and material of the MLC and the photon energy and the depth in the patient body itself. Our current study includes all the parameters of Elekta Precise medical LINAC, which can be compared to similar medical LINACs, even though it is preferable to investigate the neutron contaminations separately for individual LINACs.

The neutron dose equivalence dependent on the dose rate, monitor units, patient treatment plans using 15 MV are not reported in this paper; however, it is in process of publication in near future.

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Conflicts of interest

There Are No Conflicts of Interest.

 > References Top

Limitation of Exposure to Ionization Radiation. National Council on Radiation Protection and Measurement Report No. 116. Washington DC; 1993.  Back to cited text no. 1
Neutron Contamination from Medical Accelerators National Council on Radiation Protection and Measurements Report No. 79. Washington, DC; 1984.  Back to cited text no. 2
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Howell RM, Ferenci MS, Hertel NE, Fullerton GD. Investigation of secondary neutron dose for 18 MV dynamic MLC IMRT delivery. Med Phys 2005;32:786-93.  Back to cited text no. 4
Laughlin JS. Physical considerations in the use of a 23 MeV medical betatron. Nucleonics 1951;8:5-16.  Back to cited text no. 5
Pena J, Franco L, Gómez F, Iglesias A, Pardo J, Pombar M. Monte Carlo study of Siemens PRIMUS photoneutron production. Phys Med Biol 2005;50:5921-33.  Back to cited text no. 6
Zanini A, Durisi E, Fasolo F, Ongaro C, Visca L, Nastasi U, et al. Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems. Phys Med Biol 2004;49:571-82.  Back to cited text no. 7
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Naseri A, Mesbahi A. A review on photoneutrons characteristics in radiation therapy with high-energy photon beams. Rep Pract Oncol Radiother 2010;15:138-44.  Back to cited text no. 10
Followill DS, Stovall MS, Kry SF, Ibbott GS. Neutron source strength measurements for Varian, Siemens, Elekta, and General Electric linear accelerators. J Appl Clin Med Phys 2003;4:189-94.  Back to cited text no. 11
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Vilela E, Fantuzzi E, Giacomelli G, Giorgini M, Morelli B, Patrizii L, et al. Optimization of CR-39 for fast neutron dosimetry applications. Radiat Meas 1999;31:437-42.  Back to cited text no. 13
Triolo A, Marrale M, Brai M. Neutron-gamma mixed field measurements by means of MCP-TLD600 dosimeter pair. Nucl Instrum Methods Phys Res 2007;264:183-8.  Back to cited text no. 14
Milliken BD, Turian JV, Hamilton RJ, Rubin SJ, Kuchnir FT, Yu CX, et al. Verification of the Omni wedge technique. Med Phys 1998;25:1419-23.  Back to cited text no. 15
Awotwi-Pratt JB, Spyrou NM. Measurement of photoneutrons in the output of 15MV varian clinac 2100C LINAC using bubble detectors. J Radioanal Nucl Chem 2007;271:679-84.  Back to cited text no. 16
d'Errico F, Luszik-Bhadra M, Nath R, Siebert BR, Wolf U. Depth dose-equivalent and effective energies of photoneutrons generated by 6-18 MV X-ray beams for radiotherapy. Health Phys 2001;80:4-11.  Back to cited text no. 17
Ghasemi A, Pourfallah TA, Akbari MR, Babapour H, Shahidi M. Photo neutron dose equivalent rate in 15 MV X-ray beam from a Siemens primus linac. J Med Phys 2015;40:90-4.  Back to cited text no. 18
[PUBMED]  Medknow Journal  
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Brockstedt S, Holstein H, Jakobsson L, Tomaszewicz A, Knöös T. Be aware of neutrons outside short mazes from 10-MV linear accelerators X-rays in radiotherapy facilities. Radiat Prot Dosimetry 2015;165:464-7.  Back to cited text no. 20
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Mohammadi N, Miri-Hakimabad H, Rafat-Motavlli L, Akbari F, Abdollahi S. Neutron spectrometry and determination of neutron contamination around the 15 MV Siemens primus linac. J Radioanal Nucl Chem 2015;304:1001-8.  Back to cited text no. 22
International Electrotechnical Commission (IEC). “Medical Electrical Equipment: Particular Requirements for the Safety of Electron Accelerators in the Range 1 MeV to 50 MeV”, Document 60601-2-1. Geneva, Switzerland: IEC; 1998.  Back to cited text no. 23
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  [Figure 1], [Figure 2], [Figure 3]

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


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