Measurement of the distribution of neutrons produced by a 15 MV linear accelerator in a solid water phantom using CR-39 detectors

Main Article Content

Kasama Homkhaow
Thiansin Liamsuwan
Sawanee Suntiwong
Natch Rattanarungruangchai
Waraporn Sudchai


Background: In high energy photon therapy with >10 MV x-rays, neutrons are produced from photonuclear reactions between high energy photons and high atomic number materials in the treatment head. Although neutron dose is expected to be relatively small compared to the primary photon dose to the target, neutrons have high quality factors that are associated with the increased secondary cancer risk of the treated patient. Due to the attenuation of neutrons by the patient’s body, the distribution of neutron doses at different positions in the patient should be determined for the assessment of organ-specific secondary cancer risks.

Objectives: To determine the distribution of neutrons from a 15 MV linear accelerator at different lateral distances from the isocenter and at different depths in a solid water phantom, as a mimic for the patient body.

Materials and methods: The distribution of neutrons was measured with BARYOTRAK CR-39 detectors in term of nuclear track densities in the CR-39 detectors. A half of the detector’s surface area was covered with a boron converter and a polyethylene radiator for thermal and fast neutron measurement, while the other half had no boron converter making it only sensitive to fast neutrons. The detectors were placed in a solid water phantom at 0, 5, 10 and 15 cm lateral distances from the isocenter and at 0, 3, 7, 11, 15 and 18 cm depths from the phantom surface. The detectors were irradiated with 15 MV photon beams at 0° gantry angle for the field size of 20x20 cm2.

Results: The nuclear track density initially increased with depth, reached a maximum at 3 cm depth and decreased with depth beyond the depth of the maximum. The contributions from fast and thermal neutrons at shallow depths were competitive while at large depths most of neutrons became thermalized. The lateral distribution of the track density had a maximum at the central axis. Thermal neutrons were responsible for this behavior. In contrast, nuclear track densities generated by fast neutrons were nearly constant with the off-axis distance.

Conclusion: Nuclear track densities generated by neutrons varied with the depth and the lateral distance from the isocenter. The conversion coefficients from nuclear track density to dose should consider the energy spectrum of neutrons especially at shallow depths.

Article Details

How to Cite
Homkhaow, K., Liamsuwan, T., Suntiwong, S., Rattanarungruangchai, N., & Sudchai, W. (2021). Measurement of the distribution of neutrons produced by a 15 MV linear accelerator in a solid water phantom using CR-39 detectors. Journal of Associated Medical Sciences, 54(3), 48–56. Retrieved from
Research Articles


Kry SF, Bednarz B, Howell RM, Dauer L, Followill D, Klein E, et al. AAPM TG 158: measurement and calculation of doses outside the treated volume from external‐beam radiation therapy. Med Phys 2017; 44(10): 391-429. doi: 10.1002/mp.12462.

Biltekin F, Yeginer M, Ozyigit G. Investigating in-field and out-of-field neutron contamination in high-energy medical linear accelerators based on the treatment factors of field size, depth, beam modifiers, and beam type. Phys Med 2015; 31(5): 517-23. doi: 10.1016/j.ejmp.2015.03.015.

Nuclear Regulatory Commission and Department of Veterans Affairs. Neutron quality factor. United States: Oak Ridge Inst. for Science and Education; 1995.

Takam R, Bezak E, Marcu L, Yeoh E. Out-of-field neutron and leakage photon exposures and the associated risk of second cancers in high-energy photon radiotherapy: current status. Radiat Res 2011; 176(4): 508-20. doi: 10.1667/rr2606.1.

Schneider U, Zwahlen D, Ross D, Kaser-Hotz B. Estimation of radiation-induced cancer from three-dimensional dose distributions: Concept of organ equivalent dose. Int J of Radiation Oncology* Biol Phys 2005; 61(5): 1510-5. doi: 10.1016/j.ijrobp.2004.12.040.

Chu W, Lan J, Chao T, Lee C, Tung C. Neutron spectrometry and dosimetry around 15 MV linac. Radiat Meas 2011; 46(12): 1741-4. doi: 10.1016/j.radmeas.2011.06.029.

Montgomery L, Evans M, Liang L, Maglieri R, Kildea J. The effect of the flattening filter on photoneutron production at 10 MV in the Varian TrueBeam linear accelerator. Med Phys 2018; 45(10): 4711-9. doi: 10.1002/mp.13148.

Reft CS, Runkel‐Muller R, Myrianthopoulos L. In vivo and phantom measurements of the secondary photon and neutron doses for prostate patients undergoing 18 MV IMRT. Med Phys 2006; 33(10): 3734-42. doi: 10.1118/1.2349699.

Alem-Bezoubiri A, Bezoubiri F, Badreddine A, Mazrou H, Lounis-Mokrani Z. Monte Carlo estimation of photoneutrons spectra and dose equivalent around an 18 MV medical linear accelerator. Radiat Phys Chem 2014; 97: 381-92. doi: 10.1016/j.radphyschem. 2013.07.013.

Ipe NE, Roesler S, Jiang S, Ma C, editors. Neutron measurements for intensity Modulated Radiation therapy. Proceedings of the 22nd Annual International Conference of the IEEE Engineering in Medicine and Biology Society (Cat No 00CH37143); 2000: IEEE.

Hälg RA, Besserer J, Boschung M, Mayer S, Clasie B, Kry SF, et al. Field calibration of PADC track etch detectors for local neutron dosimetry in man using different radiation qualities. Nucl Instrum Methods Phys Res, A 2012; 694: 205-10. doi: 10.1016/j.nima.2012.08.021.

Kry SF, Howell RM, Salehpour M, Followill DS. Neutron spectra and dose equivalents calculated in tissue for high‐energy radiation therapy. Med Phys 2009; 36(4): 1244-50. doi: 10.1118/1.3089810.

Seco J, Verhaegen F., editors. Monte Carlo techniques in radiation therapy. Florida: CRC press; 2013.

Awotwi-Pratt J, Spyrou N. Measurement of photoneutrons in the output of 15 MV varian clinac 2100C LINAC using bubble detectors. J Radioanal Nucl Chem 2007; 271(3): 679-84. doi: 10.1007/s10967-007-0325-8.

d'Errico F, Nath R, Tana L, Curzio G, Alberts WG. In‐phantom dosimetry and spectrometry of photoneutrons from an 18 MV linear accelerator. Med Phys 1998; 25(9): 1717-24. doi: 10.1118/ 1.598352.

Yücel H, Çobanbaş İ, Kolbaşı A, Yüksel AÖ, Kaya V. Measurement of photo-neutron dose from an 18-MV medical linac using a foil activation method in view of radiation protection of patients. Nucl Eng Technol 2016; 48(2): 525-32. doi: 10.1016/

Dawn S, Pal R, Bakshi A, Kinhikar R, Joshi K, Jamema S, et al. Evaluation of in-field neutron production for medical LINACs with and without flattening filter for various beam parameters-Experiment and Monte Carlo simulation. Radiat Meas 2018; 118: 98-107. doi: 10.1016/j.radmeas.2018.04.005.

Farhood B, Ghorbani M, Goushbolagh NA, Najafi M, Geraily G. Different methods of measuring neutron dose/fluence generated during radiation therapy with megavoltage beams. Health Phys 2020; 118(1): 65-74. doi: 10.1097/HP.0000000000001130.

Kumar V, Sonkawade R, Dhaliwal A. Optimization of CR-39 as neutron dosimeter. Indian J Pure Appl Phys 2010; 48(7): 466-9.

Oda K, Miyake H, Michijima M. CR39-BN detector for thermal neutron dosimetry. Nucl Sci Tech 1987; 24(2): 129-34. doi: 0.3327/jnst.24.129.

Mameli A, Greco F, Fidanzio A, Fusco V, Cilla S, D’Onofrio G, et al. CR-39 detector based thermal neutron flux measurements, in the photo neutron project. Nucl Instrum Methods Phys Res Section B: Beam Interactions with Materials and Atoms 2008; 266(16): 3656-60. doi: 10.1016/j.nimb.2008.05.122.

García M, Amgarou K, Domingo C, Fernández F. Neutron response study of two CR-39 personal dosemeters with air and Nylon converters. Radiat Meas 2005; 40(2-6): 607-11. doi: 10.1016/j.radmeas.2005.04.017.

Pal R, Nadkarni VS, Naik D, Beck M, Bakshi AK, Chougaonkar MP, et al. Development and dosimetric characterization of indigenous PADC for personnel neutron dosimetry. Nucl Technol Radiat Prot 2015; 30(3): 175-87. doi: 10.2298/NTRP1503175P.

Romero-Expósito M, Martínez-Rovira I, Domingo C, Bedogni R, Pietropaolo A, Pola A, et al. Calibration of a Poly Allyl Diglycol Carbonate (PADC) based track-etched dosimeter in thermal neutron fields. Radiat Meas 2018; 119: 204-8. doi: 10.1016/j.radmeas.2018.11.007.

Thomas D, Bedogni R, Méndez R, Thompson A, Zimbal A. Revision of ISO 8529—reference neutron radiations. Radiat Prot Dosim 2018; 180(1-4): 21-4. doi: 10.1093/rpd/ncx176.

Oonsiri P, Vannavijit C, Wimolnoch M, Suriyapee S, Saksornchai K. Estimated radiation doses to ovarian and uterine organs in breast cancer irradiation using radio‐photoluminescent glass dosimeters (RPLDs). J Med Radiat Sci 2020; 68: 167-174. doi: 10.1002/jmrs.445.

Rezaian A, Nedaie HA, Banaee N. Measurement of neutron dose in the compensator IMRT treatment. Appl Radiat Isot 2017; 128: 136-41. doi: 10.1016/j.apradiso.2017.06.013.

Martinez-Ovalle S, Barquero R, Gomez-Ros J, Lallena A. Neutron dose equivalent and neutron spectra in tissue for clinical linacs operating at 15, 18 and 20 MV. Radiat Prot Dosim 2011; 147(4): 498-511. doi: 10.1093/rpd/ncq501.

Brkić H, Ivković A, Kasabašić M, Sovilj MP, Jurković S, Štimac D, et al. The influence of field size and off-axis distance on photoneutron spectra of the 18 MV Siemens Oncor linear accelerator beam. Radiat Meas 2016; 93: 28-34. doi: 10.1016/j.radmeas.2016.07.002