**3. Results and discussion**

*Use of Gamma Radiation Techniques in Peaceful Applications*

by adjusting the gain value of MCA. This enables for spectra registration up to the

*ReGe gamma spectrometer used in present study: (a) multichannel analyzer, (b) front view of high-purity germanium detector with carbon-composite entrance window, and (c) measurement configuration 50 cm from* 

Energy calibration of spectrometric system has been performed for standard MCA gain (5.0) with the use of radioisotope sealed sources (1 cm Ø) having activity of the order of 10 kBq. Subsequently, it was checked that the calibration scales

Spectrometric efficiency has been modeled in In Situ Object Counting Software (ISOCS™, Canberra Inc.), applying full factory characterization of a given detector performed with the use of NIST-traceable sources and MCNP Monte Carlo modeling code, supplied by the manufacturer. The geometry of rectangular complex plane for calculating the detection efficiency has been chosen from the ISOCS predefined templates as best matching to the experimental conditions, giving the possibility to include the multilayer design of the entrance door. The energydependent photon detection efficiency (*ε*) of spectrometric system has been finally

+ e·ln(E)4

] <sup>=</sup> *Peak*\_*net*\_*area*(*E*) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *ε*(*E*) <sup>⋅</sup> *SGe*[*cm*2] <sup>⋅</sup> *LT*[*s*]

+ f·ln(E)5

, with the fitting

(1)

+ d·ln(E)3

−1

The analysis of registered spectra (photopeaks' identification and net areas counting) has been performed using unidentified second differential and nonlinear LSQ fit in Genie™ 2000 software. The sources of gamma radiation (activated nuclides) have been identified on the base of photopeaks' energies, whereas areas under these photopeaks were used for photon flux density (*Φ*) assessment on the basis of Eq. (1), for a defined detector front surface (*SGe*) and life time (*LT*) of each

**164**

energy of 3–10 MeV.

**Figure 4.**

parameters of a–f.

measurement.

linearly inversely with the spectrometric gain.

*the entrance door to the linac radiotherapy room.*

described with the following function: ln(ε) = a + b·ln(E) + c·ln(E)<sup>2</sup>

Φ(*E*) [*cm*−2 *s*

Gamma radiation spectrometry system was placed 50 cm in front of linac treatment room entrance door, at the height of 1 m above the floor (see **Figure 4**), at a localization representative for dose rate assessment for the staff waiting for the end of patient irradiation.

The spectra were registered during emission of 6–18 MV photon beams with a dose rate of 450 MU/min. The gantry angle of 90 or 270° and irradiation field size of 40 × 40 cm2 were set to achieve maximal intensity of radiation reaching the entrance door, to study the worst scenario of occupational hazard.

#### **3.1 Comparison of spectra for various beam energies**

The detailed characteristics of gamma ray spectra acquired in standard and extended energy range near the linac room door are presented in **Table 1**. The energies of photons in 6 MV beam are too low to trigger nuclear reactions; therefore, induced radionuclides are not observed on the spectrum outside the door in this case, as shown in **Figure 5**. Nevertheless, the increase in low-energy continuous part of the registered spectrum in comparison with natural background radiation indicates that part of scattered radiation from therapeutic beam penetrates the door.

Spectra registered during high-energy beam emission (10–18 MV), as shown in **Figure 5**, are dominated by two processes: positron creation, since annihilation peak at 511 keV is clearly visible, and neutron capture in hydrogen-rich material inside the door, due to the presence of a peak at 2224.6 keV, which is the neutron-binding energy in deuterium nucleus. The intensity of these processes could be correlated not only with neutron source strength of particular linac working at defined accelerating potential, but also with the amount of hydrogen-rich material used in door construction. The peak at 477.6 keV is due to the presence of boron (mostly in the form of borax–sodium tetraborate decahydrate) and is a consequence of 10B(n,α) 7 Li reaction, where 477.6 keV is the deexcitation energy of lithium nucleus. The broadening of this peak has Doppler effect—origin, widely discussed in [56]. Since door construction is not unified and the usage of paraffin/polyethylene (as hydrogen-rich materials) or borax as neutron absorption agents depends on the construction concept, the intensity of the 477.6 keV line should not be directly connected with the therapeutic beam energy (see **Figure 5**: 15 MV vs. 18 MV cases) or even may not occur at all, whereas hydrogen capture of neutron is present for all linacs studied by us.

The gamma ray spectra are dominated by the abovementioned interactions; however, the minor contributions come from:

• (n,n'γ) and (n,γ) interactions in germanium crystal of HPGe spectrometer, which proves that neutrons contribute to the door-leakage radiation outside the treatment room;

#### *Use of Gamma Radiation Techniques in Peaceful Applications*


#### **Table 1.**

*Characteristics of gamma ray spectra registered in the energy range of 10 keV–10 MeV near the door to linac therapy room during emission of 10–18 MV photon beams.*


Neutron capture nuclear reaction is accompanied by prompt gamma rays but also decay gamma radiation might be observed, when originated nucleus is radioactive. The first mentioned radiation type is observed only during emission of highenergy therapeutic beam, but observed energies are mostly above 2 MeV, whereas the second group of gammas has energies up to about 2 MeV and contributes to the increased background after the end of therapeutic beam emission, with a characteristic half-life.

Spectra registered in extended energy range prove that neutron capture process on light elements (mainly concrete) occurs intensively. The main differences between linac room shielding properties from occupational hazard point of view (for defined therapeutic beam) are due to the diverse construction of the door and specific material used, for example: borated polyethylene or paraffin alone will result in the presence of 477.6 keV line or not, which will affect the intensity of

**167**

**Figure 5.**

**Figure 6.**

*differences.)*

at the registered spectra.

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room*

2224.6 keV line. The comparison of spectra presented in **Figure 6** demonstrates the differences between thick and thin door constructions designed for (a) higher beam

*Comparison of spectra registered for (a) 10 MV photon beams and (b) 15 MV photon beams, when thick (black line) and thin (gray line) door constructions are used. (See text above for the explanation of* 

*The comparison of spectra registered behind the treatment room door during emission of linac photon beams. Natural background radiation is presented for reference. Only the most intense lines (mentioned in the text* 

*above) are marked for clarity of presentation. Detailed analysis is presented in* **Table 1***.*

In the present study, the effective dose in front of linac room entrance door has been estimated on the base of photon flux density obtained according to the Eq. (1) and using conversion coefficients for AP geometry given in ICRP report 116 [54]. Specific values of these coefficients for energies registered on gamma ray spectra were calculated using Lagrange interpolation formula of third degree. The average total uncertainty of calculated effective dose values of 25% includes the accuracy of conversion coefficients as well as the uncertainty of photopeaks' area determination

energy than currently used and (b) maximum beam currently in use.

**3.2 Spectrometry—Based dose assessment**

*DOI: http://dx.doi.org/10.5772/intechopen.82726*

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room DOI: http://dx.doi.org/10.5772/intechopen.82726*

**Figure 5.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

72Ge(n,γ)

74Ge(n,γ)

70Ge(n,γ)

73Ge(n,γ)

56Fe(n,γ), 54Fe(n,γ)

27Al(n,γ)

1

10B(n,α) 7

Concrete 39K(n,γ) 770.3;

Germanium detector

Metal elements of door construction

Shielding elements of door construction

**Source Origin Photon energy [keV]**

73mGe 66.7

75mGe 139.7

74Ge\* 595.9 74Ge(n,n'γ) 595.9; 608.3; 867.9; 1204.2

72Ge(n,n'γ) 689.6

71mGe 174.9; 198.4; 499.9

28Al 1779.0; 3033.9 12C(n,γ) 1261.8; 4945.3

H(n,γ) 2224.6

23Na(n,γ) 834.7; 2517.8

24Mg(n,γ) 3413.1; 3916.8 31P(n,γ) 3899.9

Li 477.6 207Pb(n,γ) 7367.8

352.4; 7631.1; 7645.5

2129.2; 2425.7; 2469.2; 2526.5; 2721.3; 3185.2; 3436.6; 3854.3; 4218.3; 4406.1; 5357.4; 5920.4; 6018.5; 7278.8; 7631.1; 7645.5; 9297.7

4082.8; 4298.6; 4979.9; 5204.5; 6110.8; 6619.6; 6627.8; 7414.0; 7790.3

56Fe(n,γ) 570.0; 810.6; 920.5; 1019.0; 1358.6; 1612.8; 1358.6; 1725.3; 1972.3;

35Cl(n,γ) 516.7; 575.8; 1164.9; 1951.1; 1959.3; 2863.8; 2876.9; 3061.8; 3195.4;

40Ca(n,γ) 707.7; 1942.7; 2001.3; 3610.2; 4418.5; 6419.6

28Si(n,γ) 1273.3; 2092.9; 3101.8; 3539.0; 3723.1; 4933.9; 6379.8

• (n,γ) reactions in concrete elements: 23Na,24Mg,28Si,31P,35Cl,39K,40Ca;

*Characteristics of gamma ray spectra registered in the energy range of 10 keV–10 MeV near the door to linac* 

Neutron capture nuclear reaction is accompanied by prompt gamma rays but also decay gamma radiation might be observed, when originated nucleus is radioactive. The first mentioned radiation type is observed only during emission of highenergy therapeutic beam, but observed energies are mostly above 2 MeV, whereas the second group of gammas has energies up to about 2 MeV and contributes to the increased background after the end of therapeutic beam emission, with a character-

Spectra registered in extended energy range prove that neutron capture process on light elements (mainly concrete) occurs intensively. The main differences between linac room shielding properties from occupational hazard point of view (for defined therapeutic beam) are due to the diverse construction of the door and specific material used, for example: borated polyethylene or paraffin alone will result in the presence of 477.6 keV line or not, which will affect the intensity of

• (n,γ) reactions in metals: 27Al and 56Fe.

*therapy room during emission of 10–18 MV photon beams.*

**166**

istic half-life.

**Table 1.**

*The comparison of spectra registered behind the treatment room door during emission of linac photon beams. Natural background radiation is presented for reference. Only the most intense lines (mentioned in the text above) are marked for clarity of presentation. Detailed analysis is presented in* **Table 1***.*

**Figure 6.**

*Comparison of spectra registered for (a) 10 MV photon beams and (b) 15 MV photon beams, when thick (black line) and thin (gray line) door constructions are used. (See text above for the explanation of differences.)*

2224.6 keV line. The comparison of spectra presented in **Figure 6** demonstrates the differences between thick and thin door constructions designed for (a) higher beam energy than currently used and (b) maximum beam currently in use.

#### **3.2 Spectrometry—Based dose assessment**

In the present study, the effective dose in front of linac room entrance door has been estimated on the base of photon flux density obtained according to the Eq. (1) and using conversion coefficients for AP geometry given in ICRP report 116 [54]. Specific values of these coefficients for energies registered on gamma ray spectra were calculated using Lagrange interpolation formula of third degree. The average total uncertainty of calculated effective dose values of 25% includes the accuracy of conversion coefficients as well as the uncertainty of photopeaks' area determination at the registered spectra.

Effective doses in studied location depend on the neutron source strength Q of particular linac as well as on the construction of the treatment room door. For 18-MV photon beam, more important is the first factor (linac construction), ranging the doses from 30.6 ± 7.7 to 56.2 ± 14.1 μSv/h. The second factor plays the crucial role for 10 MV photon beams, for which neutron generation is of the lowest intensity, ranging the doses from 1.8 ± 0.4 μSv/h (for flattening filterfree (FFF) beam), through 3.4 ± 0.8 μSv/h (for thick door construction) up to 10.5 ± 2.6 μSv/h (for thin door construction). However, also in this case, neutron production intensity in linac head plays significant role, what is concluded from the differences between FFF beam and conventional linac, since flattening filter takes part in neutron production [29, 30]. Effective dose rates measured during 15-MV beam emission using Geiger-Mueller radiometer (calibrated on 60Co source) and the result obtained using spectrometry analysis presented here are 13.5 ± 3.0 μSv/h and 22.2 ± 5.5 μSv/h, respectively. This comparison shows that even 60% of dose could be omitted in the first case when excluding high-energy component of radiation leakage through the door due to prompt gamma rays accompanying the neutron capture process.

Production of neutron secondary radiation during emission of high-energy photon therapeutic beams is generally known and widely studied issue [29–46, 48]. Also, the phenomenon of high-energy X-rays and secondary neutron-induced radioactivity is well recognized [57]. However, the impact of photon radiation connected with neutron interaction in treatment room shielding materials on occupational safety is still difficult to assess experimentally in clinical conditions due to limited availability of high-resolution extended-energy range spectrometry systems, which often require special operating conditions (e.g., nitrogen cooling) and time-/labor-consuming data analysis. Nevertheless, recommendations concerning design of linac rooms [17] refer to publications devoted to this issue [58]. The use of a spectrometer (the usefulness of which has been demonstrated in presented study) is advised by IAEA [59] as a supplementary method for workplace monitoring, and its usage to characterize the energy spectrum of a given radiation type is recommended to support the performance of routinely used monitoring instruments.

## **4. Conclusion**

The qualitative analysis performed by us has shown that the major component of gamma radiation field near the treatment room door comes from prompt photons emitted during neutron capture reaction and is common in door construction as well as in concrete materials. Comprehensive study of this issue requires extended energy range of spectrometric system, as demonstrated in presented investigations. High-energy gamma rays above 3 MeV (omitted in standard spectrometric measurements) contribute to the effective dose values from 26 to 58%, for low (10 MV FFF beam) and for high (18 MV beam) neutron source strength linacs, respectively.

Reactions intended for neutron capture in door construction: 10B(n,α) 7 Li and 1 H(n,γ) 2 H contribute to the effective dose of 0–17% and 4–19%, respectively. Borated inner layer of the door is not always used, whereas hydrogen-rich material is the commonly used neutron absorber.

Presented study proves the correctness of radiation protection guidelines to avoid the vicinity of treatment door during therapeutic beam emission and additionally provides the justification in terms of dose values and mechanisms of gamma ray production.

**169**

**Author details**

Kinga Polaczek-Grelik1

and Łukasz Michalecki1

provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 Nu-Med Cancer Diagnosis and Treatment Centre, Tomaszów Mazowiecki, Poland

\*, Aneta Kawa-Iwanicka1

1 Nu-Med Cancer Diagnosis and Treatment Centre, Katowice, Poland

\*Address all correspondence to: kinga.polaczek-grelik@nu-med.pl

, Marek Rygielski<sup>2</sup>

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room*

This work was possible due to one of the authors (KPG) involvement in scientific activity in University of Silesia in Katowice, Poland. Therefore, the authors express their gratitude for the opportunity to use in situ gamma spectrometric

The authors certify that they have no affiliations with or involvement in any organization or entity with any interest in the subject matter or materials discussed

*DOI: http://dx.doi.org/10.5772/intechopen.82726*

**Acknowledgements**

**Conflict of interest**

in this manuscript.

system.

*Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room DOI: http://dx.doi.org/10.5772/intechopen.82726*
