Photo neutron dose equivalent rate in 15 MV X-ray beam from a Siemens Primus Linac.

Fast and thermal neutron fluence rates from a 15 MV X-ray beams of a Siemens Primus Linac were measured using bare and moderated BF3 proportional counter inside the treatment room at different locations. Fluence rate values were converted to dose equivalent rate (DER) utilizing conversion factors of American Association of Physicist in Medicine's (AAPM) report number 19. For thermal neutrons, maximum and minimum DERs were 3.46 × 10(-6) (3 m from isocenter in +Y direction, 0 × 0 field size) and 8.36 × 10(-8) Sv/min (in maze, 40 × 40 field size), respectively. For fast neutrons, maximum DERs using 9" and 3" moderators were 1.6 × 10(-5) and 1.74 × 10(-5) Sv/min (2 m from isocenter in +Y direction, 0 × 0 field size), respectively. By changing the field size, the variation in thermal neutron DER was more than the fast neutron DER and the changes in fast neutron DER were not significant in the bunker except inside the radiation field. This study showed that at all points and distances, by decreasing field size of the beam, thermal and fast neutron DER increases and the number of thermal neutrons is more than fast neutrons.

Introduction

Radiation therapy (RT) is an established way of cancer treatment. It delivers high doses of radiation to a tumor target. The main challenge in cancer treatment using RT is the application of high doses to the tumor volume without harming of the healthy tissue. Nowadays, several treatment modalities have been established. The selection depends on the type of tumor to be treated. Irrespective of the used technique, accurate dosimetry of the radiation fields prior to the treatment of a patient is required from radiation protection view point.[1]

One of the modalities in RT is the use of a medical linear accelerator (LINAC) generating bremsstrahlung photon beams. Photon beams with energies higher than 10MV are preferred if doses should be delivered to larger depths and to enhance the skin sparing.

For photon energies above a threshold of approximately 7MeV (depending on the atomic number of the material being irradiated in the beam, this threshold will generally be in the range of 6.7–10.8 MeV photon energy), photonuclear reactions of the heavy metals of the accelerator head lead to production of unwanted neutrons that contaminate the photon field.[2]

The importance of neutron contamination at medical LINACs has been recognized in NCRP reports 79 and 102.[34] Neutrons have a substantially higher biological effectiveness than photons; therefore, even a small number of neutrons can lead to a nonnegligible dose component. Therefore, precise knowledge of these small dose components is necessary. In this case, special radiation protection methods have to be implemented in order to prevent the exposure of patient and medical staff. In conventional RT, unwanted neutrons are usually considered negligible; however, accounted for when designing radiation protection for treatment rooms.

Many researches have been performed regarding neutron contamination of different medical electron accelerators and various ways have been used to report the contamination including; fluence, dose equivalent, and absorbed dose measurements using various devices.[25678910]

Fox and McAllister[11] and McGinley et al.,[7] have reported contamination levels of 4.8 × 106 and 13.6 × 106 fast neutrons/cm2/Gy of X-ray at beam energies of 18 and 25 MV, respectively. The value of fast-neutron contamination of 15 × 106 fast neutrons/cm2/Gy of X-ray was reported by Axton and Bardell[12] for a 16 MV X-ray beam of a LINAC. Some studies have investigated the effect of different parameters (related to system head and treatment room) in neutron contamination. Some of them have shown that neutron fluence varies with field size for different medical LINACs.[13141516]

Chibani and Ma found that for the 18MV photon beam of a Siemens Primus Linac, the dose value of neutrons at a given depth in the field increased with increasing field size.[16] Al-Ghamdi et al.,[13] showed that for fast neutrons the intensity does not vary significantly with field size for the measurements outside the primary beam and it decreases linearly with field size in the field. The thermal neutron intensity at any location in the treatment room (i. e., in and out of the field) was found to be almost independent of field size.[13] In another study by Hashemi et al., it was shown that neutron dose equivalent (NDE) increases with field size for both open and wedged beams.[14] Reft et al., reported that both the in vivo and Bonner sphere results show different secondary fast neutron dose among the three different therapy machines.[15] However, it is difficult to form conclusions based on these studies considering the effect of field size on the photoneutron contamination.

Due to the variation reported in the literature, the differences in the geometry of various treatment rooms and LINACs (even with the same type), further measurements of neutron contaminations are needed. Therefore, for each LINAC and treatment room, it is better to investigate neutron contaminations separately. It seems in addition to machine's components, other parameter such as bunker design and its materials’ composition are also responsible factors for the production of photoneutrons.

In this study, by using a BF3 proportional counter, fast and thermal neutron fluence rate from an 15 MV X-ray beam of a Siemens Primus LINAC was measured inside the treatment room (i. e., at different distances from isocenter and special points) in various radiation field sizes. In order to convert the fluence rate values to dose equivalent rate (DER), conversion factors of American Association of Physicist in Medicine's (AAPM) report number 19 and Cossairt and Vaziri[17] applied formula were utilized. The effect of variation of the distance from isocenter and radiation field size in thermal and fast neutrons DER was investigated in this research.

Materials and Methods

In this study the external therapy machine was a Siemens Primus LINAC with two photon modes (6 and 15 MV). Since the threshold of photoneutron reaction is approximately more than 7 MeV, neutron contamination for 15 MV photon beam with dose rate of 1.8 Gy/min was measured using a BF3 proportional counter (BDPN-07, ECOTEST, Ukraine). It has cylindrical shape with two spherical moderators (9” and 3” diameter) for measuring fast neutron fluence rate and bare BF3 counter for measuring the thermal neutrons (in terms of n/cm2 min). In this study, fast and thermal neutrons fluence rate at 0, 1, 2, and 3 m from isocenter in two direction (−X and +Y) and in points A, B, C, D, and E inside the treatment room and maze [Figure 1] were measured at three different field sizes. Figure 2 shows experimental set-up at isocenter.

Figure 1

Schematic diagram of the treatment room and the location of the studied points

Figure 2

Experimental set-up at isocenter with two spherical moderators (9” and 3” diameter)

BF3 counter was calibrated at Secondary Standard Dosimetry Laboratory (SSDL) of the Iranian Atomic Energy Organization (IAEO) and all data about thermal and fast neutrons are available for different neutron energies.

Main relative permissible error limit of thermal and fast neutron flux density measurement of this device when calibrated for PuBe with 0.95 confidence probability was 20 + 200/N, where N is a numeric value of measured neutron fluence density.

Results and Discussions

In all measurements, the maximum standard variation in fluence rate values was less than 2%.

In this study, for calculating thermal neutrons’ DER (~ Ē < 1eV), Cossairt and Vaziri[17] applied formula was used (i. e., for ~ Ē < 1eV dose equivalence is 10.2 pSv/n/cm2). Table 1 shows thermal neutron DER (Sv/min) at various locations and different field sizes inside the treatment room of LINAC.

Table 1

Thermal neutron dose equivalent rate (Sv/min) at various locations and different field sizes inside the treatment room of LINAC

For thermal neutrons, maximum and minimum DERs were 3.46 × 10-6 ( 3 m from isocenter in +Y direction, 0 × 0 field size) and 8.36 × 10-8 Sv/min (in maze, 40 × 40 field size), respectively.

According to the results; by increasing the distance from the center of the radiation field in the +Y direction, thermal neutron DER per dose rate of X-ray at isocenter increases at different field sizes (e. g., from 3.31E-06 to 3.46E-06 Sv/min, 0 × 0 field size). This increase was observed until 1 m in X direction (e. g., from 3.31E-06 to 3.35E-06 Sv/min, 0 × 0 field size). It seems that in opposite to +Y direction in which the bunker wall is closer to isocenter, in −X direction, the large distance of bunker wall is the main reason for reduction of thermal neutron fluence from 1 to 3 m. For points A to E inside the treatment room and maze, thermal neutron DER had the largest value in A (3.05E-06 Sv/min, 0 × 0 field size) and the least value in E (8.36E-08 Sv/min, 40 × 40 field size). In all locations, by decreasing field size of the beam, thermal neutron DER increases. This is because of increasing the (γ, n) interaction probability (due to hitting to the heavy metals in the beam path) by decreasing the field size.

Measurement of fast neutron fluence rate in this study was performed by means of two moderators (3” and 9” diameter, spherical shape) specific for BF3 proportional counter. These moderators thermalize the fast neutron and help to count them by BF3 counter.

Using available moderators (i. e., 3” and 9”) just the mean energy of fast neutrons could be estimated.[10] For deriving conversion factors for fast neutrons, the AAPM 19[18] report was used. According to this report fluence-to-dose equivalent and fluence-to-absorbed dose conversion factors as a function of neutron average energy are 4.4 × 1010/Ē0.735 n/ cm2/Sv and 4.5 × 1010/Ē0.5 n/ cm2/Gy, respectively.

Lin et al.,[10] reported mean neutron energy (MeV) at different points in treatment room of various LINACs. Table 2 shows the 9”/3” ratio and the mean neutron energy at different points and different field sizes for this study. Since the 9” moderator is most sensitive to fast neutrons above 200 keV and the 3” is most to fast neutrons below 100–200 keV, the 9”/3” ratio is then a strong function of the relative fast neutron energies above and below ~200 keV.[19]

Table 2

The 9”/3” ratio and the mean neutron energy at various locations and different field sizes inside the treatment room of LINAC

Table 3 shows fast neutron DER in various locations and different field sizes inside the treatment room for two moderators. These values obtained using mentioned fluence-to-dose equivalent conversion factor (i. e., 4.4 × 1010/Ē0.735 n/ cm2/Sv).

Table 3

Fast neutron dose equivalent rate (Sv/min) in various locations and different field sizes inside the treatment room for two moderators

For fast neutrons, maximum DERs using 9” and 3” moderators were 1.6 × 10-5 and 1.74 × 10-5 Sv/min (2 m from isocenter in +Y direction, 0 × 0 field size), respectively. At the farthest point in the maze, where the neutron has its lowest energy, the neutron fluence values for 3” moderator (e. g., 2.75E-07 Sv/min, 0 × 0 field size) are somewhat larger than 9” moderator (e. g., 7.99E-08 Sv/min, 0 × 0 field size) [Table 3].

For both moderators, fast neutron DER increases by increasing the distance from the radiation field center until 2 m in both +Y and −X directions at different field sizes [Table 3]. Moreover, in all points and distances, by increasing field size of the beam, fast neutron DER decreases. This is because of decreasing the (γ, n) interaction probability in the beam path. However, this decrease is not significant in comparison with the variation in thermal neutron DER.

Moreover, for both moderators, in all studied field sizes, fast neutron DER had the largest value in point A and the least value in point E.

It is clear, because of large size of dosimeter; angular resolution for dose distribution has not been considered in this study and seems it is not isotropic.

From the obtained results, it could be generally noted that, by changing the field size of the 15 MV X-ray beam, the percentage of variation in thermal neutron DER is more than fast ones.

In all distances and points in all studied field sizes, the number of thermal neutrons is more than fast ones. For points A–E inside the treatment room and maze, thermal and fast neutron DER had the largest value in A and the least value in E. In all points and distances, by decreasing field size of the beam, thermal and fast neutron DER increases. However for fast neutrons, this increase is not significant in comparison with the variation in thermal neutron DER.

In the studied field sizes, by increasing the distance from isocenter to 2 m, thermal and fast neutron DERs increase. It seems that scattering component of neutron fluence from bunker's walls is the main reason. It is worth to mention that in this study, room scattering component have not been measured.

Maximum fluence rate values obtained in this study are less than ones reported by Fox and McAllister, McGinley et al., and Axton and Bardell[71112] for 18, 25, and 16 MV X-ray beams of LINACs, respectively.

Conclusion

This study showed that in all distances and points in all studied field sizes, the number of thermal neutrons is more than fast ones; and in all points and distances, by decreasing field size of the beam, thermal and fast neutron DER increases.

This research also showed larger variation of thermal neutron DER than fast neutron DER with field size and also depicted that component of wall scattering is an important part of the total neutron fluence inside the bunker.

Since the obtained maximum fluence rate values in this study differs from the others, it could be concluded that because of the differences in the structure of the various treatment rooms, the concrete materials of the walls, and the type of machines; the mechanism of neutron collisions differ from one RT center to another (even with the same machine).