Correlation between radioactivity induced inside the treatment room and the undesirable thermal/resonance neutron radiation produced by linac

Application of therapeutic linear accelerators for the production of radioisotopes used in nuclear medicine

This review paper summarizes the possibilities of the use of therapeutic linear electron accelerators for the production of radioisotopes for nuclear medicine. This work is based on our published results and the thematically similar papers by other authors, directly related to five medical radioisotopes as 99Mo/99mTc, 198Au, 186Re, 188Re, 117mSn, produced using therapeutic linacs. Our unpublished data relating to the issues discussed have also been used here. In the experiments, two types of radiation were included in the analysis of the radioisotope production process, i.e. the therapeutic twenty-megavolt (20 MV) X-rays generated by Varian linacs and neutron radiation contaminating the therapeutic beam. Thus, the debated radioisotopes are produced in the photonuclear reactions and in the neutron ones. Linear therapeutic accelerators do not allow the production of radioisotopes with high specific activities, but the massive targets can be used instead. Thus, the amount of the produced radioisotopes may be increased. Apart from linear accelerators, more and more often, the production of radioisotopes is carried out in small medical cyclotrons. More such cyclotrons are developed, built, and sold commercially than for scientific research. The radioisotopes produced with the use of therapeutic linacs or cyclotrons can be successfully applied in various laboratory tests and in research.

ESTIMATION OF THERMAL & EPITHERMAL NEUTRON FLUX AND GAMMA DOSE DISTRIBUTION IN A MEDICAL CYCLOTRON FACILITY FOR RADIATION PROTECTION PURPOSES USING GOLD FOILS AND GATE 9

The aim of this study is to characterise the neutron flux generated directly behind targets used in medical cyclotrons. The characterisation process aims at determining the feasibility of using the generated neutrons for research purposes in neutron activation analysis. The study was performed by activating gold foils placed directly behind the cyclotron targets. The thermal and epithermal neutron flux were found to be 4.5E+05 ± 8.78E+04 neutrons cm-2 s-1 and 2.13E+06 ± 8.59E+04 neutrons cm-2 s-1, respectively. The flux value is the same order of magnitude listed in the manual produced by the cyclotron manufacturer. The results are encouraging and show high potential for using the cyclotron facility as a thermal neutron source for research purposes. However, it is important radiation protection procedures be followed to ensure the safety of researchers due to the high gamma dose rate measured directly behind the target at 2.46 Sv/h using an OSL chip during the beam on time.

Technical Note: Induced radioactivity in stereotactic body radiation therapy with a flattening-filter-free 10 MV beam model

PURPOSE

The induced radioactivity in stereotactic body radiation therapy with a flattening-filter-free 10 MV beam model (10 FFF SBRT) was investigated for the risk to therapists.

METHODS

This study was performed on a Varian TrueBeam linac. The induced radioisotopes were identified by γ spectroscopy. The dose rate from the induced activity was measured for 12 treatment cycles in 4 h continuously. The impacts of the characteristic factors of 10 FFF SBRT on the dose rate were investigated, including monitor units (MU), beam rate, field size, and flattening filter. The dose rate was compared between the SBRT plans and conventional fractionation plans. A mathematical model was used to analyze the results and estimate the annual dose to therapists.

RESULTS

(a) The induced radioisotopes included ²⁴ Na, ²⁸ Al, ³⁸ Cl, ⁵⁶ Mn, ⁶⁶ Cu, ¹⁸⁷ W, and ¹⁹⁶ Au. (b) In 4 h, the total dose contribution ratios were more than 70% for ²⁸ Al, about 20% for ⁵⁶ Mn, and 10% for all other long-lived radioisotopes, combining doses at the isocenter and end of the treatment couch. (c) The dose rate showed a nonlinear growth with increasing MU and beam rate. The variation of the dose rate was complicated with the jaw field and not sensitive to the MLC field. The removal of the flattening filter reduced the dose rate by about 40%. The dose level of SBRT was two to three times that of conventional fractionation. (d) The estimated annual dose to therapists was up to 0.20 mSv/y.

CONCLUSIONS

The induced radioactivity in 10 FFF SBRT was higher compared with that in 10 MV conventional fractionation. More MU and higher beam rate were the primary factors that caused the increase. The therapists should wait longer after beam-off to reduce the occupational dose. In addition, aluminum and manganese should be less used in the treatment room.

Application of the phase-space distribution approach of Monte Carlo for radiation contamination dose estimation from the (n,γ), (γ,n) nuclear reactions and linac leakage photons in the megavoltage radiotherapy facility

Aim

The aim of this study was to characterize the radiation contamination inside and outside the megavoltage radiotherapy room.

Background

Radiation contamination components in the 18 MV linac room are the secondary neutron, prompt gamma ray, electron and linac leakage radiation.

Materials and Methods

An 18 MV linac modeled in a typical bunker employing the MCNPX code of Monte Carlo. For fast calculation, phase-space distribution (PSD) file modeling was applied and the calculations were conducted for the radiation contamination components dose and spectra at 6 locations inside and outside the bunker.

Results

The results showed that the difference of measured and calculated percent depth-dose (PDD) and photo beam-profile (PBP) datasets were lower than acceptable values. At isocenter, the obtained photon dose and neutron fluence were 2.4 × 10^-14 Gy/initial e° and 2.22 × 10^-8 n°/cm², respectively. Then, neutron apparent source strength (Q_N) value was found as 1.34 × 10¹² n°/Gy X at isocenter and the model verified to photon and neutron calculations. A surface at 2 cm below the flattening filter was modeled as phase-space (PS) file for PDD and PBP calculations. Then by use of a spherical cell in the center of the linac target as a PS surface, contaminant radiations dose, fluence and spectra were estimated at 6 locations in a considerably short time, using the registered history of all particles and photons in the 13GB PSD file as primary source in the second step.

Conclusion

Designing the PSD file in MC modeling helps user to solve the problems with complex geometry and physics precisely in a shorter run-time.

MC safe bunker designing for an 18 MV linac with nanoparticles included primary barriers and effect of the nanoparticles on the shielding aspects

Aim

The aim of this study was to design a safe bunker for an 18 MV linac in to configuration; primary barriers made from nanoparticle-containing concrete and pure concrete.

Background

Application of some nanoparticles in the shielding materials has been studied and it was shown that the presence of some nanoparticles improved radiation shielding properties.

Materials and methods

Some percentage of different nanoparticles were modeled by the MCNP5 code of MC in the megavoltage radiotherapy treatment room's primary barriers. Other parts of the designed room, such as secondary barriers and maze door, were modeled as ordinary pure concrete. A safe bunker was designed according to the MC derived spectra at primary and secondary barriers location using a modeled and benchmarked 18 MV linac in free air. Then, the thickness of the required shielding materials for the door and also concrete for the walls and primary barriers were calculated separately.

Results

According to the results, required concrete thickness in primary and secondary barriers was reduced by around 0.8% compared to pure concrete application. Additionally, required lead and BPE decreased by 25% and 15%, respectively, due to primary barriers nanoparticles.

Conclusions

It was concluded that application of some nanoparticles in the shielding materials structures in megavoltage radiotherapy can make the shielding effective.

Neutron and high energy photon fluence estimation in CLINAC using gold activation foils

Aim

The thermal neutron, epithermal neutron and high-energy photon fluence were measured in this work around the Varian 21EX 23 MV CLINAC, which is operated in Albairouni hospital in Damascus, Syria.

Background

Photoneutron measurements around medical CLINAC aim to protect both patients and staff from unwanted radiation.

Materials and methods

Neutron and photon activation techniques were applied using gold foils.

Results

It was found that high-energy photons fluence has practically a constant value in the field size. The thermal and epithermal neutron fluence along ox and oy axes has the same order of magnitude.

Conclusion

Gold foils have been used successfully to measure neutron flux and high-energy photons simultaneously using activation techniques.

A REVIEW ON THE RADIATION THERAPY TECHNOLOGIST RECEIVED DOSE FROM INDUCED ACTIVATION IN HIGH-ENERGY MEDICAL LINEAR ACCELERATORS

Despite all advantages for using high-energy photons for radiotherapy, high-energy photon beams (≥10 MV) induce photonuclear and neutron capture interactions, which result in producing radionuclide byproducts inside the Linac head and bunker, exposing radiation therapy technologists (RTTs) and patients to excessive dose. By the use of higher photon energy, greater number of monitor unit, greater field size and adding treatment accessories, induced dose rate become greater in the isocenter mainly due to activation of high-Z materials inside the Linac head. Activated radionuclides disintegrate with γ, β+ and β- rays with half-lives between 2 min up to more than 5 years. Several researches estimated additional exposure to an RTT depend on treatment strategies, beam energy, and delay time before entrance to the treatment room between 0.1 and 4.9 mSv/y and proposed at least 2 min delay before entrance to the treatment room after treatments with high-energy photon beams.

Fast, epithermal and thermal photoneutron dosimetry in air and in tissue equivalent phantom for a high-energy X-ray medical accelerator

Photoneutron (PN) dosimetry in fast, epithermal and thermal energy ranges originated from the beam and albedo neutrons in high-energy X-ray medical accelerators is highly important from scientific, technical, radiation protection and medical physics points of view. Detailed dose equivalents in the fast, epithermal and thermal PN energy ranges in air up to 2m as well as at 35 positions from the central axis of 12 cross sections of the phantom at different depths were determined in 18MV X-ray beams of a Siemens ONCOR accelerator. A novel dosimetry method based on polycarbonate track dosimeters (PCTD)/¹⁰B (with/without cadmium cover) was used to determine and separate different PN dose equivalents in air and in a multilayer polyethylene phantom. Dose equivalent distributions of PNs, as originated from the main beam and/or albedo PNs, on cross-plane, in-plane and diagonal axes in 10cm×10cm fields are reported. PN dose equivalent distributions on the 3 axes have their maxima at the isocenter. Epithermal and thermal PN depth dose equivalent distributions in the phantom for different positions studied peak at ∼3cm depth. The neutron dosimeters used for the first time in such studies are highly effective for separating dose equivalents of PNs in the studied energy ranges (beam and/or albedo). The PN dose equivalent data matrix made available in this paper is highly essential for detailed patient dosimetry in general and for estimating secondary cancer risks in particular.

Significant change in the construction of a door to a room with slowed down neutron field by means of commonly used inexpensive protective materials

The detailed analysis of nuclear reactions occurring in materials of the door is presented for the typical construction of an entrance door to a room with a slowed down neutron field. The changes in the construction of the door were determined to reduce effectively the level of neutron and gamma radiation in the vicinity of the door in a room adjoining the neutron field room. Optimisation of the door construction was performed with the use of Monte Carlo calculations (GEANT4). The construction proposed in this paper bases on the commonly used inexpensive protective materials such as borax (13.4 cm), lead (4 cm) and stainless steel (0.1 and 0.5 cm on the side of the neutron field room and of the adjoining room, respectively). The improved construction of the door, worked out in the presented studies, can be an effective protection against neutrons with energies up to 1 MeV.

Measurement of activity distribution using photostimulable phosphor imaging plates in decommissioned 10 MV medical linear accelerator

Photonuclear reactions generate neutrons in the head of the linear accelerator. Therefore, some parts of the linear accelerator can become activated. Such activated materials must be handled as radioactive waste. The authors attempted to investigate the distribution of induced radioactivity using photostimulable phosphor imaging plates. Autoradiographs were produced from some parts of the linear accelerator (the target, upper jaw, multileaf collimator and shielding). The levels of induced radioactivity were confirmed to be non-uniform within each part from the autoradiographs. The method was a simple and highly sensitive approach to evaluating the relative degree of activation of the linear accelerators, so that appropriate materials management procedures can be carried out.

Thermal and resonance neutrons generated by various electron and X-ray therapeutic beams from medical linacs installed in polish oncological centers

BACKGROUND

High-energy photon and electron therapeutic beams generated in medical linear accelerators can cause the electronuclear and photonuclear reactions in which neutrons with a broad energy spectrum are produced. A low-energy component of this neutron radiation induces simple capture reactions from which various radioisotopes originate and in which the radioactivity of a linac head and various objects in the treatment room appear.

AIM

The aim of this paper is to present the results of the thermal/resonance neutron fluence measurements during therapeutic beam emission and exemplary spectra of gamma radiation emitted by medical linac components activated in neutron reactions for four X-ray beams and for four electron beams generated by various manufacturers' accelerators installed in typical concrete bunkers in Polish oncological centers.

MATERIALS AND METHODS

The measurements of neutron fluence were performed with the use of the induced activity method, whereas the spectra of gamma radiation from decays of the resulting radioisotopes were measured by means of a portable high-purity germanium detector set for field spectroscopy.

RESULTS

The fluence of thermal neutrons as well as resonance neutrons connected with the emission of a 20 MV X-ray beam is ∼10(6) neutrons/cm(2) per 1 Gy of a dose in water at a reference depth. It is about one order of magnitude greater than that for the 15 MV X-ray beams and about two orders of magnitude greater than for the 18-22 MeV electron beams regardless of the type of an accelerator.

CONCLUSION

The thermal as well as resonance neutron fluence depends strongly on the type and the nominal potential of a therapeutic beam. It is greater for X-ray beams than for electrons. The accelerator accessories and other large objects should not be stored in a treatment room during high-energy therapeutic beam emission to avoid their activation caused by thermal and resonance neutrons. Half-lives of the radioisotopes originating from the simple capture reaction (n,γ) (from minutes to hours) are long enough to accumulate radioactivity of components of the accelerator head. The radiation emitted by induced radioisotopes causes the additional doses to staff operating the accelerators.

Induced radioactive nuclides of 10-MeV radiotherapy accelerators detected by using a portable HP-Ge survey meter

The radioactivation of linear accelerator components for radiation therapy is interest for radiation protection in general, and particularly, when decommissioning these structures. The energy spectra of gamma rays emitted from the heads of two accelerator models, EXL-15SP and Clinac iX, after 10-MeV X-ray irradiation, were measured using a high-purity germanium semiconductor survey meter. After spectrum analyses, activities of (24)Na, (28)Al, (54)Mn, (56)Mn, (57)Ni, (58)Co, (60)Co, (64)Cu, (65)Zn, (122)Sb, (124)Sb, (181)W, (187)W, (196)Au, and (198)Au were detected. One centimetre deep dose-equivalent rate of the heads of the linear accelerator was measured using the survey meter. The dose rate decreased to ∼10 % of its initial rate after 1 week. Long-term activations were few, the radioactivity level was low, and a cooling time of several days was effective for reducing dose rate to an acceptable level for decommissioning.

The impact of automatic wedge filter on photoneutron and photon spectra of an 18-MV photon beam

The effect of an automatic wedge filter on photon and photoneutron spectra of a medical linac was evaluated using the Monte Carlo method. The head of an Elekta SL75/25 was simulated using the MCNPX Monte Carlo code. The photon and photoneutron spectra for open and wedged beams were calculated at the isocentre with a source to axis distance of 100 cm. For a wedged beam, the neutron fluence was from 3.84 to 7.2 times higher for field sizes from 10 x 10 to 30 x 30 cm(2). The neutron fluence is decreased with field size for open beams and is increased with field size for wedged beams The photon beam spectra became harder and the mean energy was 6 % higher for a wedged beam, which led to a 4 % increase in relative depth dose and a better skin sparing effect. The results here recommend that the higher photoneutron fluence of the wedged beam should be taken into account in patient dosimetry and shielding calculations.