Activation processes in a medical linear accelerator and spatial distribution of activation products

Assessment of radio-activation using spectroscopy in medical linear accelerators

In radiotherapy, when photon energy exceeding 8 MV is utilized, photoneutrons can activate the components within the gantry of the linear accelerator (linac). At the end of the linac's lifecycle, radiation workers are tasked with its dismantling and disposal, potentially exposing them to unintentional radiation. This study aims to identify and measure the radioisotopes generated by this activation through spectroscopy, and to evaluate the effective dose rate. We selected nine medical linacs, considering various factors such as manufacturer (Siemens, Varian, and Elekta), model, energy, period of operation, and workload. We identified the radionuclides in the linac head by employing an in situ high-purity germanium (HPGe) detector. Spectroscopy and dose-rate measurements were conducted post-shutdown. We also measured the dose rates at the beam-exit window following irradiation with 10 MV and 15 MV photon beams. As a result of the spectroscopy, we identified approximately 20 nuclides including those with half-lives of 100 days or longer, such as 54Mn, 60Co, 65Zn, 122Sb, and 198Au. The dose rate measurements after 10 MV irradiation decreased to the background level in 10 min. By contrast, on 15 MV irradiation, the dose rate was 628 nSv/h after 10 min and decreased to 268 nSv/h after 1.5 hours. It was confirmed that the difference in the level of radiation and the type of nuclide depends on the period of use, energy, and workload. However, the type of nuclide does not differ significantly between the linacs. It is necessary to propose appropriate guidelines for the safety of workers, and disposal/move-install should be planned while taking into consideration the equipment's energy usage rate.

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.

Technical Note: Time-gating to medical linear accelerator pulses: Stray radiation detector

PURPOSE

CCD cameras are employed to image scintillation and Cherenkov radiation in external beam radiotherapy. This is achieved by gating the camera to the linear accelerator (Linac) output. A direct output signal line from the linac is not always accessible and even in cases where such a signal is accessible, a physical wire connected to the output port can potentially alter Linac performance through electrical feedback. A scintillating detector for stray radiation inside the Linac room was developed to remotely time-gate to linac pulses for camera-based dosimetry.

METHODS

A scintillator coupled silicon photomultiplier detector was optimized and systematically tested for location sensitivity and for use with both x rays and electron beams, at different energies and field sizes. Cherenkov radiation emitted due to static photon beams was captured using the remote trigger and compared to the images captured using a wired trigger. The issue of false-positive event detection, due to additional neutron activated products with high energy beams, was addressed.

RESULTS

The designed circuit provided voltage >2.5 V even for distances up to 3 m from the isocenter with a 6 MV, 5 × 5 cm beam, using a Ø3 × 20 mm³ Bi₄ Ge₃ O₁₂ (BGO) crystal. With a larger scintillator size, the detector could be placed even beyond 3 m distance. False-positive triggering was reduced by a coincidence detection scheme. Negligible fluctuations were observed in time-gated imaging of Cherenkov intensity emitted from a water phantom, when comparing directly connected vs this remote triggering approach.

CONCLUSION

The remote detector provides untethered synchronization to linac pulses. It is especially useful for remote Cherenkov imaging or remote scintillator dosimetry imaging during radiotherapeutic procedures when a direct line signal is not accessible.

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.

Monte Carlo simulation of neutron dose equivalent by photoneutron production inside the primary barriers of a radiotherapy vault

PURPOSE

To evaluate the neutron dose equivalent produced by photoneutrons inside the primary barriers of a radiotherapy vault.

METHODS

Monte Carlo simulations were performed for investigating the production of photoneutrons as well as neutron shielding requirements. Two photon beams of 15 and 18 MV struck sheets of steel and lead, and the neutron doses were calculated at the isocenter (P_iso) and at a distance of 50 cm from the inside wall (P_wall) while delivering 1 Gy to the patient. The proper thicknesses of borated polyethylene (BPE) and concrete were simulated to reduce neutron contamination.

RESULTS

When the primary barrier consisted of a concrete alone, the neutron doses at P_iso were 0.5 μSv/Gy and 12.8 μSv/Gy for 15- and 18-MV, respectively. At P_wall, the neutron doses were 15.8 μSv/Gy and 318.4 μSv/Gy for 15- and 18-MV, respectively. When 15 MV photons interacted with metal sheets, the neutron doses were 0.4-22.2 μSv/Gy at P_iso and 15.8-812.5 μSv/Gy at P_wall, depending on the thickness and material of the metal sheets and neutron shielding. In the case of 18 MV photons with the same configuration, the neutron doses were 0.9-59.5 μSv/Gy and 73.9-5006.1 μSv/Gy for P_iso and P_wall, respectively. The neutron dose delivered to the patient was reduced to the level of the dose delivered with a concrete barrier by including a 10-cm-thick BPE for each beam.

CONCLUSIONS

When the primary barrier shielding is designed with a metal sheet inside for high energy, proper neutron shielding should be constructed to avoid undesirable photoneutron dose.

On the neutron radiation field and air activation around a medical electron linac

In high-energy photon therapy, several radiation protection issues result from photonuclear reactions. In this study, the photoneutron radiation field around a Varian Clinac linear accelerator in 18 MV-X mode within two different radiotherapy bunkers was investigated by means of Monte Carlo simulations using the FLUKA code as well as ambient dose-equivalent measurements. Furthermore, the activation of the air inside the treatment room due to photonuclear reactions (13N and 15O) and the capture of photoneutrons moderated down within the bunker (41Ar) was studied by FLUKA simulations. From the simulation results, the annual effective dose to medical workers due to photoneutrons and activated air was estimated. The emission of radioactivity due to ventilation of the treatment room was found to be negligible.

Direct air activation measurements at a 15-MV medical linear accelerator

Direct radiometric determination of (14)N (γ, n) (13)N air activation was achieved at a 15-MV medical linear accelerator operating in a high-energy photon mode. (13)N was identified by irradiating a gas-tight Marinelli beaker filled with nitrogen gas and later observing the 10-min half-life of the 511-keV positron-electron annihilation line using high-resolution gamma spectroscopy. Quantitative evaluation of the spectral signal yielded a (13)N production rate of 836.8 ± 32 Bq Gy(-1) in air per 40 × 40 cm(2) field cross section at 100 cm source-surface distance.

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.

Health physics aspects in treatment rooms after 18-MV X-ray irradiations

Delayed activation products contribute to the exposure of the staff operating high-energy accelerators. Induced activity was studied in a treatment room following 18-MV X-ray irradiations using a hand-held system that allows both dose rate measurements and spectroscopic analysis. The major activation products and the corresponding nuclear reactions were identified. At the majority of the studied locations, β(+) emitters were the main short-term dose contributors. The time variation of the absorbed dose rate in a treatment room during the first 20-min post-irradiation was represented by the sum of two exponential components with half-lives of 1-2 min and either 4 or 10 min, depending on the location in the room. Components with a half-life of hours or days contribute <1 % to the initial dose rate. The activation of some accessories, such as iron filters and portal imagers, deserve special attention. The collection of such data with the proposed method allows the development of optimised working protocols at each treatment room.

Contribution of activation products to occupational exposure following treatment using high-energy photons in radiotherapy

When high-energy photon beams are used for irradiation in radiotherapy, neutrons that are the result of photonuclear reactions create activation products that affect the occupational dose of radiotherapy staff. For the assessment of activation products in situ gamma spectroscopy was performed parallel to dose-rate measurements following irradiation, by using a high-energy photon beam from a linear accelerator Elekta Precise (Elekta, Stockholm, Sweden) used in radiotherapy. The major identified activation products were the following radioisotopes: (28)Al, (24)Na, (56)Mn, (54)Mn, (187)W, (64)Cu and (62)Cu. Based on the typical workload and dose-rate measurement, the assessed additional annual occupational dose ranged from 1.7 to 0.25 mSv. As the measured dose rate arising from the activation products rapidly decreases as a function of time, the assessed additional dose is negligible after 10 min following irradiation. To keep the occupational dose as low as reasonably achievable, it is recommended to delay entrance to the therapy room at least 2-4 min, when high-energy photons are used. This would reduce the effective dose by 30%.

Comparison of activation products and induced dose rates in different high-energy medical linear accelerators

Sequences of in situ gamma spectra, accompanied by continuous dose rate measurements, have been obtained at the isocenters of four different brands of high-energy medical linear accelerators shortly after beam-off in order to study the effects of radioactivation. Spectral analysis revealed up to 20 different radionuclides per machine, with a total of 21 found isotopes having half-lives between 2.3 min and 5.3 y. Important isotopes as judged from activity, dose rate, and half-life were Al, Mn, Mn, Ni, Co, Cu, Cu, Sb, W, and Au. Dose rates at isocenter calculated from the results of spectrum analysis ranged between 0.78 and 3.16 microSv h after beam-off, decaying to values between 0.18 and 0.54 microSv h within 30 min. Measured dose rates were systematically higher by up to a factor of 2, which is attributed mainly to the effect of beta radiation. No systematic dependence on machine properties or manufacturer could be identified. Assuming realistic working scenarios, absorbed dose values for the radiotherapy technologist staff range between 0.62 and 2.53 mSv y.

Cephalometric assessment of the axial inclination of upper and lower incisors in relation to the third-order angle

BACKGROUND AND AIM

Estimating incisor inclination cephalometrically by reference lines NA and NB puts the orthodontist in the difficult position of relating these axial inclination data to the bracket's third-order prescription which refers to a perpendicular to the occlusal plane. Purpose of the present study was to evaluate the relationship between the cephalometrically-assessed incisor inclination (using the lines NA and NB for reference) and the third-order angle (syn.: torque angle, TA) according to Andrews' description, and moreover to investigate the correlation between incisor inclination data and skeletal-sagittal and skeletal-vertical findings.

MATERIALS AND METHODS

The lateral cephalograms and corresponding dental casts of 67 subjects between 10 and 25 years of age (regardless of skeletal and dental relationships) were considered in the study. All subjects were Caucasian, and none had undergone orthodontic therapy. Upper (U1) and lower (L1) incisor angulations were cephalometrically assessed in reference to the NA and NB lines and compared to third-order angles obtained from dental cast measurements with an incisor inclination-recording appliance. Incisor inclination data from the two measurements were correlated to craniofacial sagittal (angles SNA, SNB, ANB) and vertical (angles NSL-NL, NSL-ML, ML-NL) findings from the radiographs.

RESULTS

The third-order angles in the upper arch measured on the dental casts were a mean of 16.2 degrees (SD = 5.3 degrees) smaller than the axial inclination according to the NA line; the lower incisor third-order data were less than those of the axial inclination according to the NB line by a mean of 27.8 degrees (SD = 4.75 degrees). In this sample, there was a range of 42.7 degrees for the U1TA variable (mean = 5.6 degrees, SD 9.73 degrees) and 47 degrees for U1NA/ degrees variable (mean = 21.71 degrees, SD = 8.67 degrees). The L1TA variable showed a range of 29 degrees (mean = -2.95 degrees, SD = 7.17 degrees), the radiographic L1NB/ degrees range was 23 degrees (mean = 24.91 degrees, SD = 5.8 degrees). We observed a highly significant correlation (r(NA) = 0.84***, r(NB) = 0.76***) between the Andrews' angle and the inclination estimated in reference to the NA and NB lines. No significant correlation between incisor inclination and craniofacial measurements was detected.

CONCLUSIONS

Dental cast measurements seem to be more precise and more valid than lateral radiographs. The method we describe enables clinicians to get a good idea precisely and quickly of how much torque potential remains in the brackets and archwires during treatment. The inclination of the incisors can also be calculated using the regression equations provided, making additional lateral cephalograms unnecessary.