High Energy Medical Linear Accelerators Research Articles

Radiation dose to radiotherapy technologists due to induced activity in high energy medical electron linear accelerators

After delivering X-ray beam of 15 MV energy, measurable radiation level near the treatment head of medical electron linear accelerator (LINAC) and in its surrounding is observed due to induced activity generated in target, flattening filter, monitor chamber, cooling tube, collimating jaws, etc., The radiation generated in the head of LINAC has the potential of exposing the radiotherapy staff/personnel (technologists) if he/she comes in close proximity of the machine head immediately after delivering the planned treatment. A systematic study was carried out to quantify the radiation levels near LINAC head, isocenter and 0.5 m lateral to isocenter for planned beam delivery of 50, 100, 200, 300, 400, 500 and 1000 MU (Monitor Units) for field sizes 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm and 40 cm × 40 cm. The measured maximum radiation level near LINAC head, isocenter and 0.5 m lateral to isocenter after 1 min of completion of treatment was about 10 μSv/h when the accelerator was operated for 10 μSv/h when the accelerator was operated for 800-1000 MU. It was also observed that the radiation level due to induced activity increases with increasing field size and number of monitor units and shows saturation characteristics when the field size approaches to 30 cm × 30 cm. This study indicates that the radiation dose received by the radiotherapy technologists while standing below the treatment head of the LINAC is significant even after 1 min of completion of the dose delivery when the accelerator is operated for higher number of monitor units (e.g., 1000 MU) in high energy photon mode.

The neutron dose equivalent around high energy medical electron linear accelerators

The measurement of neutron dose equivalent was made in four dual energy linear accelerator rooms. Two of the rooms were reconstructed after decommissioning of 60Co units, so the main limitation was the space. The measurements were performed by a nuclear track etched detectors LR-115 associated with the converter (radiator) that consist of 10B and with the active neutron detector Thermo BIOREM FHT 742. The detectors were set at several locations to evaluate the neutron ambient dose equivalent and/or neutron dose rate to which medical personnel could be exposed. Also, the neutron dose dependence on collimator aperture was analyzed. The obtained neutron dose rates outside the accelerator rooms were several times smaller than the neutron dose rates inside the accelerator rooms. Nevertheless, the measured neutron dose equivalent was not negligible from the aspect of the personal dosimetry with almost 2 mSv a year per person in the areas occupied by staff (conservative estimation). In rooms with 15 MV accelerators, the neutron exposure to the personnel was significantly lower than in the rooms having 18 MV accelerators installed. It was even more pronounced in the room reconstructed after the 60Co decommissioning. This study confirms that shielding from the neutron radiation should be considered when building vaults for high energy linear accelerators, especially when the space constraints exist.

Out-of-Field Neutron and Leakage Photon Exposures and the Associated Risk of Second Cancers in High-Energy Photon Radiotherapy: Current Status

Determination and understanding of out-of-field neutron and photon doses in accelerator-based radiotherapy is an important issue since linear accelerators operating at high energies (>10 MV) produce secondary radiations that irradiate parts of the patient's anatomy distal to the target region, potentially resulting in detrimental health effects. This paper provides a compilation of data (technical and clinical) reported in the literature on the measurement and Monte Carlo simulations of peripheral neutron and photon doses produced from high-energy medical linear accelerators and the reported risk and/or incidence of second primary cancer of tissues distal to the target volume. Information in the tables facilitates easier identification of (1) the various methods and measurement techniques used to determine the out-of-field neutron and photon radiations, (2) reported linac-dependent out-of-field doses, and (3) the risk/incidence of second cancers after radiotherapy due to classic and modern treatment methods. Regardless of the measurement technique and type of accelerator, the neutron dose equivalent per unit photon dose ranges from as low as 0.1 mSv/Gy to as high as 20.4 mSv/Gy. This radiation dose potentially contributes to the induction of second primary cancer in normal tissues outside the treated area.

Dose estimation of the neutrons induced by the high energy medical linear accelerator using dual-TLD chips

This paper discussed the neutron doses induced by the high energy medical linear accelerator (Linac). Thermoluminescent dosimeter (TLD) pairs are usually chosen to distinguish the doses of neutrons from photons in mixed radiation field. In this work, TLD-600 and TLD-700 chips were used to investigate the dose contributions of neutrons induced by 15 MV photon beam of the medical Linac used in radiotherapy. A Rando phantom filled with these dual-TLD chips was irradiated in the 15 MV photon beam to assess the doses contribution of the induced neutrons. Besides, dual-TLD chips were also placed in the treatment room to estimate the spatial dose distribution of neutrons. In the results, dose contributions of neutrons in phantom are less than 0.1% compared with the maximum dose of photons. The dose distribution of neutrons in the treatment room was also revealed.

Measurements of the photonuclear neutron yield of 15 MV medical linear accelerator

High energy medical linear accelerators (>10 MV) are increasingly used in radiotherapy. At such high photon energies neutron production via photonuclear reactions in the heavy elements which compose the linac head is inevitable. Neutrons from linacs can contribute to an additional dose to staff, patients and the general public. Our intention is two-fold; to provide shielding against the neutron contamination and to establish the depthdose curve of thermal neutrons within human tissue, with an aim to utilise linacs in boron neuron capture therapy (BNCT). In our studies neutron measurements were undertaken, with a Varian Clinac 2100C/D linear accelerator operating at 15 MV nominal energy, by irradiating 18 cm thick 30×30 cm2 block of tissue equivalent material. Measurements were taken using indium and aluminum activation foil at the centre of the block. Our results show that by leaving the linac jaws open neutron production is increased compared to the case when these are shut, for one minute exposure at 400MU. In this work we present a comparison between our results and existing literature and attempt to explore some sharp contrasts.

The measurement of photoneutron dose in the vicinity of clinical linear accelerators

In Brazil, the replacement of rather old cobalt and cesium teletherapy machines with high-energy (E > 10 MV) medical linear accelerators (linacs) started in the year 2000, as part of an effort by the Ministry of Health to update radiotherapy installations. Since then, the contamination of undesirable neutrons in the therapeutic beam generated by these high-energy photons has become an issue of concern when considering patient and occupational doses. The walls of the treatment room are shielded to attenuate the primary and secondary X-ray fluence, and this shielding is generally considered adequate also to attenuate neutrons. However, these neutrons are scattered through the treatment room maze and might result in a radiological problem at the door entrance, an area of high occupancy by the workers of a radiotherapy facility. This paper presents and discusses the results of ambient dose equivalent measurements of neutron using bubble detectors. The measurements were made at different points inside the treatment rooms, including the isocentre and the maze. Several radiation oncology centres, which are users of Varian Clinac or Siemens machines, have agreed to allow measurements to be taken at their facilities. The measured values were compared with the results obtained through the semi-empirical Kersey method of neutron dose equivalent calculation at maze entrances, with reported values provided by the manufacturers as well as values published in the literature. It was found that the measured values were below the dose limits adopted by the Brazilian Regulatory Agency (CNEN), requiring no additional shielding in any of the points measured.

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.

The effect of field modifier blocks on the fast photoneutron dose equivalent from two high-energy medical linear accelerators

High-energy linear accelerators (linacs) have several advantages, including low skin doses and high dose rates at deep-seated tumours. But, at energies more than 8 MeV, photonuclear reactions produce neutron contamination around the therapeutic beam, which may induce secondary malignancies. In spite of improvements achieved in medical linac designs, many countries still use conventional (non-intensity-modulated radiotherapy) linacs. Hence, in these conventional machines, fitting the beam over the treatment volume may require using blocks. Therefore, the effect of these devices on neutron production of linacs needs to be studied. The aim of this study was to investigate the effect of field shaping blocks on photoneutron dose in the treatment plane for two high-energy medical linacs. Two medical linacs, a Saturn 43 (25 MeV) and an Elekta SL 75/25 (18 MeV), were studied. Polycarbonate (PC) films were used to measure the fluence of photoneutrons produced by these linacs. After electrochemical etching of the PC films, the neutron dose equivalent was calculated at the isocentre and 50 cm away from the isocentre. It was noted that by increasing the distance from the centre of the X-ray beam towards the periphery, the photoneutron dose equivalent decreases rapidly for both the open and blocked fields. Increasing the energy of the photons causes an increase in the amount of photoneutron dose equivalent. At 25 MeV photon energy, the lead blocks cause a meaningful increase in the dose equivalent of photoneutrons. In this research, a 30% increase was seen in neutron dose contribution to central axis dose at the isocentre of a 25 MeV irregular field shaped by lead blocks. It is concluded that lead blocks must be considered as a source of photoneutron production when treating irregular fields with high-energy photons.

Set-up of a passive Bonner sphere system for neutron spectrometry at mixed fields with predominant photon component based on activation detector

A passive Bonner sphere system (BSS), based on thermal neutron activation detectors, was developed to perform neutron spectrometry in pulsed and very intense (n-gamma) fields with predominant photon component, as those produced by high energy (>10 MV) medical linear electron accelerators. In this paper, a description of the new system is presented together with an experimental characterisation of a portable Sodium Iodide (NaI) detector and a fixed high-purity Germanium one, both used to measure the induced gamma-activity of the activated materials, respectively, in situ and in the laboratory. The choice of the activated materials is justified according to pre-established practical considerations and physical criteria. The response functions of the entire passive BSS were calculated using the MCNPX code. A preliminary experimental validation with a bare (252)Cf source is given as well.

Neutron dose calculation at the maze entrance of medical linear accelerator rooms

Currently, teletherapy machines of cobalt and caesium are being replaced by linear accelerators. The maximum photon energy in these machines can vary from 4 to 25 MeV, and one of the great advantages of these equipments is that they do not have a radioactive source incorporated. High-energy (E > 10 MV) medical linear accelerators offer several physical advantages over lower energy ones: the skin dose is lower, the beam is more penetrating, and the scattered dose to tissues outside the target volume is smaller. Nevertheless, the contamination of undesirable neutrons in the therapeutic beam, generated by the high-energy photons, has become an additional problem as long as patient protection and occupational doses are concerned. The treatment room walls are shielded to attenuate the primary and secondary X-ray fluence, and this shielding is generally adequate to attenuate the neutrons. However, these neutrons are scattered through the treatment room maze and may result in a radiological problem at the door entrance, a high occupancy area in a radiotherapy facility. In this article, we used MCNP Monte Carlo simulation to calculate neutron doses in the maze of radiotherapy rooms and we suggest an alternative method to the Kersey semi-empirical model of neutron dose calculation at the entrance of mazes. It was found that this new method fits better measured values found in literature, as well as our Monte Carlo simulated ones.

Neutrons and carcinogenesis: a cautionary tale

The best estimates for radiation induced cancer and leukemia are based on the Japanese survivors of Hiroshima and Nagasaki. With the earlier dosimetry systems of the 1960s, it was possible to drive an RBE (relative biological effectiveness) for neutrons from the Japanese data, because it was thought that there was a significant neutron dose at Hiroshima compared with Nagasaki. The estimated RBE of about 20 was consistent with laboratory estimates for oncogenic transformation in vitro and tumors in animals. The revised dosimetry of the 1980s [DS 86] essentially eliminated the neutron component at Hiroshima, and consequently removed the only neutron RBE estimate based on human data. However, recent neutron activation measurements indicate that there may indeed have been thermal neutrons at Hiroshima, and measurements of the ratio of inter- to intra-chromosomal aberrations in peripheral lymphocytes of survivors also tend to indicate that the biologically effective dose was dominated by neutrons. Another area in which the large biological effectiveness of neutrons assumes importance is the production of photoneutrons in high energy medical linear accelerators (Linacs). An increasing number of accelerators operating in the 18 to 20 MV range are coming into routine clinical use and at this energy, photoneutrons generated largely in the collimators result in a total body dose to the patient. The increased risk of second malignancies must be balanced against the slight improvement in percentage depth doses compared with more conventional machines operating at 6 to 10 MV, below the threshold for photoneutron production.

Photoneutrons from medical linear accelerators—Radiobiological measurements and risk estimates

To assess the oncogenic potential of the photoneutrons produced by high energy medical linear accelerators. An established line of cells of rodent origin (C3H 10T1/2) was used to assess the oncogenic potential of the radiation dose received in the breast of an anthropomorphic "randoman" phanton, while the cervix received a dose of 70 Gy. Experiments were performed at 6 MV, below the threshold for the production of photoneutrons, and at 20 MV where the dose includes about 0.01 Gy of photoneutrons as well as scattered x-rays. A significantly higher transformation incidence was observed for the 20-MV machine, consistent with the measured neutron dose of about 0.01 Gy and a quality factor of 20. An estimate can be made of the additional deaths from second malignancies that might result from the photoneutrons generated by higher energy linear accelerators (Linacs), which must be offset against the possible improvements in survival that might result from the higher tumor doses made possible by the increased percentage depth doses.

Photoneutrons from high energy medical linear accelerators: Measurement of the spectrum and dose using a miniature proportional counter

A new method of measuring photoneutron dose to the patient during treatment with high energy photon or electron beams is presented. This method has the advantage of providing not only the dose, but the microdosimetric spectrum at the same time. A miniature cylindrical gas proportional counter (0.5 mm diameter by 0.5 mm height) has been used to measure scatter radiation from a 20 MV teletherapy photon beam. At atmospheric pressure, filled with propane base tissue equivalent gas, this counter simulates a unit density tissue region of approximately 0.9 microns. We present here single event microdosimetric spectra measured outside the primary beam 1.4 m from the target. This technique allows a single measurement to determine the scattered dose due to gammas and photoneutron contamination, as well as the quality factor of the photoneutrons. Spectral components from scattered photons and the photoneutrons are easily separated, and dose contributions can be estimated. The ratio of photoneutron dose measured by the proportional counter to photon dose at isocenter is 0.75 x 10(-4). Neutron dose was also measured using a bubble neutrometer. The proportional counter and neutrometer agree within experimental errors. This type of instrument is shown to be a viable technique for determination of exposure of patient and also personnel to photoneutrons, providing not only a dose determination, but also a spectrum that can be used to estimate quality factors for equivalent dose. Its main drawback is that it requires a lengthy (several hours) measurement because of low count rate of the neutrons.