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PubMed1994

Influence of cone design on the electron beam characteristics on clinical accelerators.

Electron beam characteristics were investigated for different applicator cone designs. Low-energy scattered electrons from the cone housing can contribute to the skin dose. The depth dose was measured as a function of electron energies and field size for different cones. The measurements were compared with calculated 3-D dose distributions obtained from Monte-Carlo-derived energy deposition kernels [Phys. Med. Biol. 37, 391 (1992)]. This comparison shows that beam-defining cones can be fabricated to eliminate low-energy scattered electrons from the cone housing. Eliminating low-energy electrons from the cones provides depth dose (DD) characteristics for clinical electron beams that are comparable to those (1) from nearly monoenergetic electrons (as determined from the practical range of the electrons), and (2) for which the DD is nearly independent of source-to-skin distance. Measurements of DD for small circular fields were also performed. For very small field sizes, corresponding to the width of the lateral spread of a pencil beam, the theoretical and measured DD were found to depend on field size. Three-dimensional dose calculations of small and irregularly shaped electron fields are necessary to appreciate these effects.

Biophysical Phenomena↗

PubMed1996

Characteristics of a scintillator-based daily quality assurance device for radiation oncology beams.

A plastic scintillator based device marketed as a "DCT 444" is investigated for daily quality assurance (QA) of radiation beams. Variations in DCT responses were studied for three identical devices with beam, energy, distance, dose, dose rate, and orientations. The effects of gantry, collimator, and patient support assembly (PSA) rotations were investigated at photon energies of 6, 10, and 18 MV and electron energies in the range of 5-20 MeV from three different linear accelerators. The short and long term linearity, reproducibility, and radiation damage were also investigated in kilovoltage and megavoltage beams. Results indicate that the DCT response is linear with dose and dose rate, but shows directional dependence for all beams. The response is dependent on its orientation and the angle of rotations of collimator, gantry, and PSA of the linear accelerator. When the collimator is rotated through 0-36 degrees, the DCT response varied within +/- 4.5% and +/- 7.2% for the 6 MV and 18 MV beams, respectively. With PSA rotation, deviations up to +/- 11% were noted. The DCT response is symmetric across 0 degrees and peaks at +/- 60% PSA angle for all accelerators and beam energies. The effect of backscattering medium on DCT response is noted for all beams but significant for kilovoltage beams. A deviation as high as 30% was observed when DCT was placed on a thick polystyrene phantom. The long term stability studied over 25 months showed varied signal losses possibly due to radiation damage. However, there is no noticeable signal loss over a short period of time (2-4 weeks). The lightweight DCT device is linear with dose and dose rate, stable, and easy to operate. It stores ten readings that can be downloaded to a personal computer. This study indicates that DCT-444 is a suitable QA tool for central axis dose measurement. However, proper precaution is needed as this device exhibits directional and positional dependence. For a meaningful QA result of a radiation unit, the daily QA setup geometry should be identical to that used for calibration of the DCT device.

Biophysical Phenomena↗

PubMed1999

Evaluation of neutron dose in the maze of medical electron accelerators.

MCNP code was used to simulate neutron and prompt gamma ray transport for a range of maze geometrical parameters, wall composition, and wall surface lining. Verification measurements were performed at two medical electron accelerator facilities. A very good agreement was observed between the results of the measurements and the MCNP simulation. MCNP code results were compared with the results of analytical equations used for the calculation of maze effectiveness, derived by Kersey and McCall. A good agreement exists between the simulation results and the results of the analytical methods for maze lengths longer than 8.5 m. However, the results of the present study showed that for shorter maze lengths, Kersey's method tended to overestimate neutron dose at the door entrance, whereas McCall's method with the neutron room scattered correction applied, showed an underestimation of neutron dose. Furthermore, according to MCNP simulation results, the use of barytes concrete instead of standard concrete as room shielding material, reduced neutron dose at the door entrance by about 20%. Finally, it was shown that lining with layers of wood and borated polyethylene significantly reduced the neutron dose at the door entrance by 45% and 65%, respectively.

Gamma Rays↗

PubMed2005

Neuroscience and accelerator mass spectrometry.

Accelerator mass spectrometry (AMS) is a mass spectrometric method for quantifying rare isotopes. It has had a great impact in geochronology and archaeology and is now being applied in biomedicine. AMS measures radioisotopes such as 3H, 14C, 26Al, 36Cl and 41Ca, with zepto- or attomole sensitivity and high precision and throughput, allowing safe human pharmacokinetic studies involving microgram doses, agents having low bioavailability or toxicology studies where administered doses must be kept low (<1 microg kg(-1)). It is used to study long-term pharmacokinetics, to identify biomolecular interactions, to determine chronic and low-dose effects or molecular targets of neurotoxic substances, to quantify transport across the blood-brain barrier and to resolve molecular turnover rates in the human brain on the time-scale of decades. We review here how AMS is applied in neurotoxicology and neuroscience.

Age Factors↗

PubMed1997

Comparison of parametrization methods of the collimator scatter correction factor for open rectangular fields of 6-25 MV photon beams.

PURPOSE: To facilitate the use of the collimator scatter correction factor, Sc, parametrization methods that relate Sc to the field size by fitting were investigated. MATERIALS AND METHODS: Sc was measured with a mini-phantom for five types of dual photon energy accelerators with energies varying between 6 and 25 MV. Using these Sc-data six methods of parametrizing Sc for square fields were compared, including a third-order polynomial of the natural logarithm of the field size normalized to the field size of 10 cm2. Also five methods of determining Sc for rectangular fields were considered, including one which determines the equivalent field size by extending Sterling's method. RESULTS: The deviations between measured and calculated Sc-values were determined for all photon beams and methods investigated in this study. The resulting deviations of the most accurate method varied between 0.07 and 0.42% for square fields and between 0.26 and 0.79% for rectangular fields. A recommendation is given as to how to limit the number of fields for which Sc should be measured in order to be able to accurately predict it for an arbitrary field size.

Mathematical Computing↗

PubMed2000

Design and dosimetry characteristics of a soft-docking system for intraoperative radiation therapy.

PURPOSE: The design concept and the dosimetric characteristics of an applicator system for intraoperative radiation therapy (IORT) with special emphasis on alignment methods, the effect of a plastic scatterer in the beam, radiation leakage, and misalignment dosimetry, are presented in this paper. MATERIALS AND METHODS: A soft-docking system for a linear accelerator, which enables collimation of electron beams (4-22 MeV) for IORT has been developed. The system includes twenty-one circular polymethylmethacrylate (PMMA) treatment cones of different lengths, diameters and end angles. All in-water measurements are made using p-type silicon diode detectors. RESULTS: The effect of introducing a PMMA scatterer in the therapeutic beam includes increased surface dose values (above 83% for all nominal electron energies and for all cones) and improved dose homogeneity within the therapeutic range. Electrons scattered from the inside wall of the cone result in dose profile horns at depth of dose maximum always lower than 109%. The radiation leakage outside the cone is less than 13%. Large changes in the dose profiles occur if the intraoperative cone is misaligned more than 0.5. CONCLUSION: The alignment procedure of the soft-docking system is easy to handle and the applicator design provides adequate collimation of electron beams for IORT.

Combined Modality Therapy↗

PubMedDate not supplied

Energy distributions from a racetrack microtron measured with a magnetic spectrometer.

Energy spectra of accelerated electron beams from a racetrack microtron were measured using a magnetic spectrometer. The spectrometer utilized a 90 degrees dipole magnet. A ray-tracing program was developed to determine the slit positions of the spectrometer based on a detailed map of the magnetic field measured at field strengths corresponding to about 20 and 50 MeV. The total a priori uncertainty (previously often called systematic or class B uncertainty) of the measured most probable energy Ep is 0.22% (one approximate standard deviation) and the a posteriori uncertainty (previously often called random or class A uncertainty) is 0.04% (1 sigma). The estimated energy resolution (delta E/E) of the spectrometer is 4 x 10(-4). Spectral energy distributions of the electron beam were measured at a Ep = 21.1 and 51.6 MeV, and the obtained full width at half-maximum of the energy distributions were 53 and 34 keV, respectively. All the measurements were performed in vacuum to minimize the influence of electron energy loss and scatter.

Electrons↗

PubMed1997

[Analysis and comparison of various quality protocols for radiotherapy linear accelerators].

The main parameters determining the quality of an electron beam produced by a linear accelerator for medical use were considered in this study, particularly: flatness, symmetry and uniformity. We analyzed and compared several protocols issued by national and international associations (such as the AAPM, IPSM, ICRU, NACP), the software protocol developed for the measurement system we used (Multidata) and the measurement instructions recommended by the accelerator manufacturer (Siemens). The above associations issue quality protocols to ensure system performances suitable for medical use, to increase patient safety and to improve the treatment outcome. Radiation therapy safety and improvement depend on correct dose measurements and dose distribution in the treated volume. Once the dose value per monitor unit ratio (Gy/M.U.) is determined, controls are necessary to be sure that the value does not change in time and that the dose distribution has the same effect in the whole treated volume. Our goal is to point out the differences and the affinities in the definition of the parameters, which change slightly in the different protocols, and to study the origin of the differences found when the experimental results were compared. Another important issue is represented by the frequency of quality controls, which are definitely different from the fast checks which are often performed. In conclusion, some suggestions are provided for the choice of the quality protocol to follow.

Clinical Protocols↗

PubMed1996

Computer-aided design optimization with the use of a fast dose model for linear-accelerator-based stereotactic radiosurgery.

In order to efficiently plan non-spherical radiosurgical targets we have used computer-aided design optimization techniques with a fast dose model. A study of the spatial dose distribution for single or multiple non-coplanar arcs was carried out using a 18 cm diameter spherical head model. The dose distribution generated from the 3D dose computation algorithm can be represented by a simple analytic form. Two analytic dose models were developed to represent the dose for preset multiple non-coplanar arcs or a single arc: spherical and cylindrical. The spherical and cylindrical dose models compute dose quickly for each isocentre and single arc. Our approach then utilizes a computer-aided design optimization (CAD) with the use of two fast approximate dose models to determine the positions of isocentres and arcs. The implementation of CAD with fast dose models was demonstrated. While the fast dose models are only approximations of the true dose distribution, it is shown that this approximate model is sufficient to optimize isocentric position, collimator size and arc positions with CAD.

Computer-Aided Design↗

PubMed1997

[Conformational radiotherapy with multi-leaf collimators: one year experience at the Leon-Berard Centre].

Taking advantage of the renewal of a linear accelerator, the Radiation Therapy Department of the Centre Léon Bérard implemented, in collaboration with Philips Systèmes Médicaux, a conformal therapy set-up procedure using CT-scan for 3D treatment planning and a multileaf collimator that allows achievement of numerous irregular-shaped beams via the multileaf preparation system. The various elements of this equipment make possible well defined and structured procedures for treatment planning with different steps and essential tools used by this technique. We describe the means used and indicate future improvements that will lead to automation in order to provide good quality assurance, better security and substantial time saving. During the first year, 115 patients were treated with this new technique. They presented with central nervous system tumors (32 patients), lung cancer (29 patients), prostate cancer (20 patients), paranasal sinus tumors (14 patients) and tumors located in other sites (13 patients with soft sarcoma, hepato-bilary tumor, etc).

Dose-Response Relationship, Radiation↗

PubMed2006

An experimental investigation into the radiation field offset of a dynamic multileaf collimator.

In this study we investigate the characteristics of a rounded leaf end multileaf collimator (MLC) that is used for delivering intensity-modulated radiotherapy (IMRT) with a Varian linear accelerator. The rounded leaf end MLC design results in an offset between the radiation field edge (the physical leaf position) and the light field (the geometric leaf position). We call this the radiation field offset (RFO). The leaf position is calibrated to the leaf tip at the mid-leaf plane. There is an additional offset between the geometric leaf position and the projected leaf tip position that varies as a function of distance from the collimator central axis due to the MLC geometry. We call this the leaf position offset (LPO). There is a lack of consistency in the interpretation and implementation of the RFO and the LPO in the literature. We investigated the RFO and the LPO on Varian's 600 C/D and 21 EX linear accelerators. We used a combination of film and ion chamber measurements of static, segmental MLC (SMLC) and dynamic MLC (DMLC) fields to quantify the leaf offsets across the range of leaf positions. We were able to improve the dosimetry at large off-axis positions with minor adjustments to the vendor's LPO file. The RFO was determined to within 0.1 mm accuracy at the collimator central axis. The measured RFO value depends on whether the method is based on the radiation field edge position or on an integral dose measurement. The integral dose method results in an RFO that is approximately 0.2 mm greater than the radiation field edge method. The difference is due to the MLC penumbra shape. We propose a methodology for measuring and implementing MLC leaf offsets that is suitable for both SMLC and DMLC IMRT. In addition, we propose some definitions that more clearly describe the MLC leaf position for accurate IMRT dosimetry.

Algorithms↗

PubMed1999

A new experimental system for irradiating tumors in mice using a linear accelerator under specific pathogen-free conditions.

We developed a reliable system for the irradiation of xenografted tumors in mice which allows for accurate local irradiation under specific pathogen-free conditions. The system presented here consists of acrylic supports for mice and an acrylic box connected to a pump through 0.22 microns pore-sized filters. Mice with xenotransplanted tumors growing on their right hind legs were set on the supports and put into the box in a laminar flow hood. The tumors of 7 mice were irradiated simultaneously with X-rays of 6 and 10 MV generated by a linear accelerator at a dose rate of 3.1-4.7 Gy/min. The air was ventilated through filters during irradiation in the closed box. Microorganism tests confirmed that no bacteria entered or left the box. One of the significant characteristics of this setup is that it allows for irradiation under conditions of acute hypoxia, which is obtained using an integrated tourniquet. The dose variation among 7 tumors was less than 1%. The rest of the mouse's body was shielded effectively by a half-field technique and a lead block. As a result, the whole body dose for the mice was 0-4% of the total dose absorbed by the tumor. Due to the high dose rate and the ability to irradiate 7 mice simultaneously under specific pathogen-free conditions, this new system can be considered a time-saving and valuable tool for radiation oncology research.

Animals↗

PubMed2005

[Air kerma transmission factors of Scattered X-rays in the maze of a Linac room for lead shield].

Spectra of scattered X-rays in the maze of a Linac (X-ray energies of 4, 6, and 10 MV) room were estimated by means of the Monte Carlo simulation, and air kerma transmission factors of the X-rays scattered through a lead shield were evaluated based on those spectra. Spectra of scattered X-rays showed a maximum in the energy area below 200 keV. The higher the accelerated electron energy, also, the smaller the scattering angle that tended to spread to the higher energy area of the distribution of spectra. The air kerma transmission factor of 120 degrees scattered X-rays of 4 MV X-rays obtained in this study was larger than the transmission factors of 124 degrees scattered photons of (60)Co gamma rays through a lead shield given in ICRP. The air kerma transmission factors of 120 degrees scattered X-rays of 6 MV X-rays were smaller than the transmission factors of 90 degrees scattered photons of (60)Co gamma rays. The air kerma transmission factors of 120 degrees scattered X-rays of 10 MV X-rays was slightly larger than transmission factors of 90 degrees scattered photons of (60)Co gamma rays. Therefore, in the case of a 4 MV X-ray Linac room, the calculation method given in the "Manual of Practical Shield Calculation of Radiation Facilities (2000)" causes underestimation of leakage doses.

Monte Carlo Method↗

PubMed2003

Absorbed dose determination for high energy photon and electron beams at a PRIMUS linear accelerator using the documents DIN 6800-2 and TRS-398.

The International Atomic Energy Agency in Vienna has recently published a new Code of Practice for absorbed dose determination in external beam radiotherapy (Technical Reports Series No. 398). This code claims to fulfill the need for a systematic and internationally unfied approach to the calibration of ionization chambers, as well as to the use of these detectors in determining the absorbed dose to water for the radiation beams used in radiotherapy. In Germany, the corresponding national norms are laid down in DIN 6800-2. Therefore, it appeared necessary to compare in detail the procedures and results obtained according to these two documents. The comparison was performed for a 6 MV and 15 MV photon beam, as well as for a 12 MeV and 18 MeV electron beam. Although a series of differences could be ascertained, in particular in the numerical values of the various perturbation factors, the agreement of the final results remained well within the expected uncertainty range. Nevertheless, it is suggested to modify DIN 6800-2 in order to make it more easily applicable to a hospital environment, to incorporate the progress of knowledge in the data involved, and to achieve a better adherence to international recommendations.

Calibration↗

PubMed2002

Cognitive function in patients with cerebral arteriovenous malformations after radiosurgery: prospective long-term follow-up.

PURPOSE: To evaluate the long-term cognitive function of patients with cerebral arteriovenous malformations (AVMs) after radiosurgery. METHODS AND MATERIALS: The data of 95 AVM patients were prospectively assessed up to 3 years after radiosurgery. Of these patients, 39 had a follow-up of at least 2 years. Radiosurgery was performed using a modified linear accelerator (minimal doses to the target volume 15-22 Gy, median dose 20). The neuropsychological evaluation included testing of intelligence, attention, and memory. The effect of a preexisting intracranial hemorrhage, as well as AVM occlusion, on cognitive functions was analyzed after 1 and 2 years. RESULTS: No cognitive declines were observed during follow-up. Instead significant improvements occurred in intelligence (1 year, +6.1 IQ points; 2 years, +5.1 IQ points), memory (1 year, +18.3 percentile score; 2 years, +12.2 percentile score), and attention (1 year, +19 percentile score; 2 years, +18 percentile score). Patients without previous intracranial hemorrhage improved more than patients with intracranial hemorrhage, although this difference was not statistically significant. The role of AVM occlusion on cognitive function is not clear at present. CONCLUSION: Radiosurgery does not induce measurable deterioration of cognitive function in patients with cerebral AVMs.

Adult↗

PubMed1996

Collimator (head) scatter at extended distances in linear accelerator-generated photon beams.

PURPOSE: A calculation formalism is proposed to predict variation of head scatter as a function of field size and treatment distance. METHODS AND MATERIALS: Assuming that the head scatter for the linear accelerator studied was contributed predominantly by the flattening filter, a formalism was devised to predict beam intensity as a function of distance from the target position. The method used the concept of an equivalent collimator field in which a given field at any distance can be equated to a field at the isocenter such that the extent of the flattening filter seen at the two positions is the same. RESULTS: The equation derived from the concept of equivalent collimator field size predicated change in head scatter with distance to within 0.5% for collimator field sizes ranging from 8 x 8 to 40 x 40 cm and distances up to 300 cm from the target. CONCLUSIONS: Considering flattening filter to be the main source of head scatter, the observed deviation from inverse square law for extended treatment distances can be accounted for by an equivalent collimator field size, which sees the same extent of the flattening filter at the isocenter as the field at the given distance.

Particle Accelerators↗

PubMed2006

Dosimetric characteristics of a new linear accelerator under gated operation.

Respiratory gated radiotherapy may allow reduction of the treatment margins, thus sparing healthy tissue and/or allowing dose escalation to the tumor. However, current commissioning and quality assurance of linear accelerators do not include evaluation of gated delivery. The purpose of this study is to test gated photon delivery of a Siemens ONCOR Avant-Garde linear accelerator. Dosimetric characteristics for gated and nongated delivery of 6-MV and 15-MV photons were compared for the range of doses, dose rates, and for several gating regimes. Dose profiles were also compared using Kodak EDR2 and X-Omat V films for 6-MV and 15-MV photons for several dose rates and gating regimes. Results showed that deviation is less than or equal to 0.6% for all dose levels evaluated with the exception of the lowest dose delivered at 25 MU at an unrealistically high gating frequency of 0.5 Hz. At 400 MU, dose profile deviations along the central axes in in-plane and cross-plane directions within 80% of the field size are below 0.7%. No unequivocally detectable dose profile deviation was observed for 50 MU. Based on the comparison with widely accepted standards for conventional delivery, our results indicate that this LINAC is well suited for gated delivery of nondynamic fields.

Artifacts↗

PubMed2003

[Accelerated fractionation regimens in radiotherapy of inoperable non-small-cell lung cancer].

The results of definitive radiation treatment (1988-2000) for 375 patients with inoperable non-small cell lung cancer were analyzed. Three regimens of fractionation were used: (1) accelerated fractionation (AF)--(133), 2.5 Gy, 3 days a week, to a total of 47.5--55 Gy; (2) accelerated hyperfractionation (AHF)--(93), 1.25 Gy, daily, to a total of 60-72.5 Gy and standard fractionation (SF)--(149), 2 Gy, daily, to a total of 58-68 Gy. The advantages of AHF were established as regards complete regression rate (54.9% vs. 18.6%--SF and 18.1%--AF; p(0.001), median survival (30.5(2.4 months vs. 18.9 (1%--SF (p = 0.004) and 20.4 (2.4--AF (p = 0.004)), and 3-year survival (36.6% vs. 16.7%--SF (p = 0.005) and 15.5%--AF (p = 0.005). 17.9%, 9.0% (p = 0.11) and 8.1% (p = 0.08) have survived, respectively. Overall survival in the AHF group was superior in stages IIB--III; in stage I, the results were identical. Immediate response to radical radiotherapy appeared the only statistically significant factor of survival (p = 0.005-0.008) in all the groups.

Adult↗