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PubMed2005

The development of a new basic treatment equivalent model to assess linear accelerator throughput.

AIMS: The basic treatment equivalent (BTE) model was developed in 1996 in an attempt to improve the measurement of linear accelerator throughput in radiotherapy. This study aimed to assess the effect of treatment set-up and patient characteristics on fraction duration, to update the BTE model and to determine the better throughput measure between fields per hour and BTE per hour. MATERIALS AND METHODS: Stopwatch measurements of the duration of each radiotherapy treatment fraction delivered on each linear accelerator in participating New South Wales radiation oncology departments over a 5-day period in 2003 were undertaken. Patient, equipment and staff data were collected to assess the effect of these variables on fraction duration. A new BTE equation was derived, including the most significant variables. Statistical comparison of fields and BTE per unit time was made to assess the better predictor of fraction duration. RESULTS: Data collected on 27 linear accelerators in 13 departments included a total of 135 days of linear accelerator operation, 4316 fractions and 12 892 treatment fields. Seventeen factors significantly influenced fraction duration (P < 0.01). These accounted for 46% of the total variance in the models. The eight most influential predictors of prolonged fraction duration were included in the BTE model. These were as follows: high number of fields, high number of port films/electronic portal imaging, absence of automatic field-sequencing and multi-leaf collimation, high number of junctions, use of bolus and first fraction of a treatment course. The BTE per hour was shown to be a better predictor of throughput than fields per hour. CONCLUSIONS: The BTE model is a better measure of linear accelerator throughput. It incorporates weightings for treatment and patient factors that significantly influenced fraction duration. This measure could be routinely collected by the radiation oncology departments and included in the electronic radiotherapy information systems.

Data Collection↗

PubMed2001

Room shielding for intensity-modulated radiation therapy treatment facilities.

PURPOSE: The traditional assumptions used in room-shielding calculations are reassessed for intensity-modulated radiation therapy (IMRT). IMRT makes relatively inefficient use of monitor units (MUs) when compared to conventional radiation therapy, affecting the assumptions used in room-shielding calculations. For the same single-fraction tumor dose delivered, the total number of MUs for IMRT is much greater than for a conventional treatment. Therefore, the exposure contribution from the linear accelerator head leakage will be significantly greater than with conventional treatments. METHODS AND MATERIALS: We propose a shielding calculation model that decouples the concepts of workload, MUs, and target dose when determining primary and secondary barrier thicknesses. The workload for primary barrier calculations for conventional multileaf collimator (MLC) IMRT treatments is determined according to patient tumor doses. The same calculation for accelerator-based serial tomotherapy IMRT requires scaling by the average number of treatment slices. However, rotational therapy yields a small use factor that compensates for this increase. We further define a series of efficiency factors to account for the small field sizes employed in IMRT. For secondary barrier calculations, the patient-scattered radiation is assumed to be the same for all IMRT modalities as for conventional therapy. The accelerator head leakage contribution is proportional to the number of MUs. Knowledge of the average number of MUs per patient is required to estimate the head leakage contribution. We used a 6-MV linear accelerator photon beam to guide the development of this technique and to evaluate the adequacy of conventional barriers for IMRT. Average weekly IMRT workload estimates were made based on our experience with 180 serial tomotherapy patients and published data for both "step and shoot" and dynamic MLC delivered treatments. RESULTS: We found that conventional primary barriers are adequate for both dynamic MLC and serial tomotherapy IMRT. However, the excessive head leakage produced by these modalities requires an increase in secondary barrier shielding. CONCLUSION: When designing shielding for an IMRT facility, increases in accelerator head leakage must be taken into account for secondary shielding. Adequacy of secondary shielding will depend on the IMRT patient load. For conventional facilities that are being assessed for IMRT therapy, existing primary barriers will typically prove adequate.

Facility Design and Construction↗

PubMed2004

Peripheral neutron and gamma doses in radiotherapy with an 18 MV linear accelerator.

More and more attention is being given in radiotherapy to the doses received by organs other than the target organ. With increasing survival time of the patients, the risks of secondary malignancies need to be lowered as much as possible. So total body doses are worth estimating in radiotherapy. The introduction of intensity modulated radiotherapy (IMRT) needs an increase in the number of monitor units given to the patient. So there is a risk of increasing the peripheral doses using this technique. Another aspect, mostly neglected, is the neutron peripheral dose that occurs when LINAC energies above 8 MeV are used. We did measurements for both gammas and neutrons with an 18 MV Varian accelerator for a prostate cancer treatment. The measurements were done both free-in-air, at different depths in a plexi-phantom, and using a Rando-Alderson phantom. Effective doses for the total body outside the treatment area are estimated using these measurements.

Body Burden↗

PubMedDate not supplied

Head scatter data for several linear accelerators (4-18 MV).

Measurements were made in air on the central axis of radiation beams from linear accelerators operating in the energy range from 4 to 18 MV, to determine the magnitude and source of head scattered radiation. Machines of several manufacturers were studied. The data indicate that, except for one unique collimator design, head scatter originates primarily in the flattening filter and is relatively independent of energy and machine.

Humans↗

PubMed1994

An analytic calculation of the energy fluence spectrum of a linear accelerator.

Calculation of photon dose by convolution methods requires a knowledge of the fluence spectrum of photons produced by the linear accelerator treatment head. But this spectrum is very difficult to measure accurately, and is often derived by Monte Carlo calculations modeling the elements of the treatment head. In this paper an analytic calculation technique and the corresponding computer tool for calculating the photon energy fluence spectral distribution at any point in a bremsstrahlung field are presented. Primary bremsstrahlung photon distributions are computed by modeling electron dispersion in layers in a thick target and using the thin target bremsstrahlung cross section formulas of Schiff. The first Compton scatter from all materials in a linear accelerator treatment head is computed analytically. Higher-order Compton scatter events and pair production annihilation photons are ignored, but the attenuation of both primary and first scattered photon fluence is computed. Predictions of the computer implementation of the model are compared to measurements of bremsstrahlung production in a thick target and to Monte Carlo calculations of the energy fluence emerging from a linear accelerator. Finally, the computer tool is used to investigate the source of collimator-dependent fluence fluctuations in air. In agreement with other measurements, the principal contribution to fluence outside the geometric field is found to be from scatter in the flattening filter.

Biophysical Phenomena↗

PubMed1980

5 years of operation experience with a 4 MeV linear accelerator in radiotherapy.

The results of our experience in operating a 4 MeV linear accelerator Clinac 4 from Varian in radiotherapy are reported, especially considering the 12-hours-per-day operation for more than the last 3 years. Frequency of defects of different parts is discussed. The downtime adds up to 6 days per year, service and repair costs amount of US $ 33,--per patient.

Costs and Cost Analysis↗

PubMed1994

[Accelerated hyperfractionated radiotherapy combined with simultaneous chemotherapy in locally advanced pharyngeal and oral carcinomas].

PURPOSE: To evaluate the feasibility and effectiveness of a concurrent radio-chemotherapy regimen for locally advanced carcinomas of the pharynx and floor of the mouth. PATIENTS AND METHODS: Since January 1990, 97 patients with locally advanced carcinomas of the naso-, oro-, hypopharynx and the oral cavity in UICC stages III and IV were treated according to an accelerated hyperfractionated radiotherapy schedule with concurrent chemotherapy. The primary tumor and positive lymph nodes received a total dose of 72 Gy in 6 weeks. In the first 3 weeks, large fields were irradiated 5 times per week with 2 Gy per fraction. Thereafter, treatment was accelerated, giving 2 daily fractions of 1.4 Gy with minimal intervals of 6 hours. Target volumes were reduced after 49.6 and 59.4 Gy, excluding clinically uninvolved lymph node regions of low and high risk. On day 1, 350 mg/m2 5-FU and 50 mg/m2 folinic acid were given as intravenous bolus followed by continuous infusion of 350 mg/m2 5-FU and 100 mg/m2 folinic acid, days 1 to 5. 10 mg/m2 mitomycin C was given on day 5 and 36 of the treatment. Salvage surgery was offered for residual disease 5 to 6 weeks after the end of radiotherapy. PATIENT CHARACTERISTICS: 70 male, 27 female; age: 27 to 81 years; T2/T3/T4: 7/30/60; N0/N1/N2/N3: 20/18/53/6; nasopharynx/oropharynx/hypopharynx/oral cavity: 16/33/36/12. Median follow-up is 26 months. RESULTS: Overall survival and recurrence-free survival at 2 years were RESULTS: Overall survival and recurrence-free survival at 2 years were 68 +/- 5% and 74 +/- 5%. Multivariate analysis revealed a significant influence of the geometric mean neck node diameter or the N-stage on loco-regional control and survival. T-stage, tumor volume, or tumor localisation did not become significant. Acute toxicity of this schedule was acceptable but required optimised supportive care. Treatment related grade 4+ late toxicity of mucosa, soft tissue and bone were observed with a cumulative frequency of 13% at 2 years. Two patients died during a phase of severe leuko- or thrombocytopenia. CONCLUSION: This phase I to II trial shows favourable results using an intensified treatment radio-chemotherapy protocol. A phase III study is now planned, comparing this regime with an accelerated hyperfractionated radio-therapy alone to an increased total dose of 77.6 Gy.

Adult↗

PubMed1990

Effects of electron cutouts on absorbed dose in and outside Varian-20 electron fields.

Electron beam outputs in the presence of beam shaping cutouts were measured for 6, 9, 12, 16, and 20 MeV electrons from a Varian-20 linear accelerator. This machine was provided with a new model of electron cones, for which photon jaw sizes were set differently from those of old cones. The effects of different photon jaw sizes on the electron outputs were studied. Also measured was the absorbed dose under a cutout, which was presumably contributed by scattered electrons and bremsstrahlung photons.

Electrons↗

PubMed2004

Response corrections for solid-state detectors in megavoltage photon dosimetry.

Solid-state detectors offer high sensitivity, stability and resolution and are frequently the dosimeter of choice for on-line dosimetry and small field therapies such as stereotactic radiosurgery. The departure from tissue equivalence of many solid-state devices, including diodes and MOSFETs, has to be carefully considered at lower energies and for Compton scattered radiation where the strongly Z-dependent photoelectric effect is significant. A modification of Burlin cavity theory is proposed that treats primary and scatter photon spectra separately and this has been applied to determine the correction factors for diode detector measurements of 6 and 15 MV linear accelerator beams. Uncorrected, an unshielded diode overestimates the dose at depth by as much as 15% for the 6 MV beam. The model predicts the effect to within 1% for both energies offering a basis for the correction of diodes for use in routine dosimetry.

Calibration↗

PubMed1986

Residual radioactivity in a cyclotron and its surroundings.

Neutron-induced gamma-ray-emitting radionuclides in components and surroundings of the University of Colorado 1.3-m sector-focusing cyclotron have been measured with Ge(Li) and HPGe detectors. These measurements were made before decommissioning of the cyclotron and before approving release of the accelerator components and building space for other uses. In addition to the activities expected from previous published work, 13.3-y 152Eu and 8.6-y 154Eu were found in the concrete shielding with specific activities of tens of becquerels per kilogram (a few nanocuries per kilogram).

Environmental Monitoring↗

PubMedDate not supplied

Dosimetry of small fields for Therac 20 electron beams.

The Therac 20 medical linear accelerator produces electron beams of 6, 9, 13, 17, and 20 MeV. We measured depth dose, isodose curves, and output factors for small electron fields using an ionization chamber, film, and thermoluminescent dosimeters. Tables and graphs were generated from these measurements for accurate treatment planning of various blocked and open fields.

Electrons↗

PubMed1991

A device for experimental radiosurgery.

As radiosurgery evolves into a widely available treatment modality for a variety of intracranial lesions, the need for basic research concerning the radiobiology of high-dose single-fraction ionizing radiation becomes crucial. A device especially designed for experimental radiosurgery in the cat is described. It incorporates basic parts of the Kopf stereotactic frame for accurate target positioning. A motorized pendular movement of the machine is used to describe a radiation arc, while the radiation source (either a linear accelerator or a cobalt machine) remains stationary. The pathway of the different radiation arcs is modified by rotation of the animal platform around the machine isocenter. Mechanical accuracy tests have shown a maximal alignment error of 0.15 mm, comparing favorably with that reported for modern clinical radiosurgical systems.

Animals↗

PubMed1986

[Electron irradiation of cervix cancer using a new transvaginal tube system attached to the betatron].

A new tube system for the betatron, which normally is only applied in percutaneous electron therapy, was developed at the Radiotherapeutic Department of the Medical Faculty of Istanbul. The new system allows to use the betatron also in transvaginal irradiation of the uterine cervix cancer. The authors describe the technical features of this system, its clinical application, and the results achieved. In great tumor centres where every type of malignant disease is treated, it is indispensable to dispose of the equipment necessary for certain very special cases, even if this equipment is not used frequently. In recent years, applicators for cavitary therapy and high voltage units for percutaneous therapy have been gradually substituted for the tube system employed in former times with orthovolt therapy units in intravaginal irradiation of cervical and vaginal carcinomas. There are cases, however, where the therapy techniques used nowadays are not sufficient or where a cavitary therapy should be used. Furthermore electrons which today can easily be produced for percutaneous surface therapy with linear accelerators can also be used for transvaginal therapy. We, too, have developed a new tube system which was attached to the betatron and used in intravaginal irradiation. This study describes the tube system, its clinical application, and the results achieved.

Adenocarcinoma↗

PubMed2001

Applicability of a mobile accelerator for intraoperative radiation therapy to colorectal cancer.

PURPOSE: Intraoperative radiation therapy is reportedly effective for local control and pain relief in colorectal cancer. However, this treatment requires a large number of medical personnel, which hinders expanded use of this method. A mobile electron linear accelerator for intraoperative radiation therapy has been developed and is now commercially available. This report analyzes the applicability of this accelerator to colorectal cancer. The applicability of the mobile accelerator is analyzed based on its specifications by simulating the intraoperative radiation therapy delivered to these patients with a conventional intraoperative radiation therapy unit. METHODS: From 1987 to 1999, 49 colorectal cancer patients underwent 54 surgical resections and received intraoperative radiation therapy to 75 sites. RESULTS: The mean intraoperative radiation therapy dose for colorectal cancer with the conventional unit was 22 (range, 10-30) Gy. The mean electron energy level was 10 (range, 3-30) MEV. Applicator size ranged from 4 to 10 cm in diameter. The mobile accelerator can achieve a dose rate of 10 Gy/min and an applicator unit size range of 3 to 10 cm in diameter, facilitating intraoperative radiation therapy for colorectal cancer. The electron energy limitation (12 MEV at maximum) suggests that the indications for this machine are limited. In our experience, 30 percent of patients received intraoperative radiation therapy with electron energy levels exceeding 12 MEV. Of these cases, 81 percent had macroscopic residual tumor and 69 percent had pain. CONCLUSION: An intraoperative radiation therapy mobile accelerator can cover 72 percent of the irradiation sites covered using our conventional unit. This accelerator is useful for intraoperative radiation therapy with curative intent for patients with no or slight residual tumor. Patients with gross residual tumor and pain may not be suitable.

Adult↗

PubMed1996

Collimator-related radiation dose for different cobalt machines and linear accelerators.

PURPOSE: In previous publications (12, 13) measurements are described of the dose outside the primary beam (the peripheral dose (PD)) for 60Co gamma radiation to 25 MV photons. Comparison with data published by other investigators for different treatment machines, showed good agreement. This can only be explained when the contribution to the PD of radiation leaking through and scattering from the collimator does not differ considerably between treatment machines from different manufactures, and it is the purpose of this article to investigate whether this assumption is valid. METHODS AND MATERIALS: A request was sent out to all radiotherapy departments in The Netherlands and one in Belgium to measure the dose outside the primary beam for as many machines as possible. The following geometry was given: field sizes of 10 x 10 cm2 and 20 x 20 cm2 at distances of 30 and 50 cm, for collimator angles 0 degrees and 90 degrees at the standard source surface distance. This, therefore, resulted in a dataset of eight measurements per photon energy. RESULTS: Data were collected for four cobalt machines and 37 linear accelerators, from seven different manufacturers. All together 56 datasets were collected for 12 different photon energies. Although the variation of the leakage radiation dose is small, there can be differences of about 50% in the collimator scatter dose between collimator angles of 0 degrees and 90 degrees, depending on the collimator design or on the design of the flattening filter. For dual energy machines with a large gap between the low and the high energy, the values for the high energy are higher by about 40%. Old cobalt machines show higher leakage radiation dose than modern ones. CONCLUSION: Although there is no large variation in leakage radiation dose between different makes of accelerators, some show higher collimator scatter dose than others. The magnitude of the leakage radiation dose is well within regulatory limits. As the PD not only consists of a contribution from collimator-related radiation, but also of patient scatter, the differences are negligible when estimating the peripheral dose for an individual patient.

Cobalt↗

PubMed2004

[Output-factors for squared, rectangular, and elongated photon fields of medical linear accelerators].

For reduction of data scattering in output-factor calculation, a new correction for Stirling's formula is introduced that takes into account the position of the upper and lower collimator jaw settings of linear accelerators. This correction factor requires two simple geometrical data of the treatment head, to describe the collimator exchange effect for open- as well as wedged fields. This model also implies a reduction of output measurements to only three field sizes, the minimum, the maximum, and a reference field size, without significant loss of accuracy in output factors.

Models, Theoretical↗

PubMed1984

ISABELLE shielding criteria and design.

ISABELLE is a high-intensity 400-GeV proton-storage ring under construction at Brookhaven National Laboratory (BNL). The radiation-protection problems encountered in its design are in many ways similar to those at high-energy fixed-target facilities: both the laboratory personnel nearby and the public several kilometers away must be appropriately protected from high-energy neutron and muon radiation. These radiations will result from the routine operation of the accelerators, as well as from unplanned and possibly large losses of the stored beam. The radiation exposures resulting from the loss of beam already in the accelerator could be more severe than those resulting from a full year's normal operation. The design goals (mrem/yr) which have been adopted for different populations, exposure scenarios and radiation sources are outlined. We also describe the methods (hardware, shielding, access control) which will be used to achieve these goals.

Facility Design and Construction↗

PubMedDate not supplied

Characteristic parameters of 6-22 MeV electron beams from a 25-MeV linear accelerator.

Depth-ionization measurements were performed using a thin wall parallel plate chamber in water at nominal electron energies of 6, 9, 12, 15, 18, and 22 MeV for the standard available square field sizes. The characteristic parameters of the central axis depth-dose distributions were derived and compared to corresponding values for other accelerators. Vacuum packed therapy-verification films were used in water to obtain isodose distributions in a plane containing the central axis of the beam. The uniformity index and penumbra of the beams were measured from isodose distributions obtained in planes perpendicular to the beam central axis, at depths of 1/2R85 in water.

Electrons↗