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PubMed2003

Electron dose calculations: a comparison of two commercial treatment planning computers.

The accuracy of electron dose calculations performed by two commercially available treatment planning systems, Varian Cadplan and MDS Nordion Helax-TMS, were assessed. Three tests designed to reproduce clinical treatments likely to result in dose nonuniformity have been carried out. The tests examined oblique incidence of the electron beam; incidence on a surface containing a step shape; and incidence on a phantom containing a small air cavity. Dose calculations performed by the planning systems were compared with thermoluminescence dosimetry (TLD) measurements in a WTe electron solid water phantom. A Varian 2100C linear accelerator was used. In most situations, the discrepancy between calculated and measured dose was within the tolerance specified by the ICRU; however, some exceptions were noted. Helax-TMS produced errors of 5 mm in the position of the 10% isodose line in the penumbra of the obliquely incident beam. Both Cadplan and Helax-TMS overestimated the surface dose adjacent to a step in the beam entry surface by approximately 15%. An overestimation of 10% in dose was calculated by both systems downstream of the small air cavity. Discrepancies between the measured and calculated monitor units lay within the uncertainty limits of the measurements. In conclusion, calculations of absorbed dose from electron beams performed by Varian Cadplan and MDS Nordion Helax-TMS result in significant errors at shallow depths near surface irregularities and downstream of small air cavities.

Humans↗

PubMedDate not supplied

Dosimetry and techniques for simultaneous hyperthermia and external beam radiation therapy.

An increased biological effect is realized when hyperthermia and radiation therapy are combined simultaneously. To take advantage of this effect, techniques have been developed that combine existing hyperthermia devices with a linear accelerator. This allows concomitant delivery of either ultrasound or microwave hyperthermia with photon radiation therapy. Two techniques have been used clinically: the orthogonal technique, in which the microwave or ultrasound beam and the radiation beam are orthogonal to one another, and the en face technique, in which the ultrasound or microwave beam and the radiation beam travel into the tumour through the same treatment window. The en face technique has necessitated the development of special attachments so that the hyperthermia device can be mounted to the linear accelerator and so that non-uniform portions of the hyperthermia device can be removed from the radiation beam. For microwave therapy, applicators are mounted onto the linear accelerator using the compensating filter tray holder. For ultrasound, special reflector devices are mounted to a frame that is mounted onto the compensating filter tray holder of the linear accelerator. Because the linear accelerator is an isocentric device, the height of the radiation source is fixed, and this has necessitated specially designed devices so that the ultrasound support system is compatible with the linear accelerator. The treatment setups for both the en face technique and the orthogonal technique require the interaction of both hyperthermia and radiation therapy personnel and equipment. The dosimetry and day-to-day operations for each technique are unique. The simulation for the en face technique is much different from the simulation of a normal radiation treatment and requires the presence of a hyperthermia physicist. Also, for the en face technique, the attenuation of the microwave applicator and the thickness and attenuation of the ultrasound reflector system are taken into account for radiation dosimetry. This paper presents details of the dosimetry and logistics of the techniques for simultaneous thermoradiotherapy based on 7 years of experience treating more than 50 patients.

Cobalt Radioisotopes↗

PubMed1985

Radiotherapy technologist dose from high-energy electron medical accelerators.

Whole-body doses incurred by radiation therapy technologists working with 18- and 24-MV medical linear accelerators have been measured at two clinics under normal working conditions with a sensitive solid-state personal dosimeter. The results based on 84 treatments indicate that average monthly doses in the range 12-19 mrem can be expected for 30 daily treatments similar to those monitored. It was also found that 76% of the in-room dose was due to activated components in the treatment head, the balance of the in-room dose being due to activated concrete. Other statistics were obtained on patient in-room times and technologist times and doses outside the treatment room. Although the monthly film dosimetry records show no detectable doses for technologists working with these particular machines, in the spirit of ALARA, steps have been taken to reduce residual activities even further by choice of material less susceptible to activation.

Electrons↗

PubMed1990

Estimation of neutron dose equivalent at the mezzanine of the Advanced Light Source and the laboratory boundary using the ORNL program MORSE.

To investigate the radiation effect of neutrons near the Advanced Light Source (ALS) at Lawrence Berkeley Laboratory (LBL) with respect to the neutron dose equivalents in nearby occupied areas and at the site boundary, the neutron transport code MORSE, from Oak Ridge National Laboratory (ORNL), was used. These dose equivalents result from both skyshine neutrons transported by air scattering and direct neutrons penetrating the shielding. The ALS neutron sources are a 50-MeV linear accelerator and its transfer line, a 1.5-GeV booster, a beam extraction line, and a 1.9-GeV storage ring. The most conservative total occupational-dose-equivalent rate in the center of the ALS mezzanine, 39 m from the ALS center, was found to be 1.14 X 10(-3) Sv y-1 per 2000-h "occupational" year, and the total environmental-dose-equivalent rate at the ALS boundary, 125 m from the ALS center, was found to be 3.02 X 10(-4) Sv y-1 per 8760-h calendar year. More realistic dose-equivalent rates, using the nominal (expected) storage-ring current, were calculated to be 1.0 X 10(-4) Sv y-1 and 2.65 X 10(-5) Sv y-1 occupational year and calendar year, respectively, which are much lower than the DOE reporting levels.

Laboratories↗

PubMed2002

[Dose outside the treatment field for external irradiation with high-energy X-ray reduction of the dose of the remaining testis in postoperative irradiation of seminoma].

UNLABELLED: This study aimed to decrease the radiation dose to the disease-free testis in postoperative irradiation for seminoma patients. We consider the factors influencing the peripheral dose (PD) of 10MV X-ray radiotherapy to be the distance between the caudal edge of the irradiation field and the measuring point, the size of the therapeutic irradiation field, the thickness of the lead shield laid above and lateral to the disease-free testis, and the thickness of the lateral absorber. MATERIALS AND METHODS: We measured the scattering radiation dose coming from the accelerator head and that due to irradiation volume. We measured these doses using a testicular phantom as the non-diseased testis. RESULTS: Scattering radiation from the accelerator head mainly contributes to PD, whereas the larger the size of the irradiation field the more the scattering radiation from the irradiation volume contributed to PD. PD changed more at the surface of the phantom than at its center. PD at the testicular phantom could be reduced to less than 1% of the therapeutic dose when it was situated more than 5cm distant from the caudal limit of the irradiation field, the lead shield above the testicular phantom was 7.5cm thick, and the lateral lead shield was 2mm thick. CONCLUSIONS: PD is influenced by many factors. It is necessary to clarify the change in PD at the testicular phantom, and it is important to limit the caudal edge of the irradiation field and to lay the lead shield for the attenuation of radiation on the disease-free testis.

Humans↗

PubMed1991

Is the dose-response relationship for local control of Hodgkin's disease obscured by lung inhomogeneity corrections?

Absence of a dose-response relationship has been reported for local control of Hodgkin's disease between 30 and 42 Gy delivered to the mantle field in definitive irradiation. These dose ranges were determined at the central ray using irregular field (IF) calculations without benefit of dosimetric correction for lung inhomogeneity. Detailed analysis was performed of dose delivered to hilar and mediastinal regions with mantle field irradiation incorporating lung inhomogeneity corrections for the indicated dose ranges using 60Co and 6 MV linear accelerator. Inclusion of lung inhomogeneity (LI) corrections revealed dose heterogeneity within the treatment volume, delivering higher total doses to hilar and lateral mediastinal regions. For a prescribed dose of 40 Gy to midline with 6 MV photons, the hila and mediastinum would receive 45 Gy with IF and LI corrections. Similar increases are observed with 60Co. Mantle field calculations should be performed with CT planning including lung inhomogeneity corrections to account for anatomic irregularity of the mediastinal contour and interposed lung. As lung inhomogeneity dosimetric data become available, conclusions drawn from clinical experience should be re-evaluated based upon location and extent of mediastinal disease involvement. Implications for treatment associated late toxicity using dose-volume histogram analysis should be considered.

Cobalt Radioisotopes↗

PubMed1995

Calculation of monitor units for a linear accelerator with asymmetric jaws.

A simple approach was developed that calculates the output factors and tissue maximum ratio of an asymmetric field at any point within the open field, and specifically both at the central axis (when the jaws do not shadow it) and at the effective center of the open field, using the existing tables for symmetric fields and the multidepth profile information for the largest available field size (either open or with a wedge present). Day's method was adapted to calculate the effective values of the usual field-size-dependent parameters. This approach makes these parameters also dependent on the location of the calculation point relative to the field edges in an asymmetric field. This algorithm was tested by comparing its predictions with measurements of asymmetric and half blocked fields, with and without wedges, in a water phantom at different depths and off-axis distances. The agreement between calculated and measured dose rate is within 1%-3% even in highly asymmetric fields for both 6- and 18-MV photons.

Models, Structural↗

PubMed1999

An Australasian assessment of the basic treatment equivalent model derived from New South Wales data.

The current method of assessment of radiation oncology linear accelerator throughput is either by patients per unit time or fields per unit time. This, however, does not take into consideration the complexity of different treatment techniques or of casemix. A model has been developed in an earlier study, called 'basic treatment equivalent' (BTE), to measure patient throughput of a linear accelerator, which includes consideration of the complexity of treatment techniques. The present study compared the BTE model with the current best measure of patient throughput of fields per hour. All 37 departments in Australia and New Zealand were invited to participate in testing the model, and 36 agreed to participate. The study period for each department was a consecutive 4 weeks between August and December, 1996. The prospective data collected were the total BTE units treated per linear accelerator per day, the total number of patients and fields treated per linear accelerator per day, and the total linear accelerator hours of operation per day excluding calibration time and significant breaks of linear accelerator time such as planned meal breaks. The treatment breaks between consecutive treatment fractions were not excluded from the linear accelerator treatment time. The throughput data for 36 departments (92 linear accelerators) were collected over the 4-week study period. The average throughput for the departments was 10.8 fields per hour and 4.2 patients per hour. The average BTE per department was 5.7 BTE per hour. The average BTE per episode per department was 1.38. The BTE model was found to be a more sensitive measure of productivity compared with fields per hour (P < 0.001). Some treatment techniques were thought to be not well represented by the BTE formula, particularly those techniques where junctions were present. The BTE model is a more sensitive measure than fields per hour and better reflects the variations in complexity in techniques. Despite this result there is further refinement to be performed to make the model even more sensitive.

Data Collection↗

PubMed1985

[Dose distribution of electrons obtained from linear accelerators equipped with a beam scanning system].

Homogeneous distribution of electrons used for therapeutic purposes and obtained from accelerators, is achieved by means of Potter-Bucky diaphragms or by repeated, staggered, sawtooth-shaped sweeping movements of the electron beam (scanning) over the radiation field. The repetition of the scanning process (number of scans) can result in long measurement times for achieving a sufficiently homogeneous, dosimetrically adequate distribution of the electrons. This "time problem" makes it imperative to achieve good homogeneity while keeping the number of scans as low as possible. To solve the problem, the scanning movement of the electron beam is simulated by a computer programme and the interdependence of the homogeneity of the irradiation field and number of scans is investigated. Since changing the ratio of the two deflection rates exercises a significant influence, it is mandatory in dosimetry to pay close attention to a strict observance of the deflection rates.

Humans↗

PubMed2003

[Improvement of the accuracy of the Laplace transform method for the determination of radiotherapy spectra of clinical linear accelerators].

The present study focused on the reconstruction of the bremsstrahlung spectrum of a clinical linear accelerator from the measured transmission curve, with the aim of improving the accuracy of this method. The essence of the method was the analytic inverse Laplace transform of a parameter function fitted to the measured transmission curve. We tested known fitting functions, however they resulted in considerable fitting inaccuracy, leading to inaccuracies of the bremsstrahlung spectrum. In order to minimise the fitting errors, we employed a linear combination of n equations with 2n-1 parameters. The fitting errors are now considerably smaller. The measurement of the transmission function requires that the energy-dependent detector response is taken into account. We analysed the underlying physical context and developed a function that corrects for the energy-dependent detector response. The factors of this function were experimentally determined or calculated from tabulated values.

Models, Theoretical↗

PubMed1980

Computer calculation of the dose distribution in the electron build-up region.

A procedure has been developed for producing accurate computer-calculated dose distributions at the air-tissue boundaries for oblique incidence of a photon beam. Measurements of the dose distribution under conditions of tangential incidence are presented for a Siemens 6 MV linear accelerator. These measurements have been compared with calculated distributions using the standard Cunningham-Clarkson calculation technique. Based on this comparison, two changes are suggested for improving the accuracy of the calculation. The grid spacing for the calculation is decreased in order that the rapid dose variations that occur at the boundary may be better followed. Furthermore, the dose calculation scheme is modified to take into account the differences in the amount of electron build-up which occurs. An algorithm is presented for making this modification. Also, the calculated dose at the exit surface had to be modified to handle the deficit of scatter material behind the calculation points. The results of these changes are shown as a comparison of a typical chest wall irradiation treatment plan using two opposing tangential fields.

Computers↗

PubMed1996

Measurement of photon beam backscatter from collimators to the beam monitor chamber using target-current-pulse-counting and telescope techniques.

Backscattered radiation (BSR) arising from field-defining collimators and entering the beam monitor chamber (BMC) may contribute to observed variations in medical linear accelerator photon beam output with collimator setting. Measuring the magnitude of such contributions for particular accelerators under specified operating conditions is therefore important when attempting to understand and model accelerator head scatter. The present work was conducted to confirm some backscatter measurements for collimating jaws reported previously and to extend these to include other accelerators and a multileaf collimator (MLC). BSR reaching the BMC from the jaws of Clinac 600C, 2100C and 2300CD accelerators and from an MLC on the 2300CD was investigated using both target-current-pulse-counting and telescope methods. Our measurements show that for the Clinac 600C BSR-dependent output variations are negligible. However, for the 2100C and 2300CD BSR-dependent relative output increased in an almost linear fashion, by up to 2.4% for 15 and 18 MV beams, and by up to 1.7% for 6 MV beams, as the field size varied from 5 x 5 cm2 to 40 x 40 cm2. The magnitude of BSR dependent upon collimator location in the head, as expected, thereby contributing to the collimator exchange effect. An earlier study at our centre using the telescope method had reported higher BSR levels. This discrepancy was resolved when corrections for telescope block and room scatter, previously assumed negligible, were made.

Humans↗

PubMed2004

On empirical methods to determine scatter factors for irregular MLC shaped beams.

Multileaf collimators (MLCs) are in clinical use for more than a decade and are a well accepted tool in radiotherapy. For almost each MLC design different empirical or semianalytical methods have been presented for calculating output ratios in air for irregularly shaped beams. However, until now no clear recommendations have been given on how to handle irregular fields shaped by multileaf collimators for independent monitor unit (MU) verification. The present article compares different empirical methods, which have been proposed for independent MU verification, to determine (1) output ratios in air (Sc) and (2) phantom scatter factors (Sp) for irregular MLC shaped fields. Ten dedicated field shapes were applied to five different types of MLCs (Elekta, Siemens, Varian, Scanditronix, General Electric). All calculations based on empirical relations were compared with measurements and with calculations performed by a treatment planning system with a fluence based algorithm. For most irregular MLC shaped beams output ratios in air could be adequately modeled with an accuracy of about 1%-1.5% applying a method based on the open field aperture defined by the leaf and jaw setting combined with the equivalent square formula suggested by Vadash and Bjärngard [P. Vadash and B. E. Bjärngard, Med. Phys. 20, 733-734 (1993)]. The accuracy of this approach strongly depends on the inherent head scatter characteristics of the accelerator in use and on the irregular field under consideration. Deviations of up to 3% were obtained for fields where leaves obscure central parts of the flattening filter. Simple equivalent square methods for Sp calculations in irregular fields did not provide acceptable results (deviations mostly >3%). Sp values derived from Clarkson integration, based on published tables of phantom scatter correction factors, showed the same accuracy level as calculations performed using a pencil beam algorithm of a treatment planning system (in a homogeneous media). The separation of head scatter and phantom scatter contributions is strongly recommended for irregular MLC shaped beams as both contributions have different factors of influence. With rather simple methods Sc and Sp can be determined for independent MU calculation with an accuracy better than 1.5% for most clinical situations encountered in conformal radiotherapy.

Algorithms↗

PubMedDate not supplied

The effect of flattening filter design on quality variations within an 8-MV primary x-ray beam.

Single material flattening filters supplied by manufacturers of medical linear accelerators are designed to produce the desired primary dose profile while maintaining output at a maximum level. This design criterion tends to produce substantial quality variations within the primary beam. Quality variations, as expressed by half-value layer in brass and polystyrene, were measured for an 8-MV primary beam both unfiltered and with the flattening filter supplied by the manufacturer. Most of the quality variation was introduced by the filter. Two approaches were then used to reduce this quality variation, each at a cost of a 25% reduction in output. First, a hardening filter was added to the manufacturer's flattening filter. The second approach was to design a new composite flattening filter made from brass and lead. For both approaches, the increase in quality variation over the intrinsic (no filter) variation was reduced by one-half.

Humans↗

PubMed1997

Study of boron neutron capture therapy used neutron source with protons bombarding a thick 9Be target.

Neutron sources created by 4-, 3.5-, and 3-MeV protons striking a thick beryllium target were studied via the time-of-flight technique. Protons were accelerated by the Peking University 4.5 MV electrostatic accelerator. Two disk-shaped 9Be targets with thickness 1.5 and 3 mm were used in the measurements. The time-of-flight spectra were observed at zero degrees with respect to the incident proton beam. The analysis to these time-of-flight spectra is given. The time-of-flight spectra were converted to the energy spectra and compared to a neutron spectrum of 7Li(p, n)7 Be reaction with incident energy 2.5 MeV, which was also measured in this work. Restricted by the spectrometer itself, the threshold of the measurements is 400 keV. The results show that by using several MeV protons bombarding a thick beryllium target, reactions other than 9Be(p, n)9B produce significant contributions to the neutron yield with energy less than 1 MeV.

Beryllium↗

PubMed1990

[Role of accelerated electrons in the therapy of advanced-stage skin epithelioma advanced stage: analysis of 31 cases].

Electron beam therapy is the superior modality of treatment for advanced skin cancer or in specific anatomical situations. The authors report the results for the electron beam therapy of 31 patients with advanced skin carcinoma. There were 15 squamous cell carcinomas, 11 basal cell carcinomas and 5 mixed, basosquamous cell carcinomas. All lesions were 3 cm or greater in maximum diameter. With follow-up ranging from 1 to 6 years, 28 patients have not evidence of disease, 1 is dead with disease, 1 is alive with suspicious residual disease and 1 is alive with recurrence. Acute and late complications are acceptable; bone, cartilage and soft tissue necrosis are not reported. The treatment results are similar to those in the literature and show the advantages of the electron beam irradiation for advanced skin carcinomas.

Aged↗

PubMed1995

Study of dosimetric penumbra due to multileaf collimation on a medical linear accelerator.

PURPOSE: Investigations of dosimetric penumbra produced by multileaf collimation on a medical linear accelerator are presented. METHODS AND MATERIALS: Multileaf collimators (MLCs) can be designed with at least three different shaped leaf-end profiles: straight, divergent, and curved. Assessment of the dosimetric effects of the collimator edge profiles was implemented using a fast Fourier transform (FFT) convolution algorithm. Accelerator source intensity was considered to have a Gaussian distribution. The calculated dose profile, for a source-to-surface distance of 100 cm and at depth of 10 cm in a water phantom, was fitted to a penumbral-forming function from which the penumbral width between 80% and 20% of the central axis dose was obtained. RESULTS: Calculation performed at various field sizes showed that curved collimator leaf-end produces a wider penumbra than the diverging collimator leaf-side. Film measurements agreed with the calculations within an uncertainty of less than 2 mm. The effect of backup jaws for the MLC and of the lower pair of diverging diaphragms on dosimetric penumbra was also investigated. CONCLUSIONS: This study is useful for characterizing collimator edge effects and for optimizing new collimator designs.

Algorithms↗

PubMed2000

Intrafractional tumor position stability during computed tomography (CT)-guided frameless stereotactic radiation therapy for lung or liver cancers with a fusion of CT and linear accelerator (FOCAL) unit.

PURPOSE: To evaluate intrafractional tumor position stability during computed tomography (CT)-guided frameless stereotactic radiation therapy (SRT) for lung or liver cancers, we checked repeated CT scanning, with a fusion of CT and linear accelerator (FOCAL) unit. METHODS AND MATERIALS: The FOCAL unit is a combination of a linear accelerator (Linac), CT scanner, X-ray simulator (X-S), and carbon table, and is designed to achieve CT-guided SRT with daily CT positioning followed by immediate irradiation while patients keep reduced shallow respirations. To evaluate intrafractional tumor position stability, 50 lung or liver lesions in 20 patients were checked by repeated CT scanning just before and after irradiation, and the obtained images were compared. RESULTS: There was no case with the intrafractional error judged to be greater than 10 mm. In 68% of cases, the intrafractional positioning errors were negligible (0-5 mm). CONCLUSIONS: Using the FOCAL unit, SRT for lung or liver cancers could be performed with intrafractional positioning errors not greater than 10 mm.

Equipment Design↗