摘要:

An extrafocal source model has been developed to explain the dependence of head scatter and beam penumbra on field size. In this model, the x‐ray source of a medical linear accelerator is described by two components: a small but intense focal component; and a broadly distributed extrafocal component of low intensity. The extrafocal component is so large that it can be “eclipsed” by the field‐defining collimators. Extrafocal radiation was found to account for 12% of the energy fluence on the central axis of the 6 MV x‐ray beam from a Varian Clinac 2100c accelerator. Head scatter factors were calculated “in‐air” for symmetric, asymmetric, and half‐blocked fields. Calculations agreed with measured values to better than 0.5%, on average. However, head scatter factors for asymmetric fields were underestimated by 1.2% when one of the field dimensions was reduced to 4 cm (the minimum jaw setting that was tested). The extrafocal source model was combined with a convolution/superposition dose calculation algorithm to calculate dose‐per‐monitor‐unit calibration (output) factors and beam dose profiles in water. These dose calculations predict the degradation of the field edge as a function of field size, and calculate output factors to within 0.5%, on average. In the most extreme case of a 4 cm field width, output factors were underestimated by 2%. Dose profiles are predictedwithoutthe aid of an empirical fit to measured beam penumbra data. The extrafocal source model will be particularly useful for fields defined by independent jaw and multileaf collimation systems.

ISSN:0269-3127    

DOI:10.1118/1.597648

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Transmission through a beam‐modifying absorber consists of attenuated primary beam and scattered radiation generated by the absorber. The primary component of the transmitted beam is characterized by the narrow beam attenuation coefficient which depends upon the energy of the beam and type of the absorber. In addition to beam energy and absorber material, the scatter component also depends on field size, thickness and shape of the absorber, location of the absorber with respect to the source, and the point of calculation. Based upon Compton first‐scatter, a method has been developed to calculate effective broad beam transmission through any arbitrarily shaped absorber with variable thickness for any points on and off the central axis. The method requires predetermined narrow beam attenuation coefficients as a function of thickness. Transmission calculations for various absorbers such as wedges and attenuators were performed for cobalt‐60 and 6‐MV beams and were compared with measured data. For a cobalt‐60 beam, the measured transmission fraction through a 1.33‐cm‐thick absorber (alloy, consisting of 55% bismuth and 45% lead) for a field size of 24×24 cm2is 17% higher than the calculated value using a narrow beam attenuation coefficient. Also, for the same absorber, measured central axis transmission is as much as 3.6% higher compared to off‐axis locations. The measured transmission fraction through a 1.33‐cm absorber was found to differ by as much as 13% and 14% for Cobalt‐60 and 6 MV, respectively, as the chamber‐to‐source distance was varied from 70 to 110 cm. The agreement between calculated and measured values is within 0.5% for both energies whereas conventional narrow beam calculations would have yielded errors of 18% and 19%, respectively. Similar agreement was obtained when comparing calculated and measured wedge factors as a function of field size, with the maximum deviation being 0.7%. Measured scattered doses, due to an attenuator covering part of a beam, show a maximum for a thickness of approximately one mean‐free path. This is also predicted by calculations with an agreement of 0.3%.

ISSN:0269-3127    

DOI:10.1118/1.597650

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Non‐water solid phantoms are often used in the determination of absorbed dose to water for electron beams. Protocols have been established and widely accepted. In these procedures, several assumptions in addition to the Spencer–Attix conditions are required, and several correction factors are needed. A direct approach, in which the conversion is carried out in a single step using a modified Spencer–Attix formula, is studied in this paper. The approach is consistent with the protocols for water phantom, and the conversion factors can be calculated using Monte Carlo simulation. The behavior of the conversion factors is described by comparing the results from the AAPM protocol and experiment data for three electron energies (6, 12, and 16 MeV). This study demonstrates that for beam calibration atdmax, the results from the new approach agree with those from the protocol with a maximum discrepancy of 1% for PMMA and 1.3% for polystyrene. For the depth dose measurement from near the surface toR80, the agreement is within 1.5% for PMMA, 2.5% for polystyrene, and 2.8% for electron solid water. It also demonstrates that for electron solid water, the new approach provides better agreement with experiment data for the beam calibration atdmax.

ISSN:0269-3127    

DOI:10.1118/1.597498

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597651

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

White polystyrene (C8H8with 2 wt.% to TiO2) has been compared to water using photon beams with nominal accelerating potential of 4 MV, 6 MV, 15 MV and 25 MV. The absolute reading and hence the measured absorbed dose is found to be up to 3% lower in white polystyrene which can be taken into account by using a correction factor. From these data differences in relative measurements (TMR or percentage depth dose) can be calculated showing differences of less than 0.7% normalized to maximum dose. Since the correction factor shows a dependence on photon energy and field size the use of a common scaling factor is not possible. The effect on photon dosimetry is discusssed.

ISSN:0269-3127    

DOI:10.1118/1.597652

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new type of beam‐energy monitor is described, which has been developed to check the energies of electron beams from radiotherapy accelerators. The monitor consists of a pair of scintillating fibers and photodiode read‐out circuits that measure the energy dependence of electron transmission through a wedge‐shaped absorber. A linear energy response and 1% accuracy for energy constancy measurements are attained with the monitor. The monitor having advantages of simple mechanical and electronic constructions, small size, and low cost is suitable for practical use as a portable device.

ISSN:0269-3127    

DOI:10.1118/1.597499

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597653

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new technique is presented for determining the homogeneity of the linear activity. The method uses two x‐ray films that are firmly pressed against opposing sides of a needle containing a radioactive line source. The homogeneity of the linear activity of 68 iridium‐192 wires has been measured. About 80% of the measured linear activities showed deviations of no more than 5% from the mean; about 20% of the measured linear activities showed deviations of 5%–10% from the mean. Individual wires may contain deviations of 5% or more extended over several centimeters, which causes the dose in the prescription point to deviate 5% from the desired value. Four wires (8%) were rejected for this reason.

ISSN:0269-3127    

DOI:10.1118/1.597500

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Highly focused dose distributions for radiosurgery applications are successfully achieved using either multiple static high‐energy particle beams or multiple‐arc circular x‐ray beams from a linac. It has been suggested that conformal x‐ray techniques using dynamically shaped beams with a moving radiation source would offer advantages compared to the use of only circular beams. It is also thought that, generally, charged particle beams such as protons offer dose deposition advantages compared to x‐ray beams. A comparison of dose distributions was made between a small number of discrete proton beams, multiple‐arc circular x‐ray beams, and conformal x‐ray techniques. Treatment planning of a selection of radiosurgery cases was done for these three techniques. Target volumes ranged from 1.0–25.0 cm3. Dose distributions and dose volume histograms of the target and surrounding normal brain were calculated. The advantages and limitations of each technique were primarily dependent upon the shape and size of the target volume. In general, proton dose distributions were superior to x‐ray distributions; both shaped proton and shaped x‐ray beams delivered dose distributions which were more conformal than x‐ray techniques using circular beams; and the differences between all proton and x‐ray distributions were negligible for the smallest target volumes, and greatest for the larger target volumes.

ISSN:0269-3127    

DOI:10.1118/1.597654

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dosimetric characteristics of small diameter 6 MV photon beams for a commercial radiosurgery system have been measured in a solid water phantom with a new diamond detector and compared with measurements using ap‐type Si photon diode, small volume cylindrical and parallel plate ionization chambers, and radiographic films. Tissue maximum ratios (TMR) and total scatter factors (Sc,p) were measured with the three ionometric (diamond, diode, ion chamber) detectors for 12 circular beams ranging in diameter from 12.5 to 40 mm. The TMR values obtained with the three ionometric detectors agreed well (±1%) for all cone sizes and depths investigated when the displacement of the sensitive volume of the detector from its front surface is taken into account. TheSc,pfactors obtained with the ionometric detectors also agreed well (±1.2%) for field sizes greater than 20 mm in diameter. For smaller field sizes, the cylindrical and parallel plate chambers measure a smallerSc,pfactor, as a result of the steep dose gradients across their sensitive volumes. Cross‐beam profiles acquired with the diamond detector agree well with the measurements with the diode detector and radiographic film. A distortion in the measured profiles in terms of broadened penumbra is observed with a small volume cylindrical ionization chamber.

ISSN:0269-3127    

DOI:10.1118/1.597655

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY