摘要:

Energy spectra of accelerated electron beams from a racetrack microtron were measured using a magnetic spectrometer. The spectrometer utilized a 90° 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 totalaprioriuncertainty (previously often called systematic or class B uncertainty) of the measured most probable energyEpis 0.22% (one approximate standard deviation) and theaposterioriuncertainty (previously often called random or class A uncertainty) is 0.04% (1σ). The estimated energy resolution (ΔE/E) of the spectrometer is 4×10−4. Spectral energy distributions of the electron beam were measured at aEp=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.

ISSN:0269-3127    

DOI:10.1118/1.597018

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Surface dose measurements in therapeutic x‐ray beams are of importance in determining the dose to the skin of patients undergoing radiotherapy. Measurements were performed in the 6‐MV beam of a medical linear accelerator with LiF thermoluminescence dosimeters (TLD) using a solid water phantom. TLD chips (surface area 3.17×3.17 cm2) of three different thicknesses (0.230, 0.099, and 0.038 g/cm2) were used to extrapolate dose readings to an infinitesimally thin layer of LiF. This surface dose was measured for field sizes ranging from 1×1 cm2to 40×40 cm2. The surface dose relative to maximum dose was found to be 10.0% for a field size of 5×5 cm2, 16.3% for 10×10 cm2, and 26.9% for 20×20 cm2. Using a 6‐mm Perspex block tray in the beam increased the surface dose in these fields to 10.7%, 17.7%, and 34.2% respectively. Due to the small size of the TLD chips, TLD extrapolation is applicable also for intracavity and exit dose determinations. The technique used forinvivodosimetry could provide clinicians information about the build up of dose up to 1‐mm depth in addition to an extrapolated surface dose measurement.

ISSN:0269-3127    

DOI:10.1118/1.597019

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A two dimensional approach to the evaluation of a missing tissue compensator system has been developed and implemented. Quantitative descriptors of the dose area histograms generated by scanning films irradiated under a variety of compensated and uncompensated conditions are proposed. The method has been found to be useful during the acceptance testing of a commercial compensator system and the results of this exercise are presented.

ISSN:0269-3127    

DOI:10.1118/1.597020

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A beam spoiler for a 6‐MV x‐ray beam was designed and spoiled beam dosimetry performed. The spoiled beam quality was similar to that of the unspoiled beam. The percentage depth doses at 5 mm were at least 90% for all field sizes except 4×4 cm, while the surface dose at 0 mm varied from 22.5% for a 4×4‐cm field to 85.8% for a 22×22‐cm field. The spoiled beam showed degraded beam flatness for small field size at superficial depths; the beam flatness at 6 mm for a 4×4‐cm field was ±9.0%. The beam flatness improved as the field size increased, e.g., at same depth, the beam flatness was ±2.5% for a 22×22‐cm field. The penumbra width (90%−10%) of the spoiled beam was greater than that of the unspoiled beam for all field sizes and at all depths; e.g., at 0‐mm depth, for a 22×22‐cm field, the penumbra width was 3.8 cm for the unspoiled beam and 7.2 cm for the spoiled beam. Beyonddmaxthe difference in the penumbra widths between the spoiled and unspoiled beam was about 3–4 mm for all field sizes and at all depths. The peripheral dose was larger for the spoiled beam.

ISSN:0269-3127    

DOI:10.1118/1.597021

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

While tissue‐maximum ratios (TMR) for60cobalt treatment units have been shown to be independent of source‐to‐axis distance (SAD), high‐energy photon beams demonstrate variations in their TMR as a function of SAD. Some authors have asserted that the distance dependence of the TMR stems from electron contamination in the beams, while others have suggested low‐energy, scattered photons as the cause. Using a magnet to sweep contaminant electrons out of the photon treatment beam eliminates any variation in TMR with distance. Thus, electron contamination accounts for all of the distance dependence, and any low‐energy, scattered photons behave indistinguishably like the high‐energy photons.

ISSN:0269-3127    

DOI:10.1118/1.597022

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

LiF:Mg,Ti thermoluminescent dosimeters (TLD's) were obtained from three manufacturers with the specification that they all lie within ±5% of the mean. All were chips of size 3 mm×3 mm×1 mm. The three sample groups were compared on the basis of their energy response, linearity of the thermoluminescent (TL) response with increasing exposure, sensitivity, and reproducibility of TL response. All TLD groups showed similar characteristics in energy response and linearity. This is to be expected since the base material and dopants were the same. However, the sensitivity varied among the three groups by as much as 40% and reproducibility measurements gave standard deviations ranging from 7% to 14% among the groups. Annealing procedures were identical in all cases, as were irradiation and readout conditions.

ISSN:0269-3127    

DOI:10.1118/1.597023

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple formula is evaluated for calculating the equivalent square collimator setting that gives the same headscatter factor as a given rectangular field. The expression requires that one parameter is determined experimentally. It is found that one single value of this parameter can be used for the six x‐ray beams studied (two accelerators, two energies, with and without wedge).

ISSN:0269-3127    

DOI:10.1118/1.597024

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597025

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597026

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597027

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY