摘要:

The relative dose efficiencies (RDE) of various antiscatter grids and air gaps were determined for conditions simulating those found in pediatric radiography, using phantoms representing a newborn child, a 5‐yr‐old and a 10‐yr‐old child. Our data indicate than an air gap is best for the newborn, due to the low levels of scatter. The 8:1 fiber grid or 15.2‐cm air gap without a grid can improve dose efficiency (DE) for the 5‐yr‐old child by 20%–25% relative to the 3.3‐cm air gap and no‐grid technique, while for the 10‐yr‐old child, DE can be improved by 40% with an 8:1 fiber grid.

ISSN:0269-3127    

DOI:10.1118/1.595520

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes a light, easy to use Moire camera and a computer program written to implement it as part of a clinical compensating filters design system.

ISSN:0269-3127    

DOI:10.1118/1.595637

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A filter fluorometer suitable for endoscopic applications has been developed for detection and characterization of superficial tumors by the fluorescence of a previously injected, tumor‐specific agent, hematoporphyrin derivative. Fluorescence is excited by violet light conducted through a fiberoptic lightguide in the endoscope, and the fluorescence emission together with reflected violet are collected by another fiberoptic lightguide. The red fluorescence and violet are separated by a dichroic mirror and filter and detected in photomultiplier tubes. The ratio of the fluorescence signal to the reflected violet signal is proportional to the ratio of the fluorescence yield to the violet reflectivity but is insensitive to variations in distance, angle, and violet power. The instrument may be useful for localizing small tumors, and for quantitative measurements of the amount of hematoporphyrin derivative in the tumor, a requirement for accurate dosimetry in photoradiation therapy.

ISSN:0269-3127    

DOI:10.1118/1.595521

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo computer calculations have been made to assess the effects of small collimators (i.e., output field sizes ≤1 cm) on megavoltage x‐ray beams. Such collimators are often used in experiments to measure beam energy, half value layers, and other beam parameters. Our calculations demonstrate, however, that such procedures can introduce significant changes in these same parameters. Under certain conditions, more than 40% of the transmitted photons exiting the collimator do so with degraded energy. Optimum collimator design, however, can greatly reduce these effects. In particular, it is shown that the thickness of material required to effect a good collimator is not only dependent upon the photon energy, but also upon the size of the collimator hole.

ISSN:0269-3127    

DOI:10.1118/1.595639

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A system of computer programs has been created to allow the entry of radiotherapy treatment details as defined by the physician, the verification of the machine parameters at every treatment, and the recording of the entire course of treatment. Various utility programs are available to simplify the use and maintenance of the system. The majority of the code is written infortran‐77, the remainder inmacro‐11. The system has been implemented on a PDP 11/60 minicomputer for use with a Mevatron linear accelerator, the implementation required minor hardware changes to the accelerator.

ISSN:0269-3127    

DOI:10.1118/1.595640

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The library least‐squares method was applied to the analysis of gamma‐ray spectra obtained from an array of 54 NaI(Tl) detectors in a whole‐body counter. The analysis of spectra which were obtained over a period of 8 yr demonstrates the applicability of the method despite inherent variations encountered in large counting systems. The elements of interest analyzed were total‐body K, Ca, Na, Cl, and P. Least‐squares fits obtained with library standards derived from distributed sources were better than those obtained from library standards derived from localized sources.

ISSN:0269-3127    

DOI:10.1118/1.595522

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The presence of the Lucite blocking tray in a 45‐MV betatron beam leads to a considerable loss in the skin‐sparing properties of this unit. Measurements conducted with a parallel‐plate chamber show that the surface dose is approximately doubled (up to 58% of the maximum absorbed dose) and the depth of maximum dose is reduced by as much as 1.5 cm by the Lucite. These results suggest that separate dosimetry data should be kept for blocked and unblocked fields. The penetration of the electron contamination to the open field depth of maximum dose can lead to erroneous results if attenuation measurements are made at that depth. Studies of the effect of the lead sheet and leaded acrylic on the surface dose showed that under normal circumstances the presence of high‐Zmaterials increases the surface dose, hence these materials cannot in general be used as electron filters on very high energy x‐ray units. A Luciteless blocking tray with universal triangular corner blocks has been designed for use in special cases requiring a reduced surface dose.

ISSN:0269-3127    

DOI:10.1118/1.595523

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A single‐field rotational total skin electron irradiation technique has recently been developed at the McGill University for treatment of skin malignancies. The dose received by a given surface point during rotation in a uniform large electron field depends on the radius of rotation of the surface point, on the local radius of curvature of the contour in the vicinity of the point of interest, and on the shadows cast by limbs (arms upon trunk or head and neck, and legs upon each other). A method for calculating the surface dose distribution on a patient is presented accounting for the various parameters affecting the dose. A series of measurements were performed with polystyrene and a humanoid phantom, and an excellent agreement between measured and calculated dose distributions was obtained.

ISSN:0269-3127    

DOI:10.1118/1.595524

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Radiation Therapy Committee of the American Association of Physicists in Medicine appointed Task Group 21 to write a new protocol for the calibration of high‐energy photon and electron therapy beams. This protocol updates the physical parameters used in the calculations and is intended to account for differences in ionization chamber design and some differences between phantom materials that were not considered in previous protocols. This paper discusses how the Radiological Physics Center (RPC) intends to implement the new protocol, the changes required in the RPC calibration techniques, and the magnitude of the change in the RPC calculations of absorbed dose resulting from the implementation of the new protocol. Although the change in the RPC absorbed‐dose calculations will be only 0%–2% over the range of photon and electron energies of interest, some institutions using specific dosimetry systems may find their absorbed‐dose calculations changing by 4% or more.

ISSN:0269-3127    

DOI:10.1118/1.595525

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A procedure is described for calculating photon dose distributions using convolutions of radiation interaction kernels with calculated fluence distributions. The advantages and disadvantages of the concept are discussed.

ISSN:0269-3127    

DOI:10.1118/1.595526

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY