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W F Hanson

Publications and source records attributed to W F Hanson.

At least 37 records · Page 2Linked to original sources
PubMed1995

How water equivalent are water-equivalent solid materials for output calibration of photon and electron beams?

The water equivalency of five "water-equivalent" solid phantom materials was evaluated in terms of output calibration and energy characterization over a range of energies for both photon (Co-60 to 24 MV) and electron (6-20 MeV) beams. Evaluations compared absorbed doses calculated from ionization measurements using the same dosimeter in the solid phantom materials and in natural water (H2O). Ionization measurements were taken at various calibration depths. The Radiological Physics Center's standard dosimetry system, a Farmer-type ion chamber in a water phantom, was used. Complying with the TG-21 calibration protocol, absorbed doses were calculated using eight measurement and calculational techniques for photons and five for electrons. Results of repeat measurements taken over a period of 2 1/2 years were reproducible to within a +/- 0.3% spread. Results showed that various combinations of measurement techniques and solid phantom materials caused a spread of 3%-4% in the calculation of dose relative to the dose determined from measurements in water for all beam energies on both modalities. An energy dependence of the dose ratios was observed for both photons and electrons.

Biometry↗
PubMed1994

Dosimetry characteristics of four fast neutron generators involved in RTOG interinstitutional clinical trials.

PURPOSE: To demonstrate the consistency of dosimetry data used for four fast neutron generators involved in inter-institutional clinical trials. METHODS AND MATERIALS: Central-axis dosimetry characteristics (field size dependence, percentage depth dose, beam modifier factors) at four institutions were measured by independent physicists from the Radiological Physics Center. These measurements were made in water with a single set of dosimetry equipment using tissue equivalent ionization chambers. Measurements were made with the chamber cavities filled with stationary air and flowing tissue-equivalent gas. All measurements were performed using techniques developed by the Radiological Physics Center for conventional radiotherapy equipment. The results of our measurements were compared to the data used by the institution to determine the consistency of patient doses reported to the Radiation Therapy Oncology Group. RESULTS: The agreement of the Radiological Physics Center and institution data is similar to what is typically found with conventional radiotherapy equipment. CONCLUSIONS: The doses reported by these institutions to the Fast Neutron Working Group of the Radiation Therapy Oncology Group are consistent, within accepted clinical criteria.

Fast Neutrons↗
PubMed1990

RPC technical report #18: information that should be included in every patient's radiotherapy treatment record (external beam).

The Radiological Physics Center (RPC) has frequently been asked to develop a model daily treatment record. It became obvious that there was some minimum information that must be in patient records. However, equally obvious was the fact that practices at institutions could be very different, so organization of information in records was different. We have compiled a list of data that must be in a patient's treatment record to assure that the radiation treatment can be reconstructed at a later date. The ordering of this information in forms and records is left to the discretion of the institution.

Forms and Records Control↗
PubMed1986

Mailable TLD system for photon and electron therapy beams.

The mailable TLD system developed by the Radiological Physics Center for monitoring calibration of photon beam energies from cobalt 60 to 25 MV and electron beam energies from 6 to 20 MeV has been in use since 1977 for photons and since 1982 for electron beams. Design considerations, proper use of the system and calibration techniques are detailed. The accuracy of the system is comparable to that of ion chamber measurements made in a water phantom, although it shows less precision.

Electrons↗
PubMed1976

Use of auxiliary collimating devices in the treatment for breast cancer with 60Co teletherapy units. I. Dosimetric considerations.

The dosimetry of tangential field treatment for breast cancer using an auxiliary collimating device on 60Co teletherapy machines is reviewed. The position of the device with respect to the source axis has negligible effect on either the dose rate or the dose distribution. Measurements with thermoluminescent dosimeters in an anatomic phantom verify the accuracy of the method of correction of isodose distributions for sloping skin. Possible systematic errors in dosimetry are identified.

Breast Neoplasms↗
PubMed1976

Use of axiliary collimating devices in the treatment for breast cancer with 60Co teletherapy units. II. Dose to the skin.

Special auxiliary collimating devices are often used in radiation therapy for breast cancer in order to treat the chest wall with tangential 60Co fields. Some of these devices are manufactured with a permanently attached Lucite end plate which increases the skin dose due to electron contamination. The relative surface dose was measured on a 60Co irradiator utilizing the collimating device with and without the Lucite plate. A parallel plate extrapolation chamber was used. The surface dose was measured as a function of the distance from the end of the device, the position in the field, and the angle of incidence for typical chest wall fields. These results are used to compare the dose to the skin of the chest wall for several treatment regimes. The data indicate that if the Lucite end plate is permanently attached, the skin dose may be enhanced to the extent that the use of additional bolus may be neither necessary nor desirable.

Breast Neoplasms↗
PubMedDate not supplied

Calibration in water versus calibration in air for cobalt-60 gamma rays.

In the United States it is common practice to calibrate Cobalt-60 teletherapy machines "in air," despite recommendations by the International Commission on Radiation Units and Measurements (ICRU) and other organizations that calibration be accomplished by measurement at 5-cm depth in a water phantom. A comparison has been made between the results of ionization measurements in air at 80.5-cm distance from the source and in water at 80-cm source-skin distance (SSD) for the determination of absorbed dose at three depth (5, 10, and 15 cm) for each of three fields sizes (6 X 6, 10 X 10, and 20 X 20 cm2), for a total of 42 Cobalt-60 machines. The mean of the ratio, absorbed dose from in-water measurements to absorbed dose at the same depth calculated from in-air measurements, ranged frt 5-cm depth for a 20 X 20-cm2 field size. Reasons for the differences are offered, and compliance with ICRU recommendations is suggested.

Air↗
PubMedDate not supplied

A review of the reliability of chamber factors used clinically in the United States (1968--1976).

One of the principal concerns of a physicist responsible for calibrating megavoltage radiotherapy equipment is the validity and stability of the 60Co exposure correction factor assigned to his ionization-chamber and electrometer system. It is the practice of the AAPM Radiological Physics Center (RPC) to perform an intercomparison between the RPC chamber and electrometer system and the chamber and electrometer in use at each of the various institutions visited by the RPC. The results of 202 such intercomparisons are reviewed to determine (1) the consistency in the assignment of exposure correction factors by a calibrating agency with itself and with other calibrating agencies, and (2) the dependence of the reliability of the exposure correction factors upon the type of field instrument and the time since calibration.

Cobalt Radioisotopes↗
PubMedDate not supplied

An analysis of discrepancies encountered by the AAPM radiological physics center.

The AAPM Radiological Physics Center has reviewed 188 institutions and has evaluated such parameters as coincidence of radiation field and light field, timer error (end effect), beam flatness and symmetry, transmission through blocking trays, wedges and compensators, and central-axis depth-dose data. In previous papers these data had been presented in combination as they resulted in discrepancies in tumor dose. The individual sources of discrepancies were listed only as frequency and maximum deviation. A detailed analysis is now presented which may help define criteria of recommended practice.

Cobalt Radioisotopes↗
PubMedDate not supplied

Calculative technique to correct for the change in linear accelerator beam energy at off-axis points.

The change in energy of linear accelerator x-ray beams from the central ray to off-axis points causes errors in the dose calculated by conventional techniques for large, irregularly shaped fields. A modification of conventional calculative methods to correct for the change in beam energy is presented. The results of measurements in irregular fields on a Clinac-4 are reported which verify the validity of the calculative method. A discussion of the clinical significance will point out errors of 3% to 4% in conventional dose calculations.

Particle Accelerators↗
PubMedDate not supplied

Off-axis beam quality change in linear accelerator x-ray beams.

The effective energy of the x-ray beam from linear accelerators changes as a function of the position in the beam due to nonuniform filtration by the flattening filter. In this work, the transmittance through a water column was measured in good geometry and the beam quality characterized in units of HVL in water. Measurements were made on a variety of linear accelerators from 4 to 10 MV. The beam energy decreased with increasing distance from the central ray for all accelerators measured.

Particle Accelerators↗
PubMedDate not supplied

A review of the discrepancy between the in-air and in-water calibration of cobalt-60 machines.

Causes for the discrepancy noted by Grant et al. between the in-water and in-air calibration of 60Co are discussed. Data are presented from measurements with a set of ionization chambers with thimbles of 0.5, 1.0, and 1.5 cm outside radii. These data include measurements of percentage depth dose, backscatter factors, and displacement factors. The results show that the discrepancy noted by Grant et al. is caused by a combination of small errors both in depth dose data and in the displacement factor incorporated into C lambda.

Air Ionization↗
PubMedDate not supplied

A comparison of high-energy accelerator depth dose data.

Accurate depth dose information is necessary for the use of high-energy radiotherapy photon beam units. It would be useful, therefore, to have one set of published data available for each different type unit manufactured to which physicists can compare their measured data. Pertinent questions are raised regarding the similarity between accelerators and their central axis depth dose characteristics, the availability of adequate published central axis depth dose data, and the minimum amount of data needed to determine the applicability of published data to a particular machine. Data taken by the Radiological Physics Center (RPC) for 4-10 MV units are analyzed and compared with published data in an attempt to answer these questions.

Particle Accelerators↗
PubMedDate not supplied

The implementation of the AAPM Task Group 21 protocol by the Radiological Physics Center and its implications.

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.

Calibration↗