Physical factors affecting absorbed dose to the skin from cobalt-60 gamma rays and 25-MV x rays.

When there is an absence of scattering material adjacent to the skin on the exit side of a megavoltage beam the dose to the skin is less than would be calculated using depth dose tables or isodose distributions measured in "semi-infinite" phantoms. Ionization measurements using a thin-window parallel-plate chamber show that the dose at 4 mg/cm2 from the exit surface is 14% to 16% less for cobalt-60 gamma rays and about 8% less for 25-MV x rays compared to the dose with full backscatter. As the angle of incidence increases the skin dose increases due to radiation scattered toward the surface. A method for the calculation of skin doses from tangential therapy beams is described.

Buildup region and skin-dose measurements for the Therac 6 linear accelerator for radiation therapy.

Buildup and surface-dose measurements were taken for the 6 MV photon beam from a Therac 6 linear accelerator manufactured by Atomic Energy of Canada Limited (AECL) with and without a lucite blocking tray in place. Further measurements were made with a copper filter designed to reduce secondary electrons emitted by photon interactions with the Lucite tray. The results are discussed in relation to skin-sparing for radiation therapy patients. The measurements were made with a fixed volume PTW parallel-plate ionization chamber and corrected to zero-chamber volume. The results were found to be consistent with similar measurements taken with a variable volume extrapolation chamber.

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.

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.

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.

Electron beam central axis depth dose measurements.

Central axis depth dose measurements were made by the Radiological Physics Center on over 70 electron-producing machines used in radiation therapy. These data were consistent for each machine model and nominal energy. However, the data show that depth dose relations can vary significantly among different machine models for electron beams having the same nominal energy. Analysis shows that both the method used to achieve beam flatness and the mean incident electron energy determine the central axis depth dose curve past the depth of maximum dose. A linear relation of depth dose versus mean incident electron energy is used to predict depth dose to within 2 mm for most electron beams used clinically at depths greater than d95.

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.

Measurement of dose distribution around Fletcher-Suit-Delcos colpostats using a Therados radiation field analyzer (RFA-3).

Using a small silicon diode and the Therados RFA-3 automated dosimetry system, the relative dose distribution around Fletcher-Suit-Delcos colpostats was measured. Thermoluminescent dosimeters (LiF) were used for absorbed dose measurements. These data provide the basis for computerized dose calculations which will account for the attenuation effects of the rectal and bladder shieldings. The stainless-steel housing attenuated the radiation of a Cs-137 source by 6%. The tungsten shields caused a further dose reduction of 15%-25%.

Dose enhancement from backscattered radiation at tissue-metal interfaces irradiated with high energy electrons.

Irradiation of a tissue-metal interface with 13 MeV to 20 MeV electrons results in an increased dose to the tissue on the entrance side of the metal. Ionization measurements were made with a thin-window parallel-plate chamber to determine the magnitude of the dose enhancement as a function of incident electron energy, thickness and atomic number of metals introduced into the electron beam. The presence of a metal resulted in a dose ranging from 6% to approximately 50% greater than that measured with no metal in the beam. Most of this increase in dose may be eliminated by the addition of 1-2 g/cm2 of low Z material between tissue and metal.

Comparison of skin doses to large fields using tangential beams from cobalt-60 gamma rays and 4-MV x rays.

Excess radiation to the skin during external beam megavoltage radiation therapy has reportedly caused excessive erythema in patients treated with the Clinac 4 linear accelerator on sloping surfaces, but not for similar treatments with cobalt 60. Doses at the epidermal level were measured under geometries simulating sloping surfaces for a Clinac 4 and an Eldorado 8 cobalt-60 teletherapy machine. For equal doses to the axilla, doses to the epidermal layer were similar. When the tumor dose was calculated for the mediastinum, the dose to the skin in the axillary region was 12% higher for the Clinac 4.

Surface dose from megavoltage therapy machines.

To determine the cause of increased skin reactions near the edges of the field the authors measured doses at and near the surface of the skin with an extrapolation chamber for the gamma-ray beam of an AECL Eldorado 8 60Co irradiator and the x-ray beams of a Varian Clinac 4 linear accelerator, a Varian Clinac 6 linear accelerator, and an ARCO Mevatron 8 linear accelerator. Buildup curves were measured on each machine to locate the depth of the maximum dose (Dmax). The surface dose of the central axis was measured as a function of field size. Off-axis surface doses were measured on the Clinac 4 linear accelerator and on the Eldorado 8 60Co therapy unit. No measurable increase in surface dose was found leading the authors to believe that the observed skin reaction in the axilla for the Clinac 4 linear accelerator is due to a combination of increased dose at Dmax from the anterior mantle field and an increase in exit dose from the posterior mantle field when compared to similar treatments for 60Co.