Peter R. Almond

The Physical Measurements of Electron Beams from 6 to 18 Mev: Absorbed Dose and Energy Calibration

An aluminium calorimeter has been used to measure the G-values for the ferrous sulphate dosemeter for incident electron energies of 13·25, 15·9 and 18 Mev. The ferrous sulphate dosemeter was then used to determine the correction factor to be applied to an ion chamber reading taken in a Lucite phantom to obtain absorbed dose in water. The factor was found to vary by 2% from 6 to 18 Mev, which can be correlated to the change in the average relative stopping power of the electrons in the phantom and the gas, taking the polarization effect into account. The absorbed dose calibration using the above method is accurate to within 2%. A quick and relatively accurate method has been described for determining the energy of the electron beam. Agreement within 3% has been found between this method and the method utilizing the measured electron range.

The energy response of LiF, CaF2, and Li2B4O7: Mn to high energy radiations

The decrease in sensitivity of 7LiF to high energy electrons and X-rays when compared with 7LiF exposed to 60Co gamma -rays was investigated and the energy dependence measurements were also done for CaF2 Li2B4O7. Measurements were made at electron energies of 6 to 18 MeV, X-ray energies of 18.5 and 22 Mv, for gamma -rays from 137Cs and 60Co. The difference in response between 7LiF and the other phosphors can be considered as real, since the exposures at each energy were made using the same calibrations. To explain the results, the general cavity theory has been used.

Measurement of fast neutrons produced by high-energy X-ray beams of medical electron accelerators

The fast neutron contamination associated with the 25 MV X-ray beam of a clinical linear accelerator and with the 19 MV X-ray beam of a betatron has been measured at the patient treatment location, using both fission fragment track detectors and silicon diode dosemeters. Measurements are made of the neutron fluence and dose, both in and out of the primary photon beam, at distances up to 60 cm from the central axis. Neutron distributions are similar for the linac and the betatron, with approximately ten times greater neutron dose in the X-ray field than at 20 cm outside it.

High-energy electron dose perturbations in regions of tissue heterogeneity.

The standard electron-beam dose distributions that are used in treatment planning for high-energy electron-beam radiotherapy are generally measured in homogeneous tissue-equivalent phantoms. In actual practice, however, the tissues under treatment are not homogeneous and the dose distribution is highly sensitive to the presence of tissue heterogeneities. The perturbations of the standard dose distributions produced by variations in density and structure have been demonstrated in phantoms by a number of investigator^.-^ We have investigated these dose perturbations in living tissues using lithium fluoride thermoluminescent dosimeters, and a t the same time, we have carried out parallel studies in physical phantoms. From these studies, methods have been derived for correcting the standard dose distributions so that inhomogeneities may be taken into account during treatment planning. Our interest stems largely from a desire to minimize the radiation dose to the lung in treatment of the chest wall and internal mammary nodes while maintaining an adequate treatment to these areas and also to the shielding effect of bone upon underlying tissue. Early results from these studies have previously been reported.

The use of Lithium Flouride Thermoluminescent Dosemeters to measure the Dose Distribution of a 15 MeV Electron Beam

A comparison was made of the central axis depth-dose curves obtained for 15 MeV electrons using an ion chamber and LiF dosemeters. The two uncorrected curves were very similar and gave the same depth-dose curves within experimental limits. Corrections that must be made to the LiF curve for energy and dose dependence and to the ion chamber readings for the polarization effect are small, (a few per cent); the corrected curves are essentially in agreement within experimental errors.

The use of commercially available ionization chambers for absorbed dose calibrations of high energy x-rays and electron beams.

Procedures suggested for using commercially available ion chambers for the calibration of high energy radiotherapy machines, have been investigated experimentally. Fourteen chambers of three different types were investigated for X-rays of 18.5 and 22 MV and for electrons in the energy range 6-18 MeV. For the X-rays and electrons in the range 9-18 MeV, all chambers gave the same calibrations within acceptable limits. For 6 MeV electrons, care must be taken because of the rapidly varying dose field, but for chambers of each type, satisfactory results were obtained.

The Effects of Sternum upon the Central Axis Depth-Dose Curves for High-Energy Electrons1

In planning treatment of the internal mammary chain of lymph nodes with high-energy electron beams, we have become interested in the effect of the sternum on dose distribution. Studies have been carried out with fresh specimens of human sternum in a water phantom. Doses were recorded with an ion chamber held in an x−y plotter. Due to the curvature of the bone, the probe can be brought only as close as 1 to 2 cm to the bone. A depth-dose curve is then obtained for points behind the bone. The bone is removed, a normal curve is obtained, and all readings are expressed as percentages of the maximum for this curve. It was found that thicknesses of bone up to about 1 cm do not attenuate the beam as much as water, and that for the thicker sternum, with its relatively greater investment of cortical bone, there was some increased attenuation relative to water (Fig. 1). Previous publications have described marked reduction in depth dose due to sternum. An absorption-equivalent thickness relative to water (A.E.T.) o...

Energy calibration of high energy electrons using a Cerenkov detector and a comparison with different methods

One of the basic problems in the use of high energy therapy machines is the precise determination of the primary energy of the electron beam. A new way of estimating electron beam energy has been developed using Cerenkov radiation generated by relativistic electrons passing through a gas. Carbon dioxide and air, which have threshold values of 17.0 and 22 MeV at S.T.P. were studied. The detector consists of a gas filled pipe with a thin brass window at the front and a mirror set at 45 degrees to the beam at the far end which reflects the Cerenkov radiation onto the cathode of a photomultiplier tube. Energies of electrons from a 32 MeV linear accelerator, determined by this method agree within the limits of experimental error with those from the range energy method.

The exposure rate constant for a silver wire 125I seed

The physical characteristics of the newer silver wire 125I seed were measured with a scintillation spectrometer to compare them with those of the original gold sphere 125I seed. The exposure rate constant was determined by converting the count rate from a scintillation spectrometer into the photon-fluence rate incident upon the detector, then calculating the exposure rate from the photon-fluence rate. The exposure rate constant measured perpendicular to the long axis of the seed is 1.361 R cm2/mCi h (1.192 cGy cm2/mCi h) +/- 3.7%, a value that compares favorably with the theoretical exposure rate constant of 1.354 R cm2/mCi h (1.186 cGy cm2/mCi h) calculated from the 125I emissions data. A value of 1.309 R cm2/mCi h was previously reported for the gold sphere 125I seed using the same technique. The angular intensity distribution and anisotropy factor of the silver wire 125I seed are shown to be very similar to those of the gold sphere 125I seed, leading to the conclusion that the clinical application of the two types of 125I seeds need not change.

A Cerenkov detector for the energy calibration of electron beams from 9-22 MeV

A new way of estimating the energy of electron beams has been developed using Cerenkov radiation generated by relativistic electrons passing through different gases. Recently, the authors have been studying this method using three different gases, pentane, CO2 and air which have threshold values of 9.0, 17.0, 22 MeV respectively at STP. The detector consists of a 120 cm long gas filled pipe with a thin brass window at the front; a mirror set at 45 degrees to the beam direction at the far end reflecs the Cerenkov radiation onto the cathode of a photomultiplier tube. This work was done primarily with a 32 MeV linear accelerator for which it was difficult to vary the energy continuously and a comparison with an 18 MeV betatron was made using activation analysis and range relationship methods.