Depth dose flattening of electron beams using a wire mesh bolus.

A common problem with low-energy electron beams (< 15 MeV) is their low surface dose when the incident electrons are monodirectional. This makes it difficult to deliver a uniform dose to tumor with any precision, limiting the clinical usefulness of such beams. A practical method is presented for greatly increasing the tissue depth enclosed by the 95% isodose region, while delivering the entire dose in a single uninterrupted treatment. Beam modification is achieved by placing a wire mesh of high atomic number (Z) on the treatment surface throughout the treatment. Electron beams of energies 6, 9, and 13 MeV are modified to produce a near uniform dose from the surface to the original depth of maximum, approximately doubling the depth enclosed by the 95% isodose. These beams have a step-function-like depth dose and can be arranged to deliver a constant dose to tumor located at varying depths while simultaneously sparing deeper tissues which are also located at varying depths. A single mesh design was found to be suitable for all energies.

Low-energy imaging with high-energy bremsstrahlung beams: analysis and scatter reduction.

The contrast and zero spatial frequency signal-to-noise ratio produced by a method for radiation therapy portal imaging known as low-energy imaging with high-energy bremsstrahlung beams have been mathematically analyzed. The analysis makes extensive use of Monte Carlo techniques and incorporates the detector, the spectrum, phantom, and geometry. The analysis is validated through comparison with measured data including subject contrast measurements and the attenuation of the beam with lead. Scatter reduction is found to be potentially the most effective method to improve contrast and SNR for a film based system. A large fraction of the scatter detected is of a much higher energy than that found in diagnostic radiology. Hence, traditional antiscatter grids, such as those used in diagnostic radiology, are ineffective. The analysis and theory from the literature are applied to design a new grid which is more appropriate for this application. The grid produces a modest improvement according to a contrast-detail study.

A dose-per-pulse monitor for a dual-mode medical accelerator.

On a radiotherapy accelerator, the dose monitoring system is the last level of protection between the patient and the extremely high dose rate which all accelerators are capable of producing. The risk of losing this level of protection is substantially reduced if two or more dose monitoring systems are used which are mechanically and electrically independent in design. This paper describes the installation of an independent radiation monitor in a dual-mode, computer-controlled accelerator with a moveable monitor chamber. The added device is fixed in the beam path, is capable of monitoring each beam pulse, and is capable of terminating irradiation within the pulse repetition period if any measured pulse is unacceptably high.

Low-energy imaging with high-energy bremsstrahlung beams.

A method is described for portal imaging with low-energy (approximately less than 150 keV) photons from a radiotherapy accelerator operating in a diagnostic mode. The low-energy photons are produced in the bremsstrahlung process, but are normally filtered out by thick high atomic number (Z) target materials. This absorption can be reduced by choosing a low Z target with the minimum thickness required to stop the electrons in the target. If, in addition, the operating energy is kept low (approximately 5 MeV) and the flattening filter is removed, low-energy photon images can be produced from the broad spectrum of photon energies by using standard diagnostic radiology high-Z fluorescent screen/film systems that strongly absorb at low but not high photon energies. Motion artifacts can then be avoided since only a small dose is required for such a procedure.

Direct measurement of electron contamination in cobalt beams using a charge detector.

A method is described in some detail for measuring the magnitude and penetration of the electron contamination in photon beams using a pancake charge detector. It is shown that the response of the detector to a photon beam can be separated from the component due to the electron contamination. In the present work, the detector is used to measure the electron fluence in a 60Co photon beam. This fluence is subsequently converted to dose by comparison with the fluence and dose measured from a pure electron beam (90Sr). This study proves, within experimental error, that the observed changes in the buildup region, with the collimator opening for both filtered and unfiltered 60Co beams, are due to electron, rather than photon, contamination.

Eye sparing in high energy X ray beams.

Treatment of cancer of the antrum and nasopharynx often includes the radiation of tissues close to an uninvolved eye. One treatment method consists of using an anterior high energy X ray beam directed to the tumor through the eye. To maintain a high dose adjacent to and behind the eye while reducing the entrance dose to the eye, build-up material is placed on the skin and a tunnel cut through to the eye. When the build-up material is tissue-like, the tunnel can be several centimeters in height and scattered radiation from the tunnel walls will largely offset the build-up properties of the beam. Using higher density build-up material, the dose to the superficial layers of the eye can be reduced almost to the limit set by the open beam characteristics. This technique has been used successfully for 8 years.

Theoretical and experimental investigation of dose enhancement due to charge storage in electron-irradiated phantoms.

Recent measurements have shown that significant errors in radiation dosimetry can arise by the use of insulating plastic phantoms which have been exposed to electron beams. The effect has been attributed to the generation of large electric fields in the phantom by charge storage causing alteration of electron trajectories and an increase in the measured dose. In this report, we examine this hypothesis theoretically by calculating the change in response to radiation of an ion chamber in a cylindrical cavity in an electron-irradiated polymethylmethacrylate phantom. The electric field distribution is determined using a model which allows for charge leakage by radiation-induced conductivity, and the dose in the cavity is determined by a Monte Carlo simulation using the EGS (electron gamma shower) code modified to account for electron trajectories in the electric field. The theoretical results are shown to agree well with new and previously published experimental dose enhancement data. The agreement is taken as confirmation of the reported explanation of the effect. The use of conducting phantoms in radiation dosimetry is advocated.

Dose errors due to charge storage in electron irradiated plastic phantoms.

Commercial plastics used for radiation dosimetry are good electrical insulators . Used in electron beams, these insulators store charge and produce internal electric fields large enough to measurably alter the electron dose distribution in the plastic. The reading per monitor unit from a cylindrical ion chamber imbedded in a polymethylmethacrylate (PMMA) or polystyrene phantom will increase with accumulated electron dose, the increase being detectable after about 20 Gy of 6-MeV electrons. The magnitude of the effect also depends on the type of the plastic, the thickness of the plastic, the wall thickness of the detector, the diameter and depth of the hole in the plastic, the energy of the electron beam, and the dose rate used. Effects of charge buildup have been documented elsewhere for very low energy electrons at extremely high doses and dose rates. Here we draw attention to the charging effects in plastics at the dose levels encountered in therapy dosimetry where ion chamber or other dosimeter readings may easily increase by 5% to 10% and where a phantom, once charged, will also affect subsequent readings taken in 60Co beams and high-energy electron and x-ray beams for periods of several days to many months. It is recommended that conducting plastic phantoms replace PMMA and polystyrene phantoms in radiation dosimetry.

Partial bolussing to improve the depth doses in the surface region of low energy electron beams.

In many low energy electron beams the surface dose is considerably less than the maximum dose, making them unsatisfactory for clinical application. A method is described for producing better surface dose uniformity in such beams. The method makes use of bolus applied to the patient for a fraction of each daily electron treatment. The technique is shown to be simple and practical. This approach is compared to more conventional techniques of using bolus in electron beams.