Duncan M. Galbraith

An integral quality monitoring system for real‐time verification of intensity modulated radiation therapy

PURPOSE To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy. METHODS The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX. RESULTS For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%. CONCLUSIONS The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

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.

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.

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.

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.

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.

Application of small 60Co beams in the treatment of malignant melanoma at the optic disc.

An easy and convenient method for the treatment of a malignant melanoma located near the optic disc by means of small /sup 60/Co beams is presented. With an existing treatment unit, a small beam is produced by a secondary collimator 7 cm above the patients skin surface. A uniform dose (+-5 percent) over one third of the volume of the eye can be obtained by a special arrangement of a pair of oblique beams. The disadvantages shared by all the implantation techniques are not encountered in our method. The uncertainty inherent in the large dose gradient and high contact dose obtained when one uses an implant have been eliminated. It is shown that the dose to the patients skin and lens can be kept well below the limits of normal tolerance.

A flatness and calibration monitor for accelerator photon and electron beams

A flatness monitor has been built to quickly and accurately check accelerator beam flatness and dose calibration. Consisting of a 7 X 7 ion chamber array, the unit operates in photon beams from 60Co energies to 25 MV and electron beams (scattered or scanned) from 6 MeV to 25 MeV.

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.

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.