Gideon Barnea

Monte Carlo optimization of metal/phosphor screens at megavoltage energies

The physics of imaging with metal/phosphor (Gd2O2S:Tb on brass) screens at megavoltage energies has been investigated using Monte Carlo simulation. It has been found that pair production is a significant contributor to energy deposition for Bremsstrahlung beams with energies greater than 6 MV. The effects of different thicknesses of phosphor and metal have been studied, and it is shown that the metal plays a significant role in establishing electronic equilibrium in the phosphor. The transport of optical photons through the phosphor has been modeled, and was found that only 10% to 20% of the light created in the phosphor escapes from the surface, with much of the loss being due to total internal reflection at the surface. Calculated results have been compared with experimental measurements of screen brightness for different phosphor and metal thicknesses. The SNR of a video electronic portal imaging device (VEPID) has been calculated as a function of x-ray and optical photon detection efficiency. The non-Poisson distribution of energy deposition in the phosphor is an important contributor to the SNR. The results of this paper should serve as a useful guide to the engineering design of future electronic portal imaging systems.

Measurement of the source size of a 6‐ and 18‐MV radiotherapy linac

The quality of portal imaging is strongly affected by the source size of the radiotherapy machine. The effective source size of the dual-energy clinac 1800 (6 and 18 MV) was measured with a 50 microns wide and 120 mm long slit formed by two tungsten-copper alloy blocks. A series of slit images were obtained by translating the slit horizontally. The images were analyzed using a microdensitometer. The measured data was simulated using an analytical model of the source and its size was derived by a best-fit analysis. For both energies the FWHM was found to be 1.5 +/- 0.1 mm.

Electron beam therapy with transverse magnetic fields

Detailed Monte Carlo electron transport simulations were carried out for the purpose of investigating the possibility of improving electron dose distribution for therapeutic applications, by using transverse magnetic fields. The case studied here is that of a 15 MeV electron beam of 6 cm diameter. The electrons pass through 4 cm of field-free tissue and a transverse magnetic field is applied for depth greater than 4 cm. A field of 3 T was found to improve the skin sparing factor by a factor of 2, when compared to field-free irradiation. A field of 2 T could also have a significant effect although less pronounced than 3 T while, for the case at hand, a magnetic field of only 1 T is not effective. The results here include detailed energy deposition contours in three dimensions.

A study of multiple scattering background in Compton scatter imaging

Abstract The multiple scattering background in Compton scatter imaging at 662 keV is studied, both experimentally and by Monte Carlo radiation transport calculations, as a function of the scattering angle, scattering material (aluminium, brass and tin) and object thickness. A double-peak structure was observed in the pulse-height distribution for the thicker brass and tin objects and at the larger scattering angles (90° and 120°). In addition to the Compton peak, a second peak appeared at a higher energy. Monte Carlo transport simulations have revealed the origin of the second peak: photons that have scattered exactly twice before reaching the detector. The ratio of the multiple-scattered radiation to the total radiation detected was calculated as a function of the energy-window width around the Compton peak and scattering angle. The results of this study may help in the design of future Compton scatter imaging apparatus.

Use of storage phosphor imaging plates in portal imaging and high-energy radiography : the intensifying effect of metallic screens on the sensitivity

The sensitivity of storage phosphor imaging plates (SPIP) at megavolt photon energies (60Co, 6-, 10-, and 18-MV radiotherapy beams) is studied both experimentally and by Monte Carlo radiation transport calculations. In addition, the same techniques are used to investigate the intensifying effect of metal screens on the sensitivity of the SPIP. The results provide evidence that the sensitivity of the SPIPs is proportional to the absorbed energy in the phosphor layer per cGy. The spectral sensitivity is calculated for photon energies between 10 keV and 20 MeV for various SPIP-screen combinations.

PENUMBRAL IMAGING MADE EASY

A new and most direct method of penumbral imaging is presented. It is shown that for square (or polygonal) apertures, the source distribution is given by the second‐order mixed Cartesian derivative of the penumbral image intensity distribution. Use of this approach is illustrated by computer simulation. Two different source sizes (3.2 mm and 10 μm) of high‐energy gamma sources were reconstructed. The effect of noise in the recording system is studied and a simple noise reduction technique is shown to be quite effective.

Real-time measurement of the focal-spot intensity-distribution for megavolt flash X-ray machines

A new method for measuring the 2D intensity-distribution of the focal-spot of megavolt flash X-ray machines is demonstrated. A position-sensitive plane detector views the radiation source through an X-ray-opaque large square aperture. The square aperture was introduced in a previous publication [1] and it was shown that in this case, the focal-spot intensity-distribution is given by the second order mixed Cartesian partial derivative of the penumbral image intensity distribution. This technique is demonstrated by measuring the intensity distribution of the focalspot of a pulsed X-ray machine (1 MV, 40 kA, 50 ns). The results are compared with the physical damage visible on the metal target.

Analysis of dose to patient, spouse/caretaker, and staff, from an implanted trackable radioactive fiducial for use in the radiation treatment of prostate cancera)

PURPOSE A fiducial tracking system based on a novel radioactive tracking technology is being developed for real-time target tracking in radiation therapy. In this study, the authors calculate the radiation dose to the patient, the spouse/caretaker, and the medical staff that would result from a 100 microCi Ir192 radioactive fiducial marker permanently implanted in the prostate of a radiation therapy patient. METHODS Local tissue dose was calculated by Monte Carlo simulation. The patients whole body effective dose equivalent was calculated by summing the doses to the sensitive organs. Exposure of the spouse/caretaker was calculated from the NRC guidelines. Exposure of the medical staff was based on estimates of proximity to and time spent with the patient. RESULTS The local dose is below 40 Gy at 5 mm from the marker and below 10 Gy at 10 mm from the marker. The whole body effective dose equivalent to the patient is 64 mSv. The dose to the spouse/caretaker is 0.25 mSv. The annual exposures of the medical staff are 0.2 mSv for a doctor performing implantations and 0.34 mSv for a radiation therapist positioning patients for therapy. CONCLUSIONS The local dose is not expected to have any clinically significant effect on the surrounding tissue which is irradiated during therapy. The dose to the patient is small in comparison to the whole body dose received from the therapy itself. The exposure of all other people is well below the recommended limits. The authors conclude that there is no radiation exposure related contraindication for use of this technology in the radiation treatment of prostate cancer.

Electron beam therapy with coil-generated magnetic fields.

This paper presents an initial study on the issues involved in the practical implementation of the use of transverse magnetic fields in electron beam therapy. By using such magnetic fields the dose delivered to the tumor region can increase significantly relative to that deposited to the healthy tissue. Initially we calculated the magnetic fields produced by the Helmholtz coil and modified Helmholtz coil configurations. These configurations, which can readily be used to generate high intensity magnetic fields, approximate the idealized magnetic fields studied in our previous publications. It was therefore of interest to perform a detailed study of the fields produced by these configurations. Electron beam dose distributions for 15 MeV electrons were calculated using the ACCEPTM code for a 3T transverse magnetic field produced by the modified Helmholtz configuration. The dose distribution was compared to those obtained with no magnetic field. The results were similar to those obtained in our previous work, where an idealized step function magnetic field was used and a 3T field was shown to be the optimal field strength. A simpler configuration was also studied in which a single external coil was used to generate the field. Electron dose distributions are also presented for a given geometry and given magnetic field strength using this configuration. The results indicate that this method is more difficult to apply to radiotherapy due to its lack of symmetry and its irregularity. For the various configurations dealt with here, a major problem is the need to shield the magnetic field in the beam propagation volume, a topic that must be studied in detail.

Use of computed radiography for portal imaging

This paper compares image quality of conventional film based portal images with the quality of portal images based on computed radiography (CR). The paper concludes that portal imaging with CR imaging plates instead of the conventional film provides images which have a significantly higher image quality. CR based portal images permit better visualization of important landmarks.