Robert J. Schulz

Radiation dose distributions in three dimensions from tomographic optical density scanning of polymer gels: I. Development of an optical scanner

A new method of dosimetry of ionizing radiations has been developed that makes use of tissue-equivalent polymer gels which are capable of recording three-dimensional dose distributions. The dosimetric data stored within the gels are measured using optical tomographic densitometry. The dose-response mechanism relies on the production of light scattering microparticles which result from the polymerization of acrylic comonomers dispersed in the gel. The attenuation of a collimated light beam caused by scattering in the irradiated optically turbid medium is directly related to the radiation dose over the range 0-10 Gy. An optical scanner has been developed which incorporates an He-Ne laser, photodiode detectors, and a rotating gel platform. Using mirrors mounted on a translating stage, the laser beam scans across the gel between each incremental rotation of the platform. Using the set of optical density projections obtained, a cross sectional image of the radiation field is then reconstructed. Doses in the range 0-10 Gy can be measured to better than 5% accuracy with a spatial resolution approximately 2 mm using the current prototype scanner. This method can be used for the determination of three-dimensional dose distributions in irradiated gels, including measurements of the complex distributions produced by multi-leaf collimators, dynamic wedge and stereotactic treatments, and for quality assurance procedures.

Radiation therapy dosimetry using magnetic resonance imaging of polymer gels

Further progress in the development of polymer gel dosimetry using MRI is reported, together with examples of its application to verify treatment plans for stereotactic radiosurgery and high dose rate brachytherapy. The dose distribution image produced in the tissue-equivalent gel by radiation-induced polymerization, and encoded in the spatial distribution of the NMR transverse relaxation rates (R2) of the water protons in the gel, is permanent. Maps of R2 are constructed from magnetic resonance imaging data and serve as a template for dose maps, which can be used to verify complex dose distributions from external sources or brachytherapy applicators. The integrating, three-dimensional, tissue-equivalent characteristics of polymer gels make it possible to obtain dose distributions not readily measured by conventional methods. An improved gel formulation (BANG-2) has a linear dose response that is independent of energy and dose rate for the situations studied to date. There is excellent agreement between the dose distributions predicted using treatment planning calculations and those measured using the gel method, and the clinical practical utility of MRI-based polymer gel dosimetry is thereby demonstrated.

Dose-response curves for Fricke-infused agarose gels as obtained by nuclear magnetic resonance

The radiation-response characteristics of agarose gels prepared with Fricke dosemeter solution have been studied. The response mechanism is an increase in the NMR longitudinal relaxation rate of protons caused by ferric ions. It has been observed that: (i) oxygen saturation assures consistent and maximum sensitivity; (ii) agarose concentrations in the range 1.0-2.0% have no effect upon sensitivity; (iii) the initial G value is 150 Fe3+/100 eV for gels containing 0.5 mM Fe2+ ions; (iv) increasing NMR frequencies only causes a moderate increase in sensitivity; (v) the gel dosemeters are dose rate independent in the range 4.7-24.2 Gy min-1; (vi) sensitivity is pH dependent, being zero at pH 7; (vii) freshly prepared gels are slightly more sensitive than those more than 24 h old; and (ix) the diffusion coefficient for ferric ions in a 1.0% agarose gel containing 0.0125 M H2SO4 is 1.83 x 10(-2) cm2 h-1, and this will require consideration for the NMR imaging of dose distributions.

Calculated response and wall correction factors for ionization chambers exposed to 60Co gamma rays

The response and wall correction factors for various ionization chambers in a cobalt-60 gamma-ray field have been calculated using a Monte Carlo photon-electron transport code. Among the chamber parameters studied are chamber wall material and its thickness, central electrode material and its dimensions, and the shape and size of the sensitive volume. The calculations show that the response and wall correction factors are sensitive to the shape and volume of the ionization chamber, but relatively independent of the choice of material for the chamber wall and electrode when these are compared on the basis of electron density. Data are presented for cylindrical, plane-parallel, and spherical ionization chambers constructed from carbon, magnesium, aluminum, water, Lucite, polystyrene, and ICRU muscle, as well as for a number of commerically available ionization chambers.

Assessment of the accuracy of stereotactic radiosurgery using Fricke-infused gels and MRI

The treatment plans for stereotactic radiosurgery employ small, circular, noncoplanar fields applied in a series of arcs, or with synchronous rotation of the accelerator gantry and patient support assembly. Primary or metastatic brain tumors and arterial-venous malformations are localized in relation to a stereotactic head frame using CT, MRI, and angiography. As x-ray doses in the range of 20-40 Gy are delivered in a single treatment, it is critical that the dose distribution produced by the accelerator accurately reflect the one developed by the treatment planning computer. Until the advent of Fricke-infused gels, whose NMR characteristics are changed by irradiation, there was no practical method for assessing the accuracy of x-ray beam positioning on a target that was localized by both CT and MRI. A stereotactic head frame was attached to a hollow glass head filled with a Fricke-infused gel. A 2-mm target point at approximately the center of this manikin was localized by CT and MRI. The head frame was then mounted to the patient support assembly of a linear accelerator, and given a dose of 40 Gy to the isocenter from 6-MV x rays using a modified version of the dynamic stereotactic radiosurgery plan developed in Montreal. Subsequent MRI showed the target point at the center of the dose distribution, thus confirming the accuracy of the stereotactic radiosurgery procedure. This demonstrated the unique characteristics of the Fricke-infused gel for the simultaneous localization of x-ray beams in three dimensions.

Fraction of ionization from electrons arising in the wall of an ionization chamber

The accuracy of high-energy x-ray dosimetry can be improved by taking account of differences between the compositions of the chamber wall and the buildup cap or dosimetry phantom. The fraction of the ionization due to secondary electrons arising in the chamber wall has been determined as a function of wall thickness for 60Co gamma rays and x rays in the range of 2-25 MV for Farmer-type chambers. Secondary electrons arising in the accelerator head were removed from the x-ray beams by a magnetic field placed just in front of the ionization chamber. For 60Co gamma rays, the fraction increases from 40% to 100% as the wall thickness increases from 0.05 to 0.55 g cm-2. For a 0.05 g cm-2 wall, fraction decreases from 60% to 10% as the x-ray energy is increased from 2 to 25 MV. Limited data obtained with different chambers suggest that the fraction is independent of chamber wall composition when the thickness is expressed in g cm-2.

Enhancement of electron beam dose distributions by longitudinal magnetic fields: Monte Carlo simulations and magnet system optimization

A Monte Carlo electron-photon transport code was developed in order to determine the effects of static, longitudinal, magnetic fields on dose distributions produced by high-energy electron beams, and to optimize the design of a superconducting magnet system. As a result of these simulations, a 20-cm-i.d., 30-cm-o.d., 15-cm-tall, single-coil, magnet system was designed that could be incorporated into a mobile treatment table for use with a standard radiation therapy accelerator. Operating at a current density of 18 kA/cm2, the magnet would produce field strengths of 1-4 T in the phantom and 0.01 T at the accelerator exit window. Magnetically enhanced dose distributions, calculated for 20- and 30-MeV electron beams, show a pronounced Bragg peak, steeper gradients to the sides and rear, and a roughly fourfold increase in the peak dose to entrance dose ratios relative to those similarly calculated without a magnetic field. These magnetically enhanced dose distributions have the potential for sparing intervening tissue when high-energy electrons are used for the treatment of deep-seated tumors.

On the role of intensity-modulated radiation therapy in radiation oncology.

Physicists are critical members of the radiation therapy team, and rightfully so. Therefore, it is not unreasonable that they be acquainted with the broader aspects of the management of patients who receive radiation treatments, as well as the roles played by surgeons, medical oncologists and other members of the treatment team. The spate of recent technical developments openly embraced by medical physicists, many of whom appear unconcerned by questionable benefits and very high costs, leads the authors to believe that this acquaintance is not as widespread as it should be. The present paper provides a brief review of clinical considerations in radiation oncology, statistics for the most prevalent cancers, and how those cancers that account for over 90% of mortality are currently treated. With these data as background, it then considers the extent to which one of the more widely promoted new technologies is likely to impact upon survival. By providing this modicum of perspective, physicists will be in a better position to evaluate these new technologies in more fundamental clinical terms, and thereby enhance their contributions to the overall care of the cancer patient.

Water calorimeter dosimetry for 160 MeV protons

The absorbed dose to water from a 160 MeV proton beam as determined by a flexible, temperature regulated, sealed glass core, water calorimeter was compared to that determined from ionization chambers used in accordance with AAPM Report 16. The ratios of these doses as obtained from two experiments done over four months apart, are 0.992 +/- 0.004 and 0.990 +/- 0.004. As there are no radiation dependent parameters required for the water calorimeter, these data add to the growing body of evidence which supports the use of the calorimeter as a reliable absorbed dose standard. They also support the use of 60Co-calibrated ionization chambers used in accordance with AAPM Report 16 for the dosimetry of proton beams.

Modification of electron‐beam dose distributions by transverse magnetic fields

By applying a transverse magnetic field to a dosimetry phantom, an incident high-energy electron beam is made to follow a spiral path in the course of slowing down. Certain levels, determined by the electron energy and the magnetic field strength, will be traversed several times by the same electrons. The net result of this process is an enhancement of the depth dose in relation to the entrance dose, and a more sharply defined depth of penetration. Experiments with 50- and 55-MeV electrons traversing a 20.5-kG field are shown to support the predictions of a detailed Monte Carlo calculation.