Jacek G. Wierzbicki

DEVELOPMENT OF HIGH-ACTIVITY 252CF SOURCES FOR NEUTRON BRACHYTHERAPY

The Gershenson Radiation Oncology Center of Wayne State University (WSU), Detroit, Michigan, is using 252Cf medical sources for neutron brachytherapy. These sources are based on a 20-year-old design containing < or = 30 micrograms 252Cf in the form of a cermet wire of Cf2O3 in a palladium matrix. The Radiochemical Engineering Development Center (REDC) of Oak Ridge National Laboratory has been asked to develop very compact, high-activity 252Cf neutron sources for use with remote afterloading equipment in order to reduce treatment times and dose to clinical personnel and to expedite treatment of brain and other tumors. To date, the REDC has demonstrated that 252Cf loadings can be greatly increased in cermet wires and with much smaller diameters. Equipment designed for hot cell fabrication of these wires is being tested. A parallel program is under way to relicense the existing source design for fabrication at the REDC.

Calculated dosimetric parameters of the IoGold 125I source model 3631‐A

Basic dosimetric parameters as recommended by the AAPM Task Group No. 43 (TG-43) have been determined for recently available IoGold 125I brachytherapy seeds. Monte Carlo methods (MCNP) were used in the calculation of these parameters in water, and results compared with soon to be published experimental parameters also for 125I IoGold seeds as well with parameters for model 6702 and 6711 125I seeds. These parameters were the radial dose function, anisotropy factor and constant, and the dose rate constant. Using MCNP, values for the radial dose function at 0.5, 2.0, and 5.0 cm were 1.053, 0.877, and 0.443, respectively. The anisotropy factor was 0.975, 0.946, 0.945, and 0.952 at 0.5, 1.0, 2.0, and 5.0 cm, respectively, with an anisotropy constant of 0.95. The IoGold dose rate constant was determined by excluding the low energy titanium characteristic x rays produced in the IoGold titanium capsule. Using this post TG-43 revised NIST air kerma methodology, the IoGold dose rate constant was 0.96 cGy h-1 U-1. These calculatively determined parameters for IoGold seeds were compared with those determined experimentally for IoGold seeds, and also compared with parameters determined for model 6702 and 6711 seeds as presented in TG-43.

Measurement of augmentation of 252Cf implant by 10B and 157Gd neutron capture

252Cf has been used as a brachytherapy source since the early 1970s. The dominant mechanism of interactions of 252Cf neutrons with tissue is elastic scattering. The scattered neutrons lose part of their energy, which is released as kinetic energy of the recoiling nuclei. By multiple scattering, neutrons lose their energy and eventually become thermalized (in energetic equilibrium with tissue atoms with an average energy of 0.025 eV) and do not play any role in radiotherapy. These thermal neutrons may interact with hydrogen nuclei or with nitrogen, but the cell killing effects by these reaction products are negligibly small compared to the elastic scattering by fast neutrons or by photons emitted by californium. Nonetheless, these thermal neutrons are still potentially usable for neutron capture therapy and can be used to enhance californium brachytherapy effects. Neutron capture therapy is a two-part therapy relying on the selective loading of tumor cells with compounds containing 10B or 157Gd and subsequent irradiation with thermal neutrons. To calculate neutron capture doses one has to know thermal neutron flux. This paper presents results of an experimental study of thermal neutron flux and calculations of boron neutron capture and gadolinium neutron capture doses in the vicinity of 252Cf sources.

Large discrepancies between calculated Dmax and diode readings for small field sizes and small SSDs of 15 MV photon beams

Clinical observations have revealed that diode readings from 15 MV photon beams with small field sizes and small SSD values were consistently 5%-10% higher than doses calculated at the maximum depth. Dose at depth depends on the collimator scattering factor and the phantom scattering factor--both field size dependent. Diodes are placed on the skin surface, and therefore the phantom scatter factor is different than at depth. In addition, inverse square corrections should be calculated with respect to the location of the diode itself rather than the location of Dmax within the tissue. This may account for about 8% difference between calculated Dmax and diode readings for very small radiation fields and small SSDs, e.g., a prostate boost.

Radiographic characterization and energy spectrum of the IoGold 125I source Model 3631-A

Despite the growing popularity of permanent implants using encapsulated 125 I seeds, only two types were commercially available, Models 6702 and 6711. Recently, the Food and Drug Administration (FDA) approved a new type of 125 I seed, Model 3631-A, called IoGold (apparent activity 0.28–0.63 mCi). Due to the large demand for 125 I seeds for prostate implants, IoGold seeds will be available in January 1998. In this paper, the basic properties of the IoGold seeds have been evaluated in comparison to the established Models 6702 and 6711.

Mass attenuation coefficients of clear-Pb for photons from 125I, 103Pd, 99mTc, 192Ir, 137Cs and 60Co.

The mass attenuation coefficients, mu/rho, for Clear-Pb for photon energies ranging from 10 keV to 10 MeV were determined using Monte Carlo methods and simple equations used to manipulate elemental mass attenuation coefficients. It was determined that the effectiveness of Clear-Pb as a radiation shielding material was greater than plain acrylic for all photon energies, especially those less than 150 keV, and for deep penetration problems where the differences in mu/rho between Clear-Pb and acrylic became more significant. Finally, the usefulness of Clear-Pb as a shielding material when compared with acrylic was determined for the following commonly used radionuclides: 125I, 103Pd, 99mTc, 192Ir, 137Cs, and 60Co.

Physics and Dosimetry of Clinical 252Cf Sources

Californium-252 is the most useful neutron emitter out of all the ~ 3000 radionuclides. Though isotopes like 254Cf and 260Md have higher rates of spontaneous fission, their half-lives are too short, weeks, to permit large scale fabrication. The majority, 96.9%, of 252Cf decays are through alpha decay, but due to the nature of the encapsulation, these 4He nuclei do not escape the confines of the source. A small, 3.092%, but significant proportion of 252Cf decays go via spontaneous fission which produce fission fragments as well as a neutron yield of 3.768. These neutrons have an energy spectrum which may be modeled as either a Maxwellian or a Watt fission spectrum, are shown in Figure 1, and are readily thermalized in hydrogenous media such as human tissue. Other 252Cf emissions include prompt gammas and also photons from the fission products [1]. Properties of 252Cf emissions; neutron and photon energy spectrum is presented in tables 1,2, and 3.

Routine verification of strength of 137Cs brachytherapy sources using a NaI detector

An inexpensive and easy method of calibration of brachytherapy 137Cs tubes is described. Cesium brachytherapy sources are calibrated relative to an NIST-calibrated source. The detection system is a NaI detector and single channel analyzer with the window set on the photo peak.

Californium-252 in the Treatment of Soft Tissue Sarcoma

Soft tissue sarcomas are a heterozygous group of malignancies with various treatment related outcomes that are based on factors such as grade, histology, primary site among others [1–3]. Recent series including a randomized trial from Memorial Sloan Kettering has attempted to define the role of brachytherapy in the treatment of STS. However, as with most reviews this and other reported series are limited in the traditional use or radioactive source such as I-125 or Ir-192 [4–6]. In an effort to help overcome some of the limitations placed on traditional low dose rate brachytherapy, attempts at high dose rate or pulsed high dose rate brachytherapy have been reported [7–8]. During the past thirty years, a remarkable radioactive source (Cf-252) introduced which may offer preferential benefits when compared to the traditionally utilized radiation sources. Although it has been clinically used and described in various sites such as cervix, head and neck, and brain, little has been described in its use for the treatment of STS [9–11].

157Gd Photon Dose Enhancement from a Neutron Emitting 252Cf Point Source

Though the efficacy of the 157Gd(n,γ)158*Gd neutron capture reaction (GdNCR) has yet to be demonstrated in humans, there is merit to studying the potential benefits of this modality. In this study, the photon dosimetry of GdNCR enhanced 252Cf fast neutron brachytherapy was examined.