Patrick D. Higgins

Influence of hip prostheses on high energy photon dose distributions.

Radiotherapy treatment of patients having a hip prosthesis is a common problem facing dosimetrists and physicists when the treatment plan requires irradiation of the pelvic area. To quantify the perturbation of these devices, attenuation studies were done with 6 and 18 MV photon beams using various hip prostheses models with varying size and composition. These studies have shown that an attenuation of as much as 50% can be found in a single beam profile under the prosthesis. We have studied the capability of a dose planning system to predict the transmission of these devices as compared with measurements.

Three-dimensional conformal pancreas treatment: comparison of four- to six-field techniques.

PURPOSE We compare practical conformal treatment approaches to pancreatic cancer using 6 and 18 MV photons and contrast those approaches against standard techniques. METHODS AND MATERIALS A four-field conformal technique for treating pancreas cancer has been developed using nonopposed 18 MV photons. This approach has been extended to 6 MV photon application by the addition of one to two fields. These techniques have been optimized to increase sparing of normal liver and bowel, compared with opposed-field methods, to improve patient tolerance of high doses. In this study we compare these techniques in a simulated tumor model in a cylindrical phantom. Dose-volume analysis is used to quantify differences between the conformal, nonopposed techniques with conformal, opposed field methods. This model is also used to evaluate the effect of 1-2 cm setup errors on dose-volume coverage. RESULTS Dose-volume analysis demonstrates that five-to-six field conformal treatments using 6 MV photons provides similar or better dose coverage and normal tissue sparing characteristics as an optimized 18 MV, four-field approach when 1-2 cm margins are included for setup uncertainty. All approaches using nonopposed beam geometry provide significant reduction in the volume of tissue encompassed by the 30-50% isodose surfaces, as compared with four-field box techniques. CONCLUSIONS Three-dimensional (3D) conformal treatments can be designed that significantly improve dose-volume characteristics over conventional treatment designs without costing unacceptable amounts of machine time. Further, deep intraabdominal sites can be adequately accessed and treated on intermediate energy machines with a relatively moderate increase in machine time.

Phosphorus metabolites and the distribution of cell cycle phase of RIF-1 tumors in response to 14 Gy irradiation

Simultaneous measurements of DNA cell phase cycle distributions and in vivo 31P NMR spectroscopy were performed on 40 RIF‐1 murine tumors irradiated with 14 Gy of X‐radiation. Diploid and tetraploid tumor populations were observed. The cells blocked in G2/M phase were measured as a function of the ratios of tetraploid cell number in G2/ M phase versus total cell population measured. The G2/M population reached a maximum at 32 h post irradiation, dropping to control values by 72 h, while the ratio of inorganic phosphate to β‐nucleotide triphosphate dropped significantly at 32 h and remained significantly lower than control up to 72 h post irradiation. Measurements of PME, PDE, PCr, and pH showed no significant variations at any time point. No significant change in host cell population could be observed. Since the measured G2/M population never increased to more than 3% of the total cell population, the change observed in the 31P NMR spectra were not simply the result of possible differences in NMR profiles of the different cell phase populations but were more likely due to a change in the metabolic characteristics or environment of a majority of the Cells.

Scatter factor corrections for elongated fields.

Measurements have been made to determine scatter factor corrections for elongated fields of Cobalt-60 and for nominal linear accelerator energies of 6 MV (Siemens Mevatron 67) and 18 MV (AECL Therac 20). It was found that for every energy the collimator scatter factor varies by 2% or more as the field length-to-width ratio increases beyond 3:1. The phantom scatter factor is independent of which collimator pair is elongated at these energies. For 18 MV photons it was found that the collimator scatter factor is complicated by field-size-dependent backscatter into the beam monitor.

Use of ferroelectric-crystal detectors for electron dosimetry

A ferroelectric detector system was designed for measurement of dose rate in high-energy electron beams. It was found that a simple lead zirconate-titanate ceramic responds reproducibly to a chopped electron beam by ferroelectric conversion of the resulting pulsed temperature rise to a proportional output voltage. Response as a function of dose rate is linear, the rate of response being a function of incident energy. As a calorimetric dose-rate meter the detector is responsive with response improving as energy is reduced. The detector can be used over a broad range of energies not usually accessible with a single detector.<<ETX>>

Ionizing Radiation Response Effects On Optical Fibers In Radiation Therapy Dosimetry Applications

Radiation effects on optical fibers result in two different bffects: accumulated radiation damage resulting in a substantial loss of transmission and transient radiation response in which light is generated in the fiber material itself during exposure .too radiation. In various medical applications, optical fibers are often exposed to radiation during certain clinical procedures which makes their radiation response an effect of concern to the medical community. Thus, this effect needs to be studied and quantified in that kind of environment. This study involves radiation-resistant fibers and the quantification of their light emission response as a function of dose rate and irradiation field size for photon and electron beams at the various energises used in radiation therapy.