Abstract
The current methodology of internal dose assessment used by the Society of Nuclear Medicine's Medical Internal Radiation Dose (MIRD) Committee and the International Commission on Radiological Protection (ICRP) is based, among other things, on the calculation of absorbed fractions of photon energy. These fractions are derived through the use of computer programs utilizing Monte Carlo sampling techniques. Although these codes are very useful and easily configured to a wide variety of scenarios, they may suffer from inherent statistical errors. Therefore, Monte Carlo estimates of absorbed dose should always, whenever possible, be compared to actual measurements to verify their accuracy. This study concentrated on the use of a gelatin-based volumetric dosimeter to measure dose profiles in target organs exposed to a known photon field. The dose profiles were then used to calculate an average dose for the entire target along with measured values of absorbed fractions (AF) and specific absorbed fractions (SAF). Finally, the measured values of AF and SAF in gelatin targets were compared to calculated results obtained through a Monte Carlo simulation of the various exposure configurations. All simulations done using the Monte Carlo technique were performed for both gelatin and adipose tissue targets. Ratios of measured-to-calculated values of AF and SAF (R) were then computed for each of the exposures at edge-to-edge distances of 1 cm, 2 cm, 5 cm, and 15 cm from the source volume. R values obtained for AF in gelatin targets at these distances were 0.8008, 0.7448, 0.7647, and 1.848, while R values for SAF were 0.8271, 0.7722, 0.7840, and 1.905 respectively. The final set of R values provided a direct comparison of measurements made in gelatin targets to calculations done for simulated adipose tissue. R values obtained for AF in this scenario were 0.5913, 0.5544, 0.5903, and 1.7600, with corresponding figures of 0.6108, 0.5748, 0.6052, and 1.8140 for SAF.
Walker, Scottie Wayne (1995). Measurements of photon absorbed fractions and dose profiles using a gelatin-based volumetric dosimeter. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1995 -THESIS -W35.