Internal microdosimetry of alpha-emitting radionuclides.

At the tissue level, energy deposition in cells is determined by the microdistribution of alpha-emitting radionuclides in relation to sensitive target cells. Furthermore, the highly localized energy deposition of alpha particle tracks and the limited range of alpha particles in tissue produce a highly inhomogeneous energy deposition in traversed cell nuclei. Thus, energy deposition in cell nuclei in a given tissue is characterized by the probability of alpha particle hits and, in the case of a hit, by the energy deposited there. In classical microdosimetry, the randomness of energy deposition in cellular sites is described by a stochastic quantity, the specific energy, which approximates the macroscopic dose for a sufficiently large number of energy deposition events. Typical examples of the alpha-emitting radionuclides in internal microdosimetry are radon progeny and plutonium in the lungs, plutonium and americium in bones, and radium in targeted radionuclide therapy. Several microdosimetric approaches have been proposed to relate specific energy distributions to radiobiological effects, such as hit-related concepts, LET and track length-based models, effect-specific interpretations of specific energy distributions, such as the dual radiation action theory or the hit-size effectiveness function, and finally track structure models. Since microdosimetry characterizes only the initial step of energy deposition, microdosimetric concepts are most successful in exposure situations where biological effects are dominated by energy deposition, but not by subsequently operating biological mechanisms. Indeed, the simulation of the combined action of physical and biological factors may eventually require the application of track structure models at the nanometer scale.

Thermoluminescent dosimeters (TLD-100) for absorbed dose measurements in alpha-emitting radionuclides.

Early works that used thermoluminescent dosimeters (TLDs) to measure absorbed dose from alpha particles reported relatively high variation (10%) between TLDs, which is undesirable for modern dosimetry applications. This work outlines a method to increase precision for absorbed dose measured using TLDs with alpha-emitting radionuclides by applying an alpha-specific chip factor (CF) that individually characterizes the TLD sensitivity to alpha particles. Variation between TLDs was reduced from 21.8% to 6.7% for the standard TLD chips and 7.9% to 3.3% for the thin TLD chips. It has been demonstrated by this work that TLD-100 can be calibrated to precisely measure the absorbed dose to water from alpha-emitting radionuclides.

Characterization of a segmented printed circuit board (PCB) as a standard for absorbed dose to water from alpha-emitting radionuclides.

BACKGROUND: Our previous work introduced and evaluated a standard for surface absorbed dose rate per unit radioactivity to water from unsealed alpha-emitting radionuclides used in targeted radionuclide therapy (TRT). An overall uncertainty over 4.0% at k = 1 was reported for the absorbed dose to air measurements, which was partially attributed to the rotational alignment uncertainty in the geometrical setup. PURPOSE: A printed circuit board (PCB) with a segmented guard was constructed to align the extrapolation chamber (EC) and the source plates using a differential capacitance technique. The PCB EC aimed to enhance the repeatability of the ionization current measurements. The PCB EC was evaluated using a thin film 210Po source. The measured absorbed dose to air cavity was compared with the Monte Carlo (MC) calculations. Using the extrapolation method, the surface absorbed dose rate to water was calculated. METHODS: The PCB EC was constructed with a 4.50 mm diameter collector surrounded by four sectors and a guard electrode. The sectors were isolated for rotational alignment and later connected to the guard for ionization current measurements. A bridge circuit measured differential capacitance between opposing sectors, and a hexapod motion stage rotated the source substrate to minimize the differential capacitance. The EC was evaluated using a 210Po source with a 3.20 mm diameter and 1.253 µ $\mu $ Ci radioactivity. MC simulations were performed to calculate the k p o i n t ${k}_{point}$ , k b a c k s c a t t e r ${k}_{backscatter}$ , and k d i v ${k}_{div}$ correction factors. Ionization current measurements were performed for air gaps in the 0.3-0.525 mm range and surface absorbed dose rate to water was calculated. RESULTS: Rotational offsets of up to 3.0° were found and the current repeatability was found to increase with the absorbed dose to air uncertainty calculated to be ∼2.0%. Using the capacitance method, the effective EC diameter was measured to be 4.53 mm. The recombination, polarity, and electrometer corrections were reported to be within 1.00% across all measurement trials. The MC-calculated correction factors were calculated to be much larger than the recombination and polarity correction factors. The average k p o i n t ${k}_{point}$ , k b a c k s c a t t e r ${k}_{backscatter}$ , and k d i v ${k}_{div}$ corrections were calculated to be 1.063, 0.9402, and 2.136, respectively. The MC-calculated absorbed dose to air was found to overestimate the absorbed dose by over 4.00% when compared with the measured absorbed dose to air. The surface absorbed dose rate to water was calculated to be 2.304 × 10 - 6 $2.304 \times {10}^{ - 6}$ Gy/s/Bq with an overall uncertainty of 4.07%. CONCLUSIONS: The constructed PCB EC was deemed suitable as an absorbed dose standard. A repeatable rotational alignment was achieved using the differential capacitance technique. The metal electrodes on the PCB made a difference of < 1.00% on the backscatter correction when compared to the EC comprised of polystyrene-equivalent collector. A 20% difference in the surface absorbed dose rate to water was found between the two ECs, which is attributed to the cavity diameter differences leading to different magnitudes of dose fall-off along the lateral direction.

Innovation, Impact, and Strategic Importance of Alpha-Emitting Radionuclides.

This talk briefly reviews the earlier work in the field and highlights the contributions of colleagues whose clear vision paved the road for success of TAT. The talk primarily will focus on the development of the radioisotopes for application in TAT. The challenges regarding release of daughter radioisotopes will be briefly discussed, and a summary of the alternative approaches for production of 225Ac will be presented.

Determination of specific alpha-emitting radionuclides (uranium, plutonium, thorium and polonium) in water using [Ba+Fe]-coprecipitation method.

The indicative dose (ID) is one of the parameters established in the current European directive for water intended for human consumption. To determine the ID, it is necessary to know the activity concentration of: 238U, 234U, 226Ra, 210Po, 239,240Pu and 241Am. The existing methods to determine these radionuclides involve complex radiochemical separations (ionic exchange columns, extraction chromatography, etc.), followed by measurements with a semiconductor detector, laboratory procedures that are time-consuming and costly. As a lower cost alternative that reduces measuring and preparation times, avoids the need for a self-absorption correction and the use of tracers, and above all that can be used in any laboratory, methods based on liquid-liquid extraction and selective co-precipitation were developed. These methodologies offer high separation recovery and selectivity, and the measurements are made using a gas proportional counter or a solid ZnS(Ag) scintillation counter. The separation factor ranged between 91.4% and 100.0% for all alpha-emitting radionuclides across the different methods. The activity concentration for each method was computed through linear equations that represent the relationship between the activity and selectivity of the different alpha-emitting radionuclides. This mathematical procedure simplifies the radiochemical separations and provides more accurate activity concentrations. The results of the internal and external validation studies proved that the proposed method is suitable for determining 241Am, 226Ra, uranium, plutonium, thorium and 210Po in water samples.