Determination of Activity Deposited in the Axillary Lymph Nodes by Direct, In vivo Radiation Measurements

A. Determine the confounding influence of ²⁴¹Am deposited in organs adjacent to the axillary lymph nodes by simultaneous solution of response functions for measurement of ²⁴¹Am deposited in the liver, lungs, and skeleton.

Hypothesis: A series of direct, organ-specific measurements can be used to account for measurement interference for the axillary lymph nodes from activity deposited in other organs.

Radioactive material deposited in multiple organs of the body is likely to confound a result of an in vivo measurement performed over the lungs for routine occupational exposure monitoring. The significance of this interference was evaluated by measuring anthropometric torso phantoms containing lungs, liver, skeleton and axillary lymph nodes, each with a precisely known quantity of ²⁴¹Am uniformly distributed in the organs. Arrays of multiple high resolution germanium detectors were positioned over organs within the torso phantom containing ²⁴¹Am or over proximal organs without activity to determine the degree of measurement confounding due to photons emitted from other source organs. A set of four mathematical response functions describe the measured count rate with detectors positioned over each of the relevant organs and ²⁴¹Am contained in the measured organ or one of the other organs selected as a confounder. Simultaneous solution of these equations yields the activity deposited in each of the relevant organs. The matrix solutions described represent a technically valid method for adjusting a result of ²⁴¹Am measured in one organ for interferences that may arise from ²⁴¹Am deposited elsewhere, so internal dose from radioactive materials known to deposit in multiple organs may be evaluated based upon in vivo measurements.

B. Select the size and type of detector that offers the greatest sensitivity for ²⁴¹Am in axillary lymph nodes measurements with the least measurement interference from ²⁴¹Am in other organs or tissue.

Hypothesis: Detector selection can optimize sensitivity and decrease confounding for measurement of ²⁴¹Am deposited in the axillary lymph nodes.

The detection efficiency and interference susceptibility of four different types of low energy photon detectors, each with a unique geometric arrangement, were compared for direct measurement of ²⁴¹Am deposited in the axillary lymph nodes. Although the most efficient detector was a single 23,226 mm² square phoswich detector, it was also the most susceptible to confounding from activity deposited in adjacent organs. The array of two 2,800 mm² high purity germanium detectors exhibited the highest efficiency per unit detector area with some resistance to confounding from activity deposited in the lungs. The array of two 4,560 mm² NaI(Tl) detectors was the least susceptible to confounding and nearly as efficient per square millimeter as the high purity germanium detector array. Selection of a detector system for measurement of activity deposited in the axillary lymph nodes should consider the likelihood for activity deposited in other organs, such as the lungs, to create an interference that will confound the measurement result.