A CALIBRATION PHANTOM FOR DIRECT, IN VIVO MEASUREMENT OF 241Am IN THE AXILLARY LYMPH NODES

Abstract

A calibration phantom was developed at the University of Cincinnati (UC) to determine detection efficiency and estimate the quantity of activity deposited in the axillary lymph nodes of a worker who had unknowingly sustained a wound contaminated with 241Am at some distant time in the past. This paper describes how the Livermore Torso Phantom was modified for calibrating direct, in vivo measurements of 241Am deposited in the axillary lymph nodes. Modifications involved milling a pair of parallel, flat bottom, cylindrical holes into the left and right shoulders (below the humeral head) of the Livermore Torso Phantom in which solid, 1.40-cm-diameter cylindrical rods were inserted. Each rod was fabricated using a muscle tissue substitute. One end of each rod contained a precisely known quantity of Am sealed in a 1-cm-diameter, 2.54-cm-deep well to simulate the axillary lymph nodes when inserted into the modified Livermore Torso Phantom. The fixed locations for the axillary lymph nodes in the phantom were determined according to the position of the Level I and the combined Level II + III axillary lymph nodes reported in the literature. Discrete calibration measurements for 241Am in the simulated axillary lymph nodes located in the right and left sides of the thorax were performed using pairs of high-resolution germanium detectors at UC and Lawrence Livermore National Laboratory. The percent efficiency for measuring the 59.5 keV photon from Am deposited in the right and left axillary lymph nodes using a pair of 3,000 mm2 detectors is 2.60 +/- 0.03 counts gamma-1 and 5.45 +/- 0.07 counts gamma-1, respectively. Activity deposited in the right and left axillary lymph nodes was found to contribute 12.5% and 19.7%, respectively, to a lung measurement and 1.2% and 0.2%, respectively, to a liver measurement. Thus, radioactive material mobilized from a wound in a finger or hand and deposited in the axillary lymph nodes has been shown to confound results of a direct, in vivo measurement of the lungs.

In Vivo Measurement of 241Am in the Lungs Confounded by Activity Deposited in Other Organs

“…Efficiency was determined by measuring the Livermore Torso Phantom containing precisely known quantities of 241 Am in the liver, lungs, and axillary lymph nodes (Spitz et al 1994;Spitz and Lodwick 2000;Zeman et al 2009). A head phantom, fabricated at the University of Cincinnati by Kellar (1995), containing 241 Am applied on the inner and outer surfaces of an imbedded skull, was measured to determine activity in the skeleton.…”

“…Uptake from a contaminated wound on the hand or finger may result in a fraction of the radioactive material being transported to one or more of the many lymph nodes located in the axilla region of the upper thorax, where the activity may interfere with an in vivo measurement of the lungs (NCRP 2007;Zeman et al 2009). Regardless of intake route, activity deposited in multiple organs of the body complicates analysis of in vivo monitoring results.…”

“…Regardless of intake route, activity deposited in multiple organs of the body complicates analysis of in vivo monitoring results. Fisher et al (2007), Zeman et al (2009), andCarbaugh et al (2010) have reported that activity deposited in the axillary lymph nodes creates an interference with routine lung counting. The conventional method for performing an in vivo measurement involves arranging one or more detectors on the body so that the solid angle of photons emitted from the deposition site subtends the maximum surface area of the detector entrance window.…”

“…For example, following accidental inhalation of weapons-grade plutonium containing 241 Am, the activity will first deposit in the lungs with a fraction eventually being transported to the liver and skeleton. Zeman et al (2009) described how the Livermore Torso Phantom was modified to accommodate axillary lymph nodes in the right and left shoulders. Recognizing and adjusting for measurement contributions from activity deposited in proximal organs ensures accurate radiation dose estimates.…”