Evaluation of the Clearance of Particles Deposited on the Conducting Airways of Beagle Dogs

ABSTRACT

The new respiratory tract dosimetry model of the International Commission on Radiological Protection incorporates long-term retention of radioactive particles in conducting airways in its radiation dosimetry calculations. Values for long-term retention must be realistic to gain acceptance of the model and allow meaningful airway calculations. In the present study, Beagle dogs were used to evaluate particle clearance and determine if particles were retained in conducting airways of the lung. After anesthesia, a fiberoptic bronchoscope was used to position a microspray nozzle in specific airways having internal diameters of about 4,8, and 15 mm. The dogs were made temporarily apneic, and the dosing volume was expelled through the microspray nozzle using 1 ml of air. In the clearance experiments, dogs were administered either about 8 × 10⁶ radiolabeled 3–4 μm polystyrene latex microspheres or 3 × 10⁵ radiolabeled 1.5-μm fused aluminosilicate particles in 20-μl of saline suspension. External counting of photons from the radioactive particles was used to evaluate clearance for as long as 42 days. In some cases, almost all of the administered particles cleared rapidly; in other cases, a substantial portion of the administered particles was retained for longer than 3 days. Histological examination of lung tissue sampled near and distal to the deposition sites indicated that the retained particles were essentially all in alveoli. Particle number and dosing volume were reduced in subsequent work to determine if these parameters influenced alveolarization of some of the administered particles. Additionally, initial deposition patterns were evaluated to determine if the dosing procedure itself was a factor. To determine initial deposition patterns, particle suspensions were sprayed into lung airways in dead dogs and the lungs were removed about 30 min after dosing and processed to produce slides of lung tissue from the vicinity of the particle deposition sites. We conclude that (1) the dosing procedure is unlikely to place particles directly into alveoli, (2) a local airway application of 20 μl of saline suspension containing 8 × 10⁶ particles can result in the alveolarization of some of the particles after they are deposited in airways, whereas 6 μl of saline containing less than 3 μ 10⁵ particles does not, and (3) at least 80% of the particles not subjected to alveolarization are physically cleared from the conducting airways within 3 days after dosing.