EVALUATION OF THE CLEARANCE OF PARTICLES DEPOSITED ON THE CONDUCTING AIRWAYS OF BEAGLE DOGS

The new respiratory tract dosimetry model of the International Commiss ion on Radiological Protection incorporates long-term retention of rad ioactive particles in conducting airways in its radiation dosimetry ca lculations, Values for long-term retention must be realistic to gain a cceptance of the model and allow meaningful airway calculations, In th e 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 abou t 8 X 10(6) radiolabeled 3-4 mu m polystyrene latex microspheres or 3 x 10(5) radiolabeled 1.5-mu m fused aluminosilicate particles in 20-mu l of saline suspension, External counting of photons from the radioac tive particles was used to evaluate clearance for as long as 42 days, In some cases, almost all of the administered particles cleared rapidl y; 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 numb er and dosing volume were reduced in subsequent work to determine if t hese parameters influenced alveolarization of some of the administered particles, Additionally, initial deposition patterns were evaluated t o determine if the dosing procedure itself was a factor, To determine initial deposition patterns, particle suspensions were sprayed into lu ng airways in dead dogs and the lungs were removed about 30 min after dosing and processed to produce slides of lung tissue from the vicinit y of the particle deposition sites, We conclude that (1) the dosing pr ocedure is unlikely to place particles directly into alveoli, (2) a lo cal airway application of 20 mu l of saline suspension containing 8 X 10(6) particles can result in the alveolarization of some of the parti cles after they are deposited in airways, whereas 6 mu l of saline con taining less than 3 x 10(5) particles does not, and (3) at least 80% o f the particles not subjected to alveolarization are physically cleare d from the conducting airways within 3 days after dosing.