Evaluation of 99mTc-HMPAO scintigraphy for irradiated lung in rabbits: detection of pulmonary microvascular injury.

The pulmonary microvascular endothelium manifests structural and functional alterations following irradiation. It is important to determine a suitable method for detecting the endothelial damage. For the assessment of the initial phase of this pulmonary injury, 99mTc-hexamethylpropylene amine oxime (99mTc-HMPAO) scintigraphy as an indicator of early endothelial injury induced by irradiation was employed. Japanese white rabbits were exposed to 50 Gy irradiation toward the hemithorax, and 99mTc-HMPAO scintigraphy was performed at 24 hrs, 2 days, 14 days, and 28 days following irradiation. Twenty of 24 rabbits (83.3%) showed abnormal 99mTc-HMPAO uptake in the irradiated lung during this period. Of 10 rabbits examined 24 hrs after irradiation, nine (90%) already showed abnormal 99mTc-HMPAO uptake. The time-activity curves revealed that high uptake of this agent occurred rapidly within the first 1 min after injection. Chest radiography in all these 20 rabbits with abnormal 99mTc-HMPAO uptake exhibited no abnormal opacity. Histological studies performed in 7 of these 20 rabbits revealed no remarkable changes in the irradiated lung compared to the contralateral non-irradiated lung, while electron microscopic study showed scattered vacuolation and edematous changes in the endothelium. These results indicate that 99mTc-HMPAO scintigraphy may detect early lung microvascular endothelial injury induced by irradiation.