Grade 4 asbestosis does not extend directly from the respiratory bronchiole to the peripheral lung

Aims To confirm whether or not grade 4 asbestosis progresses from the respiratory bronchiole to the peripheral lung. Methods and results We examined retrospectively the autopsy or lobectomy specimens from 31 cases (29 males; mean age 64 years) satisfying the pathological criteria of grade 4 asbestosis. Asbestos bodies ( AB s) were quantified in samples of dissolved lung and in tissue preparations on glass slides. Respiratory bronchiolar lesions were graded as 0, 1 and ≥2. Grade 4 asbestosis was subdivided into an atelectatic induration ( AI ) and usual interstitial pneumonia pattern ( UIP pattern). Five, 10, and 16 cases had grades 0, 1 or ≥2 lesions, respectively, with mean respective numbers of AB s in dissolved lung of 117 000/g dry lung, 468 000/g and 968 000/g; and in specimens on glass slides of seven AB s/cm 2 of tissue slice, 34 AB s /cm 2 and 195 AB s /cm 2 . The differences were significant. Fifteen and 16 cases showed AI and UIP patterns, respectively, with mean respective numbers of AB s in dissolved lung of 1 006 000/g dry lung and 354 000/g, and 186 and 56 AB s/cm 2 on glass slides. The differences were significant. AI patterns originated in subpleural lobules or subpleural zonal areas and UIP patterns originated in subpleural, peripheral lobules. Conclusions Grade 4 asbestosis does not start in the respiratory bronchiole. The presence of a grade 1 lesion is not required for the diagnosis of grade 4 asbestosis.