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

To confirm whether or not grade 4 asbestosis progresses from the respiratory bronchiole to the peripheral lung.

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 (ABs) 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 ABs in dissolved lung of 117 000/g dry lung, 468 000/g and 968 000/g; and in specimens on glass slides of seven ABs/cm² of tissue slice, 34 ABs /cm² and 195 ABs /cm² . The differences were significant. Fifteen and 16 cases showed AI and UIP patterns, respectively, with mean respective numbers of ABs in dissolved lung of 1 006 000/g dry lung and 354 000/g, and 186 and 56 ABs/cm² 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.

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.