Applied Radiology

Diagnosis

Bronchiolitis obliterans organizing pneumonia (BOOP)

Discussion

Bronchiolitis obliterans can be categorized histologically into two morphological types: constrictive bronchiolitis and proliferative bronchiolitis obliterans.1 Constrictive bronchiolitis involves concentrically scarred or obliterated bronchioles with less involvement of the distal alveolar ducts and alveoli.1 Proliferative bronchiolitis obliterans involves organizing granulation tissue polyps filling the lumen of terminal and respiratory bronchioles. It usually extends contiguously into alveolar ducts, and sometimes into distal alveoli resulting in organizing pneumonia.1,2 While primary or idiopathic BOOP is the most common cause of proliferative bron-chiolitis obliterans, it may also be secondary to rheumatologic or connective tissue disorders, adult respiratory distress syndrome (ARDS), infection, aspiration pneumonia, hypersensitivity pneumonitis, toxic fume exposure, eosinophilic pneumonia, post-obstructive pneumonia, post-bone marrow or lung transplantation, or other miscellaneous conditions (HIV infection, radiation therapy, cancer, inflammatory bowel disease, textile printing dye).1

BOOP is defined pathologically as granulation tissue plugs within the lumens of small airways that may result in their complete obstruction, with granulation tissue extending into alveolar ducts and alveoli1,2 (figure 3). Additional findings include connective tissue proliferation resulting in intraluminal polyps (proliferative bronchiolitis obliterans), fibrinous exudates, alveolar walls, and evenly spaced rounded balls of myxomatous connective tissue.1 The lung architecture is maintained.1 In this case, a video-assisted thorascopic wedge biopsy of the right lower lobe revealed polymorphonuclear leukocytes in the airways obliterating the bronchioles in a background of an organizing pneumonia, with inflammation and fibrosis.

Clinically, patients with BOOP most often present with a 2- to 12-week history of cough, dyspnea, and malaise (each present in over 80% of patients).3,4 Fever and weight loss are common. Crackles (over 80% of patients)3,4 and tachypnea (over 60% patients)4 are the most frequent findings on examination. Men and women are affected at equal rates,1,3,4 and no relationship to smoking exists.1 Pulmonary function studies show decreased vital capacity, normal flow rates (except in smokers), and decreased diffusing capacity.1-3 Common laboratory abnormalities include hypoalbuminemia, leukocytosis, and an increased erythrocyte sedimentation rate.3 Bronchoalveolar lavage reveals an increase in all cell types, with lymphocytes predominating.1 Definitive diagnosis is established pathologically following open lung biopsy or video-guided thoracoscopy.1

Radiographically, bilateral patchy alveolar infiltrates that may be migratory are the most common finding.1 Cavities and effusions are rare.1,2 In idiopathic BOOP, the infiltrates typically enlarge gradually, or new infiltrates appear as the disease progresses.1 On CT, BOOP typically presents as bilateral non-segmental consolidations (present in 79% of patients),5,6 masses,7 or areas of ground-glass opacity that are usually subpleural or peribronchovascular in location. Peripheral patchy infiltrates may form a characteristic triangular shape, with the base of the triangle directed toward the pleura.8 A frequent CT finding is nodules (found in 30% of patients) with well-defined, smooth margins, that are distributed randomly.5 A pulmonary vessel leading into the nodule (the "feeding vessel sign") or an air bronchogram entering the nodule (the "bronchus sign") may be observed.9

In this case, numerous small, ill-defined nodules with irregular margins were found in association with irregular peripheral masses and evidence of airway disease on CT. Although the patient in this case had a history of a primary pulmonary tumor, the CT findings made metastatic disease unlikely.

Most BOOP patients respond to corticosteroid therapy, and prednisone remains the recommended therapy.1,4 The prognosis for BOOP is good. Total and permanent recovery is seen in 65% to 80% of patients treated.1,2,7 The mortality remains approximately 5%.1

Acknowledgement: The authors would like to thank Mark D. Brownell, MD, for his kind assistance with interpretation and photography of the lung biopsy specimen.

REFERENCES

1. Epler GR: Bronchiolitis obliterans organizing pneumonia. Semin Respir Infect 10(2):65-77, 1995.

2. Epler GR, Colby TV, McLoud TC, et al: Bronchiolitis obliterans organizing pneumonia. N Eng J Med 312:152-158,1985.

3. Lohr RH, Boland BJ, Douglas WW, et al: Organizing pneumonia: Features and prognosis of cryptogenic, secondary, and focal variants. Arch Intern Med 157:1323-1329, 1997.

4. Boots RJ, McEvoy JD, Moway P, Le Fevre I: Bronchiolitis obliterans organising pneumonia: A clinical and radiological review. Aust NZ J Med 25(2):140-145, 1995.

5. Lee KS, Kullnig P, Hartman TE, Muller NL: Cryptogenic organizing pneumonia: CT findings in 43 patients. AJR 162:543-546, 1994.

6. Muller NL, Staples CA, Miller RR: Bronchiolitis obliterans organizing pneumonia: CT Features in 14 patients. AJR 154:983-987, 1990.

7. Akira M, Yamamoto S, Sakatani M: Bronchiolitis obliterans organizing pneumonia manifesting as multiple large nodules or masses. AJR 170:291-295, 1998.

8. Costabel U, Teschler H, Schoenfeld B, et al: BOOP in Europe. Chest 102:14S-20S, 1992.

9. Bouchardy LM, Kuhlman JE, Ball WC, et al: CT findings in bronchiolitis obliterans organizing pneumonia (BOOP) with radiographic, clinical and histologic correlation. J Comput Asst Tomogr 17:352-357, 1993.

1. Epler GR:Bronchiolitis obliterans organizing pneumonia. Semin Respir Infect 10(2):65-77, 1995.
2. Epler GR, Colby TV, McLoud TC, et al: Bronchiolitis obliterans organizing pneumonia. N Eng J Med 312:152-158,1985.
3. Lohr RH, Boland BJ, Douglas WW, et al: Organizing pneumonia: Features and prognosis of cryptogenic, secondary, and focal variants. Arch Intern Med 157:1323-1329, 1997.
4. Boots RJ, McEvoy JD, Moway P, Le Fevre I: Bronchiolitis obliterans organising pneumonia: A clinical and radiological review. Aust NZ J Med 25(2):140-145, 1995.
5. Lee KS, Kullnig P, Hartman TE, Muller NL: Cryptogenic organizing pneumonia: CT findings in 43 patients. AJR 162:543-546, 1994.
6. Muller NL, Staples CA, Miller RR:Bronchiolitis obliterans organizing pneumonia: CT Features in 14 patients. AJR 154:983-987, 1990.
7. Akira M, Yamamoto S, Sakatani M: Bronchiolitis obliterans organizing pneumonia manifesting as multiple large nodules or masses. AJR 170:291-295, 1998.
8. Costabel U, Teschler H, Schoenfeld B, et al: BOOP in Europe. Chest 102:14S-20S, 1992.
9. Bouchardy LM, Kuhlman JE, Ball WC, et al:CT findings in bronchiolitis obliterans organizing pneumonia (BOOP) with radiographic, clinical and histologic correlation. J Comput Asst Tomogr 17:352-357, 1993.