Anatomic Pathology / ATYPICAL ADENOMATOUS HYPERPLASIA IN LUNG ADENOCARCINOMA

We assessed the occurrence of atypical adenomatous hyperplasia (AAH) in whole lung lobes with primary cancer lesions. Following surgical resection, tissue specimens were sliced to a thickness of 4 mm (3,641 specimens from 61 cases; mean = 59.7 specimens per case). A total of 119 AAH foci were found and an association was evident in 25 (57%) of 44 adenocarcinomas, 3 (30%) of 10 squamous cell carcinomas, and 2 (29%) of 7 other lung cancers. Histologic evaluation showed that 108 AAH foci were categorized as low-grade and the other 11 as highgrade AAH. These 11 foci of high-grade AAH were present in 7 patients with adenocarcinoma, and in 1 patient there was a synchronous double primary lung adenocarcinoma. High-grade AAH was closely associated with bronchioloalveolar carcinoma (BAC) type adenocarcinoma, and low-grade AAH with nonBAC adenocarcinoma. The mean ± SD Ki-67 labeling index in high-grade AAH (3.5% ± 2.9%) was significantly higher than for the low-grade index (1.4% ± 1.6%). We propose that foci of highbut not lowgrade AAH may be potential precursor lesions of lung adenocarcinoma, especially with the BAC component. Generally, atypical adenomatous hyperplasia (AAH) has been detected by chance during histopathologic studies on surgically resected lung specimens or for autopsy. AAH is a solitary alveolar lesion with proliferation of type II alveolar pneumocyte-like and/or Clara cell–like cells with a varied cellular atypia. Miller et al1 suggested that AAH might be an adenomatous lesion as there was an analogy to colonic tumor, and subsequent studies supported this hypothesis.2-11 AAH may be a precursor lesion of peripheral lung adenocarcinoma, especially of bronchioloalveolar carcinoma (BAC), based on morphologic2 and morphometric studies,3,10 flow cytometry,11 immunohistochemical assessment of abnormal oncogene and tumor suppressor gene expression,4,8,9 and K-ras gene mutation5,6 in cases of resected lungs in patients with adenocarcinoma. Therefore, in the current lung tumor classification of the World Health Organization, AAH is now categorized as a precursor lesion to adenocarcinoma.12 It is difficult to accurately identify AAH macroscopically as AAH foci are lesions usually smaller than 5 mm in diameter.2 The frequency of AAH found in lungs with adenocarcinoma has been reported to range from 9.3% to 30%13-18; however, these studies were done using more than 5 mm of the sliced lung, a portion near the main cancer, or alveolar lesions identified macroscopically. For a more precise examination to characterize AAH, using thinner and whole resected lung specimens seems preferable. Since a 5-mm diameter has been considered one of the critical points to differentiate AAH and adenocarcinoma,10,13 we chose to examine 4-mm intervals of thickness for each surgically resected lung specimen. We then compared the incidence of AAH and the clinicopathologic background including age, sex, habits, and prognosis to clarify the significance of AAH as a precursor of lung Anatomic Pathology / ORIGINAL ARTICLE Am J Clin Pathol 2002;117:464-470 465 © American Society for Clinical Pathology adenocarcinoma. Our findings indicate that AAH is likely to be more frequent than noted in previous studies. We also suggest that high-grade AAH may be a potential precursor lesion of lung adenocarcinoma, especially with the BAC component. Materials and Methods Cases and Tissue Preparation The 61 cases we examined consisted of 44 adenocarcinomas, 10 squamous cell carcinomas, 2 small cell carcinomas, 2 large cell carcinomas, 1 adenosquamous carcinoma, and 2 mucoepidermoid carcinomas. These Japanese patients underwent surgery at Kyushu University Hospital, Fukuoka, Japan, between 1997 and 1998. The 61 surgically resected lung specimens were obtained by lobectomy in 55 cases, pneumonectomy in 4 cases, and segmentectomy in 2 cases. The patients were not prescribed chemotherapy or irradiation before surgery. The mean ± SD age was 67.9 ± 8.6 years, and the male/female ratio was approximately 1.3. According to the TNM staging system of the International Union Against Cancer,19 36 patients were in pathologic stage I, 7 in stage II, and 18 in stage III. The resected specimens were fixed by the intrabronchial instillation of 10% buffered formalin to avoid alveolar collapse and incompleteness of fixation; then they were inflated using the same formalin. After fixation for a few days, the whole resected lungs were sliced at 4-mm-thick intervals. Lung blocks were obtained from alternate sliced specimens, and the remainder of the specimens were preserved for use in other examinations. Of the 3,641 blocks, the mean ± SD value was 59.7 ± 25.7 specimens per case. The 4-μm-thick sections obtained from the paraffin-embedded lung blocks were stained with H&E. The histologic diagnosis of AAH was based on the following criteria2,3,7,10,13,20: (1) localized and well-defined boundary; (2) consisted of atypical epithelial cells that were cuboidal to low columnar or peg-shaped, and either type II pneumocytes or Clara cells were proliferating along the slightly thickened alveolar wall with mild chronic inflammation but without scar formation; and (3) cell atypia of the epithelium in AAH was apparent, but nuclear size and atypia were less prominent than in cases of adenocarcinoma. AAH was classified into 2 grades based on histologic atypia.2,21,22 Low-grade AAH consisted of a single layer of intermittent or rather uniform and continuous proliferation of mildly atypical cells lining the alveolar septa ❚Image 1A❚, ❚Image 1D❚. High-grade AAH had more increased cellularity and cellular atypia than did low-grade AAH, such as enlarged and hyperchromatic nuclei and an increased nuclear/cytoplasmic ratio ❚Image 1B❚, ❚Image 1E❚. A “heaping-up” formation of atypical cells and multinucleation were focal. Cellular polymorphism, cell density, and a nuclear area of high-grade AAH were less than in cases of adenocarcinoma ❚Image 1C❚, ❚Image 1F❚. The histopathologic classification of primary cancer was done based on the criteria of the World Health Organization but with some modifications in adenocarcinoma, as described,12,23 and was determined by 3 pathologists (Drs Koga, Matsuo, and Sueishi). The diagnosis of multiple primary lung cancer was based on documented criteria.24 Patients with a history of tobacco use were divided into smokers, including current smokers and those with a history of smoking, and nonsmokers with no history of tobacco use. Immunohistochemical Analysis To evaluate the growth potential of AAH, we used a mouse monoclonal anti–Ki-67 antibody (MIB-1, Immunotech, Marseille, France) ❚Image 1G❚, ❚Image 1H❚, and ❚Image 1I❚. For antigenic retrieval, the sections were autoclaved for 5 minutes at 121°C in a 0.1-mol/L citrate buffer solution, pH 6.0. After treating the sections with a 1.5% skim milk solution to reduce nonspecific absorption of antibodies, the sections were reacted overnight at 4°C with primary monoclonal antibodies diluted to 1:100. These tissue sections were treated with biotin-labeled antimouse antibody and then with a 0.1% hydrogen peroxide–methanol solution, followed by use of the streptavidin-biotin-peroxidase complex method.25 The Ki-67 labeling indices of AAH cells were estimated among all the AAH cells in each focus by counting the number of cells with a positive nucleus. Statistical Analysis All values are expressed as mean ± SD. To estimate the correlation between the histologic type and the frequency of AAH, the chi-square, the Fisher exact probability test, and the Mann-Whitney U test were used. All P values were based on 2-hypothesis testing, and statistical significance was assumed at a level of P less than .05. Survival curves were obtained using the Kaplan-Meier method, and the statistical significance of differences was calculated using the log-rank test.