Pulmonary papillomas can be classified according to the number of lesions, location or histology [
2]. With regard to the number of lesions, pulmonary papillomas are divided into
two types, multiple and solitary. Multiple papillomas, representing papillomatosis, are usually related to infection with papillomavirus, most often occurring in children and young adults, and involving both the upper and lower respiratory tracts. Solitary papillomas are rarer and predominantly affect adults [
8‐
11]. According to location, pulmonary papillomas can be classified as central endobronchial or peripheral bronchiolar types. Flieder
et al. reviewed 14 cases of solitary pulmonary papillomas, describing 13 as central endobronchial papillomas and only 1 as
a peripheral bronchiolar papilloma [
2]. The majority of peripheral bronchiolar papillomas are small, asymptomatic and only discovered incidentally on chest radiography, as in both our cases. In contrast,
a central endobronchial papilloma often causes
a persistent, paroxysmal and/or productive cough. Screening for lung cancer using chest radiography has recently gained popularity among healthy people in Japan, so opportunities to discover asymptomatic peripheral papillomas may increase. Pulmonary papillomas are histologically divided into
three categories: squamous cell
, glandular and mixed types [
2,
12]. Squamous cell papillomas are the most common. Glandular papillomas of the peripheral lung seem to be rare, with only 20 cases reported to date in the English literature [
2,
4‐
7]. Peripheral bronchiolar papillomas sometimes grow along alveolar walls and display an appearance similar to peripheral adenocarcinomas of the bronchioloalveolar or papillary type. For differential diagnosis, it is noteworthy that endobronchiolar papillomatous fronds are constantly present and spread along the alveolar walls is limited in alveoli adjacent to peripheral papillomas. The presence of ciliated cells and basal cells is considered an important finding for confirming the diagnosis [
4].
In both of our cases, the patient was successfully treated by local excision. To the best of our knowledge, malignant transformation has only been reported with the squamous variant [
13]. The glandular variant does not appear to recur locally after local excision and has no proven malignant potential. We therefore recommend local excision for solitary glandular papilloma.