[The histogenesis and malignant transformation of inverted papilloma on the sino-nasal cavity--in comparison with inverted papilloma on the urinary bladder].

Inverted papilloma on the sino-nasal cavity (IP on SN) is a rare benign tumor. However, local recurrence and distant metastasis do occasionally occur. The histogenesis and histological prospects for the biological behavior of this tumor are still unknown. To clarify these issues, we histologically studied 25 cases of IP on SN and 10 cases of IP on the urinary bladder (IP on UB), which were biopsied or resected in the National Shikoku Cancer Center Hospital. Since, with regard to aspects of histogenesis such as metaplasia, dysplasia and malignant transformation, there were some similarities and differences between these two benign tumors. Biopsied or resected specimens were microscopically examined with hematoxylin-eosin (HE), periodic acid-Shiff (PAS), and Azan stains. Sixteen of 25 IP on SN presented a typical inward growth Pattern and 9 cases were simultaneously associated with an exophilic proliferation pattern. In cases of IP on UB, a typical inverted growth Pattern was observed in 3 cases. Seven of 10 IP on UB showed a complex of inward and exophilic patterns. Subsequently, we focused on metaplasia of IP, probably caused by certain physiological, chemical and inflammatory stimuli. All IP on SN bore squamous metaplasia and 5 cases simultaneously showed features of transitional metaplasia. Often, these two types of metaplasia were observed in the submucosal glands. In IP on UB, 7 of 10 cases showed squamous metaplasia and 9 cases presented intestinal metaplasia. These metaplastic changes were also found in Brunn's nest. Furthermore, the metaplastic epithelium in 24 IP on SN demonstrated stratified proliferation, atypical nuclei and/or mitosis which were considered to be evidence of malignancy. In fact, 9 of 25 IP on SN partially contained a component of squamous cell carcinoma and 7 cases out of 10 IP on UB showed features of transitional carcinoma in a portion of the papilloma lesion. This evidence suggests that both IP on SN and IP on UB originate in submucosal glands, specifically exocrine glands in the sino-nasal region and Brunn's nest in the urinary bladder. Also, in these glands, metaplastic change was commonly seen, and occasionally revealed malignant features such as squamous cell carcinoma in the sino-nasal cavity and transitional carcinoma in the urinary bladder. We finally concluded that the process of growth and the mechanism of malignant transformation might be similar between IP on SN and IP on UB and metaplastic change might play an important role in the malignant transformation of IP on SN and IP on UB.