To our knowledge, this is the first report of a napsin A-positive metastatic lung tumor originating from colon cancer. Napsin A is generally expressed in lamellar bodies of type II pneumocytes [1]. Therefore, napsin A is highly expressed in primary lung adenocarcinomas, but not metastatic lung adenocarcinomas [2]. The specificity of napsin A expression is reliable especially as compared with colon adenocarcinomas. Renal cell carcinomas and a few other adenocarcinomas are known to be occasionally positive for napsin A; however, no positive colon adenocarcinoma cases have been reported [3].
It has been reported that polyclonal napsin A antibody sometimes reacts nonspecifically to non-pulmonary adenocarcinomas [4]. In our case, immunohistochemical examination was performed with various antibodies: cocktailed, polyclonal, and monoclonal. Reexamination was performed at a third-party laboratory, and all the results were positive in the both the lung tumor and rectal cancer.
The diagnosis in our case was metastatic lung cancer originating from primary rectal cancer. The histological features of the lung tumor were typical of a primary rectal adenocarcinoma. The radiological features of the lung tumor were also typical of a metastatic lung tumor. Considering the clinical course of the patient, it is irrational to believe that a primary lung cancer appeared 18 months after resection of a metastatic rectal cancer.
The lung cancer and the rectal cancer could not have been independent of each other. The results of a hematoxylin and eosin stain were similar in both lesions, as was the protein expression pattern; as such, both adenocarcinomas likely share a common origin. No other malignant lesions have been found 21 months after the last surgery.
In this patient, the colon adenocarcinoma was positive for napsin A. Some thyroid carcinomas have also been reported to be positive for napsin A, which were all of the papillary type with tall cell morphology [3]. Our patient’s rectal cancer also exhibited these features. The reasons for positive staining of napsin A in our case remain unknown. The rectal cancer may have produced some proteins that cross-react to napsin A [5] or there may be issues with the sensitivity of the antibodies used for immunohistochemical analysis.
One case of TTF-1-positive metastatic lung cancer originating from colon cancer has already been reported [6]. The expression of TTF-1 in primary lung adenocarcinomas is high but lower than napsin A. TTF-1 expression is rarely positive in cancers of the colon, uterus, ovaries, and so on [7].