Metastatic Lung Adenocarcinoma Mimicking a Colonic Polyp

Dear Editor,

Colonic polyps on colonoscopy are a common situation. Polypectomy with histologic assessment is done as standard of care. Our case represents the need for special attention to be given to patients who have a primary cancer away from the colon. Primary cancer can present as a simple colonic polyp clinically. Histology is crucial in recognition of the lesion and immunohistochemical studies must follow to determine the primary site.

We report a case of 59-year-old African American man who was diagnosed with lung cancer in 2004 presented for surveillance colonoscopy. He had two previous colonoscopies. The first colonoscopy was performed in 2005 and revealed a tubular adenoma in the sigmoid colon and a tubulovillous adenoma in the rectum. The second colonoscopy was performed in 2009 and revealed a tubulovillous adenoma in the transverse colon. Past medical history revealed hypertension and depression which were being treated with antihypertensive and antidepressant medications. He was diagnosed with a lung tumor. He had multilevel degenerative disease in his spine.

The third surveillance colonoscopy was performed in April 2012. This time colonoscopy revealed a polyp in the ascending colon which was removed and submitted for histologic examination to rule out colorectal carcinoma. Figure 1 represents the colonoscopic views of polyp. Histologic analysis demonstrated large pleomorphic cells with hyperchromasia, prominent nucleoli, and brisk mitotic activity in a polypoid fragment of colon. The tumor cells invaded around, but not into, the preexisting crypts. Figure 2a and b represent the histological slides of the colonic polyp. An extensive immunohistochemical panel was performed [Figure 2c]. The lesional cells were strongly and diffusely immunoreactive to immunohistochemicals like cytokeratin AE1/AE3, cytokeratin 7, cytokeratin 19, and thyroid transcription factor-1 with brisk Ki-67 activity. The lesional cells were focally immunoreactive to napsin A. The lesional cells were negative for cytokeratin 20, CDX-2 (a homeobox gene encoding a nuclear transcription factor), CA19-9 (carbohydrate antigen), S-100 (100% soluble protein in ammonium sulfate), melan-A (melanocytic differntiation marker), HMB-45 (human melanoma black – tumor marker), CD68 (cluster of differentiation), calretinin, renal cell carcinoma antigen, and CD10.

Figure 1.

Polyp on colonoscopy

Figure 2.

(a) Low power image with normal colonic structures replaced by malignant cells. (b) High power image demonstrating large, pleomorphic tumor cells with hyperchromasia. Mitotic figures are present. (c) The immunoblastic features of the biopsy specimen of colonic polyp

In terms of patient follow-up and further management patient declined to have treatment due to side effects and complications of chemo therapy. Patient had a life expectancy of 3-6 months. The patient died under hospice and palliative care due to progression of metastatic lung adenocarcinoma in 12 weeks.

Lung cancer has been identified as the second most common cause of cancer related deaths in both men and women in the United States. Lung cancer is responsible for 27 percent of all new cancers.[1] Lung cancer usually metastasizes to many different sites including regional lymph nodes, brain, bone, adrenal and heart.[2]

It is unusual for primary lung cancer to metastasize to the gastrointestinal tract (GIT) and highly unusual to metastasize to the colon. The small intestine is the most common site of primary lung cancer metastasis in the GIT with slightly more cases identified in the jejunum compared to duodenum. The pathogenesis of metastasis to the small intestine is thought to be due to the spread of tumor cells by hematogenous or by lymphatic route or by both.[3] Esophageal metastasis has also been reported and has been shown to be due to direct extension of tumor on autopsy.[4] A few cases of gastric metastasis from pulmonary adenocarcinoma have also been reported.[5]

Lung cancers rarely metastasize to the colon. Only a few cases of primary lung carcinoma metastasizing to the colon have been reported.[6,7] Metastases usually present as obstruction or perforation,[6] but in rare cases it may present as a fistula.[7] In our case, the patient presented with a colonic polyp, which is rare.

Colonic polyps may be of three types: Hyperplastic polyps, adenomas, and polyps associated with polyposis syndromes. 90 percent of all polyps are hyperplastic, and they are usually less than 0.5 cm. Adenomas represent 10 percent of all polyps. Histologically adenomas are of four types: Tubular, tubulovillous, villous, and serrated. Malignant potential for adenomas depends upon both size and histology; polyps greater than 1 cm in size and villous histology have greater malignant potential. Metastatic polyps can be found as a manifestation of extracolonic cancers. Metastatic polyps found secondary to extracolonic cancers include breast, ovary, skin (melanoma), stomach, esophagus, and renal cancers.[8]

In our patient, on colonoscopy, the polyp was grossly consistent with a hyperplastic polyp. However, the biopsied specimen contained clearly malignant cells, which after immunohistochemical work-up were consistent with lung adenocarcinoma. There is remarkably little evidence or literature available for such a presentation. As stated earlier, scattered cases of the small intestine and gastric metastases from lung primaries are reported, but the presentation as a colonic polyp is unusual. Colonic polyps are typically benign. Colonic polyps that contain high-grade dysplasia or microinvasive cancer confined to the mucosa are difficult to differentiate from the metastatic polyp on gross examination as in our patient. This case demonstrates the importance of the histologic and immunohistochemical examination in the assessment of an unusual polyp in a patient.