Meade, Charles

INTRODUCTION: Primary lung cancers infrequently metastasize to the gastrointestinal tract. We describe one case of isolated colonic metastasis identified at diagnosis of the primary lung malignancy and the associated immunohistochemical pattern. CASE DESCRIPTION/METHODS: An 80-year-old man presented for evaluation of subacute onset shortness of breath. CT imaging showed a spiculated mass with associated ipsilateral intrathoracic lymphadenopathy. PET-CT was performed for staging and revealed a hypermetabolic focus in the left colon with recommendation for follow up colonoscopy. No other significant lesions or evidence of distant metastatic disease were seen and subsequent biopsy of a subcarinal lymph node demonstrated primary pulmonary adenocarcinoma. The colonoscopy visualized a sessile 12 mm polyp in the recto-sigmoid colon (Figure 1). Prior colonoscopy within 5 years of presentation was normal. Colon polyp morphology showed moderately differentiated adenocarcinoma. Immunohistochemical staining of resected polyp was positive for TTF-1 and negative for CDX2. Oncology recommended tyrosine kinase inhibitor therapy for Stage IV disease and alectinib was initiated with good response at 6 month follow up. DISCUSSION: Non-small cell lung cancer metastatic to the colon is uncommon but can present as a spectrum from asymptomatic mucosal involvement to bulky obstructive disease. Autopsy series and published case reports suggest broad estimates of asymptomatic involvement in between 1-10% of lung primaries, with symptomatic gastrointestinal involvement in <1% of cases. 1 Endoscopic presentation of metastatic disease to the gastrointestinal tract is often non-characteristic and differentiating adenocarcinoma tissue of origin is aided by immunohistochemical staining. TTF-1 staining has been shown to be highly specific to pulmonary adenocarcinomas. 2 CDX2 expression is an important intestine-specific transcription factor, and CDX2 negativity suggests adenocarcinoma of non-GI origin. 3 Used in conjunction, distant metastatic disease to the GI tract can be identified.