Tumour-to-tumour metastasis: metastatic rectal adenocarcinoma within adenocarcinoma of the lung, and a review of the literature

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Background: Tumour-to-tumour metastasis (TTM) is a rare but well-recognised phenomenon. When presented with a case of synchronous colon and pulmonary adenocarcinoma in the lung, one must consider the differential diagnosis of TTM and exclude a tumour with divergent phenotype because of difference in clinical management. Aims: Herein, we present a rare case of TTM, in which a rectal adenocarcinoma has metastasised to a pulmonary adenocarcinoma. We illustrate how morphologic, immunohistochemical, and molecular analyses can help in the pathological diagnosis of TTM. Methods: A case of TTM with a review of the literature is presented. Results and conclusions: A 67-year-old male with a history of metastatic rectal adenocarcinoma to the liver has been in regular follow-up since 2012. Post-operative colonoscopy was normal and a computer tomography scan in 2015 revealed a 0.6 cm subpleural nodule in the left upper lobe of the lung. For the next 3 years, the nodule only showed mild interval enlargement, until a follow-up CT scan in 2019 revealed sudden increase in size to 1.5 cm. Suspicious of lung metastasis, the nodule was resected and histopathological analysis unexpectedly revealed a malignant tumour composed of a pulmonary and metastatic rectal adenocarcinoma. IHC for Napsin A, TTF-1, and CK7 was positive in the pulmonary adenocarcinoma, while CK20 and CDX-2 were positive in the metastatic rectal adenocarcinoma. KRAS mutation was identified in the pulmonary but not in the metastatic carcinoma. The latter was wild-type for KRAS and had TP53 mutation similar to the original rectal tumour. A review of the literature identified some 165 cases, of which only one previous case of colon adenocarcinoma metastasizing to lung adenocarcinoma has been reported. Diagnosis of TTM requires histomorphologic assessment and is aided by immunohistochemical and molecular analysis. Awareness of TTM is important because of the significant difference in clinical management.