A 42-year-old man was diagnosed with rectal cancer (cT2N2bMx [UICC 8th edition]) with a solid nodule measuring 21 x 20 mm in diameter at the superior segment (S6) of the left lower lobe on the chest computed tomography (Fig. 1). The lung nodule was considered to be either primary lung cancer or metastatic lung tumor originating from the rectal cancer. Firstly, the patient underwent laparoscopic ultralow anterior resection with lateral lymph node dissection for the rectal cancer (moderately differentiated tubular adenocarcinoma, pT4aN2bMx). Six weeks after rectal surgery, we performed robotic-assisted thoracoscopic S6 segmentectomy to both diagnose and treat the tumor. As the intraoperative pathological diagnosis of the lung nodule was metastatic tubular adenocarcinoma resembling rectal adenocarcinoma, we did not perform further resection. The patient had an uneventful postoperative course and was discharged on the 4th postoperative day.
We conducted detailed pathological examinations of the obtained tumor samples, including IHC analysis of markers for differentiating metastatic and primary lung adenocarcinoma. Hematoxylin-eosin staining of the lung tumor demonstrated columnar epithelial cancer cells consisting of tubular structures (Fig. 2a); these morphologies resembled rectal adenocarcinoma (Fig. 2i). However, contrary to our expectations, TTF-1 (clone: SPT24, Novocastra, San Ramon, CA, USA) was positive in the lung tumor sample (Fig. 2b), prompting us to retrospectively review the rectal adenocarcinoma specimen. Surprisingly, TTF-1 was also positive in the rectal adenocarcinoma specimen (Fig. 2j). Further results of IHC were as follows: the lung tumor sample was napsin A (-) (clone: IP64, Novocastra, San Ramon, CA, USA), cytokeratin 7 (CK7) (-) (clone: SP52, Ventana, Tucson, AZ, USA), cytokeratin 20 (CK20) (+) (clone: SP33, Ventana, Tucson, AZ, USA), Caudal-related homeobox transcription factor 2 (CDX2) (weakly +) (clone: CDX2-88, Biocare Medical, Pacheco, CA, USA), special AT-rich sequence binding protein 2 (SATB2) (+) (clone: EPNCIR130A, abcam, Cambridge, UK), and beta-catenin (+) (clone: 17C2, Leica Biosystems Newcastle Ltd, Newcastle, UK) (Fig. 2c-h); the rectal adenocarcinoma sample was napsin A (-), CK7 (-), CK20 (partly +), CDX2 (weakly +), SATB2 (+), and beta-catenin (+) (Fig. 2k-p). Hence, we diagnosed the lung tumor as metastatic rectal adenocarcinoma based on the histological findings and the IHC results. We applied MEBGEN RASKET -B kit (RASKET-B) (Medical & Biological Laboratories Co., Tokyo, Japan) for the pulmonary lesion and confirmed BRAF wild and KRAS mutation (G12R). The pathological stage of the rectal cancer was confirmed as pT4aN2bM1 stage IV (UICC 8th edition), and the patient initiated systemic chemotherapy based on the results of RASKET-B.