Benign notochordal cell tumor (BNCT) arises from notochordal cell remnants and has recently been recognized as a benign counterpart and possible precursor to chordoma. Chordal tumors generally occur in the medial bone where chordal remnants are present, although rare chordal tumors also occur primarily in soft tissue and visceral organs. Interestingly, to date, the only organ-derived chordal tumor has been seen in the lung. Since such tumors are rare, the cause of this phenomenon remains to be further elucidated. Recently, Takahashi et al. in Medicine, Japan, published a case of extraosseous BNCT of pulmonary origin, and the diagnosis was confirmed by pathological findings and immunohistochemical expression of brachyury, a reliable marker of chordal tumors. The case was reported in a 57-year-old woman with no symptoms and no smoking history who presented to Komagome Hospital in Tokyo, Japan, with a nodular shadow in the middle lobe of the right lung on chest plain radiograph (Figure 1A). A well-defined, round, solid nodule with a diameter of 10 mm was seen on chest CT in the 6th segment of the right lung (Figure 1B).PET-CT did not reveal any other lesions such as bone and soft tissue. Physical examination and laboratory examination did not show any significant abnormalities. On further examination, serum tumor markers were not elevated. Imaging findings suggested that the pulmonary nodules were adjacent to the dirty pleura, although it was difficult to perform histological examination by bronchial endoscopic biopsy. Therefore, a thoracoscopic wedge resection of the right lower lobe of the lung was performed for a definitive diagnosis, and no other nodules were seen in the lung intraoperatively. In general, there was a homogeneous, clear jelly-like tumor in the resected lung parenchyma with a maximum diameter of 10 mm (Figure 2A). On histological examination, the tumor was well-defined and sparsely cellular, consisting of large cells with a low nucleoplasmic ratio and significant intracellular vacuolation (Figure 2B). The tumor lacked a fibrous mesenchyme and had minimal vascularity and lymphatic vessels. Instead, the tumor cells proliferated in a marked mucinous-like background. Fatty, double vacuolated and multivacuolated cells were scattered within the tumor, the latter resembling the vacuolated cells typically seen in chordoma. Lymphocyte sleeve-like arrangement and mild disruption of alveolar structure were seen in the periphery of the tumor (Figure 2C, Figure 2D). In addition, most of the tumor cells had small round nuclei with fine chromatin and very few prominent nucleoli. Some tumor cells were irregular in shape with large nuclei (Figure 2E); nuclear division and vascular invasion were not observed. Mucus-like material was seen in the cytoplasm of vacuolated tumor cells, and small vesicles were visible in Asin blue staining, and the surrounding alveolar lumen was also stained (Figure 2F). In immunohistochemical staining, brachyury, a highly specific diagnostic marker of spinal cord differentiation, was focally and strongly positive in the nuclei of the tumor cells (Figure 3A). The proportion of brachyury-positive cells was compared in this case and in three cases of classic chordoma occurring in the mid-axis bone by counting 100 nuclei. The study was approved by the unit ethics committee (KH-1460) and written consent was obtained from the patient. The results showed that the proportion of brachyury-positive cells in this case (14%) was much lower than in the control chordoma cases (90-98%, mean 93.3%) (Figure 3B). Chordoma occurring in the median bone and BNCT positive immunohistochemical markers were also positive in this case, such as CK, AE1/AE3 (panCK), CAM5.2 (low molecular weight CK) vimentin, S-100, CK7, CK18, CK19 (Figure 3C, Figure 3D). The immunohistochemically negative markers were CK5/6, TTF-1, napsinA, CD68, CD163 (Figure 3E, Figure 3F). The percentage of MIB-1 (Ki-67)-positive cells in the tumor was extremely low (2.2%). To rule out metastasis of the tumor to the lung from an unknown primary site, extensive imaging studies such as whole-body CT, MRI of the whole spine, and bone scan were done. The result was that no tumor was seen throughout the body. Five years after surgery, the patient remains free of tumor recurrence or metastasis. Chordal tumors, including chordoma and BNCT, are generally thought to originate from the remnants of the spinal cord. In contrast, BNCT is a benign tumor of spinal cord origin that is similar in location to chordoma, but the incidence is not known; BNCT tends to present as a small tumor, approximately 4 mm in size, and cases of coexistence between chordoma and BNCT have been reported, suggesting that BNCT can progress to chordoma. Although rare, chordomas can occur outside of the mid-axis, most commonly in the soft tissues. In addition to soft tissues, a few cases of pulmonary origin have been reported. In all reports, the lung is the only visceral organ in which chordoma occurs outside the bone. Interestingly, Kikuchi et al. recently reported two cases of incidental chordoma of the lung with brachyury expression suggestive of BNCT. in addition, Lee et al. reported a case of BNCT in both lungs. The histological and immunohistochemical findings of this case reported here are very similar to those of the previously reported cases, but in this case there was no formation of a central cystic zone. The tumor in this case had mucus production, which was confirmed by Asin blue staining, but this was uncommon in typical BNCT and common in chordoma. However, the slight destruction of alveolar structures, mild nuclear atypia, and minimal nuclear division suggest that this case is still a benign tumor. The histologic and immunohistochemical expression of the three pulmonary BNCTs, plus the present case, were nearly identical to the features of the BNCT of the mid-axis bone with brachyury expression. In contrast, no soft tissue BNCT has been reported to date. Highly developed thoracic imaging techniques can detect occasional small tumors, such as BNCT, which could partially explain this, rather than the lung being a specific site for notochordal cell tumors. brachyury has been suggested to be a key transcription factor in early posterior mesodermal development, a period of development that includes the development of vascular-forming cells as well as the notochord. Recently, brachyury has been identified as a highly sensitive and specific immunohistochemical marker of notochord differentiation, allowing clear differentiation of notochord tumors from other histologically similar lesions. Although brachyury was positive in this case, the positive expression was only focal. Almost all chordoma cells have been reported to be diffusely brachyury positive, so the authors compared this case with 3 cases of classic chordoma with brachyury positive cells, in which the percentage of positivity was significantly lower. To the authors’ knowledge, no other study has compared the difference in brachyury positive expression between BNCT and chordoma. It should be noted, however, that brachyury positivity is only focally expressed in at least some BNCTs. It is unclear whether this phenomenon is unique to this case or a common feature of all BNCT. Further studies are needed to confirm this pattern of brachyury expression in spinal cord tumors. After excluding the possibility of metastatic tumors, the differential diagnosis of the primary tumor includes several possibilities: chordoma, extraosseous mucinous chondrosarcoma, myoepithelioma/mixed tumor, benign peripheral nerve sheath tumor with mucinous changes, chondrosarcoma, mucinous liposarcoma, primary mucinous sarcoma of the lung with EWSR1-CREB1 fusion, pleomorphic adenoma, chordoid meningioma, and mucinous carcinoma. Based on histopathological analysis, malignant tumors could be excluded because of their characteristic benign manifestations such as slight destruction of glandular follicular structure, absence of nuclear atypia, and minimal nuclear division of tumor cells. In addition, the tumor showed no expression of other specific immunohistochemical markers, while expressing brachyury, a highly specific marker for chordoma and BNCT. in fact, the tumor cells in this case were almost certainly a manifestation of bone BNCT in terms of histologic presentation. Consistent with previous reports, BNCT of the lung is a benign tumor detectable by imaging; however, it is unclear whether it can be considered a precursor lesion to chordoma. In addition, it is difficult to distinguish these tumors from other primary or metastatic mucinous tumors based on imaging findings alone. Therefore, brachyury testing may be appropriate on post-tumor resection pathology. In addition, the presence of primary BNCT in the lungs suggests that chordal tumors can occur in organs unrelated to known chordal development.