The notochord is a midline axial structure formed after the second to third week of embryonic development. It extends along most of the length of the embryo, inducing the formation of nerves and spines, and providing structural support for axial bone development. During development, the notochord degenerates in the vertebral body and intervertebral disc area, generally disappearing at 1–3 years old. However, some individuals still harbor notochord residues, which may lay the foundation for chordoma occurrence and development [5].
Ribbert first named the Notochordoma in the 1890s. It is a malignant tumor often occurring in the axial skeleton, exhibiting local invasion and potential metastasis. Although chordomas have been known for over a century, only recently has there been interest in defining and describing the biological properties of benign chordal lesions [6].
Benign chordomas usually grow slowly and do not invade surrounding tissues or metastasize to other sites. Such tumors can occur at any age but are common in adults [7]. BNCT comprises well-defined but unencapsulated single or isolated tumor cell sheets, which can be less vacuolated cells containing eosinophilic cytoplasm. Unlike classical chordomas, BNCT has no expansive lobular growth pattern, a syncytial cord or tumor cell chain with mucous extracellular matrix, and apparent nuclear atypia in BNCT cells [6]. Notochordomas exhibit aggressive and destructive imaging and histological features not visible on BNCT. Some authors have reported the coexistence of these two tumors in patients, indicating a specific relationship between BNCT and chordomas [8]. However, histologically confirmed BNCT patients did not show progression of these lesions during long-term follow-up. Therefore, more convincing studies are needed to prove that BNCT can be transformed into chordoma [9].
BNCT is a rare intraosseous lesion that can appear along the entire spine and may be asymptomatic or present with moderate pain. The differential diagnoses include hemangioma, osteoblastoma, osteonecrosis, and chordoma [10]. BNCT is usually considered insidious, rarely progresses clinically. Furthermore, there was no extension to the soft tissue, lack of noticeable enhancement, bone destruction, cortical penetration, or delicate tissue components, and it was very small (< 35mm). BNCT is usually concealed on X-ray films and is often accidentally detected using magnetic resonance imaging (MRI). Computed tomography (CT) may show mild sclerosis without evidence of bone destruction. On MRI, they showed bone marrow replacement on T1-weighted images with high T2-weighted signal intensity. Typically, there is no enhancement or lack of soft tissue extension. In contrast, chordoma imaging reveals various malignant and osteolytically destructive lesions with cortical destruction, quiet tissue expansion, and different contrast enhancements. MR imaging of the chordoma showed multilobulated areas of bone marrow replacement on t1-weighted images and high signal intensity on t2-weighted images. The imaging differences between BNCT and chordomas often lead some scholars to believe that biopsy or surgical resection of BNCT is usually unnecessary, but imaging monitoring may be required. [7, 11].
However, we propose new perspectives on the cases introduced in this study. Some tumors such as hemangiomas can show trabecular thickening on plain radiographs, and the punctate sclerotic area on CT may involve either part of the vertebral body or the whole vertebral body [8]. This is similar to the performance observed in this study. Because the lesions in this case involved the vertebral body and its appendages, and there have been no previous reports of BNCT cases with such a wide range of lesions, chordoma has been reported to involve the vertebral appendages [12]. Although the patient's imaging data were comprehensive, doubts persisted regarding the nature of the tumor. Therefore, we believe that diagnosing BNCT solely based on imaging data is challenging, at least in this case. Consequently, we relied on the biopsy results to confirm the diagnosis. However, we need to perform a more detailed immunohistochemical examination of the patients’ living tissue specimens based on sufficient imaging data, selecting only the most relevant items.
In the current WHO classification, BNCT and chordomas include the entire spectrum of chordoma-derived tumors. BNCT and chordomas have clear radiological and histological features that facilitate differentiation. According to the WHO standards, BNCT lacks cortical penetration and soft tissue extension. However, some scholars have pointed out that BNCT may have slight and limited penetration into the cortex or even extraosseous smooth tissue extension, proposing a cautious classification method for these problematic tumors and designating them as atypical notochordal cell tumors [7]. However, it is challenging to confirm these differences histologically. Identifying BNCT and chordoma is clinically valuable. Patients with asymptomatic BNCT can use a series of imaging techniques to track tumor changes dynamically, and symptomatic BNCT can be conservatively treated to alleviate symptoms. Chordoma is characterized by aggressive behavior and usually requires radical resection, including adjuvant radiotherapy [10]. It has been reported that the patient underwent surgical treatment owing to the misdiagnosis of BNCT as chordoma and died of surgical complications. Therefore, patients should be prevented from engaging in excessive medical behavior [13].