2 Introduction

Osteosarcoma is a malignant bone tumor predominantly affecting adolescents and young adults, though it can occur across all age groups. It is characterized by the production of osteoid matrix by malignant mesenchymal cells.¹ The most common subtype, conventional osteosarcoma, typically arises in the metaphysis of long bones such as the distal femur, proximal tibia, and proximal humerus.¹ Other subtypes, including telangiectatic osteosarcoma, exhibit distinctive features such as prominent vascularity. Epidemiologically, osteosarcoma constitutes about 3 % of all primary bone tumors, peaking in incidence during adolescence.¹ Clinically, it presents with localized pain, swelling, and sometimes pathologic fractures. Despite advancements in treatment, the aggressive nature of osteosarcoma and its tendency for metastasis, particularly to the lungs, make it a challenging malignancy to manage.¹

Chondroblastoma-like osteosarcoma (CBLOS) is an exceptionally rare variant of osteosarcoma that complicates diagnosis due to its striking histological resemblance to chondroblastoma, a benign cartilage-producing tumor.² While chondroblastoma accounts for less than 1 % of all primary bone tumors and typically affects younger individuals in the epiphyseal regions of long bones, CBLOS presents a unique diagnostic challenge.² Unlike chondroblastoma, which generally has a benign course, CBLOS exhibits more aggressive cellular behavior, including pronounced cytological atypia and osteoid matrix production. The overlap in radiological and histological features between CBLOS and chondroblastoma often makes differentiation difficult, requiring careful evaluation by an experienced orthopedic oncologist.²

The objective of this article is to address the diagnostic challenges and treatment implications associated with CBLOS by presenting a detailed case study. This case, focusing on a patient with CBLOS in the pelvic region, highlights the complexities of distinguishing CBLOS from benign chondroblastoma and other osteosarcoma variants. Through this case, the article aims to provide insights into the clinical presentation, diagnostic approach, and management strategies for CBLOS, ultimately contributing to better understanding and treatment of this rare and aggressive tumor.

The report standards

This study follows a retrospective case series design, adhering to the PROCESS (Preferred Reporting of Case Series in Surgery) guidelines.³

3 Case presentation

3.3

3.3 Imaging findings

Radiographs of the pelvis revealed a lytic lesion in the quadrangular area of the right acetabulum, which disrupted the ilioischial line, a key indicator of possible bone pathology. The CT scan with PET-CT demonstrated intense FDG uptake with a maximum standardized uptake value (SUVmax) of 21.19, suggesting a highly metabolically active lesion. There were no other lesions detected on PET-CT, which helped rule out metastatic disease. MRI imaging showed an aggressive and destructive mass measuring approximately 7x6x5 cm in the right acetabulum and inferior pubic ramus. The mass displayed central cystic necrotic areas with intense peripheral enhancement, indicative of its aggressive nature and involvement of adjacent structures (Fig. 1).

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Fig. 1 Coronal T2 image.

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3.4

3.4 Histopathological findings

Initial histopathological analysis from an ultrasound-guided biopsy suggested features consistent with chondroblastoma. However, due to the clinical suspicion of a more aggressive pathology, a mini-open biopsy was performed but did not conclusively establish the diagnosis. The presence of atypical cells raised concerns, prompting a third CT-guided biopsy. This biopsy ultimately confirmed the diagnosis of chondroblastoma-like osteosarcoma. Histological examination revealed chondroblastic differentiation and a cellular stroma, which were key features in distinguishing it from other conditions such as chondroblastoma and giant cell tumor (Fig. 2).

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Fig. 2 Immunohistochemically, SATB2 is strongly positive and H3F3B is negative (Immunohistochemical analysisX20).

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3.5

3.5 Treatment approach

The patient was treated with neoadjuvant chemotherapy, though the specific regimen details were not provided. The chemotherapy was aimed at reducing the size of the tumor before surgical intervention (Fig. 3).

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Fig. 3 Postneoadjuvan T2.

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Following chemotherapy, the patient underwent extensive surgical resection, including type 2 + 3 pelvic resections with extra-articular hip resection. The surgical team performed reconstruction using an Ice-cream cone tumor prosthesis and mesh to restore functionality and structural integrity. No additional adjuvant therapies, such as radiation or targeted therapy, were noted in the treatment plan (Fig. 4).

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Fig. 4 Macroscopic resection material.

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4 Discussion

The study of CBLOS is of paramount importance due to its rarity, aggressive behavior, and diagnostic complexities. As an atypical variant of osteosarcoma, CBLOS poses significant challenges in distinguishing it from both benign chondroblastoma and other malignant bone tumors, which can lead to delays in appropriate treatment.^1,2 Early and accurate diagnosis is crucial for effective management, as CBLOS often necessitates more intensive treatment strategies compared to its benign counterparts.² Understanding the unique radiological and histopathological features of CBLOS enhances diagnostic accuracy and informs the development of tailored treatment protocols.^4–8 Furthermore, this study contributes valuable insights into the clinical presentation, management approaches, and outcomes for patients with CBLOS, ultimately aiming to improve patient care, optimize treatment strategies, and enhance survival rates for this rare and aggressive tumor type. By addressing these critical aspects, the research supports the advancement of clinical knowledge and the refinement of therapeutic practices in oncology.

The role of radiologists and pathologists is critical in diagnosing chondroblastoma-like osteosarcoma (CBLOS), given its complex radiological presentation.^4,5 Chondroblastoma, a benign cartilage-producing tumor, typically appears as a well-circumscribed, eccentric radiolucent lesion with a thin sclerotic rim. In contrast, CBLOS presents with more aggressive radiological features, including cortical destruction, soft tissue invasion, and a mixed lytic-sclerotic pattern.^4,5 This distinction is crucial because the presence of these aggressive characteristics can significantly impact treatment strategies and prognosis. In the case of CBLOS located in the acetabulum, the initial radiographs showed a lytic lesion disrupting the ilioischial line.^4,5 MRI further revealed an aggressive, destructive mass with central cystic necrosis and intense peripheral enhancement. These findings are consistent with the known radiological profile of CBLOS, which often includes a mixed lytic and sclerotic pattern, expansile radiolucent areas, and focal punctate calcifications.^4,5 The challenge lies in differentiating these features from those of other bone tumors, as the aggressive nature of CBLOS can mimic the appearance of various malignant and benign bone conditions.

Several radiological mimics pose challenges in distinguishing CBLOS from other bone tumors.^4,5 Chondroblastoma, with its benign appearance, often shares similarities with CBLOS, such as its location in the epiphysis and the presence of a well-defined, radiolucent lesion.^4,5 However, CBLOS differs by exhibiting more aggressive features like cortical destruction and soft tissue invasion. Giant cell tumors, another common mimic, also present with aggressive bone destruction and may share some radiological characteristics with CBLOS. Similarly, aneurysmal bone cysts, known for their expansile and lytic appearance, can resemble CBLOS in imaging. The PET-CT findings in our case, showing intense FDG uptake (SUVmax: 21.19), highlighted the high metabolic activity of the tumor, which is instrumental in distinguishing CBLOS from these other entities. Previous reports emphasize that such radiological features are pivotal in differentiating CBLOS from its mimics, underscoring the critical role of comprehensive imaging in accurate diagnosis and effective management of this rare malignancy. The extensive bone involvement and destruction observed in our case further illustrate the importance of advanced imaging techniques in guiding biopsy, confirming diagnosis, and planning complex surgical interventions.^4,5

Histologically, chondroblastoma-like osteosarcoma (CBL-like OS) is characterized by cellular regions that mimic chondroblastoma (CB) but with distinctive abnormalities.^6,7 The tumor displays an infiltrative growth pattern with small cells featuring ovoid, folded, or grooved nuclei and pale eosinophilic cytoplasm, reminiscent of CB. However, it also shows areas with larger, atypical cells, mitotic figures, and malignant osteoid deposition. The presence of osteoid production, increased and atypical mitoses, cytologic atypia, tumor necrosis, and destructive infiltration of surrounding tissues further differentiates CBL-like OS from CB. Histopathologic diagnosis is confirmed by biopsy, revealing chondroblasts arranged in a “chickenwire” pattern with foci of chondroid matrix and multinucleated giant cells. Immunohistochemistry helps differentiate CB from CBL-like OS, as CB is H3K36M positive, while CBL-like OS is negative. Conventional chondroblastic osteosarcoma and chondrosarcoma are unlikely to be confused with CBL-like OS due to their distinct histological features, such as high-grade hyaline cartilage deposition and marked cellular atypia. Additionally, while CBL-like OS and giant cell tumors may appear similar, CBL-like OS is differentiated by the presence of osteoid formation, atypical mitoses, and nuclear atypia, absent in giant cell tumors.^6,7

In the management of CBLOS, radical resection is crucial due to the aggressive nature of the tumor and its potential for local recurrence. The primary goal of surgical intervention is to achieve complete tumor removal while preserving functional integrity. In our case, the patient underwent a type 2 + 3 pelvic resection with extra-articular hip resection. This approach was necessary due to the extensive involvement of the acetabulum and the inferior pubic ramus, which required removal of the tumor along with adjacent affected tissues. The surgical planning included careful preoperative imaging to delineate the tumor boundaries and assess involvement of surrounding structures. Reconstruction was performed using an Ice-cream cone tumor prosthesis and mesh to restore pelvic stability and function. The extensive nature of the resection and reconstruction underscores the importance of meticulous surgical planning to optimize outcomes and minimize complications.

Postoperative outcomes in our case were favorable, with no complications reported. The patient's recovery was closely monitored, and regular follow-up was initiated to detect any signs of recurrence or complications early. Potential complications of such extensive pelvic resections include wound infections, prosthesis-related issues, and functional impairments. Long-term follow-up is essential to ensure that any complications or tumor recurrences are promptly addressed. The use of a tumor prosthesis in reconstructive surgery provides stability but requires careful monitoring to ensure its proper integration and function.

For chondroblastoma-like osteosarcoma (CBLOS), neoadjuvant chemotherapy is often employed to reduce tumor size and facilitate surgical resection.^8,9 Effective chemotherapy regimens typically include a combination of agents such as methotrexate, doxorubicin, and cisplatin, which are standard in the treatment of osteosarcoma. In our case, the patient received neoadjuvant chemotherapy before the surgical intervention. This approach aims to decrease the tumor's volume and improve surgical resectability, potentially enhancing outcomes and reducing the likelihood of residual disease.

The response to chemotherapy in CBLOS, as with other osteosarcoma variants, is generally evaluated through imaging studies and clinical assessment. Response rates can vary, but successful chemotherapy usually leads to a significant reduction in tumor size and metabolic activity. Potential side effects of the chemotherapy regimens include nausea, myelosuppression, and increased susceptibility to infections. Monitoring and managing these side effects are crucial to maintaining the patient's overall health and optimizing treatment efficacy.

A multimodal approach involving both surgery and chemotherapy is essential for the management of CBLOS. The combination of preoperative chemotherapy followed by radical surgical resection offers the best chance for long-term disease control. In our case, neoadjuvant chemotherapy helped to reduce the tumor burden, making it amenable to extensive surgical resection. Postoperative chemotherapy may also be recommended to address any residual disease and prevent recurrence. This integrated approach maximizes the chances of successful treatment and minimizes the risk of relapse.

While radiotherapy is not typically a first-line treatment for CBLOS, it may be considered in cases with residual disease or unresectable tumor components.¹⁰ Emerging treatments, including targeted therapies and immunotherapy, are areas of ongoing research and may offer additional options for managing this rare variant of osteosarcoma. The potential benefits of these treatments are currently being evaluated in clinical trials and could provide new avenues for improving patient outcomes in the future.

The prognosis for patients with CBLOS can be challenging to determine due to its rarity and the limited number of reported cases. However, survival rates for osteosarcoma in general are influenced by factors such as tumor size, location, and response to chemotherapy. For CBLOS, factors such as the extent of surgical resection and the presence of residual disease play a significant role in prognosis. Early detection and aggressive treatment are critical for improving survival outcomes.

CBLOS, as an exceptionally rare subtype of osteosarcoma, is often compared with more common variants such as conventional osteosarcoma and telangiectatic osteosarcoma. While conventional osteosarcoma has a more established prognosis and treatment protocol, CBLOS requires a nuanced approach due to its atypical presentation and aggressive behavior. Comparing outcomes across these subtypes highlights the importance of tailored treatment strategies and underscores the need for continued research into effective management and prognostic factors for rare osteosarcoma variants.

Lessons learned.❖Early and accurate diagnosis of CBLOS is critical: Due to its histological similarity to benign chondroblastoma, careful radiological and histopathological evaluation is essential to distinguish CBLOS and initiate appropriate aggressive treatment.❖Comprehensive imaging is pivotal: Advanced imaging techniques, such as MRI and PET-CT, play a crucial role in identifying the aggressive features of CBLOS, guiding biopsy, and planning surgical interventions.❖Multimodal treatment approach is key: Combining neoadjuvant chemotherapy with radical surgical resection significantly improves the chances of complete tumor removal and better long-term outcomes in CBLOS patients.

5 Conclusion

In summary, CBLOS is a unique and challenging subtype of osteosarcoma that demands a comprehensive and tailored approach to diagnosis and treatment. By advancing our understanding of its clinical presentation, diagnostic features, and treatment strategies, we can improve patient care and outcomes for those affected by this rare and aggressive tumor.

Authors' contributions

H.B. conceived and designed the study. E.K. extracted the data. H.B. analyzed the data. H.B., E.K., and A.Y. interpreted the data and wrote the manuscript. K.O. and S.G.B contributed to the critical revision of the manuscript for important intellectual content. All authors approved the final version to be published and agreed to be accountable for all aspects of the work, ensuring that any questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Ethics approval and consent to participate

For this type of study, consent to participate and ethical approval are not required.

Not applicable.

Consent for publication

Not applicable; for this type of study, consent for publication is not required.

Not applicable.

Availability of data and material

The data for this study were sourced from the our hospital database, it is available upon a request.

Not applicable.

Funding

This study did not receive any funding.