World health organization classification of bone tumors (fifth edition): What a radiologist needs to know?
How to cite this article: Saran S, Phulware RH. World Health Organization classification of bone tumors (fifth edition): What a radiologist needs to know? Indian J Musculoskelet Radiol 2022;4:73-86.
Since 1967, the World Health Organization (WHO) classification of tumors is regarded as a reference standard and practical guide and provides a precious resource not only for the histopathologists and oncosurgeons but also for the radiologists involved in cancer care. Primary bone tumors are a rare and heterogeneous group of neoplasms that have a broad spectrum of morphological, biological, genetic, and radiological features. Appropriate imaging workup and accurate histopathological diagnosis are crucial for appropriate management and prognostication. The fourth edition of the WHO classification of tumors of soft tissue and bone was introduced in 2013. In the past 7 years, there have been considerable advances in the understanding of this large and diverse group of tumors. With technological advances and the introduction of new molecular and genetic data about some bone tumors, there has been reorganization in the classification and introduction of a few new entities. The new WHO classification of soft tissue and bone tumors introduced in 2020 (fifth edition) has made essential refinements in the classification and has also introduced many new entities. Newly identified genetic alterations and corresponding immunohistochemical markers are included in the new classification, and this has helped in the reclassification of the existing tumor entities. These novel genetic alterations not only help in prognostication but are a target for potential therapeutic options which can bring a paradigm shift in the chemotherapeutic regimen for these entities in the future. The sole basis for the classification of bone tumors is histopathological. Although radiologists are not expected to know about the exquisite pathological details of bone tumors, a broad knowledge of the recent updates, including the reclassification of a few entities or the introduction of some, is vital for narrowing the differentials in imaging. A multidisciplinary approach including an orthopedic oncologist, radiologist, pathologist, surgical, and medical oncologist is required for accurate diagnosis and management of primary bone tumors. We hereby present a simplified review for the radiologists comprising the relevant details of the updates in bone tumors along with a simplified diagnostic algorithm to characterize these lesions.
World Health Organization classification of bone tumors
Primary bone tumors are a rare and heterogeneous group of neoplasms that account for 0.2% of all human neoplasms, and these neoplasms have a broad spectrum of morphological, biological, genetic, and radiological features. Primary bone tumors frequently affect younger age groups, and often the etiology is unknown. Benign tumors often present as incidental findings, whereas malignant tumors are often diagnosed at a late stage. Appropriate imaging workup and accurate histopathological diagnosis are crucial for appropriate management and prognostication. The fourth edition of the World Health Organization (WHO) classification of tumors of soft tissue and bone was introduced in 2013. In the past 7 years, there have been considerable advances in the understanding of this large and diverse group of tumors. The new WHO classification of soft tissue and bone tumors introduced in 2020 (fifth edition) has made essential refinements in the classification and has also introduced many new entities [Table 1].
|1.||Chondrogenic tumors||Subungual exostosis
Bizarre parosteal osteochondromatous proliferation.
Atypical cartilaginous tumor
|Chondrosarcoma, grades 1
Chondrosarcoma, grades 2
Chondrosarcoma, grades 3
Clear cell chondrosarcoma
|2.||Osteogenic tumors||Osteoma NOS
Osteoid osteoma NOS
|Low-grade central osteosarcoma
High-grade surface osteosarcoma
|3.||Fibrogenic tumors||Desmoplastic fibroma||Intermediate|
|4.||Vascular tumors of bone||Hemangioma NOS||Benign|
|Epithelioid hemangioendothelioma NOS
|5.||Osteoclastic giant cell-rich tumors||Aneurysmal bone cyst
|Giant cell tumor of bone NOS||Intermediate|
|Giant cell tumor of bone, malignant||Malignant|
|6.||Notochordal||Benign notochordal cell tumor||Benign|
|Chordoma NOS (Chondroid chordoma)
Poorly differentiated chordoma
|7.||Other mesenchymal tumors of bone||Chondromesenchymal hamartoma of the chest wall
Simple bone cyst
|Osteofibrous dysplasia-like adamantinoma
Adamantinoma of long bones (Dedifferentiated adamantinoma)
Pleomorphic sarcoma, undifferentiated
|8.||Hematopoietic neoplasms of bone||Plasmacytoma of bone
Hodgkin disease NOS
Malignant lymphoma, non-Hodgkin, NOS; Diffuse large B-cell lymphoma NOS; Follicular lymphoma NOS; Marginal zone B-cell lymphoma NOS; T-cell lymphoma NOS; Anaplastic large cell lymphoma NOS; Malignant lymphoma, lymphoblastic, NOS: Burkitt lymphoma NOS
Langerhans cell histiocytosis NOS
Langerhans cell histiocytosis, disseminated
|9.||Undifferentiated small round cell sarcomas of bone and soft tissue||Ewing sarcoma
Round cell sarcoma with EWSR1–nonETS fusions
Sarcoma with BCOR genetic alterations
Since 1967, the WHO classification of tumors is regarded as a reference standard and practical guide and provides a precious resource not only for histopathologists and oncosurgeons but also for the radiologists involved in cancer care. The sole basis for the classification of bone tumors is histopathological. Although radiologists are not expected to know about the exquisite pathological details of bone tumors, a broad knowledge of the recent updates, including the reclassification of a few entities or the introduction of some, is vital for narrowing the differentials in imaging. A multidisciplinary approach including an orthopedic oncologist, radiologist, pathologist, surgical, and medical oncologist is required for accurate diagnosis and management of primary bone tumors.
After the introduction of the fifth edition of the WHO classification of tumors of soft tissue and bone, a few articles are published outlining the recent updates and comparison with the previous edition, but these articles are difficult to understand by the radiologists as these articles mainly focus on the histopathological details, genetic alteration, and new markers. We hereby present a simplified review for the radiologists comprising the relevant details of the updates in bone tumors along with a simplified diagnostic algorithm to characterize these lesions.
HOW TO APPROACH A BONE TUMOR?
Many factors come into play when we encounter and try to diagnose a case of bone tumor. These factors include the age of the patient, location in the skeleton system (axial or appendicular) and multiplicity. Whenever a patient complains of bone pain and swelling pointing toward a neoplastic pathology affecting bone, conventional radiography (CR) is the first imaging modality used to evaluate the lesion because it is widely available and affordable. Despite the advances in the imaging field in the form of multiplanar and functional imaging, the role of CR cannot be neglected, and it is still the most relevant first investigation. A radiograph gives us much information about the bone tumor, including location within the long bone (epiphyseal, diaphyseal, or metaphyseal), the pattern of bone destruction (geographic, moth-eaten, or permeative), zone of transition, type of periosteal reaction, matrix mineralization, and adjacent soft-tissue involvement. Most bone tumors can be diagnosed, or at least we can narrow down our differentials on plain radiographs. By seeing above mentioned criteria, bone tumors can be classified into benign, intermediate, and malignant entities on radiography.[6,7] However, there are some limitations of CR. Lesions located in the region of complex anatomies such as the pelvis, scapula, and spine are challenging to evaluate on CR. The extent of marrow and soft-tissue involvement, including neurovascular bundles, joint involvement, presence of skip lesions, and distant metastasis, is challenging to assess on CR. These factors are essential for staging the disease, and ultimately deciding the treatment and outcome. Computed tomography (CT) and magnetic resonance imaging (MRI) help to cover all these aspects. An algorithm for the radiological diagnosis of bone tumors is presented in [Figure 1].
The role of radiologists does not end with the imaging diagnosis and staging of the bone tumor, but he has a significant role to play in planning and performing image-guided biopsies (ultrasound or CT guided), which helps to draw the specimen from the most viable part of the tumor. Histopathological evaluation of samples obtained under image guidance is the final step in diagnosing and grading bone tumors. The biopsy track should always be planned after discussing with the surgeon so that the biopsy track is excised with the tumor to avoid recurrence.
WHAT’S NEW IN 2020 WHO CLASSIFICATION OF BONE TUMORS?
With technological advances and the introduction of new molecular and genetic data about some bone tumors, there has been reorganization in the classification and introduction of a few new entities. Newly identified genetic alterations and corresponding immunohistochemical markers are included in the new classification, and this has helped in the reclassification of the existing tumor entities. These novel genetic alterations not only help in prognostication but are a target for potential therapeutic options which can bring a paradigm shift in the chemotherapeutic regimen for these entities in the future. Radiologists play an essential role in the multidisciplinary team involved in the care of bone tumor patients, and therefore they need to be aware of important new development in the latest WHO classification. Major changes in the categories of the tumor, their biological potential and summary of newly added and recategorized entities in the 2020 WHO Classification of Bone Tumors with their Clinical and Imaging characteristics are summarized in [Tables 2 and 3]. In the upcoming sections, we will be discussing various categories of bone tumors and the updates in the new classification with an emphasis on their imaging appearance.
|Tumor entities||2013 WHO Classification||2020 WHO Classification|
|Chondroblastoma||Intermediate (rarely metastasizing)||Benign|
|Chondromyxoid fibroma||Intermediate (locally aggressive)||Benign|
|Synovial chondromatosis||Benign||Intermediate (locally aggressive)|
|Chondrosarcoma grade 1||Intermediate (locally aggressive)||Malignant|
|Epithelioid hemangioma||Intermediate (locally aggressive and rarely metastasizing) tumor||Intermediate (locally aggressive) tumor|
|Aneurysmal bone cyst||Tumor of undefined neoplastic nature;
Intermediate (locally aggressive)
|Osteoclastic giant cell-rich tumor;
|Non-ossifying fibroma||Fibrohistiocytic tumor||Osteoclastic giant cell-rich tumor|
|Chondromesenchymal hamartoma of the chest wall||Tumor of undefined neoplastic nature||Other mesenchymal tumor of bone|
|Simple bone cyst||Tumor of undefined neoplastic nature||Other mesenchymal tumor of bone|
|Fibrous dysplasia||Tumor of undefined neoplastic nature||Other mesenchymal tumor of bone|
|Osteofibrous dysplasia||Tumor of undefined neoplastic nature||Other mesenchymal tumor of bone|
|Osteofibrous dysplasia like Adamantinoma||Tumor of undefined neoplastic nature;
|Other mesenchymal tumor of bone;
Intermediate (locally aggressive)
|Adamantinoma||Miscellaneous tumor||Other mesenchymal tumor of bone|
|Pleomorphic undifferentiated sarcoma||Miscellaneous tumor||Other mesenchymal tumor of bone|
|Langerhans cell histiocytosis||Tumor of undefined neoplastic nature||Hematopoietic neoplasm of bone|
|Erdheim-Chester disease||Tumor of undefined neoplastic nature;
Intermediate (locally aggressive)
|Hematopoietic neoplasm of bone;
|Rosai-Dorfman disease||Tumor of undefined neoplastic nature||Hematopoietic neoplasm of bone|
|Ewing sarcoma||Miscellaneous tumor||Undifferentiated small round cell sarcoma|
|Leiomyosarcoma||Myogenic tumor||Other mesenchymal tumor of bone|
|Lipoma||Lipogenic tumor||Other mesenchymal tumor of bone|
|Benign fibrous histiocytoma||Fibrohistiocytic tumor||Removed|
|Giant cell lesions of the small bones||Osteoclastic giant cell rich tumor||Removed|
|Category||Sub-category||Common locations in the skeleton system; Most common age group affected; Radiographic features|
|Chondrogenic tumors||Periosteal chondroma||Proximal humerus and distal femur; 3rd–5th decade; Arise from the periosteum of long bones and shows saucerisationof the adjacent bony cortex with a sclerotic periosteal reactionand ring and arc pattern of calcification.|
|Enchondroma||Small tubular bones of hand/feet and long bones; 1st–3rd decade; small osteolytic lesions with no/mild endosteal scalloping and ring and arc pattern of calcification. No cortical destruction and periosteal reaction. Multiple lesions can be seen in Ollier diseaseand Maffucci syndrome|
|Chondroblastoma NOS||Epiphysis or apophysis of a long bone; 1st–3rd decade; well-defined lytic lesion (<5 cm) with geographical bone destruction and thin sclerotic margins. Rings and arcs calcificationcan be seen. Intense adjacent marrow edema is often seen on MRI. Might be associated with joint effusion.|
|Chondromyxoid fibroma||Metaphyseal region of long bones; 2nd and 3rd decade; well-defined lytic lesion with geographical bone destruction and sclerotic margins. No cortical destruction and periosteal reaction. Pseudotrabeculation can be seen.|
|Chondromatosis NOS||Knee and hip; 4th–5th decade; soft-tissue mass surrounding the joint with numerous uniform-sized calcified loose bodies demonstrating rings and arcs calcification.|
|Atypical cartilaginous tumor||Appendicular skeleton (femur, humerus, tibia, ribs); 3rd–6th decades; osteolytic lesions with no/mild endosteal scalloping and ring and arc pattern of calcification. No cortical destruction and periosteal reaction.|
|Chondrosarcoma, grades 1||Axial skeleton (pelvis, scapula, skull base); 3rd–6th decades; osteolytic lesions with no/mild endosteal scalloping and ring and arc pattern of calcification. No cortical destruction and periosteal reaction.|
|Vascular tumors of bone||Epithelioid hemangioma||Multifocal regional distribution (long and flat bones); all age groups; expansile radiolucent, lytic or cystic-appearing lesions with narrow transition zone and endosteal scalloping.|
|Osteoclastic giant cell-rich tumors||Aneurysmal bone cyst||Metaphysis of long bones, posterior elements of the spine; 1stand 2nd decade; a sharply defined, eccentric expansile multicystic lucent bone lesion, with fluid-fluid levels on MRI|
|Non-ossifying fibroma||Metaphysis of a long bone; 1stand 2nd decade; multiloculated, lucent lesion eccentrically located in the metaphysis near the physisof a long bone with a thin sclerotic rim.|
|Notochordal||Poorly differentiated chordoma||Clivus, skull base, and cervical spine; children and young adults; destructive lytic lesion with expansile soft-tissue mass. Calcification is uncommon.|
|Other mesenchymal tumors of bone||Chondromesenchymal hamartoma of the chest wall||Ribs; neonate; or infants; well-defined, expansile, partly calcified mass involving one or more ribs with fluid-fluid levels on MRI.|
|Simple bone cyst||Metaphysis of long bones (humerus>femur); 1stand 2nd decades; centrally located well defined geographic lytic lesionwith a narrow zone of transition, thin sclerotic margin with no periosteal reactionor soft-tissue component. Sometimes they are associated with a pathologic fracture. Fallen fragment signand trap door sign can be seen.|
|Fibrous dysplasia||Long bones, craniofacial bones, and ribs (monostotic or polyostotic); 1st–3rd decade; expansile or non-expansile lytic lesion with usually smooth and homogenous appearance, with endosteal scalloping and cortical thinning. Ground glass matrix is generally seen. No periosteal reaction. Rind sign at the margins is seen.|
|Osteofibrous dysplasia||Mid-diaphysis of tibia; 1st decade; a bubbly lytic lesion centered in the tibial cortex, sclerotic margins, no periosteal reaction, pseudo trabeculation, and anterior bowing can be seen.|
|Lipoma NOS||Calcaneum, femur; 4th–5th decade; benign-appearing osteolytic bone lesion with well-defined margins and occasional central calcification giving cockade sign.|
|Hibernoma||Spine and pelvis; 5th–7th decade; sclerotic (most commonly) or osteolytic with peripheral sclerosis.|
|Osteofibrous dysplasia-like adamantinoma||Mid-diaphysis of tibia; 3rd decade; a bubbly lytic lesion centered in the tibial cortex, with sclerotic margins, cortical destruction with soft tissue extension, no periosteal reaction, intramedullary extension common.|
|Mesenchymoma NOS||Long bones(metaphysis), pelvic bones; 1st and 2nd decade; lytic expansile lesions with sclerotic rim, internal ring-like calcifications, and features of cortical thinning and/or destruction with soft tissue mass.|
|Adamantinoma of long bones (Dedifferentiated adamantinoma)||Mid-diaphysis of tibia; 2nd–3rd decades; a bubbly lytic lesion centered in the tibial cortex, with sclerotic margins, cortical destruction with soft tissue extension, no periosteal reaction, intramedullary extension common. It has a propensity to metastasize to distant locations.|
|Leiomyosarcoma NOS||Long bones and flat bones; 7th–8th decades; osteolytic lesion with an aggressive pattern of bone destruction.|
|Pleomorphic sarcoma, undifferentiated||Femur, humerus and tibia; 3rd–5th decade; highly destructive with a wide zone of transition (occasionally expansile). Periosteal reactionis uncommon. The lesion usually has no matrix mineralization.|
|Hematopoietic neoplasms of bone||Langerhans cell histiocytosis NOS||Skull, pelvis and femur; 1st decade; solitary or multiple punched out lytic lesions without sclerotic rim. Hole within a holesign, button sequestrumand geographic destructionis seen in the skull. A floating tooth is seen in the mandible and vertebra plana are seen in the spine.|
|Langerhans cell histiocytosis, disseminated||Skull, pelvis and femur; 1st decade; multiple organ systems with multiple lesions are seen. Skeletal lesions are similar to Langerhans cell histiocytosis NOS.|
|Erdheim-Chester disease||Femur and tibia (Multifocal involvement); middle age; bilaterally symmetrical cortical sclerosis obliterating the cortico-medullary differentiation. Cardiac involvement and retroperitoneal fibrosis can be seen leading to arrhythmias and hydronephrosis.|
|Rosai-Dorfman disease||Femur, tibia, skull; 2nd decade; skeletal lesions are typically lytic and intramedullary, sometimes with surrounding sclerosis. Permeative destruction, Cortical thinning, and focal breakthrough can also be seen. It was previously termed “sinus histiocytosis with massive lymphadenopathy.” Bone involvement is seen in 5–10% of cases.|
|Undifferentiated small round cell sarcomas of bone and soft tissue||Ewing sarcoma||The diaphysis of long bones, flat bones; 1st and 2nd decade; large osteolytic lesion with a permeative pattern of destruction and wide zone of transition extending into adjacent soft tissues. Lamellated (onion skin) periosteal reactionis common.|
|Round cell sarcoma with EWSR1–nonETS fusions||Long bones and flat bone; broad age range; large osteolytic lesion with the permeative pattern of destruction and wide zone of transition extending into adjacent soft tissues.|
|CIC-rearranged sarcoma||Predominantly involve soft tissue with rare involvement of bones; broad age range; lytic or mixed lytic-sclerotic lesion with an aggressive periosteal reaction.|
|Sarcoma with BCOR genetic alterations||Pelvis and long bones; 1st and 2nd decade; lytic or mixed lytic-sclerotic lesion with an aggressive periosteal reaction.|
The latest classification has recategorized chondroblastoma and chondromyxoid fibroma from the intermediate to benign category. Previously, enchondroma and periosteal chondroma [Figure 2] were listed together under the term chondroma, but now they are reclassified as separate benign entities. Synovial chondromatosis (SC), which has a high recurrence after excision, is recategorized from benign to intermediate category. The SC has recurrence rates varying from 10% to 15%. It can undergo malignant transformation into synovial chondrosarcoma in 1–5% of cases in the long run. There is a significant overlap between SC and synovial chondrosarcoma in terms of clinical behavior and radiological appearance, however cortical destruction and marrow invasion in the setting of multiple recurrences raise the suspicion of malignant transformation.
The previous classification used the terms “atypical cartilaginous tumor (ACT)” and “Chondrosarcoma Grade 1 (CS1)” interchangeably and classified them in the intermediate category. In the appendicular skeleton, these lesions behave in a locally aggressive fashion and do not metastasize. Therefore, in the current classification, the term ACT is used when the lesion is located in the appendicular skeleton, whereas CS1 is used when the lesion is located in the axial skeleton even though both lesions are identical in histomorphology. Reclassifying CS1 in the malignant category benefits understanding that these lesions need more extensive surgery than the benign lesion. The radiologist needs to understand another critical difference in the terminology, that is, between central and peripheral ACT/CS1. The term “central ACT/ CS1” is used when the tumor is located in the medulla of the bone, whereas the term “peripheral ACT/CS1” is used when the tumor arises from the cartilaginous cap of a pre-existing osteochondroma. Central ACT/CS1 can further be classified into primary (de novo) and secondary (arising from preexisting enchondroma) categories. About 50% of high-grade chondrosarcoma share IHD1 or IHD2 mutations with enchondroma/ACT/CS1 suggesting the genetic connection between these entities. The location of high-grade chondrosarcoma is similar to that of ACT/CS1. It is challenging to differentiate between enchondroma and ACT/CS1 radiologically. Some imaging features which can help to differentiate enchondroma from ACT/CS1 are listed in [Table 4].
|Appearance||Clustered cartilage deposits||Confluent mass|
|Tumor size||Less than 5 cm||More than 5 cm|
|Endosteal scalloping||Less than 2/3 of cortical thickness||More than 2/3 of cortical thickness|
|Expansile bony remodeling||Generally Absent||May be present|
|Soft tissue extension||Absent||May be present|
|Radiotracer uptake on bone scan||Less or no||More|
|Presence of marrow fat signal on MRI||Present||Present/absent|
Osteochondromas are benign bony outgrowths covered by a cartilaginous cap with clear continuity of cortex and medulla. The lesions can be sessile or pedunculated, and these lesions generally point away from the joint. The growth of osteochondroma ceases after the fusion of physis. Some clinical and radiological criteria predict the development of malignancy in the cartilaginous cap. Clinical features include syndromic association, an increase in the size of mass, pain, and the development of symptoms due to compression of the adjacent neurovascular bundle [Figure 3]. Radiological criteria are very objective and include the thickness of the cap of more than 20 mm, altered appearance on sequential studies, and development of soft-tissue mass. MRI has a clear advantage over any other modality for visualization of the cartilaginous cap. MRI sequences such as diffusion-weighted imaging (DWI) and dynamic contrast-enhanced MRI (DCE-MRI) also help distinguish between benign cartilaginous cap and malignancy developing within it. Imaging helps to obtain a biopsy sample from the most viable (enhancing and with restricted diffusion) part of the cap to increase diagnostic yield. Progression to malignancy occurs in 1% of cases of solitary osteochondroma and 20% of cases of hereditary multiple exostoses. ACT/CS1 constitutes more than 90% of cases of malignancies in osteochondroma, and CS 2–3 constitutes the rest of the cases.
Osteoid osteoma (OO) and osteoblastoma are differentiated based on size and location in the skeleton system [Figure 4]. OOs are more predominantly seen in the diaphyseal or metadiaphyseal region of long bones, whereas osteoblastomas are more prevalent in posterior elements of the spine. Osteoblastoma can sometimes become aggressive and is classified in an intermediate category. Lesions <2 cm are classified as OO and lesions ≥2 cm osteoblastoma in the presence of classic clinical and radiological features. Nevertheless, both OO and osteoblastoma share the same molecular genetic alteration.[11,12]
The previous classification included secondary osteosarcoma in the conventional osteosarcoma (COS) subtype, but now, the recent classification has described it in a separate category. Now osteosarcoma not otherwise specified (NOS) includes only three subtypes: COS, telangiectatic osteosarcoma, and small cell osteosarcoma. COS accounts for the majority of osteosarcoma [Figure 5]. Osteosarcoma NOS can also be subdivided into different types based on the dominant matrix: Osteoblastic, chondroblastic, and fibroblastic; However, this subdivision has no role in predicting prognosis. COS and telangiectatic osteosarcoma are more commonly encountered in the metaphyseal region of long bones whereas small cell osteosarcoma is predominantly seen in the diaphysis. Recent classification has also removed Clear cell and chondroblastoma-like osteosarcoma subtypes from the osteogenic tumors.
In the latest classification, secondary osteosarcoma is divided into six subtypes: 
Osteosarcoma in Paget’s disease of bone,
Infarct related osteosarcoma
Chronic osteomyelitis related
Osteosarcoma secondary to fibrous dysplasia.
The prognosis of secondary osteosarcoma occurring as a result of Paget’s disease of bone and radiation treatment is poorer than COS.
The 2020 WHO classification has not updated this category of bone tumors. It includes the desmoplastic fibroma of bone in the intermediate grade and fibrosarcoma in the malignant grade. These tumors are composed of spindle cells with variable collagen.
Desmoplastic fibroma is an extremely rare tumor of bone (<0.1% of all bone tumors) that is locally aggressive but does not metastasize. It is considered the bony counterpart of desmoid tumors found in the soft tissues and is a diagnosis of exclusion. Both desmoplastic fibroma and desmoid tumors are histologically identical. Low-to-intermediate signal intensity is characteristic of MRI. Transition to fibrosarcoma or osteosarcoma is extremely rare. The recurrence rate after curettage is generally more than 70%, so the preferred management is resection with wide margins.
Fibrosarcoma is a sporadic malignant fibrogenic tumor of bone that occurs in the middle and old age groups. Initially, malignant fibrous histiocytoma and fibrosarcoma of bone were considered as a single entity, but these lesions are distinct entities.
In 2020 WHO classification, epithelioid hemangioma of bone is moved from an intermediate locally aggressive and rarely metastasizing tumor to the intermediate locally aggressive tumor category. Epithelioid hemangioma, epithelioid hemangioendothelioma, and angiosarcoma can be multifocal and involve multiple bones. Angiosarcoma accounts for <1% of malignant bone tumors. Angiosarcoma has the propensity to metastasize to other skeletal sites.
Osteoclastic giant cell-rich tumors
It comprises a heterogeneous group of tumors and tumor-like lesions rich in osteoclast type multinucleate giant cells. There has been a recategorization of the aneurysmal bone cyst (ABC) and non-ossifying fibroma (NOF) into this group in the latest classification. In the present classification, the term “benign fibrous histiocytoma” is no longer in use.
Giant cell tumors (GCT) can be categorized into benign or malignant [Figure 6]. Malignant GCTs represent 5–10% of GCT. Radiographs are not very helpful in differentiating benign from malignant GCTs. DWI and DCE MRI can help identify highly cellular components within the lesion, which can further be biopsied to obtain representable samples for histopathological evaluation. Malignant GCTs can be categorized into primary (more favorable prognosis) and secondary (more common). Denosumab-treated GCT is now recognized as a distinct variant of GCT. Giant cell lesions of the small bones are now considered a true solid variant of ABC. Terminologies such as “giant cell lesion of small bones” and “giant cell reparative granuloma of small bone” are obsolete. ABC can be categorized into primary (de novo) and secondary (developing in preexistent tumors such as GCT, chondroblastoma, fibrous dysplasia, and NOF) [Figure 7]. Distinguishing primary from secondary ABC is important as the treatment for the two is completely different. [Table 5] summarizes features that help to distinguish primary from secondary ABC.
|Primary ABC||Secondary ABC|
|Age||1st–2nd decade||3rd decade|
|Location||Metaphysis with or without epiphyseal extension||Epiphysis (preexistent GCT or chondroblastoma)
Diaphysis (preexistent NOF and FD)
|Cortex||Intact||Breached with a soft tissue mass|
|USP6 gene rearrangement||Present in 70% of cases||Absent|
Poorly differentiated chordoma (PDC) is a new entity that is added to the recent classification of bone tumors. Like conventional chordoma, PDC also has a predilection for the axial skeleton but it predominantly affects the clivus and skull base. It is generally seen in children, and young adults, with females affected slightly more than males. PDC is more aggressive than conventional chordoma. It generally demonstrates intermediate T2 signal intensity unlike conventional chordoma and shows avid contrast enhancement.
Dedifferentiated chordoma is characterized by the presence of sarcomatous elements in addition to the chordoma (biphasic). Biomorphic appearance can be visualized even on MRI.
Other mesenchymal tumors
This is the new category of bone tumor introduced in the 2020 WHO classification. Tumors included in this category in the present classification were previously categorized as tumors of undefined neoplastic nature or Miscellaneous tumors. Hibernoma of bone, mesenchymoma, and dedifferentiated adamantinoma are newly recognized entities. PUS was previously termed as “undifferentiated high-grade pleomorphic sarcoma.” PUS can be primary or secondary. Secondary PUS may arise in pre-existent bone infarcts, Paget’s disease and radiation necrosis.
Hibernoma (tumor of brown adipose tissue) is a very rare benign tumor seen in the spine or pelvis of elderly patients (female>male). Mesenchymoma, also known as Fibrocartilaginous mesenchymoma, is a very rare, locally aggressive neoplasm seen in children and affects the metaphyseal region of long bones, most commonly followed by pelvic bones, vertebrae, and ribs.
Adamantinoma is divided into three types in the recent classification: Classic adamantinoma (malignant), OFD-like adamantinoma, and dedifferentiated adamantinoma (newly introduced). OFD-like adamantinoma was previously categorized as a malignant tumor, but in recent classification, it is placed in the Intermediate (locally aggressive) category. Dedifferentiated adamantinoma is the rarest subtype and has an aggressive clinical course with metastasis seen in 2/3rd of the patients. This subtype may be associated with sarcomatoid dedifferentiation. In contrast classic adamantinoma displays a low rate of metastasis and longer survival. OFD and OFD like adamantinoma are more commonly seen in the young age group with females affected more than males, whereas classic and dedifferentiated adamantinoma is more commonly seen in males of the middle age group. OFD and all types of adamantinoma are typically located in the tibial cortex. Anterior bowing is more common in OFD and OFD like adamantinoma whereas marrow involvement and extraosseous extension are more commonly seen in classic and dedifferentiated adamantinoma.
Hematopoietic neoplasms of bone
Multiple myeloma is the most common malignant bone tumor in adults [Figure 8]. It can present in either disseminated form (more common and poor prognosis) or as solitary plasmacytoma. The disseminated form presents as multiple punched-out lytic lesions predominantly involving the axial skeleton. Sometimes disseminated form presents as mere diffuse osteopenia with no identifiable lytic lesion. Solitary plasmacytoma in the majority of patients has latent systemic involvement at the time of presentation. The term “Plasma cell myeloma” is no longer in use. Multiple myeloma is removed from the fifth edition of the WHO classification of bone tumors and is included in the fourth edition of the WHO classification of hematopoitic and lymphoid tissues published in 2017.
Langerhans cell histiocytosis [Figure 9], Erdheim-Chester disease, and Rosai-Dorfman disease were classified previously in the category of tumors of undefined neoplastic nature, but in the recent classification, these tumors are classified in the category of hematopoietic neoplasms of bone.
Primary bone lymphoma (PBL) can be unifocal or multifocal involvement of the skeletal system without evidence of systemic disease for 6 months. PBL is less common than secondary involvement from disseminated lymphoma. More than 80% of PBL are diffuse large B-cell lymphomas. Different types of bone lymphomas are indistinguishable from Imaging studies.
Undifferentiated small round cell sarcomas of bone and soft tissue
A new chapter on undifferentiated small round cell sarcomas of bone and soft-tissue tumors is introduced in the 2020 WHO classification, which includes Ewing’s sarcoma (EWS), round cell sarcoma with EWSR1–non-ETS fusions, CIC-rearranged sarcoma, and sarcoma with BCOR genetic alterations. These tumors are different from each other based on clinical features and molecular profiles. EWS is different from the other three because of the unique gene fusion involving the FET family of genes and a member of ETS transcription factors. EWS is the second most common malignant bone tumor after osteosarcoma in the pediatric and adolescent age group [Figure 10].
In this review, we have summarized major changes in the 2020 WHO classification of bone tumors with relevant points for the information and knowledge of the radiologist. Radiologists play a crucial role in the team of physicians and surgeons involved in the care of bone tumor patients; Therefore, the radiologist needs to stay updated with the latest development and advances even if the basis of these developments is majorly molecular, genetic, and pathological. We have also summarized the imaging approach and parameters used for the evaluation of patients presenting with clinical features of bone tumors. Imaging features of many new entities are yet to be discovered, and with time, we may be able to understand these entities better with the help of further research and enhancement in the available literature.
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