The pathologic diagnosis of bone tumors has become increasingly complex in light of recent advances in tumor immunohistochemistry and genetics. While some tumors may be relatively simple to accurately diagnose, others can be tremendously complex, requiring many diagnostic techniques and significant pathologic expertise. It is not the intention of this chapter to transform the reader into a bone pathology expert. Despite this, it is possible to simplify tumor diagnoses by appreciating trends along with tissue and tumor cell pattern recognition in the context of the radiographic differential diagnosis. Hopefully, this will give the reader a framework to utilize which will simplify the process of narrowing down the histologic differential diagnosis to one or a few lesions.
Bone-forming Tumors
Bone-forming tumors consist of 2 histologic components: the mineralized matrix (bony trabeculae or osteoid) and the intervening cellular stroma, which may also contain various types of acellular material known as matrix. As a basic tenet, it is the bone which tells you if the lesion is reactive or neoplastic. Once you are in a neoplastic category, it is the stromal cells that then tell you if the lesion is benign or malignant.
Reactive bone is typified by the presence of woven bone with osteoblastic rimming ultimately developing into lamellar bone. These osteoblasts lining immature reactive woven bone appear as large distinct cells lining the reactive bony trabeculae. These trabeculae, since they are reactive, are laid down in a sensible fashion since they are reacting to some stimulus and not just laid down haphazardly as occurs in neoplasms. As such, the reactive bony trabeculae look sensible. They are connected and typically appear in a fashion that would make sense in terms of resisting physical stress. Because this reactive bone is recently formed and immature, it is associated with active osteoblasts with demonstrated osteoblastic rimming as well as numerous osteoclasts. This combination is indicative of active bone deposition and resorption. Over time, bone remodeling will convert this immature woven bone to mature lamellar bone. Since reactive lesions are, by definition, not neoplasms and not cancer, the stromal cells between the bony trabeculae will appear benign without the typical changes associated with malignancy which we will describe subsequently. Figure 1 demonstrates this sensible pattern of bony trabeculae with obvious osteoblastic rimming and a benign-appearing cellular stroma consistent with a reactive lesion.
Figure 1: H&E stain of reactive bony trabeculae with osteoblastic rimming and a benign spindle cell stroma.
Neoplastic bone is typified by the presence of woven bone without osteoblastic rimming. These neoplastic bony trabeculae typically appear nonsensical as they are laid down haphazardly by the tumor without regard to resisting physical stress. As such they are typically bizarre appearing and frequently nonconnecting. While neoplastic woven bone could theoretically eventually develop into lamellar bone, this is not frequently seen clinically. The difference between benign and malignant bone neoplasms, both of which contain neoplastic woven bone, is determined by the characteristics of the cellular stroma. Benign bone neoplasms will demonstrate a stroma with lower cellularity and normal appearing cells. These cells will demonstrate minimal nuclear size, shape, and staining pleomorphism as well as the absence of visible mitoses, especially bizarre mitoses (Figure 2). Malignant bone tumors will demonstrate increased cellularity, significant nuclear pleomorphism in size, shape, and staining, the presence of mitoses (especially bizarre mitoses), and finally, the presence of spontaneous necrosis (Figure 3). So, again, the bone itself tells you whether the lesion is reactive or neoplastic while the stroma tells you if a neoplasm is malignant or benign.
Figure 2: H&E stain of neoplastic woven bone showing the absence of osteoblastic rimming, many nonsensical disconnected trabeculae and a benign spindle cell stroma.
Figure 3: H&E stain of neoplastic woven bone with malignant stroma.
There are four distinct bone-forming neoplasms which appear histologically benign. These include osteoid osteoma, osteoblastoma, fibrous dysplasia, and finally, parosteal osteosarcoma. Even though parosteal osteosarcoma is malignant, it is a low-grade tumor with a fairly benign appearing stroma. As such, it conveniently falls into the histologic recognition category of benign appearing bone tumors. Each of these tumors has the same two components, namely neoplastic woven bone and a benign appearing spindle cell stroma. Fortunately, the imaging characteristics of these lesions are quite different, making it easy to distinguish between these lesions once a benign appearing bone tumor is recognized histologically. Again, Figure 2 demonstrates these histologic findings. Also seen in many bone tumors are varying numbers of giant cells. It is important to note that giant cells can be seen in any lesion in bone. They are not usually part of the histology of the tumor itself but are background macrophages. As such, giant cells only matter if you are considering the diagnosis of a benign giant cell tumor of bone.
The combination of neoplastic bone (osteoid if very immature, or woven bone if more calcified) and a malignant spindle cell stroma define osteosarcoma. Again, Figure 3 demonstrates the presence of neoplastic woven bone with intervening spindle cell stroma showing obvious malignant characteristics of the stromal cells including pleomorphism, increased cellularity, mitoses, and necrosis.
In summary:
- Reactive bone lesions: reactive woven bone (with osteoblastic rimming) laid down in a sensible configuration along with the presence of a benign appearing spindle cell stroma.
- Benign appearing bone neoplasms: neoplastic woven bone (without osteoblastic rimming) laid down in a nonsensical configuration along with the presence of a benign appearing spindle cell stroma. Remember that parosteal osteosarcoma looks benign histologically because the stroma is not pleomorphic.
- Malignant bone neoplasm (osteosarcoma): neoplastic woven bone (without osteoblastic rimming) in a nonsensical configuration along with the presence of a malignant appearing spindle cell stroma.
So, that is bone-forming tumor histology laid out in a simple fashion that should make the recognition of straightforward bone tumors well within the reach of an orthopedic surgeon or resident.
Cartilage-forming Tumors
In order to understand cartilage tumors, it is helpful to understand that they occur in 4 patterns. The first pattern is the continuous spectrum of central cartilage tumors ranging from enchondroma to borderline abnormal histology to low-grade chondrosarcoma and then onto intermediate- and high-grade chondrosarcoma. One difficulty here that needs to be understood is that this spectrum of progressive abnormality is a complete continuum so it can become difficult to know exactly where to separate these lesions based on where along this continuum they exist. The second pattern, which is easy to discriminate, is chondroblastoma. Since this lesion is typically in the epiphysis of a child, the differential diagnosis is already significantly narrowed by the imaging studies and clinical context to include mainly chondroblastoma and infection. The third pattern is chondromyxoid fibroma, which is a very rare benign neoplasm occurring most commonly in the proximal tibia of children and young adults and which tends to have the appearance of a large non-ossifying fibroma (NOF). The fourth pattern is osteochondroma.
Central Cartilage Tumors
The main differential diagnosis of a central cartilage tumor is typically enchondroma versus some grade of chondrosarcoma. This is a good time to restate the radiologic picture of central cartilage lesions. Enchondromas are benign lesions which typically do nothing bad to the bone. They are usually metaphyseal although they may be diaphyseal less commonly. The metaphyseal lesions typically do not alter the external anatomy of the bone unless the host bone is small and do not invade the adjacent cortices. Speaking anthropomorphically, enchondromas are not smart. They do not know what bone they are in, nor do they know where they are in that bone. All they know is that they want to grow to a certain size. Like most other benign tumors, they will stop growth after reaching a certain size, which may be due to an intrinsic tumor suppression mechanism. As such, one expects no cortical alteration adjacent to an enchondroma in a large bone. The exception to this would be where the center of the lesion is quite eccentric within the cross-section of the bone so that growing to an expected size would involve some expansion of the cortex in that region. In the context of small bones or narrow bones such as phalanges, metacarpals, metatarsals, the proximal fibula, or the body of the scapula, enchondromas again want to grow to a certain size and this may well involve expansion of the adjacent cortex.
In adults, enchondromas will get more calcified over time but should not grow over time. Any measurable change in the size of an enchondroma in an adult should seriously raise the possibility of malignant transformation. It would probably make sense that a larger enchondroma with more tissue at risk would be more likely to develop a malignant clone that will begin to reproduce. Despite this, there are no firm data to show that larger enchondromas have a higher malignant transformation rate than smaller ones. There is, however, evidence that more proximal enchondromas have a somewhat higher malignant transformation rate than more distal lesions.
If you look at hyaline cartilage histologically, it has two components: cells and matrix. Normal benign cartilage is extremely hypocellular. The cells are each located in a lacunar space with a small crescent-shaped nucleus and form a small cluster called an isogenous group. The matrix should occupy most of the cross-sectional area of the tissue and be uniform in nature. This typical appearance of benign cartilage can be seen in the lower right corner of Figure 4. This contrasts with the changes seen on the left side of that same slide which demonstrate the following findings: increased cellularity, plump nuclei, binucleate cells, more than one cell per lacuna, cells outside lacunae, as well as loss of clustering architecture. Despite these cellular changes, the matrix is still high quality and uniform. These six changes along with the presence of a good chondroid matrix take one to the end of the continuum of benign lesions.
Figure 4: H&E stain of low grade cartilage, borderline low grade cartilaginous changes.
The next step as demonstrated in Figure 5 demonstrates a further increase in cellularity with plump nuclei and clonal proliferation of cells within a grouping of lacunae. Also noted is irregularity in the matrix associated with the fact that this matrix is no longer high-quality chondroid and demonstrates what is termed “myxoid change.” What is happening here is that the hyaline cartilage tissue is losing its normal chondroid phenotype by becoming more cellular with more abnormal looking cells and making poorer quality matrix. This process then continues as noted in Figure 6 which demonstrates extremely cellular tissue with malignant-appearing cells with large dark staining nuclei with only a minimum of chondroid matrix being produced as is noted in the lower portion of the figure. Again, this is further loss of normal chondroid phenotype as the cells are now making minimal to no chondroid matrix and the cells themselves are large with dark staining nuclei. This is the spectrum that is involved as cartilage goes from benign to low-grade to intermediate-grade to high-grade. In terms of the matrix produced, higher grade chondrosarcomas produce worse and poorer matrix as well as less matrix once high-grade histology is achieved. As this progressive “myxoid change” is noted histologically, clinically this may be seen as a “runny” matrix. Whenever you encounter a chondroid tumor which is not well formed and so has a runny, viscous, fluid-like matrix, this is a significant finding and indicative of malignant transformation in most cases.
Figure 5: H&E stain of higher-grade cartilage, characteristic of grade 2 chondrosarcoma with increased cellularity, plump nuclei and myxoid change in the matrix.
Figure 6: Further dedifferentiation and high cellularity consistent with grade 3 chondrosarcoma. Very little chondroid matrix is seen.
Two other forms of chondrosarcoma are worth mentioning and should be appreciated. The first is dedifferentiated chondrosarcoma. This is diagnosed by the presence of low-grade chondrosarcoma juxtaposed directly to areas of high-grade sarcoma. This high-grade component, in light of the fact that it is so undifferentiated, has lost its chondroid phenotype and commonly looks like undifferentiated pleomorphic sarcoma (UPS). This implies that the higher grade areas have developed by dedifferentiating from the lower grade lesion as seen in Figure 7. The second unusual form of chondrosarcoma is clear-cell chondrosarcoma which occurs typically in the femoral head. This rare tumor demonstrates large clear malignant chondrocytes which have the appearance of eggs fried sunny-side up as demonstrated in Figure 8. Although this tumor is quite rare, it does show up frequently in examinations concerning orthopedic pathology.
Figure 7: H&E stain of dedifferentiated chondrosarcoma, with low-grade cartilage juxtaposed against high-grade spindle cell sarcoma (Image courtesy of Dr. Gord Zhu)
Figure 8: H&E stain of clear cell chondrosarcoma
So, in summary we have:
- Benign cartilage: hypocellular, small crescent-shaped nuclei, one cell per lacuna, and well-formed uniform matrix.
- Borderline benign cartilage: increased cellularity, plump nuclei, binucleate cells, more than one cell per lacuna, cells outside lacunae, and loss of clustering architecture, but still showing well-formed uniform matrix.
- Chondrosarcoma: continuum of loss of normal chondroid phenotype with increasing cellularity, pleomorphism, and myxoid change in chondroid matrix followed by more complete loss of chondroid matrix. These tumors are acting less and less like normal cartilage and more and more like tumors of increasing grade of malignancy.
Chondroblastoma
As noted previously, chondroblastoma is easy to diagnose clinically. It is a lytic lesion in the epiphysis of a child with significant surrounding edema as noted on MRI scan. The radiographic differential for these lesions is primarily chondroblastoma and infection. Chondroblastomas are typically painful.
Histologically, the tumor has a somewhat heterogeneous spectrum of appearance due to an inconsistency in the amount of matrix, the potential presence of secondary aneurysmal bone cyst formation, and some cytologic variability. The typical chondroblasts seen in this tumor are polygonal or oval and sharply demarcated from adjacent cells. These are known as cobblestone cells because they look like individual cobblestones in a street. The matrix between these cobblestone cells may be calcified resulting in a characteristic “chicken wire” pattern of calcification.
So, in summary chondroblastomas demonstrate:
- Cobblestone chondroblasts
- Chondroid matrix
- Chicken wire calcification
Chondromyxoid Fibroma
These are very rare tumors which are benign but locally aggressive. They typically occur in individuals below the age of 30 years and are usually associated with some level of localized pain. The most common location is in the long bones, especially in the proximal tibia, but they do occasionally occur in the flat bones of the pelvis as well. Radiographically these lesions appear somewhat like a large non-ossifying fibroma with bone lysis, a benign appearing geographic margin, and surrounding sclerosis. Unlike non-ossifying fibromas, these lesions occasionally cross the growth plate and so may demonstrate epiphyseal extension.
Histologically, this tumor has a lobular pattern. It contains short spindle cells sitting in a chondromyxoid matrix but not within visible lacunar spaces. Foci of necrosis are commonly found.
So, in summary, chondromyxoid fibromas demonstrate:
- A benign appearing metaphyseal lesion with a sclerotic geographic margin usually larger than a typical non-ossifying fibroma and occasionally demonstrating epiphyseal extension.
- Histologically it is characterized by the presence of short spindle cells sitting in a chondromyxoid matrix without the presence of lacunar spaces.
Osteochondroma
An osteochondroma is a bony growth with a cartilaginous cap which, in a skeletally immature individual, grows out at an angle to the normal direction of longitudinal growth of the bone. These tumors are hamartomas which develop from aberrant cartilage growth. The hallmark of these lesions is that as they grow, the cortex of the bone becomes the cortex of the lesion and normal marrow bone is brought up into the lesion itself. This is termed cortico-medullary continuity. As such, osteochondromas never sit on an intact cortex.
The second issue involves the cartilaginous cap. The cartilaginous cap and its connection to the underlying bone of the osteochondroma below it basically mimic a normal growth plate. The chondroid cap will produce new bone beneath it by endochondral ossification and grow while the individual grows. As the normal growth plates of the individual close, so does the growth of the osteochondroma conclude.
Once skeletal growth is complete, a concern is conversion of the remaining cartilage of the osteochondroma to chondrosarcoma. This conversion is a lifelong risk but most typically happens beyond the age of 40 years. As such, in adults, any osteochondroma with documented growth or one with a cartilage cap thickness of 2 cm or greater is considered to be malignant. It should also be remembered that most osteochondromas in adults have a chondroid cap less than 5 mm thick so anything beyond that but not yet at 2 cm should still raise the suspicion of early transformation of an osteochondroma to secondary chondrosarcoma. As such, more frequent follow-up to ensure stability is reasonable.
Osteochondromas can be pedunculated in which case they have a mushroom-type shape with a narrow stalk and a larger cap. Conversely, they can be sessile in configuration where they look in profile like a hill on the side of the bone projecting away from the bone itself.
Histologically, these lesions are composed of hyaline cartilage which forms a chondroid cap over the distal tip of the lesion. Beneath this is found normal bone which may be cortical or medullary in appearance.
So, in summary we have:
- A pedunculated or sessile growth of bone at an angle to the normal bone growth.
- Because the lesion grows out of the bone, the cortex of the bone becomes the cortex of the lesion and it pulls normal marrow bone up into the lesion itself. This is termed cortico-medullary continuity.
- The cartilaginous cap is typically 5 mm or less in adults.
- Any growth in a cartilaginous cap in adults or a cap with a thickness greater than 2 cm is strongly suspicious for osteochondroma with chondrosarcomatous transformation.
Round Cell Infiltrates
While most of the tumors you will encounter are composed primarily of spindle cells, certain reactive and neoplastic lesions will be composed primarily of round cells. The following will give you a reasonable systematic approach to evaluating these lesions histologically in light of the imaging studies and clinical context.
Infection
Osteomyelitis is characterized histologically by the presence of bone destruction, cellular debris, and the presence of polymorphonuclear leukocytes (neutrophils). While polys predominate in acute infections, chronic infections may show significant numbers of lymphocytes and plasma cells as well. All of these are small round cells and obviously the key to acute diagnosis of infection is recognizing the polys. The clinical context is usually suggestive in terms of both the clinical history and imaging findings. Positive cultures establish the diagnosis of infection.
Langerhans Cell Histiocytosis
This condition includes three diseases all of which have the same pathology. These are Letterer-Siwe disease, Hand-Schuller-Christian disease, and eosinophilic granuloma. These lesions contain Langerhans cell histiocytes which are macrophage-like cells containing grooved nuclei and which with electron microscopy can be shown to contain Birbeck granules in the cytoplasm. These Langerhans cells can be difficult for the novice pathologist to recognize. Fortunately, these lesions also contain significant numbers of eosinophils. These are easy to spot as they are small round cells with a bilobed nucleus and bright red cytoplasmic granules. As such, pathologists look for Langerhans cells while the rest of us hope to find significant numbers of eosinophils.
Primary Round Cell Tumors
1. In children and young adults
a. Ewing sarcoma
This tumor is composed of sheets of uniform small round blue cells with no matrix production. These cells are typically small, round to polygonal in shape, fairly uniform, and demonstrate scant cytoplasm (Figure 9). These will be discussed further at length in the chapter dealing exclusively with this tumor.
Figure 9: H&E stain of Ewing sarcoma showing a monotonous array of uniform round cells.
b. Neuroblastoma
These tumors are typically composed of sheets of small round cells which may be divided into small lobules. The cells themselves look similar to Ewing sarcoma cells in so much as they are small, round cells with deeply staining blue nuclei and scant cytoplasm. One differentiating factor that distinguishes neuroblastoma cells from Ewing sarcoma is the presence of pseudorosettes wherein the cells form a ring with pink extracellular matrix within the rings of round cells. While there is a small subset of Ewing sarcoma tumors which can also form rosettes (neuroectodermal differentiation), this is not typical and remains beyond the scope of this discussion. So, in a child, a round cell tumor without rosettes is probably a Ewing sarcoma and if pseudorosettes are present, then this would favor neuroblastoma. When a lesion of neuroblastoma does appear in a bone, it is likely a a metatasis from the primary tumor within the chest or abdomen. Obviously, there are many other levels of investigation of these tumors which can help distinguish them and these will be further documented in the appropriate chapters to follow.
2. In older adults
a. Plasma cell (multiple) myeloma/plasmacytoma
Plasma cell tumors are the most common primary malignant neoplasms to affect bones in the older age group of patients. These tumors demonstrate sheets of mature to immature plasma cells. Plasma cells are identified histologically as a small round blue cell with a clock-face nucleus, perinuclear hof (a clear area around the nucleus which is related to the presence of the Golgi apparatus in this location), and pink cytoplasm. You must be able to recognize plasma cells so study the cells seen in Figure 10.
Figure 10: Intermediate-power H&E stain of a malignant plasma cell neoplasm demonstrating clockface chromatin within the nuclei of the plasma cells, and perinuclear clearing (“perinuclear huff”). (Image courtesy of Atlas of musculoskeletal oncology. In OrthopedicsOne- The Orthopaedic Knowledge Network. Accessed 2/15/22)
b. Lymphomas of bone
These tumors are composed of round cells which typically have more cellular variability than what we saw above in Ewing sarcoma. In line with this irregularity, some of the cells may have a fibroblastic or spindled component. Basically, if you see a round cell tumor in the bone in an adult and the cells do not look like plasma cells, then the tumor is probably either a primary lymphoma or a small round cell metastatic carcinoma, and these require immunohistochemical tests to distinguish in many cases (Figure 11).
Figure 11: H&E stained histology slide showing lymphoma of bone: sheets of large atypical lymphoid cells infiltrating bone marrow fat with vesicular chromatin, prominent nucleoli, and scant to moderate cytoplasm. Numerous mitotic figures and frequent apoptotic bodies are present. (Image courtesy of Dr. Gord Zhu)
Bone cysts (unicameral bone cyst and aneurysmal bone cyst), fibrous lesions (metaphyseal fibrous defects and fibrous dysplasia), and a few other individual lesions that do not fall into these larger categories such as giant cell tumor of bone will be discussed in their representative chapters.