Bone consists of cartilaginous, osteoid, and fibrous tissue and bone marrow elements. Each tissue can give rise to benign or malignant spindle cell tumors. Bone tumors are classified on the basis of cell type and recognized products of proliferating cells. Each tumor be considered a separate clinicopathologic entity. Radiographic, histologic, and clinical data are necessary to form an accurate diagnosis and to determine the degree of activity and malignancy of each lesion.
Cartilage tumors are lesions in which cartilage is produced. They are the most common bone tumors. Osteochondroma is the most common benign cartilage tumor; some 1% to 2% of solitary osteochondromas become malignant and Enchondroma is a benign cartilage tumor that occurs centrally; in adults, malignant transformation may occur. Chondrosarcoma, the most common malignant cartilage tumor, is either intramedullary or peripheral. Ten percent are secondary, arising from an underlying benign lesion. Most chondrosarcomas are low grade, although 10% dedifferentiate into high-grade spindle cell sarcomas or, rarely, a mesenchymal chondrosarcoma.
Osteoid tumors are lesions in which the stroma produce osteoid. The benign forms are osteoid osteoma and osteoblastoma. Osteoid osteomas are never malignant. Osteoblastomas rarely metastasize; when they do, it is only after multiple local recurrences.Osteosarcomas are the most common primary malignant tumors of the bone. Histologically, they are composed of malignant spindle cells and osteoblasts that produce osteoid or immature bone. Several variants are now recognized.60 Parosteal, periosteal, and low-grade intraosseous osteosarcoma are histologically and radiographically distinct from the “classic” central medullary osteosarcomas and have a more favorable prognosis.
Fibrous tumors of bone are rare. Desmoplastic fibroma is a locally aggressive, nonmetastasizing tumor, analogous to fibromatosis of soft tissue.Fibrosarcoma of bone appears histologically as its soft tissue counterpart. Multiple sections must be obtained to demonstrate the lack of osteoid production. If osteoid is present, the lesion is classified as an osteosarcoma. MFH, a rare lesion and the counterpart of soft tissue MFH, has been described in bone.The pathophysiologic behavior of bone and soft tissue MFH is similar, consisting of a storiform pattern with a histiocytic component. Giant cell tumors of unknown origin were originally called benign but are now considered low-grade sarcomas. They have high rates of local recurrence and malignant transformation.
Tumors presumably arising from bone marrow elements are the round cell sarcomas. The two most common are Ewing's sarcoma and the rarer non-Hodgkin's lymphoma.
RADIOGRAPHIC EVALUATION AND DIAGNOSIS
Radiographic evaluation combined with the clinical history and histologic examination is necessary for accurate diagnosis. Bone scans, angiography, CT, and MRI are generally not helpful in determining a diagnosis but are important in delineating the extent of local involvement. A systematic approach to the radiographic evaluation of skeletal lesions has been described by Madewell and colleagues, who studied and correlated several hundred radiographic and pathologic specimens. They considered the radiograph as the gross specimen from which a detailed histologic interpretation could be made and biologic activity accurately diagnosed. According to their system, a bone tumor is evaluated by five radiographic parameters:
1. Anatomic site. Specific anatomic sites of the bone give rise to specific groups of lesions. Johnson explained this by a “field” theory, which hypothesizes that the most active cells of a certain area of bone give rise to tumors that are characteristic of that area. In general, spindle cell sarcomas are metaphyseal, whereas round cell sarcomas tend to be diaphyseal.
2. Borders. The border reflects the growth rate and the response of the adjacent normal bone to the tumor. Most tumors have a characteristic border. Benign lesions (e.g., nonossifying fibromas and unicameral bone cysts) have well-defined borders and a narrow transition area that is often associated with a reactive sclerosis. Aggressive or benign tumors [e.g., chondroblastoma and giant cell tumors (GCTs)] tend to have faint borders and wide zones of transition with very little sclerosis, reflecting a faster-growing lesion. Poorly delineated or absent margins indicate an aggressive or malignant lesion.
3. Bone destruction. Bone destruction is the hallmark of a bone tumor. Three patterns of bone destruction are described : geographic, moth-eaten, and permeative. In general, these patterns are found in the tubular bone rather than in the flat bone and represent a combination of cortical and cancellous destruction. These patterns reflect a progressively increasing growth rate of the underlying tumor.
4. Matrix formation. Calcification of the matrix, or new bone formation, may produce an area of increased density within the lesion. Calcification typically appears as flocculent or stippled rings or clusters. The appearance of the new bone varies from dense sclerosis that obliterates all evidence of normal trabeculae, to small, irregular, circumscribed masses described as “wool” or “clouds.” Calcification and ossification may appear in the same lesion. Neither type of matrix formation per se is diagnostic of malignancy.
5. Periosteal reaction. Periosteal reaction is indicative of malignancy but not pathognomonic of a particular tumor. A combination of periosteal changes is often noted. In malignant tumors, periosteal reaction is noncontinuous and thin, with multiple laminations. A parallel or a perpendicular pattern may be present.
The radiographic parameters of benign and malignant tumors are quite different. Benign tumors have round, smooth, well-circumscribed borders. No cortical destruction and, generally, no periosteal reaction are found. Malignant lesions have irregular, poorly defined margins. Evidence of bone destruction and a wide area of transition with periosteal reaction are noted. Soft tissue extension is common.
NATURAL HISTORY
Tumors arising in bone have characteristic patterns of behavior and growth that distinguish them from other malignant lesions. These patterns form the basis of a staging system and current treatment strategies. These principles and their relationship to management, as formulated by Enneking and colleagues, are described here.
BIOLOGY AND GROWTH
Spindle cell sarcomas form a solid lesion that grows centrifugally. The periphery is the least mature part of this lesion. In contradistinction to a true capsule, which surrounds a benign lesion and is composed of compressed normal cells, a malignant tumor is generally enclosed by a pseudocapsule and consists of compressed tumor cells and a fibrovascular zone of reactive tissue with an inflammatory component that interdigitates with the normal tissue adjacent to and beyond the lesion. The thickness of the reactive zone varies with the degree of malignancy and histiogenic type. The histologic hallmark of sarcomas is their potential to break through the pseudocapsule to form satellite lesions of tumor cells. This characteristic distinguishes a nonmalignant mesenchymal tumor from a malignant one.
High-grade sarcomas have a poorly defined reactive zone that may be invaded and destroyed by the tumor. In addition, tumor nodules in tissue may appear to be normal and not continuous with the main tumor. These are termed skip metastases. Although low-grade sarcomas regularly demonstrate tumor interdigitation into the reactive zone, they rarely form tumor nodules beyond this area.
The three mechanisms of growth and extension of bone tumors are:
(1) compression of normal tissue
(2) resorption of bone by reactive osteoclasts
(3) direct destruction of normal tissue.
Benign tumors grow and expand by the first two mechanisms, whereas direct tissue destruction is characteristic of malignant bone tumors. Sarcomas respect anatomic borders and remain within one compartment. Local anatomy influences tumor growth by setting the natural barriers to extension. In general, bone sarcomas take the path of least resistance. Most benign bone tumors are unicompartmental; they remain confined and may expand the bone in which they arose. Malignant bone tumors are bicompartmental; they destroy the overlying cortex and go directly into the adjacent soft tissue. The determination of anatomic compartment involvement has become more important with the advent of limb-preservation surgery.
On the basis of biologic considerations and natural history, Enneking and colleagues classified bone tumors into five categories, each of which shares certain clinical characteristics and radiographic patterns and requires similar surgical procedures.
1. Benign/latent: lesions whose natural history is to grow slowly during normal growth of the individual and then to stop, with a tendency to heal spontaneously. They never become malignant and, if treated by simple curettage, heal rapidly. Surgery is not indicated unless they become symptomatic.
2. Benign/active: lesions whose natural history is one of progressive growth. Simple curettage leaves a reactive rim with some tumor. Curettage is associated with a high recurrence rate. Wide excision through normal bone results in local control in approximately 95% of all cases.
3. Benign/aggressive: lesions that are locally aggressive but do not metastasize. The tumor extends through the capsule into the reactive zone. Local control can be obtained only by removing the lesion with a margin of normal bone beyond the reactive zone.
4. Malignant/low grade: lesions that have a low potential to metastasize. Histologically, a pseudocapsule rather than a true capsule is found. Tumor nodules exist within the reactive zone but rarely beyond. Local control can be accomplished only by removal of all tumor and reactive tissue with a margin of normal bone. These lesions can be treated successfully by surgery alone.
5. Malignant/high grade: lesions whose natural history is to grow rapidly and metastasize early. Tumor nodules are often found within and beyond the reactive zone and at some distance in the normal tissue. Surgery is necessary for local control, and systemic therapy is warranted to prevent metastasis.
METASTASIS
Bone tumors, unlike carcinomas, disseminate almost exclusively through the blood; bones lack a lymphatic system. Early lymphatic spread to regional nodes has only rarely been reported.Lymphatic involvement, which has been noted in 10% of cases at autopsy, is a poor prognostic sign. McKenna and associates noted that patients (3%) with osteosarcoma who underwent amputation demonstrated lymph node involvement. None of these patients survived 5 years. Hematogenous spread is manifested by pulmonary involvement in its early stage and secondarily by bone involvement. Bone metastasis is occasionally the first sign of dissemination. With the use of adjuvant chemotherapy, the skeletal system has become a more common site of initial relapse.
SKIP METASTASIS
A skip metastasis, as previously defined, is a tumor nodule that is located within the same bone as the main tumor but not in continuity with it. Transarticular skip metastases are located in the joint adjacent to the main tumor. Skip metastases are most often seen with high-grade sarcomas. A skip lesion develops by the embolization of tumor cells within the marrow sinusoids; in effect, they are local micrometastases that have not passed through the circulation. Transarticular skips are believed to occur via the periarticular venous anastomosis. The clinical incidence of skip metastases is less than 1%. These lesions are a prognosticator of poor survival.
LOCAL RECURRENCE
Local recurrence of a benign or malignant lesion is due to inadequate removal. The aggressiveness of the tumor determines which surgical procedure is required for local control. Ninety-five percent of all local recurrences, regardless of histology, develop within 24 months of attempted removal. Local recurrence of a high-grade sarcoma decreases overall survival prospects substantially. Local recurrence in patients who have undergone therapy is associated with an even poorer prognosis.
Cartilage tumors are lesions in which cartilage is produced. They are the most common bone tumors. Osteochondroma is the most common benign cartilage tumor; some 1% to 2% of solitary osteochondromas become malignant and Enchondroma is a benign cartilage tumor that occurs centrally; in adults, malignant transformation may occur. Chondrosarcoma, the most common malignant cartilage tumor, is either intramedullary or peripheral. Ten percent are secondary, arising from an underlying benign lesion. Most chondrosarcomas are low grade, although 10% dedifferentiate into high-grade spindle cell sarcomas or, rarely, a mesenchymal chondrosarcoma.
Osteoid tumors are lesions in which the stroma produce osteoid. The benign forms are osteoid osteoma and osteoblastoma. Osteoid osteomas are never malignant. Osteoblastomas rarely metastasize; when they do, it is only after multiple local recurrences.Osteosarcomas are the most common primary malignant tumors of the bone. Histologically, they are composed of malignant spindle cells and osteoblasts that produce osteoid or immature bone. Several variants are now recognized.60 Parosteal, periosteal, and low-grade intraosseous osteosarcoma are histologically and radiographically distinct from the “classic” central medullary osteosarcomas and have a more favorable prognosis.
Fibrous tumors of bone are rare. Desmoplastic fibroma is a locally aggressive, nonmetastasizing tumor, analogous to fibromatosis of soft tissue.Fibrosarcoma of bone appears histologically as its soft tissue counterpart. Multiple sections must be obtained to demonstrate the lack of osteoid production. If osteoid is present, the lesion is classified as an osteosarcoma. MFH, a rare lesion and the counterpart of soft tissue MFH, has been described in bone.The pathophysiologic behavior of bone and soft tissue MFH is similar, consisting of a storiform pattern with a histiocytic component. Giant cell tumors of unknown origin were originally called benign but are now considered low-grade sarcomas. They have high rates of local recurrence and malignant transformation.
Tumors presumably arising from bone marrow elements are the round cell sarcomas. The two most common are Ewing's sarcoma and the rarer non-Hodgkin's lymphoma.
RADIOGRAPHIC EVALUATION AND DIAGNOSIS
Radiographic evaluation combined with the clinical history and histologic examination is necessary for accurate diagnosis. Bone scans, angiography, CT, and MRI are generally not helpful in determining a diagnosis but are important in delineating the extent of local involvement. A systematic approach to the radiographic evaluation of skeletal lesions has been described by Madewell and colleagues, who studied and correlated several hundred radiographic and pathologic specimens. They considered the radiograph as the gross specimen from which a detailed histologic interpretation could be made and biologic activity accurately diagnosed. According to their system, a bone tumor is evaluated by five radiographic parameters:
1. Anatomic site. Specific anatomic sites of the bone give rise to specific groups of lesions. Johnson explained this by a “field” theory, which hypothesizes that the most active cells of a certain area of bone give rise to tumors that are characteristic of that area. In general, spindle cell sarcomas are metaphyseal, whereas round cell sarcomas tend to be diaphyseal.
2. Borders. The border reflects the growth rate and the response of the adjacent normal bone to the tumor. Most tumors have a characteristic border. Benign lesions (e.g., nonossifying fibromas and unicameral bone cysts) have well-defined borders and a narrow transition area that is often associated with a reactive sclerosis. Aggressive or benign tumors [e.g., chondroblastoma and giant cell tumors (GCTs)] tend to have faint borders and wide zones of transition with very little sclerosis, reflecting a faster-growing lesion. Poorly delineated or absent margins indicate an aggressive or malignant lesion.
3. Bone destruction. Bone destruction is the hallmark of a bone tumor. Three patterns of bone destruction are described : geographic, moth-eaten, and permeative. In general, these patterns are found in the tubular bone rather than in the flat bone and represent a combination of cortical and cancellous destruction. These patterns reflect a progressively increasing growth rate of the underlying tumor.
4. Matrix formation. Calcification of the matrix, or new bone formation, may produce an area of increased density within the lesion. Calcification typically appears as flocculent or stippled rings or clusters. The appearance of the new bone varies from dense sclerosis that obliterates all evidence of normal trabeculae, to small, irregular, circumscribed masses described as “wool” or “clouds.” Calcification and ossification may appear in the same lesion. Neither type of matrix formation per se is diagnostic of malignancy.
5. Periosteal reaction. Periosteal reaction is indicative of malignancy but not pathognomonic of a particular tumor. A combination of periosteal changes is often noted. In malignant tumors, periosteal reaction is noncontinuous and thin, with multiple laminations. A parallel or a perpendicular pattern may be present.
The radiographic parameters of benign and malignant tumors are quite different. Benign tumors have round, smooth, well-circumscribed borders. No cortical destruction and, generally, no periosteal reaction are found. Malignant lesions have irregular, poorly defined margins. Evidence of bone destruction and a wide area of transition with periosteal reaction are noted. Soft tissue extension is common.
NATURAL HISTORY
Tumors arising in bone have characteristic patterns of behavior and growth that distinguish them from other malignant lesions. These patterns form the basis of a staging system and current treatment strategies. These principles and their relationship to management, as formulated by Enneking and colleagues, are described here.
BIOLOGY AND GROWTH
Spindle cell sarcomas form a solid lesion that grows centrifugally. The periphery is the least mature part of this lesion. In contradistinction to a true capsule, which surrounds a benign lesion and is composed of compressed normal cells, a malignant tumor is generally enclosed by a pseudocapsule and consists of compressed tumor cells and a fibrovascular zone of reactive tissue with an inflammatory component that interdigitates with the normal tissue adjacent to and beyond the lesion. The thickness of the reactive zone varies with the degree of malignancy and histiogenic type. The histologic hallmark of sarcomas is their potential to break through the pseudocapsule to form satellite lesions of tumor cells. This characteristic distinguishes a nonmalignant mesenchymal tumor from a malignant one.
High-grade sarcomas have a poorly defined reactive zone that may be invaded and destroyed by the tumor. In addition, tumor nodules in tissue may appear to be normal and not continuous with the main tumor. These are termed skip metastases. Although low-grade sarcomas regularly demonstrate tumor interdigitation into the reactive zone, they rarely form tumor nodules beyond this area.
The three mechanisms of growth and extension of bone tumors are:
(1) compression of normal tissue
(2) resorption of bone by reactive osteoclasts
(3) direct destruction of normal tissue.
Benign tumors grow and expand by the first two mechanisms, whereas direct tissue destruction is characteristic of malignant bone tumors. Sarcomas respect anatomic borders and remain within one compartment. Local anatomy influences tumor growth by setting the natural barriers to extension. In general, bone sarcomas take the path of least resistance. Most benign bone tumors are unicompartmental; they remain confined and may expand the bone in which they arose. Malignant bone tumors are bicompartmental; they destroy the overlying cortex and go directly into the adjacent soft tissue. The determination of anatomic compartment involvement has become more important with the advent of limb-preservation surgery.
On the basis of biologic considerations and natural history, Enneking and colleagues classified bone tumors into five categories, each of which shares certain clinical characteristics and radiographic patterns and requires similar surgical procedures.
1. Benign/latent: lesions whose natural history is to grow slowly during normal growth of the individual and then to stop, with a tendency to heal spontaneously. They never become malignant and, if treated by simple curettage, heal rapidly. Surgery is not indicated unless they become symptomatic.
2. Benign/active: lesions whose natural history is one of progressive growth. Simple curettage leaves a reactive rim with some tumor. Curettage is associated with a high recurrence rate. Wide excision through normal bone results in local control in approximately 95% of all cases.
3. Benign/aggressive: lesions that are locally aggressive but do not metastasize. The tumor extends through the capsule into the reactive zone. Local control can be obtained only by removing the lesion with a margin of normal bone beyond the reactive zone.
4. Malignant/low grade: lesions that have a low potential to metastasize. Histologically, a pseudocapsule rather than a true capsule is found. Tumor nodules exist within the reactive zone but rarely beyond. Local control can be accomplished only by removal of all tumor and reactive tissue with a margin of normal bone. These lesions can be treated successfully by surgery alone.
5. Malignant/high grade: lesions whose natural history is to grow rapidly and metastasize early. Tumor nodules are often found within and beyond the reactive zone and at some distance in the normal tissue. Surgery is necessary for local control, and systemic therapy is warranted to prevent metastasis.
METASTASIS
Bone tumors, unlike carcinomas, disseminate almost exclusively through the blood; bones lack a lymphatic system. Early lymphatic spread to regional nodes has only rarely been reported.Lymphatic involvement, which has been noted in 10% of cases at autopsy, is a poor prognostic sign. McKenna and associates noted that patients (3%) with osteosarcoma who underwent amputation demonstrated lymph node involvement. None of these patients survived 5 years. Hematogenous spread is manifested by pulmonary involvement in its early stage and secondarily by bone involvement. Bone metastasis is occasionally the first sign of dissemination. With the use of adjuvant chemotherapy, the skeletal system has become a more common site of initial relapse.
SKIP METASTASIS
A skip metastasis, as previously defined, is a tumor nodule that is located within the same bone as the main tumor but not in continuity with it. Transarticular skip metastases are located in the joint adjacent to the main tumor. Skip metastases are most often seen with high-grade sarcomas. A skip lesion develops by the embolization of tumor cells within the marrow sinusoids; in effect, they are local micrometastases that have not passed through the circulation. Transarticular skips are believed to occur via the periarticular venous anastomosis. The clinical incidence of skip metastases is less than 1%. These lesions are a prognosticator of poor survival.
LOCAL RECURRENCE
Local recurrence of a benign or malignant lesion is due to inadequate removal. The aggressiveness of the tumor determines which surgical procedure is required for local control. Ninety-five percent of all local recurrences, regardless of histology, develop within 24 months of attempted removal. Local recurrence of a high-grade sarcoma decreases overall survival prospects substantially. Local recurrence in patients who have undergone therapy is associated with an even poorer prognosis.
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