Cortical scalloping and cortical penetration by small eccentric chondroid lesions in the long tubular bones: not a sign of malignancy?
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- Bui, K.L., Ilaslan, H., Bauer, T.W. et al. Skeletal Radiol (2009) 38: 791. doi:10.1007/s00256-009-0675-0
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The objective of this study was to evaluate by cross-sectional imaging the prevalence and degree of cortical scalloping by small eccentric chondromas correlated with histologic diagnosis and patient history.
Materials and methods
From 122 patients with histologically proven enchondromas and two patients without histology but with radiologic and clinical follow-up, 11 patients with small, eccentrically located chondromas in the long bones had cross-sectional imaging available. The lesions were evaluated for location, size, presence, and degree of cortical scalloping. The patient’s medical charts and microscope slides were reviewed for relevant clinical history, clinical management, and histology.
The chondromas ranged in size from 1.6 to 3.8 cm (mean 2.3 cm). Two lesions were located in the proximal femoral diaphysis, two in the distal femoral diaphysis, six in the distal femoral metaphysis, and one in the proximal tibial epimetaphysis. The lesions were curetted due to diagnostic uncertainty, continued pain, marked radiologic cortical penetration, or due to patient insistence on biopsy. All 11 lesions were benign, nine histologically, and two by stability over 4 and 7 years. The prevalence of cortical scalloping among eccentric chondromas was 100%. Cortical scalloping or occupancy ranged from 50 to 100% (mean 75%).
All small eccentric chondromas in this study were associated with an appearance of cortical scalloping of varying degree. All curetted lesions were histologically bland without nuclear atypia. Based on the benign histology of nine lesions and lack of growth of two lesions over several years, the degree of cortical scalloping is felt to be a result of lesion location within the endosteum rather than biological activity or malignancy.
KeywordsEndosteal chondromaEccentricLong boneEndosteal scallopingImaging
Chondromas are benign cartilaginous neoplasms. Their presumed origin in bone serves as a descriptor of the lesion. Surface lesions are called periosteal or juxtacortical chondromas . Those confined to the cortex are termed intracortical chondromas. The common intramedullary lesion is termed enchondroma. Enchondromas are felt to arise from cartilaginous rests in the growth plate that become trapped in the metaphysis and continue to grow .
Enchondromas are common neoplasms, comprising 12–24% of all benign bone tumors and 3–10% of all bone tumors . These are generally discovered in the third to fourth decades . They are commonly located centrally within the bone, with a predilection for short tubular bones, proximal femur, and humerus . About 50% of enchondromas in long bones are incidental findings . When these are symptomatic, the pain is usually of short duration and due to small stress fractures from strenuous activity .
Benign enchondromas and low-grade chondrosarcomas can be difficult to distinguish, both radiologically and histologically . Experienced pathologists and radiologists may draw discordant conclusions in distinguishing enchondromas from low-grade chondrosarcomas, and this inconsistency was highlighted in a recent blinded study of radiologists and pathologists .
Some radiographic criteria suggestive of a chondrosarcoma in long bones include: cortical scalloping greater than two thirds the normal cortical thickness, periosteal reaction, cortical destruction and thickening, size greater than 5–6 cm, soft tissue extension or mass, pain referred to the lesion, and the presence of a pathologic fracture [6–8]. Intuitively, the more criteria that are present, the more likely a lesion is a chondrosarcoma.
We observed several small eccentric chondroid lesions with marked cortical scalloping and cortical penetration that were curetted and were unequivocal enchondromas histologically. Thus, we sought to determine the prevalence and degree of cortical scalloping in small (less than 4 cm) eccentrically located, histologically proven chondromas, and correlate with the patient’s clinical history.
Materials and methods
Institutional Review Board approval was obtained for this study. A retrospective computerized search through the pathology database for biopsied or curetted enchondromas between January 1, 1995 and January 1, 2008 was performed. Two additional patients without pathology but with follow-up imaging were discussed at our institution tumor board conference and included in the study.
Initially, 122 patients with histologically proven enchondromas were identified. We limited our study to lesions in the long tubular bones. We defined small eccentric chondromas as chondroid lesions less than 4 cm in size, eccentrically located in the medullary canal, and also located within the endosteal cortex to a variable degree, producing an appearance referred to as endosteal scalloping. Lesions in the hands and feet, lesions without cross sectional imaging, and centrally located lesions were excluded.
The magnetic resonance (MR) and computed tomography (CT) images of patients who met the inclusion criteria were re-reviewed by two musculoskeletal radiologists, in consensus. A variety of different 1 and 1.5 T MR sequences, including axial GRE, axial and coronal T1, axial and sagittal dual echo proton density/T2 images, and coronal T2 images were evaluated. The CT images varied between 1 and 2.5 mm thick axial slices in bony algorithm, some with coronal and sagittal reconstructions. The images were reviewed for location, size, presence and degree of cortical scalloping. The percentage of cortical scalloping was estimated on a viewing station by utilizing the measuring tool to measure the scalloped cortex where it was the thinnest, then measuring the adjacent normal cortex thickness on the same slice, calculating the percentage of remaining cortex and subtracting that value from 100%. The plane demonstrating the deepest scalloping was utilized. Two patients had only CT images, four patients had both CT and MR imaging, and for all four, the measurements from the CT were utilized, and five patients had only MR images. Microscope slides from all cases were re-reviewed by a tumor pathologist to confirm the diagnosis, and the medical records were reviewed for relevant clinical history and management.
Nine patients met the inclusion criteria. Two additional patients without histology but with clinical and imaging follow-up of 4 and 7 years, respectively, were also included in the study. Seven patients initially presented with pain. In six of those patients, the pain was subsequently determined to be unrelated to the radiologically identified lesion. Pain in one patient was attributed to the lesion. Two patients had lesions that were incidental findings: one was diagnosed in the proximal femur on a KUB radiograph, and the other was in the right distal femur on imaging performed for left knee pain. Two patients had no clinical history available.
Nine of the 11 lesions were curetted either because there was uncertainty if the lesion was benign or a low-grade chondrosarcoma, continued pain, and marked radiologic cortical penetration or because the patient insisted on biopsy. All patients for whom history was available were treated with curettage and bone graft. Two patients who did not undergo biopsy demonstrated stability both clinically and radiologically over 4 and 7 years. None of the curetted tumors recurred with a mean follow-up of 4.5 years (range 1 to 9 years).
Enchondromas are benign hyaline cartilage lesions most commonly intramedullary in location and most commonly affecting the hands, followed by the proximal humerus, and proximal and distal femur. They are uncommon in flat bones and rarely encountered in craniofacial bones . Primary or conventional chondrosarcoma is defined by the World Health Organization (WHO) as an intramedullary malignant tumor with pure hyaline cartilage arising centrally in previously normal bone . Their favored sites of predilection are the pelvis, femur, and humerus. Because enchondromas and low-grade chondrosarcomas occur with equal frequency in long tubular bones, they present diagnostic problems for both the radiologist and pathologist . A recent study using multiple expert musculoskeletal pathologists and radiologists showed less than optimal concordance and inter-observer reliability in separating enchondromas from low-grade chondrosarcomas .
It has been recognized that one of several criteria used to help radiologists differentiate a low-grade chondrosarcoma from an enchondroma is marked cortical scalloping, with greater than two thirds scalloping of the normal cortical width favoring a chondrosarcoma [6–8]. The findings from this study show that eccentrically located enchondromas can have marked endosteal scalloping, oftentimes greater than two thirds the normal cortex, and that this finding in isolation should not be considered a sign of malignancy warranting biopsy or curettage.
Of 11 patients in this series, eight (all women) with eccentrically located benign chondroid lesions showed greater than two thirds endosteal scalloping (Figs. 4a and 5), seven in the femur, and all less than 4 cm in size. Of these, three lesions demonstrated outer cortical penetration (Fig. 6a, b), with the remainder of the lesion in the cancellous portion of the bone. Despite the small lesion size in our patients, in every instance, there was cortical scalloping of varying degree, which is one of the imaging signs usually associated with a low-grade chondrosarcoma. Histologically, the lesions were bland with few signs of increased cellularity or nuclear atypia (Figs. 4b, c, 6c, d), the presence of which has been an explanation for some of the endosteal effects of enchondromas in the phalanges .
Although it would appear that deep endosteal scalloping and even cortical penetration are not signs of malignancy in these eccentrically located small chondromas, it is not clear why this is so. We speculate that the appearance of endosteal scalloping is not due to the lesion eroding the endosteum from the cancellous bone but is due to the lesion partially originating in the endosteal aspect of the cortex and adjacent medullary canal. The exquisite detail provided by current cross-sectional imaging showed all lesions to be intimately related to the endosteum; i.e., always occupying it and with a component of the lesion in the cancellous bone. We, therefore, propose that the endosteum is at least in part the origin of these lesions, analogous to intracortical and periosteal chondromas, which are named for their presumed sites of origin. Given the benign histology with relatively low cellularity and without nuclear atypia and the lack of growth for the two lesions followed for 4 and 7 years, we suggest that the appearance of endosteal and cortical penetration is not a consequence of growth into the endosteum from the cancellous bone but a consequence of the lesion having at least partially originated in the endosteum.
There are two types of enchondromas whose widely used and accepted nomenclature are based entirely on their relationship to the cortex, i.e., periosteal chondroma and intracortical chondroma, the latter with fewer than ten cumulative case reports. We believe that the lesions described in our series belong to the spectrum of chondroid lesions that are intimately related to the cortex, which in our patients was the endosteum. In proposing the term “endosteal chondroma,” we hope to convey that deep endosteal scalloping, a feature common to all of our cases, is not a consequence of morphologic growth or cellularity but a manifestation of location that mimics growth.
We did not attempt to clarify the radiologic or clinical features of chondrosarcomas that show cortical erosion, a subject of repeated and continuing discussion [2, 4–8, 10]. A study of 10 years ago evaluated the distinguishing features on imaging between enchondromas and chondrosarcomas in the appendicular skeleton . Two cross-sectional imaging signs that strongly suggested chondrosarcoma in that study were deep cortical scalloping (greater than two thirds of the normal cortex) and cortical penetration . The former sign was found in eight of our 11 patients and the latter sign in three patients. Other signs in that study that favored chondrosarcoma not seen in our patient population were average lesion size of 5 cm, cortical remodeling, pathologic fracture, and soft tissue extension . Finally, that study evaluated all intramedullary lesions and found that chondrosarcomas were centered in the metaphysis or subarticular portion of the long bone and enchondromas in the diaphysis . Perhaps the most striking imaging difference between our cases with endosteal involvement and that of Murphey et al. is that all of our lesions showed this finding with no lesion greater than 4 cm in length. In their study, Murphey et al. found that the longer the extent of scalloping in relation to lesion length, the more likely for the lesion to represent a chondrosarcoma 67–79% . The 100% cortical involvement to a degree disproportionate to their size and the bland monotonous histology without nuclear atypia lead us to believe that the lesions we report are not conventional intramedullary enchondromas but rather endosteal chondromas mimicking an enlarging enchondroma or chondrosarcoma. With the high quality of imaging detail afforded in current clinical practice, it seems to us that a reasonable alternative to curetting this subset of endosteal chondroid lesions would be to follow them clinically and by imaging.
The limitations of this study are the small series, perhaps a reflection of the rarity of the lesion and not dissimilar in prevalence to the relatively uncommon periosteal and intracortical chondromas, and the two patients who had follow-up imaging without histology.
Eccentrically located benign chondroid lesions smaller than 4 cm, partially occupying the endosteal aspect of the cortex and cancellous bone, produce an appearance of endosteal scalloping. In this subset of lesions, the appearance of scalloping was not associated with biological activity, growth, or malignancy. The appearance of scalloping is likely the result of the origination of the lesion within the endosteum and adjacent cancellous bone.