—  SPECIALTY CONFERENCE  —

Bone and Soft Tissue Pathology

Case 1 - What is New in Pathology of Osteochondroma?

David R. Lucas
Clinical Associate Professor of Pathology
University of Michigan


Click on each slide thumbnail image for an enlarged view
Clinical History
A 21-year-old man with hereditary multiple exostosis (HME) presented with new-onset pain and swelling in his left leg. Radiographs disclosed a large osteochondroma that formed a synostosis between the proximal fibula and tibia. The gross resection specimen contained a 10 cm ostechondroma with a lobulated cartilage cap, which varied from 0.2 to 2.0 cm in thickness. Destructive involvement of the stalk was not present, and there was no gross evidence of peripheral soft tissue invasion.


Case 1 - Figure 1 - Low-power photomicrograph depicting residual osteochondroma consisting of perichondrium on top, columns of chondrocytes reminiscent of the normal growth plate in the middle, and endochondral ossification at the base.
Case 1 - Figure 2 - Low-power view showing the lobular architecture of the cap consisting of lobules of cartilage separated by slit-like spaces as well as by thick fibrous bands at the bottom.


Case 1 - Figure 3 - Occasional cartilage lobules appeared to separate from the cap and invade beyond the perichondrium into skeletal muscle, as depicted in this low-power view.
Case 1 - Figure 4 - High-power photomicrograph illustratrating the bland cytological features of the tumor cells.
Case 1 - Figure 5

Introduction
A lot of new and exciting information about osteochondroma has emerged in the past few years. Much comes from molecular and cytogenetic studies relating to the etiology and pathogenesis of HME and sporadic osteochondromas, and to genetic progression of osteochondroma to peripheral chondrosarcoma. These findings are discussed later in this presentation. But first, let's review some basics.

General
Osteochondroma is the most common primary bone tumor, accounting for approximately 35% of all benign tumors [1] It typically presents during the first two decades of life, and most commonly involves the metaphyseal region of a long bone. Flat bones, with the exception of the calvarium, are also commonly affected. Osteochondromas are uncommon in the small bones of the hands and feet. The male-to-female ratio is 1.7 to 1 [2].

Gross Pathology
Osteochondromas can be pedunculated or sessile, and vary greatly in size, with some achieving great dimension. Grossly and radiographically osteochondromas have continuity with the medullary cavity of the parent bone, with the cortex of the bone flaring into the stalk of the osteochondroma. The cartilage cap can be smooth or bosselated and usually measures around 0.5 cm in thickness. However, the thickness is quite variable. For instance, in a growing child the cap can sometimes measure up to 3 cm [3, 4, 5] , while in a skeletally mature adult the cap can be almost entirely absent. The surface of an osteochondroma is covered by perichondrium. A bursa can sometimes form around the cap, and osteocartilaginous loose bodies can form within the bursa [6].

Histopathology
Microscopically, the cartilage cap of an osteochondroma resembles the physis, replete with chondrocytes arranged in columns and endochondral ossification. The hyaline cartilage typically forms a diffuse sheet without substantial interstitial lobulation. The bone underlying the cap is formed by endochondral ossification, which is most active in skeletally immature individuals. However, unlike in normal bone, endochondral ossification in osteochondroma is less orderly and often fails to completely mature, producing large areas of heavily calcified cartilage.

Hereditary Multiple Exostosis
HME is an autosomal dominant condition with a prevalence of 1 to 50,000 [7]. Although most studies report a male predominance, others claim both sexes to be equally affected [7, 8] . The osteochondromas in HME are identical to sporadic osteochondromas. However, HME patients have additional skeletal abnormalities such as short stature, limb length discrepancies, dislocations, and bowing deformations of the long bones, especially in the forearm [9, 10, 11, 12] . Synostosis can form between bones due to fusion of juxtaposing osteochondromas [9], as was present in our case. Multiple osteochondromas are also found in other, more complex syndromes such as Langer-Giedion syndrome [13, 14, 15] and Deletion 11 syndrome [16, 17] , which have additional abnormalities including mental retardation, and craniofacial and other skeletal deformations.

Etiology and Pathogenesis
In 1891, Virchow [18] proposed that osteochondromas derive from displaced growth plate cartilage, which, after rotating 90%, forms a cartilage-capped bony protuberance that grows perpendicular to the axis of the parent bone. This hypothesis was supported by experimental evidence from a rabbit model [19], in which osteochondromas developed from transplanted epiphyseal cartilage. Alternative hypotheses are those of Muller [20], who proposed that osteochondromas derive from nests of cartilage formed within the periosteum (ecchondroses), and Keith [21] who believed they derive from defects in the periosteal ring surrounding the growth plate. Jaffe [9] concluded that these last two hypotheses are mutually compatible and re-enforcing. Other theories proposed that osteochondromas derive from focal accumulations of embryonic connective tissues at sites of tendon attachment, which are transformed into tumorous growths by tensile force [22]; or that they result from defects in mucopolysaccharide maturation [23]. Osteochondromas are also known to arise in radiation fields [24]. For instance, 5% of children who underwent total body irradiation prior to bone marrow transplantation developed osteochondromas in one study [25]. The most recent evidence supports a neoplastic origin, as elaborated below.

Genetic Model of Pathogenesis
HME is a genetically heterogeneous disease associated with mutations in a least 3 different genes: EXT1 located at 8q24.1, EXT2 located at 11p11-p12, and EXT3 located at 19p [26, 27, 28, 29] . Of note, germline deletions at 8q24.1 and at 11p11-p12 characterize Langer-Giedion and Deletion 11 syndromes, respectively [16, 17] . These are continuous gene deletion syndromes caused by massive deletions that include the genes for HME [23]. Three other homologous genes, termed "EXT-like genes," EXTL1, EXTL2, and EXTL3 have also been identified [30, 31, 32] . Mutations of EXT1 and EXT2 account for approximately one-half and one-third of HME pedigrees [33], respectively, and have been the most extensively studied.

EXT1 and EXT2 mRNA is expressed at high levels in developing limb buds of mice [34]. The gene products, exostocin 1 and 2, are ubiquitous in musculoskeletal connective tissues. These proteins are homologous transmembrane glycosyltransferases [35, 36] located in the endoplasmic reticulum, which catalyze the synthesis and modification of heparan sulfate [37]. Heparan sulfate is a complex polysaccharide and a key component of cartilage. It plays important roles in cell adhesion, growth factor signaling, and cell proliferation at the growth plate.

Exostocin 1 is also responsible for the normal diffusion of Indian hedgehog (IHh) [38, 39] in cartilage. IHh is produced by growth plate chondrocytes and diffuses to the perichondrium where it inhibits chondrocyte differentiation via IHh/PTHrP signaling. EXT1 mutation leads to decreased IHh diffusion, thus inhibiting this important negative feedback loop in chondrocyte differentiation. In summary, EXT mutations affect normal growth plate function by decreasing the activity of heparan sulfate (in particular by inhibiting FGF signaling), and by inhibiting IHh/PTHrP signaling. In addition, EXT mutations appear to induce cytoskeletal abnormalities (actin accumulation) in osteochondroma chondrocytes [40].

EXT1 and EXT2 function as tumor suppressor genes [31, 39, 41, 42, 43] . Both EXT1 and EXT2 HME pedigrees have loss-of-function germline mutations [39, 44, 45, 46] , often stop codons, that result in truncated proteins with diminished biological activity. Subsequent loss of the wild-type allele in the cartilage cells of the physis may then lead to osteochondroma formation [47, 48, 49, 50] .

Cytogenetic studies have demonstrated clonal chromosomal aberrations in both sporadic and hereditary osteochondromas. Mertens et. al. [47] reported structural rearrangements leading to loss of 8q in 3 of 8 sporadic osteochondromas, and hypothesized that somatic mutation of 8q24 in both chromosomes is an important developmental mechanism in sporadic tumors. Bridge et. al. [48] confirmed 8q24.1 abnormalities in sporadic osteochondromas, but also demonstrated deletion of 11p11-13 in one case. They concluded that sporadic osteochondromas, like hereditary osteochondromas, are genetically heterogeneous and that loss of EXT1 and EXT2 are important in the pathogenesis of sporadic osteochondromas as well. And recently, using FISH, Feely et. al. [49] found 8q24.1 loss in a high percent (79%) of hereditary and sporadic osteochondromas, highlighting the importance of loss of EXT1 in the pathogenesis of both.

Based upon its clonal origin, along with evidence of aneuploidy and LOH [33] in the cartilage cap, osteochondroma is now regarded as a neoplastic growth as opposed to a developmental aberration. And finally, regarding radiation-induced osteochondromas, it has been speculated that radiation could induce mutations of EXT in the physis, and thus facilitate neoplastic transformation [23]. However, this has not been studied.

Malignancy in Osteochondroma
Malignant transformation in osteochondroma is rare. Although, difficult to determine the true incidence, since many osteochondromas never come to clinical attention, the estimated risk is probably less than 1% for sporadic osteochondromas and less than 5% in HME [8]. However, in HME the reported incidence of malignancy is highly variable, ranging from 0.5 to 25%, reflecting genetic heterogeneity in predisposition to malignant degeneration among kindreds [7]. Low grade chondrosarcoma is by far the most common secondary tumor in osteochondroma, and most patients can be successfully treated with wide local excision [51]. High grade malignant transformation, such a spindle cell [52], osteosarcomatous [53], or dedifferentiated chondrosarcomatous [54] transformation is rare.

Some authors believe that all peripheral chondrosarcomas arise from osteochondromas. However, this remains contentious since residual osteochondroma is not identified in most peripheral chondrosarcomas. On the other hand, histologic evidence of a pre-existing osteochondroma is found in only half of secondary chondrosarcomas [51]. A more important problem for the pathologist is to identify the early malignant lesions using objective criteria, which is the reason I chose this particular case.

What are the criteria for diagnosing early chondrosarcomatous transformation in osteochondroma?
When I reviewed the major textbooks in bone tumor pathology, I realized that the criteria for malignancy in osteochondroma were quite variable and not always clear. For instance, the thickness of the cartilage cap has traditionally been regarded as an important feature of malignancy. However, the level at which thickness becomes important in defining malignancy varies. Other criteria that have been espoused include: nuclear atypia and binucleation, mitotic activity, separate peripheral lobules that invade soft tissue, stalk invasion, lobular architecture, thick fibrous bands that encase cartilage lobules, cystic/myxoid change in the cartilage matrix, and necrosis. Clinical features, such as age of the patient and pain, and radiographic features are also included in the criteria by some. Here are excerpts from some of the major texts:

Lichtenstein (1977) [55]
"If the cartilage cap measures as much as 1 cm or more in thickness, then one has cause for serious concern over the possibility of chondrosarcomatous change." He then indicates that search for atypical nuclei should be undertaken. He also states that, "In dealing with smaller peripheral tumors, the most important single pathologic observation is measurement of the width of the cartilage cap. If this exceeds 1 cm (and the patient is an adult), then the cartilage has been actively growing and, as such, represents chondrosarcoma."

Dahlin (1986) [56]
"Irregularity and thickening of the cap, especially in an adult, demand careful histologic study because of the likelihood of secondary chondrosarcoma. Although a rare, thinner cap may be associated with malignancy, secondary chondrosarcomas are usually at least 2 cm in thickness. Cystic change within a cartilage cap is also cause for concern." He also states that, "If exostosis is present, the finding of a cartilaginous cap, irregularly thickened to more that 1 cm, must be viewed with the suspicion of malignancy; cartilaginous masses of 3 cm or 4 cm usually indicate a chondrosarcoma."

Mirra (1989) [4]
"In my experience, the few lesions with cartilage caps above 2.0 cm in an adult patient have demonstrated microscopic evidence most consistent or absolutely confirmatory of malignant transformation." He then states, "Considering this fact in conjunction with all of the clinical, radiologic, and pathologic features, a diagnosis of malignancy need not be based solely on the caprice of a ruler." He claims that any signs of malignant permeation patterns within the cortex or medullary bone, or bands of fibrosis between the lobules are diagnostic of malignancy.

Huvos (1991) [3]
"If the thickness of the cap exceeds 1 cm, serious consideration should be given to the likelihood of the lesion becoming, or already being, a chondrosarcoma…. it is important to remember that in cases in which the cartilage cap is irregular and more than a few millimeters thick, it should definitely be suspected as being a chondrosarcoma."

Fechner and Mills (1993) [5]
"areas (of chondrocyte atypia in an osteochondroma) are not of importance, as long as the cap is less than 3 cm in thickness, there are no masses beyond the periosteum, and there is no radiographic evidence of destruction of the underlying bone."

Schajowicz (1994) [57]
"There is no absolute rule that serves as a reliable criterion for malignancy. However, the lesion should be studied carefully in order to rule out a secondary chondrosarcoma." He also states, "In this writer's experience (unlike Lichtenstein 1977) and others, a width of 1 cm alone, especially in pelvic lesions, is not an indication of malignancy. However, a large cap, increasing in size at the end of the growth period and often accompanied by irregular calcification that becomes indistinct or fuzzy beyond its previous borders or by pain of a relatively sudden onset in a previously painless ostechondroma, is highly suspicious for malignant transformation." He then emphasizes, "The histologic criteria for the diagnosis of malignancy are the same as those applied for conventional central chondrosarcoma (which emphasize atypical nuclear features in his book)."

Dorfman (1998) [1]
"The pathologic criteria for diagnosis of low-grade chondrosarcoma arising in an osteochondroma are based on gross (thickness of the cap) and microscopic features. Increased thickness of the cartilage cap with the formation of discrete, grossly detectable peripheral lobules that may extend outside the fibrous perichondrium is present in the majority of typical cases. The thicken cap usually exceeds 2 cm in the area of lobular proliferation. Microscopically, the proliferating lobular areas show increased cellularity and clustering of plump chondrocytes that contain enlarged nuclei with an open chromatin structure. Mild to moderate nuclear atypia with frequent binucleate and multinucleate cells are usually present."

WHO Classification (2002) [58]
"A thick cartilage cap (greater than 2 cm) may be indicative of malignant transformation" and that microscopically, "Loss of the architecture of the cartilage, wide fibrous bands, myxoid change, chondrocyte celluarity, mitotic activity, significant chondrocyte atypia, and necrosis are all features that may indicate secondary malignant transformation." The authors also note that in secondary chondrosarcoma the cap is thick (>2 cm), lobulated, and usually shows cystic cavities.

Thus, the criteria for recognizing early malignancy in osteochondroma, aside from clear-cut malignant features of invasion, high cellularity and nuclear atypia, which are not always evident [51, 59] , appear to have evolved from those that emphasize the measured thickness of the cartilage cap, to those that emphasize architectural features, especially lobularity, thick fibrous bands, and cystic/myxoid matrix. And finally, to my knowledge there have been no clinical studies limited to early lesions, thus these criteria remain untested.

Genetic Progression to Secondary Peripheral Chondrosarcoma
Although research into the biological mechanisms of chondrosarcoma is still in the early stages, a number of important observations have been made regarding peripheral chondrosarcomas that arise from osteochondromas. Osteochondromas acquire genetic alterations during malignant transformation different from those found in central chondrosarcomas. For example, they show greater frequencies of LOH in multiple EXT and EXTL genes [43, 46, 60, 61] , more frequent involvement of TP53 and RB1, wider variation in ploidy status, and higher rates of proliferation [61]. Near-haploidy is more frequent in these tumors, and may be regarded as a rather specific marker [61]. Also, Bcl-2 and PTHrP expression appear to be markers of malignant transformation in osteochondroma [50, 62] .

Histologic Findings
In our case, the cartilage cap was thickened (up to 2.0 cm) and lobulated. The base of the cap showed areas of endochondral ossification, with no evidence of invasion into the stalk. Most of the cap was lobulated, consisting of rounded cartilage lobules separated by slit-like spaces as well as by thick fibrous bands in some areas. At the surface, a few lobules appeared to separate from the cap, and to invade beyond the perichondrium into adjacent skeletal muscle. Focally, cystic change was present within the cartilage matrix. The cartilage was hypocellular throughout. Unequivocal malignant nuclear atypia was not present; there was no mitotic activity, and only minimal binucleation.

Differential Diagnoses
Osteochondroma
Osteochondroma with osteocartilaginous loose bodies
Osteochondroma with atypical cartilage cap
Peripheral chondrosarcoma, grade I

My Diagnosis
Secondary chondrosarcoma, grade I, arising in osteochondroma

Concluding Remarks
Diagnosing early chondrosarcomatous transformation in an osteochondroma can be very challenging for the pathologist. When called upon to rule-out malignancy in a pre-operative biopsy specimen, it is essential to know the clinical and radiographic findings. For example, new onset pain and swelling in a preexisting tumor in an adult patient, or radiographic evidence of invasion into soft tissue, would be supportive of malignancy. MRI is particularly useful for determining the thickness of the cap and for detecting soft tissue invasion [63]. It can also highlight lobular architecture. However, once the tumor is removed, which is frequently not preceded by a biopsy in my experience, gross and microscopic evaluation becomes paramount, and clinical and radiographic findings are irrelevant.

The published criteria for diagnosing chondrosarcoma in osteochondroma are not consistent and not always entirely clear. The measured thickness of the cap, although important, is not an absolute criterion of malignancy. However, a thick cap (> 2 cm) in an adult patient is more likely to harbor malignant cytoarchitectural features than a thin one [4]. In the absence of unequivocal soft tissue or stalk invasion, the most important microscopic features for malignant transformation are loss of the usual orderly architecture of the cartilage cap, presence of wide fibrous bands that separate lobules of cartilage, and of course nuclear atypia when present.

Because of slow growth and very low metastatic potential, the recurrence rate for a completely-excised, grade I secondary peripheral chondrosarcoma is low [51]. Thus, recognizing early malignant transformation in osteochondroma, becomes most important when the tumor has not been adequately excised, since it then has greater potential to recur as a more aggressive tumor. Molecular diagnostic information such as gene profile data may prove very useful for recognizing early malignant transformation in future cases. Finally, after more than a century of debate regarding the etiology of osteochondroma, the scientific evidence now supports a neoplastic origin.

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