Benign Lytic Lesions

Fegnomashic

FEGNOMASHIC is defined in “Funk and Wagner’s” unabridged dictionary, 13th edition, as “one who uses mnemonics.” It serves as a nice starting point for discussing possibilities that appear as benign lytic lesions in bone. That mnemonic has been in general use for many years, but I have never heard a claim as to who first coined it. The first mention of it that I saw in print was in 1972 in a radiology article by Gold, Ross, and Margulis. By itself, it is merely a long list—about 14 entities—and needs to be coupled with other criteria to shorten the list into manageable form for each particular case. For instance, if the lesion is epiphyseal, only three to five entities, depending on how accurate you care to be, need to be mentioned. If multiple lesions are present, only half a dozen entities need to be discussed. Ways of narrowing the differential are discussed later in this chapter.

The next step after learning the names of all the lesions is getting some idea of what each one looks like. This is where experience becomes a factor. For the medical student or first-year resident, it’s difficult to go beyond saying that they all look lytic or bubbly and benign. However, the fourth-year resident should have no trouble differentiating between a unicameral bone cyst and a giant cell tumor because he or she has seen examples of each many times before and knows what each looks like. The fourth-year resident may have a hard time verbalizing the differences but should be able to tell them apart.

A novice can quickly gain experience by looking at the examples of each of these lesions in a major skeletal radiology text. In fact, I highly recommend that you compare my description and differential points on each lesion with multiple examples in other books. Some of these lesions can only be diagnosed radiologically on a “pattern identification” or “Aunt Minnie” basis. In other words, there are no hard and fast criteria to differentiate some of the lesions from the others.

After getting a feel for what each lesion looks like radiographically and overcoming the frustration that builds when you realize that many of them look alike, you should try to learn ways to differentiate each lesion from the others. I have developed a number of keys that I call “discriminators” that will help to differentiate each lesion. These discriminators are 90% to 95% useful (I will mention when they are more or less accurate, in my experience) and are by no means meant to be absolutes or dogma. They are guidelines but have a high confidence rate.

Textbooks rarely tell you that a finding “always” or “never” occurs. They temper their descriptions with “virtually always,” “invariably,” “usually,” or “characteristically.” I have tried to pick out findings that come as close to “always” as I can, realizing that I will often only be 95% accurate. That’s good enough for me. If it’s not good enough for you, you’ll have to get your own differential criteria or discriminators. Try these and see if they work for you. If they don’t, modify them as necessary. But whatever you do, develop reasons for including things in your differential. Have concrete criteria of some kind for including or excluding each entity.

I will give a brief description of each entity, as complete descriptions are readily available in any skeletal radiology text. What I will dwell on, however, are the points that are unique for each entity, thereby enabling differentiation from the others. Table 2.1 is a synopsis of these discriminators.

Fibrous Dysplasia

It is unfortunate that this differential starts with fibrous dysplasia because fibrous dysplasia can look like almost anything. It can be wild looking, a discrete lucency, patchy, sclerotic, expansile, multiple, and a host of other descriptions. Unless it is a classic example, it can be difficult to look at a bubbly lytic lesion and unequivocally say it is or is not fibrous dysplasia. When assessing such a lesion, radiology residents usually say, “I suppose it could be fibrous dysplasia, but I’m not sure.” The resident is feeling insecure and becomes defensive right off, setting the tone for the entire differential diagnosis. It would be better if the differential started on a positive note, say, with giant cell tumor or chondroblastoma, where there are some hard and definite criteria. That way the resident would set the tone of self-assurance and decisiveness rather than appear wishy-washy. Then, when mentioning fibrous dysplasia and using the same words, “I suppose it could be fibrous dysplasia, but I’m not sure,” it’s looked upon as thoughtful deliberation rather than insecurity or ignorance. It’s pure gamesmanship, but it makes a difference in how you are perceived.

How do you know whether to include or exclude fibrous dysplasia if it can look like almost anything? Experience is the best guideline. In other words, look in a few texts and find as many different examples as you can; get a feeling for what fibrous dysplasia looks like. A few examples are shown here ( Figs. 2.1 to 2.6 ), but poring over another text for 10 to 15 minutes will be time well spent.

Fibrous dysplasia will not have periostitis associated with it; therefore if periostitis is present, you can safely exclude fibrous dysplasia. It would be possible to have a pathologic fracture through an area of fibrous dysplasia, which then had periostitis, but I have never seen this occur. Fibrous dysplasia virtually never undergoes malignant degeneration and should not be a painful lesion in the long bones unless there is a fracture.

Fibrous dysplasia can be either monostotic (most commonly) or polyostotic and has a predilection for the pelvis, proximal femur, ribs, and skull. When it is present in the pelvis, it is invariably present in the ipsilateral proximal femur ( Figs. 2.3 and 2.4 ). I have seen only one case in which the pelvis was involved with fibrous dysplasia and the proximal femur was spared. The proximal femur, however, may be affected alone, without involvement in the pelvis ( Figs. 2.5 and 2.6 ).

The classic description of fibrous dysplasia is that it has a ground-glass or smoky-appearing matrix. This description confuses people rather than helps them, and I do not recommend using “ground-glass appearance” as a buzz word for fibrous dysplasia. Fibrous dysplasia is often purely lytic and becomes hazy or takes on a ground-glass look as the matrix calcifies. It can go on to calcify quite a bit, and then it presents as a sclerotic lesion. Also, I often see other lytic lesions that have a distinct ground-glass appearance; therefore the ground-glass quality can be misleading.

When a lesion in the tibia that has fibrous dysplasia in the differential diagnosis is encountered, an adamantinoma should also be mentioned ( Fig. 2.7 ). An adamantinoma is a malignant tumor that radiographically and histologically resembles fibrous dysplasia. It occurs almost exclusively in the tibia (for unknown reasons) and is rare. Because it is rare, you may choose not to include it in your memory bank—you won’t miss more than one or two in your life, even if you’re a busy radiologist.

Polyostotic fibrous dysplasia occasionally occurs in association with café au lait spots on the skin (dark-pigmented, freckle-like lesions) and precocious puberty. This complex is called McCune–Albright syndrome. The bony lesions in this syndrome, and even in the simple polyostotic form, often occur unilaterally (that is, in one-half of the body). This doesn’t happen often enough to be of any diagnostic use in differentiating fibrous dysplasia from other lesions. The presence of multiple lesions in the jaw has been termed “cherubism,” which relates to the physical appearance of the affected child. Such children present with puffed-out cheeks, producing an “angelic” look. The jaw lesions in cherubism regress in adulthood.

Fibrous dysplasia often has areas of chondroid matrix which, when biopsied, can resmble chondrosarcoma (as can any chondral tissue). I have seen sampling errors result in inappropriate radical surgery when the diagnosis should have been made on the plain films with no biopsy performed.

Discriminator:

No periosteal reaction.

Enchondroma and Eosinophilic Granuloma

Enchondroma

The most common benign cystic lesion of the phalanges is an enchondroma ( Fig. 2.8 ). Enchondromas occur in any bone formed from cartilage and may be central, eccentric, expansile, or nonexpansile. They invariably contain calcified chondroid matrix ( Fig. 2.9A ) except when in the phalanges. If a cystic lesion is present without calcified chondroid matrix anywhere except in the phalanges, I will not include enchondroma in my differential.

Occasionally it can be difficult to differentiate between an enchondroma and a bone infarct. Although some of the following criteria are helpful in separating an infarct from an enchondroma, they are not foolproof. An infarct usually has a well-defined, densely sclerotic, serpiginous border, whereas an enchondroma does not ( Fig. 2.9B ). An enchondroma often causes endosteal scalloping, whereas a bone infarct will not.

It is difficult, if not impossible, to differentiate an enchondroma from a chondrosarcoma. Clinical findings (primarily pain) serve as a better indicator than radiographic findings, and indeed pain in an apparent enchondroma should warrant further investigation. Periostitis should not be seen in an enchondroma either. Trying to differentiate an enchondroma from a chondrosarcoma histologically is also difficult, if not impossible, at times. Therefore biopsy of an apparent enchondroma should not be performed routinely for histologic differentiation. Magnetic resonance imaging (MRI) criteria for benign versus malignant includes lack of a soft tissue mass and no surrounding T2 high-signal edema in benign enchondromas.

Because an enchondroma can histologically mimic a chondrosarcoma, painless chondroid lesions of all types should not be routinely biopsied. Most tumor surgeons prefer to watch them with serial imaging (every 6 months to a year) and close clinical supervision. Radiologists should refrain from putting in a formal report “cannot exclude chondrosarcoma” when looking at a benign appearing chondroid lesion, even though that is technically correct. The problem is there are few surgeons who will not biopsy a lesion if the dictation mentions chondrosarcoma as a possibility. I have seen multiple examples of radical surgery for benign cartilaginous lesions because a biopsy was incorrectly interpreted as chondrosarcoma (including a forequarter shoulder amputation). The correct dictation for an enchondroma should simply say “benign appearing chondroid lesion with no aggressive features noted”—no differential diagnosis should be given.

Multiple enchondromas occur on occasion, and this condition has been termed Ollier disease ( Fig. 2.10A ). It is not hereditary and does not have an increased rate of malignant degeneration. Older books say that Ollier disease has a high rate of malignant degeneration; that is because any chondroid lesion can look “malignant” when biopsied and needs to be correlated radiographically and clinically. The presence of multiple enchondromas associated with soft tissue hemangiomas is known as Maffucci syndrome ( Fig. 2.10B ). This syndrome also is not hereditary; however, it is characterized by an increased incidence of malignant degeneration of the enchondromas. Mafucci syndrome is quite rare, whereas Ollier disease is not.

Discriminators:

• 1.

  Must have calcification (except in phalanges).

• 2.

  No periostitis.

Eosinophilic Granuloma

Eosinophilic granuloma (EG) is a form of histiocytosis X, the other forms being Letterer–Siwe disease and Hand–Schüller–Christian disease. Although these forms may be merely different phases of the same disease, most investigators categorize them separately. The bony manifestations of all three disorders are similar and are discussed in this text simply as eosinophilic granuloma, or EG.

EG, unfortunately for radiologists, has many appearances. It can be lytic or blastic, it may be well-defined or ill-defined ( Figs. 2.11 and 2.12 ), it may or may not have a sclerotic border, and it may or may not elicit a periosteal response. The periostitis, when present, is typically benign in appearance (thick, uniform, wavy) but can be lamellated or amorphous. EG can mimic Ewing sarcoma and present as a permeative (multiple small holes) lesion.

How, then, can one distinguish EG from any of the other lytic lesions in this differential? Let me say right out that it is difficult to exclude EG from almost any differential of a bony lesion. Although some authorities say that up to 20% of EG occurs in patients over the age of 30, others claim that it is rare in older age groups. I have seen only one or two cases of EG in someone over the age of 30 and have seen at least 100 cases in children and young adults. Therefore I find the 30-year cutoff point quite useful and am willing to exclude the diagnosis of EG in anyone over the age of 30. (I accept the fact that I will miss the diagnosis in older age groups, but I hate to clutter up every differential with EG.)

EG is most often monostotic, but it can be polyostotic and thus has to be included whenever multiple lesions are present.

EG may or may not have a soft tissue mass associated with it, so the presence or absence of a soft tissue mass will not help in the differential diagnosis. In fact, I know of no entity in which presence or absence of an associated soft tissue mass will warrant inclusion or exclusion of a processs from a differential. It is important to note the presence of a soft tissue mass (or its absence), but it will do little to narrow your differential diagnosis.

EG occasionally has a bony sequestrum ( Fig. 2.13 ). Only three other entities have been described that, on occasion, have bone sequestra: osteomyelitis, lymphoma, and fibrosarcoma. Therefore when a sequestrum is identified, these four entities should be considered ( Table 2.2 ). (Another entity, osteoid osteoma, can sometimes have an appearance that mimics a sequestrum—this is discussed in Chapter 8 , Miscellaneous Conditions.)

TABLE 2.2

Entities that can present with a sequestration

Osteomyelitis

Eosinophilic granuloma

Malignant fibrous histiocytoma (includes fibrosarcoma and desmoids)

Lymphoma

(Osteoid osteoma can mimic a sequestration)

Clinically, EG may or may not be associated with pain; therefore clinical history is noncontributory for the most part.

Discriminator:

Must be under age 30.

Giant Cell Tumor

Giant cell tumor (GCT) is an uncommon tumor found almost exclusively in adults in the ends of long bones and in flat bones.

It is important to realize that one is unable to tell, regardless of its radiographic appearance, if a GCT is benign or malignant. In fact, histologically, a GCT cannot be reliably divided into either a benign or a malignant category. Most surgeons curettage and pack the lesions and consider them benign unless they recur. Even then, they can still be benign and recur a second or third time. About 15% of GCTs are said to be malignant based on their recurrence rate. Only rarely will GCTs metastasize. They metastasize to the lungs and then should be characterized as malignant.

I use four radiographic criteria for diagnosing GCTs ( Figs. 2.14 and 2.15 ). If any of these criteria are not met when looking at a lesion, I discard GCT from my differential diagnosis.

Number one: GCT occurs only in patients with closed epiphyses; this is valid at least 98% to 99% of the time and is extremely useful. I will not entertain the diagnosis of GCT in a patient with open epiphyses.

Number two: The lesion must be epiphyseal and abut the articular surface. There is disagreement over whether GCTs begin in the epiphysis or metaphysis, or from the physeal plate itself; however, except for rare cases, when radiologists see the lesions, they are epiphyseal and are flush against the articular surface. The metaphysis also has some of the tumor in it because the lesions are generally very large. I’m not terribly interested in the embryogenesis of a lesion when I’m looking at it as an unknown and a handful of surgeons are breathing down my neck wanting a differential diagnosis. I (and they) want to be able to intelligently say what it is or is not. Therefore I don’t get caught up in the argument of where the lesion began. When you see a GCT, it will be epiphyseal. Perhaps more important, it should be flush against the articular surface of the joint. This occurs in 98% to 99% of GCTs; therefore if I have a lesion that is separated from the articular surface by a definite margin of bone, I will not include GCT in the diagnosis.

Number three: These lesions are said to be eccentrically located in the bone, as opposed to being centrally placed in the medullary cavity. I don’t find this to be a terribly helpful description, but it is one of the classic “rules” of a GCT. It is accurate; however, occasionally the lesion is so large that it’s difficult to tell whether or not it is really eccentric.

Number four: The lesion must have a sharply defined zone of transition (border) that is not sclerotic. This is a very helpful finding in GCT. The only places this does not apply is in flat bones, such as the pelvis, and the calcaneus.

Using these four “rules” will allow one to eliminate GCT from a list of differential possibilities with accuracy and assurance when otherwise it would have to be included. Unwarranted inclusion of a lesion in a differential tends to clutter up the list and make it unnecessarily long.

It is important to realize that the four criteria for a GCT apply only to GCTs and to no other lesion. For instance, I know of no other lesion that is dependent on whether the epiphyses are open or closed. No other lesion in any of my lists can be defined by whether or not the zone of transition is sclerotic (many lesions, such as nonossifying fibromas, will usually have a sclerotic margin, but it doesn’t occur enough to include as a differential point). No other lesion must always abut the articular surface, and no other lesion has the classic description of always being eccentrically placed (although several lesions, including nonossifying fibroma and chondromyxoid fibroma, are, in fact, eccentric in greater than 98% of cases).

So although these four criteria work nicely for GCT, they don’t work at all for any other lesions. Residents have a tendency to apply these criteria to every lytic lesion encountered for the simple reason that they’ve learned the four criteria. Once one of the criteria is violated, the remainder don’t even have to be used to eliminate a GCT. For instance, if a lytic lesion is found in the middiaphysis of a bone, GCT can be excluded. There’s no need to check further to see if it’s eccentric, if it has a nonsclerotic margin, or if the epiphyses are closed.

Again, these “rules” will be greater than 95% effective and, in my experience, close to 99% effective, but only in long bones. Flat bones such as the pelvis and the calcaneus are exceptions. If one or two cases are found that don’t fit the criteria, another pathologist should probably review the slides. Many pathologists refer to aneurysmal bone cysts (ABCs) as GCTs; hence they have “GCTs” that don’t obey any of the criteria. These pathologists may be correct, but they are not in the mainstream of what most people use for GCT criteria, both radiographically and histologically.

Discriminators: (long bones only)

• 1.

  Epiphyses must be closed.

• 2.

  Must abut the articular surface.

• 3.

  Must be well defined with a nonsclerotic margin.

• 4.

  Must be eccentric.