CHAPTER 206 Skull Tumors and Fibrous Dysplasia
Scalp and skull masses are often initially evaluated because of their abnormal cosmetic appearance or discovery of a symptomatic mass by a child’s parents. Probably for the reason that these lesions are easily accessible, a wide variety of physicians (including general surgeons, dermatologists, plastic surgeons, and neurosurgeons) can manage these lesions. However, because such a disparate array of physicians has been reporting these cases, the range of initial symptoms reported has also varied widely; for example, plastic surgeons concentrate on the aspect of cosmetically deforming lesions, whereas dermatologists are inclined to focus on skin structures. Neurosurgeons have tended to draw referrals that are more likely to be associated with midline abnormalities or intracranial involvement.
Because such disparate disciplines have been reporting the clinical series, epidemiologic studies have varied considerably in the incidence of the different types of lesions. Variations are noted in the clinical findings associated with the lesion, location, proximity to the skull base, multiplicity of lesions, and degree of intracranial involvement. Therefore, reports on the incidence and types of scalp and skull masses may differ not only because of the focus of a particular institution but also because of the specialty of the physicians collecting the series (Table 206-1).
Approximately 50% to 60% of all scalp and skull lesions found in pediatrics are classified as either dermoids or epidermoids (see Table 206-1).1,2 These lesions involve the calvarial bone (especially the outer table), and in some cases the inner table is eroded and the outer leaf of the dura is involved. Dermoids occur more frequently, at a range of 15% to 58%, whereas epidermoids are noted in 3% to 5% of patients. Pathologic analysis differentiates strictly between the two lesions (Fig. 206-1). Epidermoids are composed of only keratin-producing epithelial cells and are characterized by cystic contents with white cheesy keratin debris. Dermoid cysts, in contrast, have walls that are more complex, including the cutaneous adnexal cells of the lower layers of the skin combined with sebaceous and sweat secretions produced by exocrine glands.
FIGURE 206-1 The epidermoid (A) lining is made of squamous epithelium, with the inner cyst being composed of keratin debris, which clinically appears to be cheesy and is also referred to as a pearly tumor. In contrast, the lining of a dermoid cyst (B) contains cutaneous adnexa and in this figure shows a corrugated pattern with more complex epithelium, but it may also contain a variety of keratin debris, along with sebaceous secretions from the skin adnexa.
Although inclusion cysts may be found anywhere in the scalp, dermoid cysts develop more frequently along the midline in the area covered by the anterior and posterior fontanelles, and epidermoids tend to occur laterally at suture sites between the pterion and the asterion along the squamosal suture.3
The etiology of these inclusion cysts appears to be misdirected epithelial and dermal tissue originating from the scalp that embeds itself downward into the skull. Cyst size may range from less than 1 cm up to 4 or 5 cm. These lesions tend to be immobile and rubbery, and sinus tracts might extend intracranially through the dura.
On plain radiographs, a lucent center is identifiable and accompanied by an eroding outer table or by involvement of the full thickness of the skull and a rim of hypersclerosis. Computed tomography (CT) will confirm that the lesion is extradural by displaying evidence of bone erosion. Magnetic resonance imaging (MRI) often shows the cyst to have a hypodense or hyperdense core, and the cyst wall may also enhance (Fig. 206-2).4,5
FIGURE 206-2 On magnetic resonances images in the sagittal (A) and coronal (B) planes, a midline dermoid is identified on T1 weighting, with the signal being hypointense in comparison to brain tissue.
In managing these lesions surgically, a curvilinear incision around the lesion is preferable to an incision on the vertex of the lesion. A curvilinear incision minimizes the risk of incising into the cyst and thereby contaminating the surgical field. Ideally, the cyst should be excised without rupturing it. Although separating the cyst from the surrounding subcutaneous tissue and bone is usually quite simple, a small fibrous or vascular pedicle arising from the suture may require curettage or cauterization. These lesions can occasionally be manifested as a dumbbell configuration affixed to the underlying dura. If the bone defect is very large, reconstruction with split-thickness cranial bone may be necessary. Sharp and scalloped bony margins should be smoothed with a bur, drill, or curet. Incomplete removal of these lesions may lead to recurrence.6–8
Historically, this entity has been referred to variously as histocytosis X or eosinophilic granuloma; Hand-Schüller-Christian disease and Letterer-Siwe disease are sometimes classified as systemic forms of histiocytosis related to Langerhans cell histiocytosis. Accounting for 7% to 10% of all reported skull lesions, this entity is a non-neoplastic lesion believed to be an inflammatory response to an unknown stimulus.9 It may be a solitary lesion classically referred to as an eosinophilic granuloma, or it may extend to multiple lesions, especially in children younger than 2 years. In its systemic manifestations, the process can disseminate into the central nervous system and through the viscera, thereby generating skin lesions, pulmonary infiltrates, hepatosplenomegaly, pancytopenia, and lymphadenopathy. An intermediate chronic form of this entity features lesions involving the calvaria in the granulomatous process, as well as exophthalmos and diabetes insipidus (obviously affecting the neurohypophysial structures). A recent study has associated degenerative central nervous system histiocytosis with this chronic process.10
The lesions are characterized by the presence of Langerhans cells, which are immunoreactive to cluster differentiating protein (CDIa) and S-100 protein. Tinctorial histologic stains reveal a mixture of pleomorphic histocytes, eosinophils, lymphocytes, and plasma cells.
Skull radiographs show an osteolytic lesion with unequal involvement of the outer and inner tables, accompanied by a hyperostotic or sclerotic rim (Fig. 206-3A). CT demonstrates this expansile lesion to best advantage, especially with reconstructed bone formats. Even though the lesion usually enhances moderately, it can have a hemorrhagic core that is hyperdense on non–contrast-enhanced images. Although MRI will confirm the aforementioned findings in soft tissue, it defines the bone changes less clearly than CT does (Fig. 206-3B).
FIGURE 206-3 A, Plain skull radiograph showing the punched-out appearance of eosinophilic granuloma and the margins lacking in sclerosis. This is further imaged on a computed tomography scan (B), where greater involvement of outer table than the inner table is appreciated.
Management of these lesions involves wide surgical resection to normal bone margins (Fig. 206-4). The skull lesions can be attached to the outer leaf of the dura or to subcutaneous tissue, from which the lesion can usually be safely removed by curettage. These friable lesions are quite vascular and expansile within the outer and inner tables of the calvarial bone. Despite occasional location over the venous sinuses, vascular invasion is not seen in radiologic studies of these lesions. Persistence or recurrence of these lesions may require further treatment, including low-dose chemotherapy or, rarely, low-dose radiation therapy.
FIGURE 206-4 A, Intraoperative photograph demonstrating the appearance of an eosinophilic granuloma before surgical resection but after the scalp has been peeled back. B, The surgeon is now peeling the lesion away from the skull, where a clear border is identified between the granulomatous tissue and the lytic skull margin.
Hemangiomas account for approximately 3% to 20% of reported skull and scalp lesions. They are classified into capillary and cavernous forms. An unusual osseous form that has been noted in adults is rarely seen in children. Hemangiomas tend to occur in the midline but may arise in the diploic spaces, where they extend outward and rarely involve the inner table of the skull, unless their blood supply originates from the dura.11
These lesions may initially be noted primarily as a simple cosmetic deformity or secondarily after trauma as a source of bleeding (Fig. 206-5). In some cases, compression, necrosis, and infection have been identified. In older children, hemangiomas have been accompanied by focal headaches, presumably caused by stretching of the surrounding pericranium.
FIGURE 206-5 This photograph of a child shows the midline location of an anterior fontanelle scalp hemangioma, with the appearance of varices coming through in a very superficial fashion on the outer layer of the scalp.
Scalp hemangiomas are best imaged with MRI, which should confirm that the lesions are confined extradurally, especially when they are located in the area of the anterior fontanelle (Fig. 206-6). Either plain radiographs or CT scans provide the clearest images of the bony form of these lesions with sunburst-radiating trabeculae. The vertical orientation of this trabecular honeycombed arrangement of the skull is especially well visualized on tangential cross sections (Fig. 206-7).
FIGURE 206-7 This histologic section of a scalp hemangioma demonstrates the vascular lining of this malformation. The lack of muscularis in this vascular lesion suggests that it is dominantly a venous abnormality, and its ability to extend and compress is readily appreciated.
Treatment requires en bloc circumferential excision to margins of normal bone and pericranium. This surgical approach not only minimizes blood loss but also prevents recurrence of the lesion. When the lesion involves the superficial aspect of the scalp, the operation may require excising the involved skin area and reapproximating the skin margins. Hemangiomas occurring in the scalp alone frequently involute spontaneously, and plastic surgical consultation in such cases is often helpful.
A wide spectrum of congenital cranial dysraphic abnormalities (from overt encephalocele to simple cutis aplasia, also known as a sequestered/occult meningocele) can be manifested as scalp lesions. Retention of the heterotopic brain tissue associated with these lesions and even direct central nervous system connections may occur to varying degrees.
Cutis aplasia tends to be found over the cranial vertex along the sutural lines (Fig. 206-8). It may vary from less than 1 cm in size to very large scalp lesions (>10 cm). The resulting scalp defect may include the absence of bone formation and even a dural defect. Although smaller lesions (<3 cm) will spontaneously re-epithelialize, larger lesions (especially those with bone and dural defects) will require surgical reconstruction. Involvement of a plastic surgeon for skin rotation flaps, skin grafts, or tissue expanders is highly recommended for these larger lesions.
FIGURE 206-8 In this photograph, the scalp has been shaved and an area of epithelial abnormality over an occult meningocele is appreciated. In the unshaven state there was minimal hair directly over this abnormal epithelium. Adjacent to this area were heterotopic meninges without glial structures.