Sonographic diagnosis of intraosseous epidermoid cysts of the calvarium

Abstract

While dermoids and epidermoid cysts represent the most commonly occurring cutaneous lesions in children, intraosseous epidermoid cysts of the calvarium are rare. They are thought to be derived from a proliferation of abnormally situated ectodermal cells and, while benign, have the potential to cause significant erosive damage to the calvarium.1 Ultrasound is often the initial imaging modality for the investigation of soft tissue masses in children. This case report follows three cases of intraosseous epidermoid cysts of the skull and highlights the important role ultrasound plays in the investigation of a non-painful soft tissue mass of the scalp. Ultrasound can provide valuable information on location, perfusion and the extent of calvarium erosion, this information can help facilitate clinical management and timing of any surgical intervention. The following cases report the embryology, aetiology and ultrasound characteristics of intraosseous epidermoid cysts of the calvarium. An 8-year-old-male presented for an ultrasound examination of a subcutaneous lesion on his left temple. A discussion with the child and his mother revealed that the lump had appeared about 12 months prior to presentation and had slowly increased in size. There had been no trauma to the region of interest, and the lump was not and had never been painful. The mother, a nurse, felt it was a superficial lymph node and had vague memories of a lump in a similar location during his infancy. She was not worried about it until a recent family cancer scare. The child was clinically well, and on clinical examination there was a palpable soft spongy lesion over the left temporal bone with no skin discoloration. Imaging was performed on a Philips iU22 ultrasound machine, a MSK superficial preset, and a linear 17-5 MHz transducer (Philips Healthcare, Bothell, WA, USA). The region of interest was evaluated in longitudinal and transverse planes using B-mode, power and colour Doppler imaging. Ultrasound demonstrated a large well-circumscribed soft tissue mass in the left temple region. It was a bilobar solid lesion with complex echogenicity and deep extension that indented and disrupted the inner table of the left temporal bone. The lesion measured 16 × 15 × 27 mm. The bony margin appeared discontinuous and irregular. While the mass protruded intracranially through the bony defect, it appeared to remain separate from intracranial contents and to be contained by the fragmented pieces of the temporal bone (Figure 1). Colour Doppler and power Doppler imaging demonstrated no vascularity within the lesion (Figure 2). In light of the patient's age and clinical presentation, the ultrasound images resulted in a primary differential diagnosis of Langerhans cell histiocytosis. Other differential diagnoses in the paediatric age group would include osteomyelitis, Ewing's sarcoma, intraosseous epidermoid cysts and metastatic neuroblastoma. Further imaging studies were performed which included a skull X-ray and a nuclear medicine whole body bone scan to assess skeletal involvement and a magnetic resonance imaging brain scan to determine the extent of bone marrow involvement. The skull X-ray showed a well-defined osteolytic lesion with sclerotic margins (Figure 3), the bone scan showed no additional bony involvement and the magnetic resonance imaging demonstrated mild displacement of the underlying dura and cortex but no infiltration of the cerebral parenchyma. Blood tests were unremarkable with no features suggestive of inflammation or infection. The child was managed by the oncology department, and once the lesion was confirmed as solitary, surgical excision was chosen as the best treatment. Microscopic histopathology reported a proliferation of laminated keratin surrounded by stratified squamous epithelium with lyosis of the underlying bone. These findings were not consistent with a diagnosis of Langerhans cell histiocytosis but instead intraosseous epidermoid cysts. The patient had a good post-surgical recovery, and a follow-up bone scan showed no recurrence and good bony regeneration at the surgical defect in the left temporal bone. A 2-year-old female presented for an ultrasound examination of a subcutaneous lesion on the left temple. The child was clinically well with no history of trauma. The patient's mother had noticed the lump two weeks prior while washing her hair. It had not changed in size and did not appear painful. Clinical examination revealed a discrete firm mobile lesion in superficial tissues of left temporal bone. Initial imaging was performed on a Philips iU22 ultrasound machine, a MSK superficial preset, and a linear 17-5 MHz transducer (Philips Healthcare, Bothel, WA, USA). The follow-up examinations were performed on a Toshiba Aplio 500 ultrasound machine, a MSK preset and a linear 18 L7 transducer (Toshiba Medical Systems, Otawara, Tochigi, Japan). The region of interest was evaluated in longitudinal and transverse planes using B-mode, power and colour Doppler imaging. Ultrasound demonstrated a well-circumscribed soft tissue mass in the left temple region. The lesion was solid with indentation of the left temporal bone cortex. The inner table appeared scalloped but intact. There was convexity of the outer table over the lesion (Figure 4). The lesion measured 12 × 7 × 10 mm. Colour Doppler imaging demonstrated no vascularity within the lesion. In this case as the lesion appeared to be within the diploic space of the temporal bone, a primary differential diagnosis of intraosseous epidermoid cyst was reported. In light of the child's age, no further imaging was performed. The child was referred to a paediatric neurosurgeon where the normal pattern of progression of these lesions was explained to the parents. Surgical excision was recommended as the best treatment but as the lesion was on the child's face the parents chose to monitor the lesion growth instead. Serial ultrasounds at six monthly intervals were organised to determine if the lesion would become destructive. Serial ultrasounds showed continued growth but no bone destruction for the next 18 months. At the 2-year follow-up screening ultrasound, bony destruction of the inner table of the temporal bone was demonstrated but the lesion did not appear to protrude intracranially (Figure 5). The parents again declined surgery and opted to continue with six monthly serial screening studies. At the next ultrasound, the lesion had continued to grow now measuring 19 × 7 × 17 mm, and the bony destruction had worsened. A skull X-ray was performed which demonstrated an oval-shaped lytic bone lesion with sclerotic margins on the left temporal bone (Figure 6). The child, now 4.5 years of age, underwent surgical excision of the intraosseous epidermoid cyst which was confirmed histologically. She had an uneventful post-surgical recovery and has been lost to follow-up. A 6-week-old female presented with two lesions on the left parietal region of the skull for investigation. The patient had been delivered by an uncomplicated vaginal delivery and neither forceps nor vacuum extraction had been required. Clinical examination determined two discrete firm lesions several centimetres apart overlying the left parietal bone. Imaging was performed using a Toshiba Aplio 500 ultrasound machine, a MSK preset and a linear 18 L7 transducer (Toshiba Medical Systems, Otawara, Tochigi, Japan). The region of interest was evaluated in longitudinal and transverse planes using B-mode, power and colour Doppler imaging. Ultrasound demonstrated two well-circumscribed hypoechoic lesions in the subcutaneous tissues which both indented the bony cortex. The inner table of the parietal bone was reported to be scalloped but intact (Figure 7). Both lesions were in close proximity to sutures and each other, lesion one was near the sagittal suture and measured 9 × 4 × 10 mm. Lesion two was near the left lambdoid suture and measured 12 × 3 × 14 mm (Figure 8). Neither lesion demonstrated vascularity under colour or power Doppler investigation. A diagnosis of intraosseous epidermoid cysts was made. No further imaging was performed. The patient was referred on to a paediatric neurosurgeon. Surgical excision was recommended as the best treatment, and this was performed at 12 weeks of age. Histology confirmed that both lesions were intraosseous epidermoid cysts. The infant has had an uneventful recovery. Dermoid and epidermoid cysts in children are benign developmental lesions arising from abnormally situated embryonal cells. Often referred to in the literature as congenital inclusion cysts, they are thought to occur when the surface ectodermal tissue fails to separate from the underlying structures and is included within the fusion of branchial arches.2 This occurs between the third and fifth week of gestation as the neural groove closes.3, 4 Dermoid cysts contain both ectoderm and mesoderm tissue; epidermoid cysts contain ectodermal tissue only. Differentiation is not possible via ultrasound. They can occur subcutaneously anywhere along the branchial arches. Their growth is usually considered slow and is the result of a progressive accumulation of debris within the cyst, thus explaining the solid appearances under ultrasound. Management of subcutaneous lesions is often conservative as 49% will undergo forms of regression.5 There is no gender bias.3 All three of our cases were histologically proven to be intraosseous epidermoid cysts. Intraosseous (intradiploic) epidermoids of the calvarium are a rare subcategory of epidermoid cysts. Arana et al. (1996) state that epidermoid cysts account for less than 1% of all cranial tumours, 25% of those being intradiploic cysts. In this situation the lesion develops within the diploic space of the calvarium. The calvarium or skull is composed of the flat bones, frontal, parietal and occipital and the squamous portion of the temporal bones. The diploic space of the calvarium is situated between the two layers of cortical bone, the inner and outer tables and only exists in flat bones. The diploe is composed of bone marrow, blood vessels and cancellous bone. As in our first two cases when an epidermoid lesion expands within the diploic space, it erodes both the inner and outer tables of the bone and may expand intracranially.4, 6, 7 Our clinical presentations were typical with painless semi firm to firm mobile lumps although late presenters (adults) may report tenderness and headaches. Ultrasonically they appear as a well-circumscribed, thin walled lesion which is hypoechoic to surrounding subcutaneous soft tissue and usually have no vascularity unless inflamed. In small or non-erosive lesions, the inner and outer layers of bone cortex should be able to be identified thus helping with diagnosis. This was evident in the last two of our cases1, 5, 8, 9 (Figure 7). All three of our patients ultimately had a cranioplasty with gross resection of their lesions. This is recommended for intraosseous lesions because of the erosive nature with the risk of osteolyosis, intracranial involvement and the potential, although rare, of malignant degeneration into squamous cell carcinoma.1, 3 Intraosseous epidermoid cysts, while benign, are a rare insidious lesion causing significant bony erosion if left untreated. While the literature suggests that definitive diagnosis can only be made histologically, ultrasound is an important first step in the imaging of painless lumps on the scalp. In children where the use of radiation should be limited, ultrasound is able to establish vital information on the location, perfusion and extent of erosion whereby aiding in timely surgical management. The clinical presentation and the ultrasound features, in particular the identification of a lesion between the inner and outer tables of the flat bones, significantly improve the differential diagnosis.