The series of calvarial lesions presented here shows a variety of different entities (Table 1). Their incidence correlates with the cases already published [8, 14, 17, 9]. Depending on the author, the lesions primarily originate from the bone are divided into three or four subcategories: primary neoplasia (in individual papers already divided into good and malignant primary neoplasia), secondary neoplasia and tumor-like lesions [17, 22].
Primary osseous neoplasia come from the bone and are distinguished into benign and malignant lesions. Primary malignant lesions such as chordomas or sarcomas are not represented in our series, since they are frequently located at the skull base. Because a complete resection is not always possible, it is advisable to perform at least one biopsy or extended removal to ensure the diagnosis and initiate adjuvant treatment [9, 10]. The most common benign lesions include meningiomas (Fig. 2a–d), osteomas (Fig. 2f) and intraosseous hemangiomas (Fig. 2e). Osteomas (Fig. 2f) grow slowly and are located on the tabula externa. The main complaints are the cosmetically disturbing swelling, less than the pain. If they are localized at the orbita, diplopia can occur [1]. Imaging shows a densely compact bone growth. No further treatment is necessary after a complete resection and there is an extremely low risk of recurrence [8, 9, 14]. Intraosseous hemangiomas (Fig. 2e) are described in the literature more in women. As long as they are limited to the bone, they are often an asymptomatic random result [15, 16, 18]. Further growth can cause painful swelling. A typical “sunburst” phenomenon is shown in the CT [18]. In the MRI a variable arterio-venous contrast enhancement is described [8]. Complete resection down to healthy bone is recommended to minimize the recurrence risk [8, 17, 18, 9, 14, 15]. Meningiomas (Fig. 2a–d) are among the most common benign intracranial tumors. However, about 2% of meningiomas grow ectopic in the bone and infiltrate the adjacent tissue only secondarily [25, 5]. Typical imaging findings are calcifications, homogeneous contrast enhancement, edema and a dural tail [8, 9, 14]. Our series includes different WHO grade of meningiomas, which also have significant different infiltrating growth. Adequate preoperative imaging is essential to minimize the intraoperative and postoperative risk for the patient. A complete resection is also recommended for meningiomas. If this is not possible, appropriate adjuvant therapy should be evaluated depending on the histology [5].
The most common entities of secondary neoplasia are metastasis (Fig. 3a–b) of different primary carcinomas. In most cases, they are asymptomatic and stand out as random findings in the regular staging examinations. Patients can develop pain when the periost is infiltrated. By intracranial growth, the patients can also develop neurological deficits. Typical of the imaging are irregular osteolysis with remodeling of the bone, a variable contrast enhancement, and signs of a co-reaction of the surrounding tissue, for example, a perifocal edema [8, 9, 14, 22]. A complete resection must be sought and subsequently, an adjuvant treatment should be started depending on the primary carcinoma [17, 13, 22]. The osteoclastic defects of a plasmacytoma (Fig. 3d) also fall into the category of secondary neoplasia. Typical are pure osteolysis without remodeling of the bone [8]. Surgical resection with cover of the bone defect is recommended. In the event of systemic infestation, further treatment of the disease is necessary. In the scenario of a solitary manifestation and complete surgical resection, close-meshed follow-ups are necessary. In this case, further treatment is controversial discussed in the literature [3, 11, 6, 19].
In the group of tumor-like lesions, there are, in particular, histiocytosis (Fig. 4a–b), especially eosinophilic granuloma, and metabolic disorders, such as fibrous dysplasia (Fig. 4c–e). Histiocytosis (Fig. 4a–b) is a group of diseases with an increased proliferation of macrophages. Typical imaging findings are osteolysis with singular bone fragments within the lesion, as a sign of irregular bone destruction. MRI shows a homogeneous contrast enhancement limited to the bone without a co-reaction of the dura or perifocal edema (Table 1) [8, 9, 20]. The diagnosis can only be secured histologically. After this, a staging should been performed. If the lesion is isolated at the skull, no further therapy is necessary, but for multifocal manifestations, chemotherapy and radiotherapy are recommended [2, 20]. Another known bone malformation is fibrous dysplasia (Fig. 4c–e), which is due to overactivity of osteoclasts and leads to disturbed bone differentiation [7]. The malformation can affect all bones of the body, including the skull. Patients often complain of severe pain and have swelling or fractures. A distinction is made between the monostotic form (Fig. 4c–d), which can be treated by surgical resection and covering the defect [4], from the polyostotic form (Fig. 4e), in which symptomatic treatment with analgesics and bisphosphonates takes place. In the polyostotic form, only partial removal makes sense if important structures are compressed and patients develop complaints [7]. A typical finding in the CT-scan are ground-glass opacities [8, 17, 4].
In our series, secondary the bone-infiltrating lesions also occurred, such as squamous cell carcinoma and malignant melanoma (Fig. 3c). These, too, have an osteolytic growth; depending on the stage of the disease, the infiltration can range from the skin to intracranial. Surgical resection and cover of the defect with subsequent systemic adjuvant therapy must be planned [24, 21].
Based on the imaging, suspicions of the underlying lesion can be obtained (Table 1 and Figs. 2–4). The limits of a lesion can suggest conclusions about the speed of growth: smooth edges and a homogeneous internal structure mark more of a benign, slowly growing process; irregular edges with inhomogeneous internal structure point, rather, to a malignant, fast-growing process. Also, slowly growing lesions are usually limited to the bone, while fast-growing show an infiltrating growth in adjacent tissue. With the administration of a contrast agent, further conclusions can be drawn about the dignity of the lesion. While reactive or scarring changes in the dura have a linear, homogeneous contrast enhancement, tumor infiltration is often characterized by irregular, inhomogeneous contrast enhancement. If the dura looks distant from the bone and, thus, the epidural space is visible, a dura infiltration is unlikely [8, 9, 14, 17, 12].
The management of patients with calvarial lesions should be interdisciplinary, as with other oncological diseases. The general practitioner is usually the first contact for the patient, as the lesions are often a random results and there are no complaints. To draw conclusions about the possible entity of the lesion, a targeted image is required [8, 9, 14, 17, 12]. The suspected diagnoses can be limited and the further procedure planned. In any case, an surgical extraction of a tissue sample is necessary to ensure the diagnosis. If properly planned, the operation is a low-risk procedure without significant morbidity and mortality [12, 17, 22]. Important imaging aspects are, in particular, the extension of possible infiltration and the course of the large blood vessels, especially intracranially of the sinus. Therefore, we recommend the implementation of a CT and MRI with and without contrast agent. Some lesions can have a similar extension on the bone, but differ significantly in their intracranial spreading (Fig. 2a–b vs 2c–d). The medical history, clinical complaints and differential diagnoses must be taken into account together to evaluate the urgency of histological examination. While an underlying carcinoma requires a timely confirmation of a possible metastasis to initiate a targeted adjuvant treatment, in the case of a purely intraosseous lesion with a homogeneous internal structure and without any co-reaction of the adjacent tissue and without any complaints, an initial follow-up observation is also acceptable and surgical care is not urgent. If an operation takes place, a complete resection with cover of the defect must be planned.