Macroscopically, HPC is a firm and nodular tumor attached to the meninges with a clear margin to the cortical surface. It cannot be distinguished from meningioma, except for a high bleeding tendency. Microscopically, a characteristic staghorn pattern of thin-walled vessels, which is the hallmark of HPC tumors, is observed (Middleton et al. 1998). Tumor cells are uniform and polygonal to spindle shaped, often with vesicular nuclei. Reticulin staining reveals a typical pattern of fine fibers surrounding the individual tumor cells (Middleton et al. 1998). HPCs, like meningiomas, are positive for vimentin and CD34. Epithelial membrane antigen (EMA) is generally negative, but focal reactivity for EMA is sometimes found in HPC (Rajaram et al. 2004). Immunohistochemical expression of the vascular endothelial growth factor receptor (VEGFR) has been reported in HPC tumor cells and in the endothelium, with overexpression of vascular growth factors and receptors such as VEGFR1 and VEGFR2 in the endothelium (Hatva et al. 1996). A study by Dietzmann et al. (1997) showed that HPCs frequently overexpress the platelet-derived growth factor receptor (PDGFR), which is detectable by immunohistochemistry.

On computed tomography (CT) imaging, hyperostosis or calcification, which are frequently observed in meningiomas, are seldom observed in HPCs (Chiechi et al. 1996; Barba et al. 2001). Instead, bony erosion is often found (Chiechi et al. 1996).

In magnetic resonance (MR) studies, HPCs are typically isointense to grey matter on T1- and T2-weighted images (Sibtain et al. 2007). The intratumoral flow voids suggest high vascularity, which is sometimes found in meningiomas but is more frequent in HPCs (Chiechi et al. 1996; Sibtain et al. 2007).

On angiography, some angioarchitectural patterns have been shown to be common in HPCs and are useful for distinguishing them from meningiomas [19]. HPC is supplied from the internal carotid artery (ICA) or vertebral artery (VA), and external carotid arteries (ECA), with dominant supply from the ICA branches rather than ECA, which is commonly seen in meningiomas. There are many intratumoral, irregular, corkscrew vessels arising from a main feeder; intense fluffy tumor staining rather than the sunburst pattern observed with meningiomas; no early draining veins; and a prolonged tumor circulation time (Marc et al. 1975; Sibtain et al. 2007).

HPCs have a peculiar biological behavior and prognosis with local aggressiveness, a high rate of recurrence, and a propensity to metastasize to numerous extracranial locations.

Rutkowski et al. (2010) analyzed 277 patients with HPC and reported an overall median survival of 13 years, with 1-, 5-, 10-, and 20-year survival rates of 95, 82, 60, and 23%, respectively. Schiariti et al. (2011) reported overall survival rates of 93, 67, 45, and 23% at 5, 10, 15, and 20 years, respectively. Ecker et al. (2003) reported that the 5-year Kaplan–Meier survival rate among patients treated since 1990 was 93%. The 5-year disease-free survival rate was 89%.

For local control, which is influenced by treatment strategy, the 5-, 10-, and 15-year recurrence rates after surgery without radiation have been reported to be 54, 92, and 100%, respectively (Schiariti et al. 2011).

Many patients develop distant metastases. Because of the small number of cases, the rate of metastasis varies. Though some authors have reported around 20% metastasis (Rutkowski et al. 2012; Olson et al. 2010; Schiariti et al. 2011), Soyuer et al. (2004) reported 5-, 10-, and 15-year distant metastasis-free survival rates of 80, 46, and 21%, respectively. Adjuvant radiation seems to decrease local recurrence, as is discussed later; however, it has no effect on the incidence of metastasis (Dufour et al. 2001; Rutkowski et al. 2012; Schiariti et al. 2011). Distant metastasis was correlated with shorter survival (Kano et al. 2008). Until date, there has been no report of an effective prophylaxis for preventing distant metastasis. The incidence of recurrence or metastasis increases with duration of follow-up (Olson et al. 2010), emphasizing the necessity of long-term follow-up.

HPC usually has multiple feeders and plenty of intratumoral vascular networks, and thus, presurgical embolization is required. In general, feeder occlusion and intratumoral embolization must be differentiated. Proximal feeder occlusion by detachable coils or concentrated glue is generally safe and easy, especially in ECA feeders. However, proximal feeder occlusion of ECA feeders, which is sometimes useful in meningioma surgery, has little effect on decreasing intraoperative blood loss in cases where cortical (ICA) or vertebral–basilar artery (VA-BA) feeders dominate supply to the tumor. A combination of embolization of intracranial feeders (ICA, VA-BA) and surgical detachment of ECA feeders might be useful, although embolization of intracranial feeders carries greater risks than that for ECA feeders. Ideally, embolization of the tumor bed by particles or liquid materials is required to decrease intraoperative blood loss. However, in cases of hemangioblastomas, which are also rich in vascularity and sometimes contain arteriovenous (AV) shunt-like intratumoral vessels, tumor bleeding after particle embolization has been reported (Cornelius et al. 2007). As in our case 1 (Fig. 4.1), almost complete embolization can make surgical resection safer and more comfortable and successful. However, tumor bleeding and swelling that worsens the preexisting mass effect of the tumor could sometimes lead to a disastrous situation requiring emergent decompression.

Thus, presurgical embolization can be considered in the case of proximal occlusion of a feeding artery that cannot be accessed in the early stages of surgery or tumor bed embolization can be achieved using liquid formulations such as n-butyl cyanoacrylate (NBCA) or Onyx which rarely induces recanalization after embolization. And care should be taken not to avoid pressure injection of contrast materials or embolic agents. Preparations should also be made for emergency situations requiring surgery for worsening peritumoral edema or tumor bleeding after embolization.

Surgery represents the mainstay treatment for HPC, especially in the initial setting. It not only offers immediate alleviation of the mass effect but also allows pathological confirmation, thus facilitating differential diagnosis of HPCs from meningiomas.

Most importantly, the greater the extent of resection, the better the prognosis. In a previous study, local control rates [5-year local control rates for patients treated with gross total removal (GTR) and subtotal removal (STR)] were 84 and 38%, respectively (P = 0.003) (Soyuer et al. 2004). GTR was also associated with increased overall survival (log-rank, P < 0.05) (Rutkowski et al. 2012) compared with STR (Rutkowski et al. 2010). Although some reports have not reported any benefit from radiation (Rutkowski et al. 2012), consistent benefits have been reported for resection with GTR (Schiariti et al. 2011).

As mentioned previously, obtaining a definitive preoperative diagnosis from radiologic findings alone is sometimes difficult as discriminating meningioma from HPC is not easy. Thus, in practice, the strategy for initial surgery is based on that for meningioma. However, we have to bear in mind that HPC is richer in vascularity and is fed by ICA or VA-BA in addition to ECA; furthermore, it has a higher rate of local recurrence than meningioma. HPC surgery is therefore sometimes more challenging than meningioma surgery. The surgical approach chosen must ensure both a wider operative field and easy accessibility to the majority of feeders. In meningioma surgery, the shortest and most direct approach to the point of tumor attachment should be the priority.