Introduction

Petroclival meningiomas (PCM) account for about 2% of all intracranial meningiomas and are more common in women (3: 1) [1—3]. In most cases, these benign tumors are characterized by slow growth. In some cases, even large neoplasms can cause only moderate neurological symptoms [2, 4—6]. Resection of PCM is desired, but not always achievable goal of neurosurgeon. Total resection of tumor is often impossible without neurological deficit de novo or exacerbation of preoperative disorders. This is due to deep localization of PCM, its dimensions and relation to surrounding neurovascular structures. The problem is aggravated by the fact that PCM, according to various authors, occurs more often in young employable people [1—5, 7].

Initial growth, classification

PCM originates from dura mater within the petroclival junction, the top of temporal pyramid and upper 2/3 of clivus with or without spread to cavernous sinus. Anterior border of the matrix of these tumors is the back of sella turcica, posterior border — anterior edge of internal auditory canal. Thus, the matrix of typical PCM is placed on the top of temporal pyramid and extends towards the clivus. The main difference of PCM from other meningiomas is medial localization of the matrix in relation to cranial nerves 5, 7 and 10—11 [2]. Considering usual benign nature and slow growth of PCM, these neoplasms reach large dimensions, overgrow and/or displace skull base vessels and cranial nerves and also cause brain stem compression.

There are about 25 different classifications of meningiomas of posterior cranial fossa including PCM [2, 4, 5].

Our compatriot I.Ya. Razdolsky suggested the first classification of meningiomas of posterior cranial fossa in 1936 [2]. Classifications of H. Cushing [8], F. Castellano [9] and others were proposed later. The term “petroclival meningiomas” itself was first proposed by the neurosurgeon M. Yasargil in 1980 [10]. In 1996, L. Sekhar et al. proposed the most comprehensive classification of PCF meningiomas. This system is still used in the American school of neurosurgeons. He divided PCF meningiomas into 6 groups depending on place of initial growth [11].

In 2005, Shimansky V.N. [2] reported a classification of skull base meningiomas of posterior cranial fossa. This system combines anatomical and neuroradiological data, reflects localization of tumor matrix and is valuable to determine optimal surgical approach. The classification includes:

1 – petroclival meningiomas (with predominant supra-, subtentorial or even spread of meningiomas in relation to cerebellar tentorium).

2 – meningiomas of large occipital foramen.

3 – meningiomas of posterior surface of temporal pyramid.

4 – meningiomas of jugular foramen.

This classification is currently used at the Burdenko Neurosurgery Center of the Ministry of Health of the Russian Federation.

In 2008, S. Ichimura et al. [12] divided PCM into 4 subgroups considering x-ray and intraoperative findings. PCMs were classified taking into account localization of matrix and trigeminal nerve deviation into meningiomas with spread towards upper part of the clivus, cavernous sinus, cerebellar tentorium and the apex of temporal pyramid. The authors found different neurological symptoms in patients with PCM from different subgroups. Moreover, they concluded that anterior transpetrosal approach could be used for resection of any tumor. It was also noted that anterior transpetrosal approach is advisable for tumors spreading towards upper part of the clivus and cerebellar tentorium.

Another classification of petroclival meningiomas is classification depending on their dimensions. This approach is based on measurement of meningiomas considering their clear boundaries with medulla and homogeneous structure. In 1990, Sekhar et al. [13] reported 4 groups of PCMs: small (up to 1 cm), medium (1—2.4 cm), large (2.5—4.4 cm) and giant (over 4,5 cm). This classification is successfully used at the present time [2, 14].

The volume of PCM is usually evaluated considering 3 largest dimensions in axial, sagittal and frontal planes to schedule surgery. Contrast-enhanced MRI is applied for these purposes [2]. Radiologists more often evaluate PCM volume in cubic centimeters. This is due to the need to calculate irradiation dose depending on tumor volume.

In our opinion, PCMs classification depending on severity of brain stem compression proposed by Pirayesh et al. in 2016 is important. Contrast-enhanced axial T1-weighted image with the greatest brain stem compression is selected. The next stage is brain stem contouring. The obtained oval is divided into 4 equal quadrants by intersecting lines. Tumor occupies less than 1 quadrant in case of mild brain stem compression and up to 2 quadrants in case of moderate compression. Severe brain stem compression is followed by involvement of more than 2 quadrants. Thus, 3 grades of brain stem compression are distinguished [1].

According to the latest WHO classification, the vast majority of PCM are benign (Grade I). Atypical PCMs (Grade II) are less common [15, 16].

Biology of meningiomas

Analysis of genetic features of meningiomas is currently ongoing although meningioma is one of the most studied human tumor from molecular-biological point of view. Mutation in neurofibromatosis II gene is found in more than 60% of all sporadic meningiomas in all groups according to the WHO classification. This gene is localized in the long arm of chromosome 22. In recent studies devoted to biological analysis of meningiomas, the authors emphasize neurofibromatosis II gene alteration or mutation in the genes AKT1, CMO, TRAF7, PIK3CA, KLF4, POLR2A, SUFU and SMARCB1 associated with Grade I meningiomas. Tumors Grade II—III are characterized by lost gene of neurofibromatosis II and uncommon mutations, although these neoplasms may be associated with larger mutational abnormalities. TERT gene mutations are more common in tumors with progression in their histological class, SMARCE1 gene alteration is characteristic of clear cell meningiomas and BAP1 is typical for rhabdoid meningiomas (Grade III). These mutations and alterations may be used for pharmacological inhibition of Grade II—III meningiomas. The authors conclude that further analysis of genetic and immunological features of meningiomas may be useful to supplement conventional surgery and radiotherapy with various drugs [17].

V.A. Byvaltsev et al. [18] analyzed current literature devoted to violation of intracellular signaling pathways as a leading factor in the development of meningiomas and formation of their genetic and molecular profiles. The authors conclude that meningeal tumors are characterized by various molecular genetic lesions. Tumor progression is associated with activation of various cellular signaling pathways. Meningiomas are characterized by impaired function of pRb/p53-, Hh- and WTN/β-catenin signaling pathways responsible for cell cycle regulation and apoptosis. The pathogenesis of meningiomas also includes cascades associated with various growth factor receptors (VEGF A, EGF, TGF, IGF, etc.). These cascades result activation of proliferation, invasion and neovascularization. Perhaps, genotyping of patients with meningiomas will be important for differentiated inclusion of patients into clinical trials of targeted drugs. Biological aspects of the development of meningiomas is very important part in the study of pathogenesis of these tumors and searching for targets drugs considering molecular genetic injuries. Thus, the main objective of modern genetic researches is development of molecular-biological model of meningioma. It is necessary for analysis of its genetic and immunological features with subsequent clinical application of these patterns.

Clinical symptoms

Clinical symptoms of PCMs include the main syndromes typical for tumors of posterior cranial fossa: cerebellar disorders, cerebral, brain stem symptoms, bulbar disorders, symptoms of cranial nerve injury (cranial nerves 3—11 as a rule), hypertensive hydrocephalic syndrome, pyramidal disorders. Severity of syndromes depends on several factors. These are volume and spread of the tumor. These factors determine compression and destruction of neurovascular structures of posterior cranial fossa [2].

In 2014, H. Morisako et al. [19] reported surgical treatment of 60 patients with petroclival meningiomas. The authors used the so-called “Petroclival meningioma impairment scale” to assess postoperative condition of patients after resection of tumor. This scale included 8 points. These are functional impairment of cranial nerves 5, 6, 7, 8, 9, 10 nerves, petrosal disorders and level of consciousness.

Condition of patient with PCM was assessed using Karnofsky scale. Various authors propose other scales including Glasgow coma scale, modified Rankin scale and SF-36 questionnaire [5, 19].

Diagnosis

Magnetic resonance imaging (MRI) is the main method for PCM visualization. There are several reports devoted to X-ray features of PCMs. In 2016, A. Pirayesh et al. [1] reported surgical treatment of 18 patients with PCM. The authors carefully studied X-ray features of PCM in each case: dimensions, the presence of calcifications, relationship of tumor edge and brain stem, the presence of “cerebrospinal fluid fissure” between tumor edge and brain matter, brain stem edema and compression, tumor signal intensity in T2-weighted images. Brain edema was the only predictor of long-term postoperative deterioration. Significant factors affecting total resection of the tumor were the absence of “cerebrospinal fluid fissure” between tumor edge and brainstem and “scalloped structure of tumor edge”. These characteristics indicated close adherence of PCM and pia mater. Radical resection of these tumors was possible only in 22% of cases. The authors proposed to classify these neoplasms as high risk tumors and initially schedule these patients for subtotal resection followed by radiotherapy.

Another diagnostic method for PCM is computed tomography (CT). CT can demonstrate destruction of bone structures besides petrificates in tumor stroma. These data may be useful to choose surgical approach and schedule quality of resection [20—23].

Cerebral angiography is the main method for assessing blood supply of petroclival meningiomas [24]. In contrast to cerebral angiography evaluating tumor vascularization, modern non-invasive techniques such as MR perfusion and CT perfusion quantify blood supply of tumor tissue [25, 26].

CT perfusion is relatively recently used in the diagnosis of skull base tumors and allows you to get additional unique information about hemodynamic features of tumor and brain tissue. These data significantly increase specificity of preoperative diagnosis of skull base tumors, in particular, petroclival meningiomas. In contrast to MR perfusion, CT reveals a linear relationship between tissue density and contrast agent concentration in the absence of the artifacts from the bones. Therefore, CT perfusion is more valuable method in assessing hemodynamics (volumetric blood flow in a tumor) and blood-brain barrier (microvascular permeability).

CT perfusion with structural and functional assessment of hemodynamic changes inside the tumor may be preferred for non-invasive assessment of pathological processes in the areas inaccessible for direct surgery. This is especially important for giant petroclival meningiomas, when the volume of surgical intervention is being determined. This technique may also be successfully used for differential diagnosis of benign and malignant lesions of skull base [27].

Great number of modern reports are devoted to positron emission tomography (PET-CT) in the diagnosis of meningiomas. Interesting trial was reported by Japanese authors who applied C-methionine and fluorodeoxyglucose PET-CT in 17 patients with recurrent or continued growth of meningiomas Grade 2—3. The authors conclude that C-methionine PET-CT is significantly more effective to diagnose these meningiomas [28].

F. Stade et al. [29] successfully used PET-CT Ga68 Dotatoc (radiopharmaceutical drug with Ga68 isotope) for planning radiotherapy in patients with meningiomas along with MRI.

In 2017, N. Galldiks et al. [30] analyzed the use of PET-CT with various radiopharmaceuticals in patients with intracranial meningiomas. The authors conclude that PET-CT may be successfully used for differential diagnosis of intracranial meningiomas, in particular, determination of post-radiation changes, invasion grade in the areas with complex anatomy (skull base) or in poorly contrasted areas (in case of bone invasion) for scheduling resection or radiotherapy [30].

There are also researches devoted to preoperative assessment of the density of intracranial meningiomas including PCM. Standard MRI can give only indicative information about density of meningiomas in T1W1 mode. Increased signal in this mode corresponds to milder tumors. CT may be only used to analyze the presence of petrificates in tumor stroma. This examination does not carry information about the density of the tumor. There are reports about rarer diagnostic method for analysis of tumor density. It is MR elastography [31]. This technique was described in 1995 and allows you to quantify tissue density. MR elastography is based on assessment of tissue response to harmonic mechanical excitation and able to record structural displacement of tissue with an accuracy of less than 200 nanometers. However, the use of this technique in neuroscience is still limited mainly by experimental works. In 2013, Murphy et al. demonstrated a fundamentally new quantitative approach to assessment of density of intracranial meningiomas. They found significant correlation of preoperative data (MR elastography) and intraoperative findings [32].

Surgical treatment

Development of surgery for petroclival meningiomas is inextricably associated with evolution of skull base surgery. Various surgical approaches including those accompanied by advanced resection of skull base structures were aimed to improve visualization of tumor and surrounding neurovascular structures. Some authors considered better visualization of these structures as a prerequisite for more radical and safe resection of tumor [33, 34]. Since the 70s of the XX century, mortality rate in PCM surgery has been reduced from 50% to less than 10% in recent studies. However, postoperative morbidity is still high despite the routine use of physiological monitoring of cranial nerves and evoked potentials [21, 33—38]. Many authors indicate that quality of resection of PCM largely depends on characteristics of tumor, blood supply, density and the presence of dissection plane between neurovascular structures of the skull base rather surgical approach [39—41].

In 1982, Konovalov A.N. et al. [42] reported outcomes of resection of 23 clival meningiomas including petroclival meningiomas. The authors used appropriate surgical access depending on tumor spread. Suboccipital retrosigmoid approach was used in 9 cases, subtemporal transtentorial — in 9 cases, combined approach — in 5 cases. Fatal outcome was observed in 1 case. The authors came to the conclusion that total resection of clival meningioma is usually possible in case of adequate surgical approach associated with microsurgical techniques.

Some authors are inclined to believe that relatively safe suboccipital retrosigmoid approach may be used instead of traumatic basal approaches [2, 39, 40, 41]. This method may be supplemented by intradural resection of the apex of petrosal part if it is necessary. The authors propose two-stage surgery in patients with significant supratentorial spread of the tumor. The first stage implies brain stem decompression through resection of adjacent part of meningioma via suboccipital retrosigmoid approach [42].

Most authors use the Simpson scale to assess quality of resection of meningiomas [43, 44]. This scale comprises of 5 groups. The first grade includes complete resection of meningioma with dura mater and underlying bone, the fifth grade — decompression with or without biopsy.

In 2016, D. Li et al. [45] reported surgical treatment of 199 patients with medium and large PCMs. Retrosigmoid suboccipital, transpetrosal, subtemporal or far lateral approaches were used depending on localization of tumor and surgeon's preferences. Total resection was achieved in 111 (55.8%) patients, subtotal — in 65 (32.7%), partial — in 23 (11.6%) patients. The most common complication was cranial nerve dysfunction (n=133, 66.8%). Mortality rate was 2%. Multivariate analysis showed that MR signs of brain stem edema and tumor dimension over 4 cm increase the likelihood of unfavorable prognosis (Karnofsky score <80). The authors conclude that surgical treatment of medium and large PCMs may be performed with good results and low mortality. However, overly aggressive tumor resection often results neurological deficit.

Shimansky V.N. [2] emphasizes postoperative impairment of Karnofsky score regardless quality of PCM resection. Uncomplicated course of postoperative period was observed in 68 patients (52.3%). There was no postoperative neurological deficit de novo and postoperative aggravation of preoperative symptoms. Complicated postoperative period was observed in 62 cases (47.7%). Mortality after PCM resection was 5.39% (7 out of 130 patients). Postoperative complications included early postoperative hemiparesis in 3 patients (6.9%) and delayed postoperative hemiparesis in 2 patients (2.3%), cerebellar disorders (0 and 1 (1.1%) patient, respectively), bulbar disorders in 7 (7.9%) and 6 (6.7%) patients, hearing impairment in 14 (15.7%) and 13 (14.6%) patients, facial paresis in 30 (33.7%) and 12 (13.5%) patients, trigeminal nerve lesion in 9 (10.1%) and 20 (22.5%) patients, oculomotor disorders in 15 (16.9%) and 3 (3.4%) patients. Total resection was performed in 6.7% of patients, subtotal resection — in 16 (17.9%) out of 89 patients with PCMs. The author reported that mean recurrence-free period is 5 years after subtotal resection of PCM [2].

V. Seifert [46] reported one of the most extensive articles devoted to combined treatment of PCM in 2010. The author followed-up 161 patients with PCM for the period from 1988 to 2008. Follow-up period ranged from 4 to 242 months. Thirteenth patients were excluded from the study due to inability to collect anamnesis. Large and giant PCMs were diagnosed in 130 (87%) patients, medium and small tumors — in 18 (12%) patients.

Isolated resection was made in 71 patients (48%), surgery followed by radiotherapy – in 22 patients. Thus, surgery as first-line therapy was applied in 93 patients (63%). Radiosurgery was performed in 29 patients (20%), fractionated radiotherapy — in 2 patients (1%). Follow-up was preferred in 24 patients (16%).

Total resection of tumor was carried out in 34 patients (37%), subtotal resection — in 36 (39%) cases. Orbitozygomatic/pterional, combined transpetrosal and retrosigmoid approaches were used for PCM resection depending on localization and dimension of tumor. No postoperative mortality was observed. Early postoperative complications were mainly represented by cranial nerve deficiency (31% of patients). This value decreased up to 22% in long-term follow-up period.

Recurrence without need for redo surgery occurred in 2 (6%) out of 34 patients after total resection of tumor. Stabilization of residual tumor fragments was noted in 31 (74%) out of 42 patients after subtotal or partial resection without additional treatment. Continued tumor growth was absent in 15 out of 17 patients after subtotal or partial resection and subsequent adjuvant therapy. Tumor progression was observed in 11 patients (26%).

In conclusion, the author emphasizes that follow-up may be advisable only in patients with small asymptomatic tumors. Microsurgical resection is preferred in patients with growth of small PCM and surgical outcome is directly related to tumor volume. Radiosurgery for small PCM is possible in elderly patients or in patients with aggravated somatic status and diagnosed PCM growth. In patients with symptomatic medium PCMs, total resection is desirable only if quality of life will be preserved. Resection of large and giant PCMs is indicated for decompression of neurovascular structures and subsequent follow-up or radiotherapy.

M. Koutourousiou et al. [47] reported surgical treatment of 32 patients with petroclival meningiomas. Lateral approaches (suboccipital retrosigmoid or far lateral approach) were used in 11 patients, endoscopic endonasal approach — in 17 cases, combination of lateral and median surgical approaches — in 4 patients. Karnofsky score was 87.9 within the mean follow-up period of 14 months. Better outcomes were observed after primary surgery in young patients undergoing subtotal resection. Mortality rate was 3.1% (1 patient). Endoscopic endonasal approach was associated with total resection in 6 patients, subtotal — in 9 cases. The authors conclude that less aggressive approaches to skull base including retrosigmoid and endoscopic endonasal accesses is effective alternative to transpetrosal approaches. These procedures are useful to reduce the volume of petroclival meningioma and improve condition of patients with small risk of surgical complications.

J. Muto et al. [48] analyzed 10 anatomical specimens and concluded that endoscopic endonasal approach combined with anterior transpetrosal approach is appropriate for advanced PCM.

In 2016, Beer-Furlan et al. [49] reported endoscopic endonasal resection of 3 PCMs through extended transclival approach. The authors emphasize the advantages of this procedure. Surgeon is able to resect tumor matrix, devascularize neoplasm and reduce tumor volume without traction of the brain structures and traumatization of cranial nerves. Moreover, surgery is associated with resection of underlying bone and dura mater. Therefore, total resection of tumor (Simpson I) is ensured. In authors’ opinion, the main contraindication for this approach is excessive tumor lateralization. Moreover, the authors indicate formation of large defects in dura mater of skull base and, as a result, increased risk of postoperative cerebrospinal fluid leakage.

Generalized analysis of these reports shows that surgical treatment of patients with PCM should be individualized depending on dimensions and spread of the tumor, neurological and somatic status of patient, age and neuroimaging data. The main goal of treatment should be the most complete resection of these benign neoplasms. At the same time, the main reason for high incidence of recurrence in patients with meningiomas is impossibly total resection, as well as inability to influence their “biological behavior”. All neurooncological interventions are essentially cytoreductive. Surgeon resect a large or smaller part of the tumor, because the neoplasm cannot be excised along with surrounding tissues. Development of residual tumor cells depends on their number, blood supply and some insufficiently studied factors, such as molecular biological characteristics of meningioma and immune response [50].

Currently, fluorescence diagnosis and laser spectral analysis are actively used in surgery of meningiomas [51—55]. In 2018, Potapov et al. [56]analyzed the use of intraoperative fluorescence diagnosis in surgery of meningiomas. Meningioma of posterior cranial fossa was diagnosed in 11 (10.9%) out of 101 patients. Intraoperative fluorescence was observed in all cases (bright fluorescence — 7 (63.6%) cases). This highly specific and sensitive technique is valuable for better assessment of tumor boundaries and exclusion of excessive resection of surrounding tissues. The last one may be associated with postoperative complications. Intraoperative laser spectral analysis gives information on tissue photosensitizer accumulation grade. The method is especially useful for analysis of the boundaries of meninges and underlying bone infiltration, that directly affects surgical strategy and quality of resection.

Intraoperative MRI is successfully used to assess quality of resection of meningiomas including PCM [57]. However, widespread use of intraoperative MRI is limited by high cost of the scanner and equipment of operating theatre.

Analysis of large sample sizes demonstrates that long-term recurrence occurs even after radical resection of PCM [2, 45, 46]. Moreover, advanced resection is associated with injury of surrounding neurovascular structures that aggravates subsequent quality of life [2, 5, 21, 39].

Thus, surgical treatment of PCM is still one of the most difficult problems in modern skull base surgery.

Radiosurgery and radiotherapy

There is a great number of reports devoted to radiotherapy of skull base meningiomas of posterior cranial fossa including PCM [58—63]. Radiotherapy of small (<3 cm and volume <14 cm³) PCMs with moderate neurological symptoms or without any manifestations usually does not cause problems. Patients undergo a course of radiosurgery [58, 59]. Treatment is carried out on a Gamma-knife device or other Linac-based devices [60—62]. Mean irradiation dose is 14 Gy (range 12—16 Gy). Resection is usually first-line approach in patients with large or giant PCM. At the second stage, the question of radiotherapy of residual tumor fragments is decided. There are reports describing two-stage resection of giant PCM followed by radiotherapy. Either radiosurgery or fractional irradiation may be offered after surgery depending on the volume of residual tumor and other factors [58—63].

Neurosurgeons and radiologists consider various factors in planning radiotherapy of PCM including somatic status of patient, tumor dimensions, peritumorous edema, brain stem compression, linear blood flow velocity in the tumor (CT perfusion data), blood supply, tumor density and presence of hydrocephalus. It should be emphasized that proton therapy is successfully used for the management of skull base meningiomas. The difficulty lies in small number of proton therapy devices. Equipment for generating a proton beam occupies large territories. As a result, cost of irradiation is extremely high [64–66].

In 2016, Kim et al. [60] reported 89 patients with PCM. Mean volume of tumors was 6.7 cm³, mean irradiation dose — 13.2 Gy. First-line radiosurgery (Gamma Knife) was applied in 58 patients, microsurgical resection followed by radiotherapy – in 31 patients. Mean follow-up period was 68 months. Radiosurgery was followed by tumor volume decrease in 50 patients (56.2%), stable dimensions in 34 patients (38.2%) and tumor enlargement in 5 patients (5.6%). Five-year freedom from tumor progression was 94.7%, 10-year freedom — 88.2%. Microsurgical resection and subsequent radiosurgery were followed by occurrence of cyst in 3 patients, who subsequently required redo resection of tumor for brain stem decompression.

In 2010, Flannery et al. reported 168 patients with PCM who underwent radiosurgery (Gamma Knife) for the period from 1987 to 2008. Primary radiosurgery was performed in 97 patients; 71 patients underwent primary surgery. Mean tumor volume was 6.1 cm³, mean irradiation dose — 13 Gy. Mean follow-up period was 72 months. Positive neurological dynamics was observed in 44 (26%) patients, neurological symptoms de novo were absent in 98 (58%) patients, deterioration of neurological status was observed in 26 (15%) patients. Tumor volume decreased in 78 patients (46%), remained the same in 74 (44%) and increased in 16 (10%) patients. Eight patients underwent redo radiosurgery, 4 patients required resection of recurrent tumor, 4 patients underwent fractionated radiotherapy. Five- and 10-year recurrence-free survival rates were 91 and 86%, respectively. The authors conclude that first-line radiosurgery may be indicated in patients with small symptomatic PCMs [15].

In 2006, G. Park et al. [7] reported treatment of 75 patients with PCM. All patients were divided into the following groups: surgery (49 patients), radiosurgery (12 patients), fractionated radiotherapy (5 patients) and follow-up groups (9 patients). Mean follow-up was 86 months. In the surgical group, 11 patients (22.4%) had tumor progression after primary resection. At the same time, the authors note that subtotal resection followed by radiosurgery resulted recurrence only in 1 patient. Tumor growth control was higher in the group of combined treatment (90.9%). This value was 73.7% in the group of isolated surgical treatment. Mortality rate in the surgical group was 2%, aggravation of neurological deficit was observed in 28.6% (n=14) of patients. Tumor enlargement or neurological deterioration were absent in 5 patients after radiotherapy and 12 patients after radiosurgery for the entire follow-up period. In the follow-up group, 2 out of 9 patients required surgical intervention due to tumor enlargement. The authors conclude that combined treatment of PCM (surgery+radiosurgery/radiotherapy) results a long recurrence-free period. In their opinion, radiosurgery can also be used as a first-line treatment in patients with small asymptomatic PCM. However, more aggressive treatment is required in young patients with fast clinical aggravation.

As indicated above, PCMs are mostly benign tumors (GRADE I). Atypical PCMs are found in certain observations. These neoplasms are more common in patients with multiple cerebral meningiomas [15, 67].

R. Lall et al. [68] reported delayed transformation of PCM (histological diagnosis: typical meningioma, Grade I) into chondrosarcoma after stereotactic radiosurgery. Recurrence occurred in 14 years after resection of small PCM and 2 courses of stereotactic radiosurgery (Gamma Knife). Therefore, redo resection was required. Biopsy confirmed malignant transformation of meningioma to chondrosarcoma. Subsequently, proton therapy for residual tumors was applied. However, we found no more cases of malignant transformation of benign PCM after radiotherapy.

In 2011, V.N. Shimansky et al. [69] reported a rare case of meningioma with extracranial metastases. Primary surgery was performed for meningioma of posterior petrous surface (histological diagnosis: meningioma with mixed structure). Local recurrence was verified after 10 months. Moreover, new foci were found in both cavernous sinuses, upper parts of the clivus, petroclival junction, internal auditory canal. Redo surgery implied excision of multiple nodes within the skull base of posterior cranial fossa (histological diagnosis — atypical meningioma). Four months later, the patient noted dense right-sided formation on the neck within the place of previous surgical interventions. MRI did not confirm enlargement of intracranial foci. Focal lesion of soft tissues on the neck has been increased over several weeks. Resection was recognized as necessary. Two nodes of meningioma were freely excised from the cervical muscles (histological diagnosis: anaplastic meningioma infiltrating adipose and muscle tissue, Ki-67 index about 15%). Thus, there was malignant progression of tumor followed by soft tissue metastasis without previous radiotherapy.

Analysis of the outcomes of radiological treatment of PCM emphasizes difficult choice of optimal algorithm in patients with large and giant PCM. This is especially true for patients with significant concomitant diseases. Surgical treatment of large and giant PCM is associated with high risk of neurological deficiency and disability, radiosurgery is impossible due to large dimensions of tumor, fractionated radiotherapy may be followed by progression of cerebral edema and radiation-induced necrosis, progression of hydrocephalus. At the same time, radiologists and neurosurgeons have adequate treatment strategies for small and medium PCMs.

Palliative surgery, conservative therapy, endovascular treatment, follow-up

Palliative operations in patients with PCM include cerebrospinal fluid bypass surgeries (ventriculoperitoneal bypass as a rule), as well as craniovertebral junction decompression with dura mater repair with or without biopsy. In some cases, patients with large PCM have predominant occlusive symptoms (benign intracranial hypertension syndrome). In this case, ventriculoperitoneal bypass surgery is useful to stabilize patient's condition before resection or fractionated radiotherapy. The same is true for patients with large PCM complicated by severe somatic status and hydrocephalus. Ventriculoperitoneostomy is often the only possible type of surgical treatment in these patients [70].

Craniovertebral junction decompression with dura mater repair is performed in patients with severe somatic status and advanced edema of the structures of posterior cranial fossa. Decompression of craniovertebral junction is carried out as life-saving operation to prevent circulatory disorders in the brain stem and brain herniation in case of PCM resection followed by edema of the structures of posterior cranial fossa. However, there are no reports with clear indications for craniovertebral decompression in available literature.

Considering predominant benign nature of PCMs (only few cases of atypical or anaplastic PCM are described in the literature), there are currently no recommendations for chemotherapy with cytostatics, angiogenesis inhibitors and immunotherapy [50, 67]. Dexamethasone is one of the most effective adjuvants for conservative treatment of patients with PCM. Effect of this drug is associated with significantly reduced expression of vascular endothelial growth factors and other “angiogenins” by tumor cells. These molecules are important factors of tumor metabolism contributing to peritumorous edema of the brain [50, 67].

There are few reports about the use of endovascular techniques in the treatment of PCM. In most cases, selective embolization of meningeal branches of internal carotid artery supplying PCM is performed [71, 72]. Embolization is usually made as preoperative measure prior to resection of tumor. However, some authors found that this procedure can result necrosis of tumorous stroma in addition to blood-sparing role of embolization [73, 74]. Unfortunately, endovascular techniques are not widespread due to the features of blood supply to PCM. These tumors are supplied by the branches of intracavernous segment of internal carotid artery (meningohypophyseal trunk). Embolization of these arteries is associated with high risk of ischemic complications in the brain stem [75, 76].

Wait and see approach may be advisable in some cases after partial resection of PCM or in case of accidental detection of tumor and the absence of symptoms. However, any patient should be informed that PCMs is characterized by a tendency to continuous growth despite their benign nature. J. Hunter et al. [3, 4] reported 34 patients with benign PCM (Grade I) who underwent surgery for the period from 1999 to 2015 and were under follow-up. Mean follow-up was 44.5 months. Tumor growth was observed in 88.2% of patients within this period. Mean tumor enlargement rate was 2.38 cm³/year. Tumor volume, hyperintense signal in T2-mode, peritumorous edema and cerebellar symptoms were significantly associated with increased risk of tumor growth. Redo surgery was required in 12 patients with recurrences, radiotherapy – in 1 case.