Introduction

Meningioma has become the most common brain tumor [31], and management options that range from dismissal of incidental findings over follow-up with wait and scan [17] to aggressive surgery are repeatedly discussed [25]. A major determinant of the best strategy is long-term tumor control [19, 20, 26]. Studies with long-term follow-up reveal a disturbingly high recurrence rate after surgery even if apparent gross total removal was achieved [2, 8, 15, 16, 24, 30, 33, 39]. Recurrence of a previously operated tumor is an undisputable evidence that some tumor cells were left behind and have grown. Time to recurrence varies with biological qualities of a tumor [29], but the actual risk of recurrence is binary. The tumor can recur if neoplastic cells are left behind and not treated, while a completely removed tumor without remaining cells cannot recur. Meningiomas comprise a well-demarcated tumor, yet meningioma cells invade adjacent mesenchymal tissues and a macroscopically evident tumor border does not necessarily allow identification and removal of all neoplastic cells. Intraoperative or radiological visualization of individual tumor cells is not feasible; hence, surgery and definition of extent of resection (EOR) must depend on indirect measurement and proxy parameters of EOR. For systemic cancers, surgeons typically rely on pathology examinations of resection margins which provides a reproducible assessment of EOR which is reliable enough to allow stratification of adjuvant treatment and follow-up [10, 11, 13]. This classification has not been practical for meningiomas, since growth patterns may disagree with feasibility of harvesting tissue for analyses of resection margins and, moreover, because histopathology may be insufficient to distinguish between normal and neoplastic meningeal cells [9].

For meningiomas, EOR has instead relied on two different approaches and variations thereof. The classic description of EOR is a structured report of the surgeon’s assessment of whether tumor has been biopsied, subtotally removed, completely removed, or completely removed with additional management of proposed dural and bone invasion as described by Simpson in 1957 [39]. The alternative approach is postoperative MRI imaging as described in EANO guidelines [14]. The surgeon’s intraoperative assessment lacks objectively verifiable criteria and does not allow an estimate of microscopic residual cells after completed surgery [37]. A radiological assessment is not sensitive enough to rule out residual tumor masses [40]. Spatial resolution of MRI imaging can leave residual tumors with up to 1,000,000 cells undetectable. Moreover, MRI imaging is not specific. Surgery commonly leads to tissue reactions and scarring with contrast enhancement that may be impossible to differentiate from tumor on an MRI image.

An updated agreement regarding how to report the extent of meningioma resection is warranted, as also concluded by a recent RANO review [34].

Yet, immunohistochemistry has developed to improve diagnosis of meningioma and imaging with PET for somatostatin receptors, almost exclusively expressed by tumor cells in patients with meningioma, provide superior sensitivity and specificity compared to contrast enhanced MRI [3,4,5, 27].

There is a need for a traceable and reproducible system to describe extent of resection for meningiomas [37]. We propose use of a combination of the most sensitive and specific imaging modality paired with optimal immunohistochemistry.

This technical note aims to describe a novel grading system that combines analyses of resection margins and PET imaging of somatostatin receptors. The note comprises a proposal for the system and a user’s guide. Herein, we present our grading system and show the application our first six patients as a proposal to wide implementation of Copenhagen Grading for maximally traceable reporting of extent of resection of meningiomas.

Methods

Copenhagen Grading was designed to describe presence or absence of residual meningioma tissue after surgery by combining imaging and histology for tumors suspected to be meningiomas from pre-surgical scanning and confirmed by intraoperative analysis of frozen tissue. The classification combines results of immunohistochemistry from surgical biopsies and DOTATOC-PET scanning at follow-up after 3 months. Copenhagen Grading is organically implemented, and we have proposed a preliminary follow-up algorithm which is to be evaluated and adapted during continuous follow-up of included patients. Clinical data is prospectively recorded for evaluation and adaption during implementation.

Surgical biopsies

After as complete a resection as feasible, biopsies are taken from 4 different quadrants of the resection margin, marked and sent to pathology at the conclusion of surgery. Alternatively, a complete strip of dura surrounding the tumor can be sent separately if feasible (marked as “complete resection margin”). A conscious effort is made to include any area where the surgeon is least confident to have removed all tumor tissue. For intraventricular and posterior third ventricular meningiomas, biopsies are obtained from resection margins of the choroid plexus depending on the symmetry of the attachment. “Area of doubt” resembles areas outside the resection margin where the surgeon might suspect residual tumor cells, i.e., dura or bony structures. These sites are biopsied and marked as “area of doubt (AOD)” (Figs. 1, 2, 3 and 4).

Fig. 1
figure 1

Illustration of per-operative field. The aim is to obtain a circular resection margin of dura where it is feasible. Biopsies are taken from 4 different quadrants of the resection margin. “Area of doubt” biopsies are taken from the borders of the resection margin or bony structures

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Fig. 2
figure 2

Copenhagen Grading for meningiomas: score system depending on the empirical observations

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Fig. 3
figure 3

The Copenhagen Grading setup including histopathology from per-operative biopsies and imaging at 3 months follow-up

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Fig. 4
figure 4

Preliminary follow-up algorithm. The extent of follow-up is tailored depending on the Copenhagen Grading and tumor phenotype.

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The grade is 0 if the margins or any “areas of doubt” are free and 1 if the margins or any “areas of doubt” contain meningioma cells by routine histology plus immunohistochemistry for EMA and SSTR2 if needed for differential diagnostics[9].

Brain invasion is automatically considered “Copenhagen grade 1.”

It is optional to add molecular analyses of the tumor-specific signature if immuno-histopathology or histopathology remains equivocal.

Imaging

At 3 months after surgery, routine radiological follow-up is undertaken with MRI or CT and 68Ga-DOTATOC-PET CT or MRI22, depending on available spatial resolution of scanners. The grade is 0 if there is no detectable tumor and 1 if the images reveal a specific 68Ga-signal.

The Copenhagen Grade

The Copenhagen Grade is thus a combination of histopathology (0 or 1) and radiological imaging (0 or 1). It is expressed as 0/0, 1/0, 0/1, or 1/1 depending on the empirical observations. Histopathology is reported as “1” if at least one of resection margin—or AOD biopsies—is positive for tumor. Any tumor with demonstration of brain invasion from histopathology is graded as “1” from histopathology. Brain invasion is defined in agreement with WHO grading characterized by irregular groups of tumor cells infiltrating the adjacent cerebral parenchyma, without an intervening layer of leptomeninges [18, 21].

The extent of follow-up is tailored depending on the Copenhagen Grade and tumor phenotype.

Results

We present our first six patients with complete assessment of extent of resection according to Copenhagen Grading (Table 1). The preliminary experience indicated that biopsies could be obtained without technical difficulties. Biopsies from resection margins or an “area of doubt” that were sent as separate samples were analyzed during routine histopathology to either contain or not contain meningioma cells. Post-operative scanning with MRI and DOTATOC-PET was successfully obtained during follow-up 11–14 weeks after surgery.

Table 1 Preliminary results on 6 enrolled patients from December 2020 until March 2021.
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All patients had been described to have undergone gross total resection. Four tumors were perioperatively assessed to have been removed according to Simpson grade 2, and one each as grade 1 and grade 3. The histopathology showed residual tumor in three patients. MR imaging showed residual tumor in two patients and DOTATOC-PET in three. One patient was positive on histopathology but negative on MRI and DOTATOC-PET; one was positive on DOTATOC-PET but negative on histopathology and MRI (Table 1).

Typical Copenhagen Grades 0/0, 1/0, 0/1, and 1/1 are illustrated in a case example (Fig. 5).

Fig. 5
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Case 2: A 63-year-old woman with a left-sided convexity meningioma. Underwent operation with tumor removal, resection margin with 4 sites + “area of doubt” biopsy from adjacent bone. Resected dura was replaced with artificial dura substitute

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Estimated as Simpson grade II

Histopathology: WHO grade I. Dura biopsies without remaining tumor cells. “Area of doubt” biopsy showed lamellar bone structure and meningioma cells

Imaging: DOTATOC-PET and MRI without signs of remaining tumor. PET-DOTATOC and MRI co-registration showing expected activity uptake due to tissue healing in relation to the craniotomy and dura-substitute and no signs of remaining tumor

Copenhagen Grading: 1/0

Discussion

It has been possible to implement Copenhagen Grading for meningiomas. Our first cases illustrate the grading and suggest that histopathology and DOTATOC-PET imaging add complementary information to the evaluation of extent of resection. We interpret the data to indicate preliminary support for the concept of combining histology and the presently most sensitive and specific imaging biomarker to allow a dichotomized description of residual meningioma after surgery.

During routine implementation into the workflow during surgery, histopathology, and imaging, local routines must be developed and validated. Typically, surgeons must incorporate biopsies into surgical routine after removal of the main bulk of tumor and biopsies must be marked and sent separately for histopathology. These two steps are feasible within a surgical checklist and separately marked biopsies are processed routinely by pathologists who develop the routine and evaluate Copenhagen grades as part of a routine histology report.

Imaging is dependent on availability of a PET facility and easily offered to all patients who are eligible; participation in follow-up imaging is not different from routine follow-up with MRI or CT. Our assessment is that Copenhagen Grading could be implemented as a feasible and a potential objective grading system. The combination of histopathology and imaging appeared to improve sensitivity to detect residual tumor.

It may be technically problematic to obtain representative and relevant intraoperative biopsies from resection margins, particularly from tumors with difficult cranial base locations or potential invasion of mesenchymal tissues. Nonetheless, a tissue biopsy that can be diagnosed with sensitive biomarkers such as EMA and SSTR2, which provide 96% specificity, is far better than reliance only on postoperative MRI scanning or the surgeon’s subjective impression. Biopsies can be harvested during surgery and be processed for analysis without urgency. A risk of failure to obtain representative tissues must be accepted with any tissue based diagnostic method. We postulate, in agreement with our surgical experience, that even difficult cranial base locations allow that also far sides of a tumor attachment can be reached with instruments, yet any surgical attempt to obtain a representative biopsy can fail in more than 5% of cases even in diagnostic image-guided surgery [12, 22]. However, we noted that biopsies were not omitted because of technical difficulties.

We thus consider the use of histology on resection margins as feasible, promising and relevant.

Recently molecular biomarkers for aggressive phenotypes (TERT, CDKN2A) have been validated in large meta-analyses [28, 42] and found feasible for clinical implementation in meningioma grading. Furthermore, genetic and epigenetic landscapes allow novel classifications based on meningioma biology [6, 7, 23, 32, 41,42,43]. Next-generation sequencing (NGS) of a targeted meningioma gene panel could be a specific discriminator between different neoplastic and non-neoplastic meningeal cells, which we expect to correlate with the clinical course. We consider that improved biological data from molecular characteristics would be particularly powerful to predict the fate of a patient when combined with a highly traceable description of the resection such as we propose.

68Ga-DOTATOC PET is known to offer better diagnostic information of residual or recurrent meningioma than MRI scanning [4, 5]. Presently, the major drawback for wide implementation is that 68Ga-DOTATOC PET scanning requires equipment and expertise, which is mostly available in specialized centers. Still, PET scanning is widely available in national centers and scanning is undertaken electively approximately 3 months after surgery. Elective, ambulatory scanning is feasible and must then be weighed against cost. Our preliminary impression is that unequivocal data on tumor residues would change MRI follow-up and use of adjuvant therapies to an extent that may very well decrease cost of follow-up while empowering patients with less uncertainty of a potential cure of disease.

We cannot yet assess the clinical impact of grading or its cost-efficiency, but it is very likely that wide implementation of Copenhagen Meningioma Grading can increase efficiency in follow-up and save cost. We have indicated how better traceable information on EOR can produce a stratified follow-up with fewer regular MRI scans. Moreover, better information on EOR is likely to supply patients with better discrimination between potential cure or harboring a chronic condition.

The dichotomized information of presence of residual neoplastic cells is fundamental, although not always admitted. Recurrences are possible only if neoplastic cells are present and it is usually considered possible to remove a benign meningioma completely. Usually, a “risk of recurrence” is used as an amalgamated function of residual tumor volume—if any—and biological qualities such as growth rate [37]. We argue that biological qualities should be analyzed separately from extent of resection. The risk of recurrence is a function of residual cells, while the detection of a recurrence depends on how fast tumors grow. We have recently analyzed a cohort of meningiomas with true long-term follow-up and demonstrated that the biomarker Ki-67 proliferation index (Ki-67 PI) primarily correlated with time to recurrence; suggesting that any tumor may recur if neoplastic cells remain—if follow-up is long enough [29]. We suggest that optimum information on extent of resection allows a nuanced assessment of the biological qualities of any residual tumor including tailoring of follow-up and adjuvant or additional treatments. The biological qualities that affect growth rate and invasiveness must be assessed from known biomarkers such as WHO grades [21], Ki-67 PI [1, 35], TERT-promoter mutations, and homozygous deletion of CDKN2A/B [28, 38]. Such information should be used to select additional treatments and design individualized follow-up [36]. Better standardized subgroups that have been characterized from both aspects allow better trials. We have primarily targeted WHO grade 1 meningiomas but consider our grading useful for all meningiomas. The need to optimally target additional or adjuvant therapy is probably even greater for WHO grade II.

Copenhagen grading integrates different diagnostic modalities with the expectation that they may disagree; their strengths and weaknesses of sensitivity and specificity may differ and agreement between the different modalities is not primarily desirable. The grading is asymmetrical, since any objective finding that points at residual tumor shifts the patients into a classification where need for additional management must consider that the patient has residual disease and we have designed the grading to allow the best available way to detect residual tumor with certainty.

This is a technical note that does not show long-term outcomes or benefits. The primary benefit is the theoretical advantage of superior traceability, sensitivity, and specificity. Sensitivity and specificity cannot be evaluated without long-term follow-up; the lack of validation is a major weakness. Unfortunately, traditional prospective trials to validate use of Copenhagen Grading of meningiomas are not feasible. The slow growth of meningiomas limits relevance of short-term prospective trials. We suggest that Copenhagen Grading is organically implemented. Organic implementation entails continuous re-evaluation of treatment and findings so that suboptimal strategies are changed according to experience and judgment while functional strategies are maintained. Information is collected prospectively and analyzed critically. The main parameters to validate during follow-up are whether Copenhagen Grading proves to be traceable, reproducible, and support standardization for better targeted meningioma subgroup management.

Conclusion

This technical note describes Copenhagen Grading for extent of resection of meningiomas, which is based on the rationale of resection-margin analyses as gold standard and superior imaging performance of 68Ga-DOTATOC PET. The preliminary observations suggest that Copenhagen Grading is technical, feasible, and traceable. It may comprise the much-needed grading system that allows standardized clinical research and thereby improves management of follow-up, adjuvant therapy, and prognostication of meningiomas. Presentation of the grading is a prerequisite for organic development of applications and amendments.