Stereotactic Radiosurgery Meningiomas

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Chapter 104 Stereotactic Radiosurgery Meningiomas

Meningiomas arise from the arachnoid cap cells, or “meningothelial” cells, and are the most frequently reported neuro-oncologic challenge, accounting for 16% to 30% of intracranial tumors,16 and for 22% to 25% of spinal cord tumors.7,8 Furthermore, a non-negligible percentage of them is diagnosed on the basis of imaging alone,3,9,10 and an additional 2.3% in autopsy reports (Fig. 104-1).3,11 With the growing contribution of imaging and necroptic observations the relative percentage might become higher.

image

FIGURE 104-1 Autoptic finding, showing how meningioma are discrete, smooth-surfaced massed attached to the dura, with pushing of the leptomeninges.

(Courtesy of Prof. Felice Giangaspero and Dr. Manila Antonelli, Department of Experimental Medicine, University La Sapienza, Rome.)

Epidemiology

The average annual incidence is five to six new cases per 100,000,16,12,13 with the age-related risk drastically increasing from the pediatric population to a peak during the sixth and seventh decades.14,15 It is worth stressing that more aggressive clinical and histologic features have been observed in children and adolescents.14 The female/male ratio is approximately 2:1 to 3:1,3,1618 and this prevalence is presumably related to as yet poorly known progesterone- and estrogen-receptor–mediated cytoactivation.3,15,19

As to their natural history, the few reported series of conservatively managed symptomatic meningiomas have documented a consistent trend to progression in one third of the patients, although with a wide spectrum of variability.2028 In fact, in a non-negligible percentage, either spontaneously or after subtotal resection, the annual growth rate may suddenly increase up to several millimeters documenting an unexpected aggressiveness.2932 The main factors associated with growth are younger age and T2-hyperintensity, and presence of calcifications.20 Both growth and recurrence rates are higher in grades 2 and 3 meningiomas29,33,34 and this justifies the advocated multidisciplinary treatments.13 Finally, although in anecdotal cases, post-surgical or radiosurgical remnants of a “benign” meningioma have been shown to undergo spontaneous or “induced” malignant cytologic transformation, subsequently confirmed at reoperation.30,35,36

Citopathology

During the last decade, the pathologic criteria for the definition of “atypical” as well as of “anaplastic” meningiomas have been gradually expanded to include not only nuclear pleomorphism, mitoses, atypia, necrosis, and so on, but also other relevant landmarks of parenchymal invasion such as the formation of small cell infiltrates, arachnoidal disruption, sheeting, macronucleoli focal malignant macrodifferentiation (e.g., glial fibrillary acidic protein (GFAP) production, melanocytic foci, etc.), and extra-axial diffusion (Fig. 104-2).2 According to the Mayo Clinic Scheme,2,36,37 substantially endorsed by recent revisions of the World Health Organization (WHO) (Table 104-1),2 these specific parameters may be associated with malignant behavior regardless of their histologic subtype.2 In fact, the very peculiar metastatic potential of these tumors seems to confirm the mentioned cytobiological discrepancy, since histologically benign meningiomas have sometimes shown biological aggressiveness, not only in terms of craniospinal seedings, but also of extraneural colonization (lung, liver, bone, lymph nodes).2,7,38,39 However, the vast majority of these tumors are considered benign or grade 1 in the WHO classification,2,40 currently accounting for a 5-year survival rate exceeding 80%.1,41,42 Atypical (grade 2, 4.7-7.2%, prototypes: clear cell and chordoid varieties) and anaplastic or malignant (grade 3, 1.0%-2.8%, prototypes papillary and rhabdoid) forms are rarer2,40,4345 and usually bear a worse prognosis, with reported 5-year survivals of 32% to 64%.1,36,42,4650 Monosomy of chromosome 22 is the most common cytogenetic alteration in the overall oncotype population.51,52 Nearly all neurofibromatosis type 2 (NF2)–associated meningiomas have mutations of the NF2 gene on 22q12.3,51 Moreover, a stepwise change in the genetic characteristics of benign meningiomas undergoing anaplastic transformation has been observed: The loss on 22q, 1p, 6q, 10, 14q, and a gain on 9q, 12q, 15q, 17q, may frequently parallel this evolution as well as amplification on 17q and loss on 9p (CDKN2 gene).2,5355 Such a theory of correlative clinical-pathologic malignant progression is supported by atypical or anaplastic recurrences of formerly benign lesions, but the oncologic rationale is still debated.2,56

Table 104-1 Meningiomas Grouped by Likelihood of Recurrence and World Health Organization Classification

Meningiomas with Low Risk of Recurrence and Aggressive Growth Meningiomas with Greater Likelihood of Recurrence and/or Aggressive Behavior
Grade I Grade II Grade III
Meningothelial
Fibrous (fibroblastic)
Transitional (mixed)
Psammomatous
Angiomatous
Microcystic
Secretory
Lymphoplasmacyte-rich
Metaplastic
Atypical: Clear cell/chordoida
• ≥ 4 mitoses/10 HPF (≥2.5/mm2)
Or at least three of the following four features:
• Sheeting
• Macronucleoli
• Small cell formation
• Hypercellularity (≥53 nuclei/HPF, ≥118/mm2)
• Or Brain invasion
Clear cell (intracranial)
Chordoid
Anaplastic (Malignant): Rhabdoid, Papillary, etc.
Papillary
Anaplastic (malignant)a
• ≥20 mitotic figures/10 HPF (≥12.5/mm2)
Or:
• Focal or diffuse loss of meningothelial differentiation resulting in carcinoma-, sarcoma-, or melanoma-like appearance
Meningiomas of any subtype or grade exhibiting high proliferation indices or brain invasion

Note: World Health Organization meningioma grading according to aggressive behavior (i.e., probability of recurrence).

a Mayo Clinic meningioma grading scheme.

HPF, high-power microscopic fields.

Treatment Options

Surgery

There is general agreement that the optimal treatment for these lesions, whenever feasible, should be a Simpson grade I resection of the tumor—carefully pre-embolyzed if necessary—providing definitive diagnosis, reducing immediately any mass effect, and alleviating clinical signs and symptoms.2,57,59,60 Such a golden therapeutic standard is still the dominant option in the vast majority of convexity meningiomas in fronto-orbital and spinal locations, whereas surgical results are less satisfactory in intraparenchymal or cranial base lesions2,5962 and particularly grim in grades 2 and 3.3 Unfortunately, despite surgical advances, when these tumors are infiltrating the skull base, cranial nerves, or vascular structures, complete resection may not be feasible without unacceptable morbidity rates. Considering more recently published series, gross removal of basal meningiomas was achieved in 60% to 87.5% of the patients,5965 with 30% to 56% of severe complications,58,60,6567 mostly represented by newer or deteriorated pre-existing cranial nerve deficit, occurring temporarily in 20% to 44%, and permanently in 16% to 56% of the cases. Median postoperative mortality rate was 3.6% (range 0%–9%).5968 Moreover, local recurrence rates are strictly dependent on Simpson’s grade69: at 10-year follow-up, 10% to 33% (average 20% to 25%) after complete resection (Simpson 1–2); and 55% to 75% (average 60%–62%) after partial resection, that is, Simpson >2 (Table 104-2).20,32,7074 The relevance of these findings may probably explain why, particularly in atypical-anaplastic varieties, adjuvant therapies and sometimes preplanned, combined multidisciplinary approaches may be advocated, to avoid early recurrences and repeat surgery, thereby reducing morbidity.2,12,35

Fractionated Radiotherapy

External beam radiation therapy (EBRT) has been used for decades, either as a primary or as an adjuvant treatment, with documented improvement in local control and rarely in recurrences (see Table 104-2).12,41,42,7580 In more aggressive cytotypes, most authors seem to favor administering EBRT—mostly Fractionated Stereotactic Radiotherapy (FSRT) early to patients who have undergone subtotal and even total resections.2,3,12,42,43,45,81 FSRT has proven to be successful in primary (imaging defined) meningiomas,12,8287 particularly in all cases with crucial exposure of extremely radiosensitive structures such as the optic pathways.12,8891 Series reports comparing progression-free survival (PFS) for patients treated with gross/total removal and subtotal removal (STR), with or without EBRT, have consistently shown the best results with the radical surgery or STR plus radiation, whereas STR alone turned out to be less valuable.3,32,33,41,42,45,70,71,7678,85,92107

Stereotactic Radiosurgery

According to the historical definition by Lars Leksell stereotactic radiosurgery (SRS) is “a technique of closed skull destruction of a predetermined intracranial target by a single-fraction high dose of ionizing radiation using a precision stereotactic apparatus”.108 In contrast with spatially less accurate conventional radiotherapy, radiosurgery has the capacity to maximize/optimize the dose exposure on the target volume, while minimizing the irradiation of surrounding critical structures, thereby reducing collateral damages. The concept of delivering a high dose of radiation energy to treat focal pathologic lesions fits all criteria for minimal invasiveness and has gradually become a powerful and attractive therapeutic strategy for many neurosurgical disorders. Recently the impressive advances in neuroimaging, stereotactic techniques, and robotic technology have further improved results, expanding the spectrum of applications. This approach may be increasingly considered not only as potentially adjuvant to microsurgery but also as a valuable alternative option.

Lars Leksell designed the first arc centered device in 1948,109 and in 1951 he introduced the term and concept of radiosurgery.108,109 His first stereotactic instruments were suitable for replacing a probe (needle electrode) by cross firing intracerebral structures with narrow beams of radiant energy. X-rays were first tried, but both gamma rays and ultrasonic rays were included as alternatives. In close collaboration with Borje Larsson, a physicist at the synchrocyclotron unit in Uppsala, Leksell performed the original experiments with highly focused high-energy proton irradiation of human malignancies.110 As a rule, precise, well-limited lesions were produced, but the synchrocyclotron proved to be too complicated for widespread clinical use. This compelled Leksell to consider other radiation sources and he started designing a 179-source 60Co gamma unit that was fully integrated into the stereotactic system; the first unit was inaugurated in 1967.110 Radiosurgery was initially developed with the aim to offer a bloodless and less risky method, essentially for functional treatments. However, within a few years the machine proved to be extremely effective in a variety of intracranial lesions, provided that the rationale of the approach, that is, the fundamentals of the technique, had been respected. Briefly, the latter are limited target volume, well-defined imaging, compatible site, and adequate cytology. As shown in the following reported experiences, to date it is possible or nearly possible to overcome each of the following constraints: tackling larger volumes with lower dosages,111,112 crucial locations with staged procedures, and, perhaps in the near future, radioresistant oncotypes by means of radiosensitizers.

In recent decades, SRS techniques, particularly gamma knife radiosurgery (GKR), have progressively gained an unquestionable momentum in the therapeutic armamentarium for most brain tumors, particularly for well defined lesions, such as meningiomas. The main reasons for this growing role may be summarized as follows: (1) targeting update, with the advent of computerized and coregistered, morphofunctional neuroimaging; (2) the availability of newer, more powerful and precise irradiation devices; (3) the introduction of computer-guided dose planning; and (4) a deeper radiobiological experience. To date, an estimated half a million people have been treated by GKR worldwide at a continuously increasing annual rate (in 2009, roughly 50,000 patients were treated). In addition, approximately 200,000 patients worldwide have experienced SRS with other dedicated machines. Elective indications currently include metastatic brain tumors, benign endocranial tumors (meningiomas, neuromas, pituitary adenomas, etc.), low-grade neuro-ectodermal tumors, vascular malformations, and some types of functional neurosurgery (e.g., trigeminal neuralgia). Finally, it is generally accepted that the putative mechanism of action of SRS is intimately dependent on the main technical variables (dose–volume integral, timing, target cytology), as well as the goals that we are pursuing (tumor growth control [TGC], necrotic evolution, ephaptic block, etc.). Regarding meningiomas, routine protocols are focused on TGC, probably obtained through a combined mechanism, such as: 1, direct cytotoxicity, presumably promoting apoptosis; 2, damage to the vascular supply, mediated by inhibition of growth factors (vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), factor 8, etc.); and 3, inactivation/destruction of hormonal receptors (e.g., octreotide [OCT] receptor).113,114

Targeting Update

The development of SRS has closely followed the impressive evolution of neuroradiologic techniques. The best example is perhaps the exponential increase of radiosurgical indications after the introduction of computerized imaging, particularly magnetic resonance imaging (MRI). Indeed, for routine SRS procedures, stereo-MRI remains a mainstay in target localization. Coregistered multimodality acquisitions (MRI, computed tomography [CT], angiography, positron emission tomography [PET] scan) have led on one side to the development of fusion algorithms based on contour definition, signal intensity, and voxel matching that allow detailed, interpolated 3D and 4D pictures, speeding up all the preoperative procedures. On the other, advances in computerized integration of stereotactic and nonstereotactic imaging, including functional MRI and diffusion tensor imaging (DTI), are opening unexpected frontiers to SRS treatments.115 Improving safety and efficacy of radiosurgical planning is now possible via fusing the conventional sequences of a stereotactic MRI with the three Tesla pictures of corticospinal, visual, or arcuate DTI, with the aim of preserving the specific tract from undue damage (Fig. 104-3).116 CT and MRI scans sometimes may not adequately identify the regions of functional interest surrounding the planned target. Coregistration of functional MRI becomes mandatory, albeit sometimes still inadequate. Molecular imaging may be the appropriate integration. Using stereo-PET scan metabolic mapping, with FDG or a spectrum of amino acids, particularly fluorodopa and 11C-methionine,117,118 can be merged with conventional pictures, thereby helping to identify the borders of the lesion as well as the metabolically active sites of the tumor or necrotic/radionecrotic foci on the basis of the differential uptake.119,120 Moreover, PET techniques allow us to differentiate the usually octreotide-rich surfaces of meningiomas from other skull base tumors that generally lack these receptors.113,114

Irradiation Techniques and Modalities

Radiosurgery, in the absolute majority of cases, differs from conventional ab externo radiotherapy in various well-known aspects:

Currently stereotactic radiosurgery techniques may utilize different types of penetrating energy, including cyclotron- or synchrotron-generated particles such as protons or heavy-charged particles, and photon devices such as modified linear accelerators (LINACs) or the gamma knife (GK) (Fig. 104-4).

Linear Accelerator

Linear accelerator–based radiosurgery uses x-ray beams that are produced by the collision of accelerated electrons with a metal target (see Fig. 104-4). Multiple noncoplanar arcs converge at a single isocenter, where a nearly sphere-shaped dose distribution is created. These arcs are produced by gantry rotation during irradiation, and each is defined by a different couch angle. By adjusting the number, length, angles, and weights of individual arcs, irradiation of adjacent crucial structures can be reduced. Most LINAC radiosurgery units (e.g., Synergy and Axesse by Elekta, Stockholm; Trilogy by Varian, Palo Alto, CA; Novalis by Brain Lab, Feldkirchen, Germany) use tertiary circular collimators that further decrease beam divergence, thereby protecting normal tissues. For higher conformality while departing from a spherical shape, multiple isocenters are possible, at the cost of greater inhomogeneity and much longer treatment times. This technique has proved to be an effective, alternative radiosurgical option, particularly in cases of larger target volumes, or whenever fractionated radiosurgical procedures are required.2,12,121 Comparative analyses of the literature concerning LINAC versus GK results in meningiomas, however, still show on one side similar tumor control indexes, and on the other, much higher complication rates with LINAC, with comparable or shorter follow-up periods (Table 104-3).9,29,42,58,63,122159

With the increasing use of SRS, the growing interest for highly sophisticated photon linear accelerators has led to micro-multileaf (MML) technology, potentially more competitive with GK and proton beam devices, and characterized by an easy patient setup and greater possibilities of reaching any irradiation position. Basic parameters that allow complex field shaping with the LINAC include the patient couch angle, gantry arc angle, isocenter position, and collimator size. Additional factors include beam weighting and the tissue depth through which various arc segments are delivered. Manipulation of these simple elements provides significant flexibility in the conformal three-dimensional dose distribution for a single isocenter. By using multiple isocenters, it is also possible to maximize the buffer exposure of the target, and of the adjacent critical structures.

CyberKnife

This extremely refined instrument was conceived in the mid-1980s and manufactured within a decade. In its 15-year history, the CyberKnife (Accuray, Sunnyvale, CA), has clearly shown basic pros and cons of the technique.121,155,160163 Regarding the former, the main advantages include (1) easier fractionation, that is, increased protection of sensory cranial nerves,162 and so on; (2) no need of general anesthesia even in young patients; and (3) flexibility to treat lesions throughout the body. Regarding the latter, the absence of a stereotactic frame, as well as the mobility of the radiation source should somehow entail a slightly superior error margin if compared to the gamma knife. It is well known that CyberKnife operative programs are essentially based on CT scan imaging recognition. For certain targets, this might involve less sophisticated imaging.

The CyberKnife (see Fig. 104-4) combines a lightweight 6-MeV LINAC designed for radiosurgery and mounted onto a highly maneuverable robotic arm that can position and point the LINAC. Internally controlled real-time image guidance eliminates the need of skeletal fixation for either positioning or rigid immobilization of the target. This system acquires radiographs of skeletal features associated with the treatment site, uses image registration techniques to determine the target’s coordinates with respect to the LINAC and to the manipulator, which finally directs the beam to the planned point. Whenever the patient moves, internal controls detect the change, stop radiation, correct the trajectory of the beam, and start irradiating again nearly in the real time. Complex radiosurgical treatments may be performed, in which beams originate at arbitrary points in the workspace to target arbitrary points within the lesion. Even nonisocentric beams can be focused anywhere within a volume around the center. Total treatment time depends on the complexity of the plan and delivery paths but it is comparable to standard LINAC treatments.

Tomotherapy

Helical tomotherapy unit is a new modality for radiation treatments, the first dedicated to intensity-modulated irradiation using a fully integrated image-guided radiotherapy machine with on-board megavoltage CT capability. The tomotherapy system (see Fig. 104-4) uses a 6-megavoltage accelerator, and a 64-leaf binary multileaf collimator and xenon image detector array mounted on a rotating gantry. Radiation is delivered in a helical way, obtained by concurrent gantry rotation and couch/patient movements. Altogether, these components allow continuous, intensity-modulated rotational irradiation with fan beam entry from 360 degrees. A megavoltage CT scan before the treatment (nominal energy of 3.5 megavoltages) can be fused with a planning CT scan to determine the correct patient setup every time that the radiotherapist needs to check it. Finally, ultimate dose calculation is refined by means of convolution/superimposition. Tomotherapy has been used for the treatment of benign brain tumors (meningiomas and neurinomas), resulting in good target coverage with high-dose homogeneity and respecting organs-at-risk constraints.164 In addition, this technique allows treatment of larger brain lesions than GKS. Once again the most important difference seems to be that the latter still maintains a better conformity index, and non-negligible advantages in terms of the integral dose to the brain.

Gamma Knife

Depending on the model, the gamma knife (GK) (Elekta, Sweden) contains an array of 201 (Model B or C) or 192 (Perfexion) (see Fig. 104-4) individual 60Co sources aligned with a collimation system that directs each of the radiation beams to a very precise focal point. As a consequence, even very small targets can be treated by a high radiation dosage, whereas peripheral dose levels remain low. GK treatments are therefore quite heterogeneous in terms of dose distribution inside and outside of the target; tumor control and tissue sparing is achieved via the steep dose gradient at the periphery rather than exploiting the radiobiological differential between normal and pathologic tissue as in fractionated radiotherapy. At installation, GK units have an initial dose rate of approximately 4 Gy/min. Given that the 60Co half-life of 5.28 years, the primary sources must be replaced every 6 to 7 years. Due to the low energy of the isotope sources, most treatments are referred to a normalization of 50%. With the multiple isocenters routinely used, GK plans tend to be more conformal but less homogeneous than LINAC-based plans. Due to the typically steep dose gradients of radiosurgery, an accurate alignment of the plan’s isocenter with the physical isocenter of the machine is a quite challenging issue.12 Details of the structure and operation of the central body and collimation system are among the biggest differences between the Perfexion Gamma Knife (GKPx) and earlier B and C models. While the external hemispheric helmet in the B and C models is fundamental in providing the final beam collimation, in the Perfexion model the central body and collimation system are somewhat more complex because the 60Co sources are mobile and the conical collimator body is entirely internal to the unit, so that there are no external helmets (Fig. 104-5). Moreover the surgeon can modify the shooting collimator size using the sector drive motors that move the sources along their bushing to the correct position; the localization of each sector is monitored by linear and rotational encoders characterized by a positioning repeatability of less than 0.01 mm.165,166 Critical structures such as optic pathways and cranial nerves may be shielded through the application of beam blocking patterns that minimize the contribution of possibly dangerous shots.

Dose Planning

The goal of dose planning is to create an extremely conformal and selective isodose configuration, with complete covering of the tumor, and minimal exposure of the surrounding tissues (Fig. 104-6). Using a combination of multiple isocenters of different sizes, differential weighting of the crucial shots, selective blocking of the collimated beams and—typical of the Perfexion Gamma Knife—using hybrid shots,165 it is possible to produce dose plans that closely conform not only to the main shape of the meningioma, but also to all the MRI-documented dural attachments (Fig. 104-7), while sparing the neighboring neural structures. Probabilistic models, quadrature-sum analysis, and phantom studies have repeatedly confirmed the reliability of such operative models.107,108,166 These newer approaches in treatment planning have consistently improved Paddick’s conformity index,167 meanwhile reducing treatment times for both GKR and GKPx88,165,169 and for MML LINAC.88,168 To date, the recommended surface (SD), edge (ED), or peripheral (PD) doses for meningiomas range from 11 to 15 Gy at the 50% isodose; the higher levels are currently reserved for more aggressive histotypes.2,3 The “ideal”—that is, the most biologically justified—planning target volume, is still a matter of debate, with a spectrum of options, ranging from including the gross, T1 contrast–enhancing image plus a supposedly infiltrated margin of a few millimeters,3,32,41,98,103,170 to the controversial inclusion of the “dural tail” that—according to studies of extremely refined doses—should be essentially composed of hypervascular dura with none of the expected tumor colonies.3,92,171,172 Another deceptive variable in the definition of the target volume is represented by hyperostosis,3,173,174 particularly after the studies of Pieper et al.174 showing that (in a series of 26 consecutively operated patients) hyperostotic bone was almost constantly (25/26) present, even without any imaging evidence. In these cases, ablative radiosurgery on the hyperostotic bone might have the same meaning of Simpson’s grade 1 in surgical approaches. As mentioned previously, the novel and impressive advances in radiosurgical treatment strategies, particularly coregistration and fusion algorithms–dose-planning software, and automatic positioning systems are helping to overcome some of the major SRS, and specifically GKR, constraints. We can reasonably target unusually larger volumes, even in crucial sites, by using lower dosages or adopting “staged” SRS procedures, via means of dose and/or volume fractionation.111,112

Published results are interesting, and there is no evidence of increased adverse radiation effects (AREs).111,112,152 In fact, the well-known relationship between the dose–volume integral and the risk of adverse radiation effects does not seem to apply at lower-dose regimens.135,152,175,176 Nonetheless, in meningioma treatments, relevant limits, pitfalls, and risks remain to be tackled. An instructive example is represented by dosimetry planning for cavernous sinus meningiomas (Fig. 104-8). The dose heterogeneity of these treatments usually requires an extremely careful evaluation of dose-distribution algorithms,177179 because of the occasional reported cases of radio-induced vascular injury to the carotid arteries,177179 the still observed morbidity of this technique on sensory nerves,151 and the sometimes disappointing results in atypical and anaplastic lesions.2,151 Finally, a controversial issue in treatment planning pertains to the so-called “radiation-induced meningiomas.180182 These tumors usually appear at a variable interval from radiation exposure, depending on the dose and timing. They are non-infrequently deep seated, multifocal in 4.6% to 18.7% of the cases.180,181 Surgical resection is considered to be the best therapeutic option. However, when dealing with frail patients or with particularly crucial locations, radical surgery may be too risky, opening the question on the SRS alternative. The published literature is rather scanty.180,182 Eligibility criteria (limited volume, well-defined imaging, etc.) adopted in these cases do not differ from standard protocols. Clinical-radiologic results—particularly the LTC (75% to 78%) and the 5-year PFS (similar)—are satisfactory, and only slightly inferior to the published data regarding regular meningiomas. Morbidity remains low (5%) with no reported cases of newer carcinogenesis.180182

Radiobiology

Radiosurgery, like most radiation treatments, results in the formation of free radicals as electrons are freed from their atoms. Their main biological effect occurs at the DNA level: The transfer of energy results in breaks in the DNA strands (direct effect). Additional radiation damage to the DNA is mediated through reactive species of water (indirect effect). Whenever single-strand breaks occurs, the aberrations are of little eventual consequence, because the breaks are easily repaired. Conversely, lethal aberrations may occur leading to mutagenesis, or to cell death, due either to double-strand breaks or to chromosome breaks with the “sticky ends” rearranging and rejoining in grossly distorted, nonviable formations. The number of lethal aberrations and subsequent killed cells is closely related to several conditioning factors: the specific oncotype and a complex series of cellular parameters (“alpha–beta ratio,” superoxide-enzyme characterization, etc.) defining the specific radiosensitivity, the radiation dose, the tumor volume, and the microscopic model of energy deposition. On the basis of these features, meningiomas mostly belong to relatively radiosensitive, late-responding tissues.

Effective dosages are in the lower range, not far from normal-cell thresholds, while the time interval for the effect is close to the maximum in-vivo doubling time.21,22,183185 Moreover, all kinds of ionizing radiations are currently identified, not only according to dose level, but also characteristic pattern of energy transfer and consequent relative biological effectiveness (RBE).186 Linear energy transfer (LET) is defined as the average energy that is locally delivered by a particle in an absorbing medium divided by unit length of the crossed distance. Larger particles are correlated with higher LET values, with increasing ionization density and greater RBE. The differential cytotoxicity of low- versus high-LET radiation beams is particularly emphasized in hypoxic tumors. Indeed, some of the main reactive molecules produced by radiation beams (i.e., ion pairs and free radicals) are represented by superoxide compounds with unpaired valence electrons, breaking DNA-protein chemical bonds. Therefore, in most cases the permanent effect of this free radical–mediated injury is dependent on the presence of oxygen.

When tumor cells rapidly divide, they outgrow their vascular supply and become chronically hypoxic. With increased LET beams, this close dependence on oxygenation for biological effect decreases. On the contrary, low-LET sources such as x-rays and gamma-rays rely significantly on local oxygen levels for their biological effect. Hence, the importance sometimes associated with the radiosurgically typical dose inhomogeneity that may somehow compensate for this disadvantage is the hot spot in the “core” of a tumor, which may be desirable for several reasons. First, it offsets the relative protection offered by the poor oxygenation of the tumor core; second, it may increase the cell kill in the surrounding of the hot spot due to the “penumbra effect.” Fractionated radiation results in the distribution of lethal and sublethal effects within a wider targeted field. In contrast, the typically small targeted size and sharp dose fall-off of radiosurgery allows the delivery of high radiation levels to a limited area.

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