Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)

Indications

SRS/SBRT is appropriate for patients with brain metastases and certain primary brain tumors, eg, meningiomas, vestibular schwannomas, pituitary tumors, and hemangioblastomas, as well as those tumors considered resistant to conventional fractionated or whole-brain radiation therapy (WBRT).[6][8][9][10][11]

Brain metastases are the most common intracranial tumors in adults and can develop in up to 30% of patients with cancer, although this incidence is at present increasing.[12] SRS/SBRT can be used to treat single or multiple lesions, often deep-seated and surgically inaccessible due to their location.[4] SRS/SBRT can be used as an adjunct to surgery for postoperative residual disease and in cases of tumor recurrence. This treatment is ideal for patients with a limited number of cerebral metastases that would not be amenable to complete surgical resection.[13] Multiple, sequential SRS/SBRT treatment sessions can also be performed, with a median overall length of survival of 5 to 6 months for those undergoing 1, 2, 3, or more than 4 SRS/SBRT treatments.[14] Uncertainty exists regarding the use of SRS/SBRT combined with WBRT for brain metastasis, where no clear clinical consensus has been established. Trials for brain metastasis treatment have shown advantages in local control with combination therapy; however, conflicting outcomes have been reported for length of survival and the effect on cognition. Although most studies indicate that less cognitive deterioration occurs when SRS/SBRT is used alone, the effect on cognition remains unclear.[15][16][17][18][19][20]

Meningiomas and vestibular schwannomas are extra-axial tumors with clear, defined borders that are well-suited for SRS/SBRT treatment. Meningiomas are commonly treated with 12 to 13 Gy in a single radiation fraction, achieving growth control rates of 90% to 98% with a potential for toxicity of less than 10%.[21] SRS/SBRT provides a tumor control equivalent for Simpson grade 1 resection for tumors <3.5 cm in diameter over long-term follow-up.[21] Adjuvant SRS/SBRT following subtotal resection has a superior 15-year progression-free survival rate compared to patients with gross total resection who do not receive SRS/SBRT.[21]

Vestibular schwannomas are commonly treated with 12 to 13 Gy in a single fraction.[22] The 5-year local control rates for vestibular schwannomas range from 92% to 100%, with hearing preservation rates being superior to those achieved with surgery (32% to 71%), and cranial nerve V and VII preservation rates exceeding 95%.[23][24] Hearing preservation rates of 63% to 94% have been reported with fractionated SRS/SBRT; however, there is a lack of long-term follow-up, and no randomized controlled trials have been conducted comparing the 2 modalities.[23]

For pituitary adenomas with expansion of the intrasellar region or invasion into the cavernous sinus, surgery may only achieve a subtotal resection. In such cases, SRS or fractionated SBRT is an appropriate treatment option.[25] The reported rate of tumor control ranges from 91% to 99%. Endocrine disturbances occur in 1% to 22% of cases, and optic neuropathy results in 1% to 2% of cases, depending on the tumor volume.[26] The highest remission rates for endocrinopathies are observed in patients with Cushing disease, and the lowest are in those with prolactinomas.[27]

For primary gliomas, studies have evaluated the impact of SRS combined with conventional fractionated SBRT as a means of dose escalation and to decrease treatment time in patients with limited life expectancy.[22] Prospective and retrospective studies reveal a median survival of 6 to 18 months following salvage SRS for recurrent glioblastoma multiforme (GBM) and 6 to 14 months with fractionated SBRT.[22] SRS delivered to previously irradiated brain tissue increases the risk of future radionecrosis; therefore, some studies have evaluated the use of bevacizumab for treating GBM and have found positive results.[28][29] 

SRS can be used to treat vascular pathologies, eg, arteriovenous malformations (AVMs), dural arteriovenous fistulas, and cavernomas.[30][31][32] Rates of complete obliteration for AVMs have been determined to be 76% for low-grade Spetzler-Martin I and II, 69% for grade III, and for high-grade IV-V AVMs, obliteration rates range from 0 to 61%.[33][34][35] Volume-staged SRS, where distinct geometrical parts of AVM are treated over time, has higher obliteration rates and similar complication rates compared to dose-staged fractionated SBRT or repeat SRS for large AVMs that are not amenable to single-session SRS.[36] SRS may have a role in patients with dural arteriovenous fistulas that are not amenable to surgery or endovascular embolization with an acceptable success rate and adverse effect profile.[37]

SRS was originally developed to treat functional disorders that include intractable pain, epilepsy, and movement disorders. The first SRS procedure performed was a thalamotomy for a patient with trigeminal neuralgia.[1] SRS is a treatment option in movement disorders for patients who are not suitable for open neurosurgery or deep brain stimulation (DBS). SRS has achieved comparable rates of tremor control to those of DBS placement and radiofrequency lesion generation, with an 85% to 90% rate of clinical improvement.[38] SRS thalamotomy has focused on treating the nucleus ventralis intermedius.[38] For trigeminal neuralgia, 50% to 80% of patients achieve complete pain relief, although several patients have required repeat SRS treatment, with associated trigeminal dysfunction in the form of facial numbness.[39][40]

SRS/SBRT are useful for seizure control in patients with mass lesions, including tumors, AVMs (with a mean seizure remission rate of 70%), and cavernous malformations (with a mean seizure remission rate of 50%).[41] The results of SRS/SBRT for treating mesial temporal lobe epilepsy show a wide range of efficacies in the literature, ranging from 0% to 86%.[41] Possible mechanisms include neuromodulation and ablation.[42] The recent ROSE trial suggests that open surgery has an advantage over SRS/SBRT for seizure remission, but that SRS/SBRT is a safe and appropriate alternative.[43]

SRS is an option for spinal and paraspinal lesions due to its noninvasive and targeted approach, including intramedullary and intradural spinal tumors.[44][45] The most common indication is vertebral metastasis, causing cancer-related pain.[46] Other indications include primary treatment for metastases, radioresistant pathologies, failure of radiotherapy, and as adjuvant treatment for residual or recurrent disease.[46] SRS can yield high rates of pain relief (85% to 92% within a few days to weeks) and tumor control (77% to 94%) and is an indicated therapy option for patients who cannot tolerate surgery, have residual disease, or as a palliative treatment.[46][47]

SBRT has been in use since the 1990s and includes treating pulmonary and hepatic tumors.[44][48][49]

Contraindications

Contraindications to using SRS/SBRT include extensive target lesions or multiple lesions that are too numerous to be effectively treated. Single-fraction SRS is usually limited to small lesions that are typically <3 cm, although fractionated SBRT is being used increasingly for larger lesions.[50]

For newly diagnosed glioblastoma, a randomized trial comparing SRS followed by external beam radiotherapy and carmustine chemotherapy versus only SBRT and carmustine showed no difference in overall survival, improvement in local control, or quality of life.[51] Reports of late toxicity were 4%.[51] Further studies, trials, and systematic reviews support this data. Currently, insufficient evidence supports the use of SRS/SBRT.[52]

SRS pallidotomy yielded a high complication rate, including visual field defects, and is not routinely in use for this indication.[38]

Equipment

Radiation Therapy Types

Various devices support SRS/SBRT, each differing in the type of radiation delivered and the technique used to focus beams on the target.

Ionizing radiation includes both electromagnetic and particulate forms capable of producing ions as they pass through matter. A photon, defined as a discrete packet of electromagnetic energy, spans a range of wavelengths, including radio waves, infrared, visible light, ultraviolet, and x-rays, depending on its energy level. Among these, only x-rays possess sufficient energy to produce ions, making them suitable for use in SRS/SBRT. Photon beams originate either from radioisotope sources, eg, cobalt-60, or from x-ray generating machines like linear accelerators. Radioactive isotopes consist of atoms with unstable nuclei that emit ionizing radiation through radioactive decay as they transition to a more stable state.

Focused gamma beams

Focused gamma beams are the simplest and oldest SRS device, first used on in 1967.[2][44] This device is a cobalt-60 radioisotope-based device with 192 individual sources of radiation, arranged in a conical tungsten shell.[2][6] The radioactive decay of cobalt leads to the emission of ionizing radiation in the form of beta particles and 2 strong gamma radiations: one with 1.17 MeV of energy and one with 1.33 MeV of energy.[5] This approach means that the effective energy of these focused beams is 1.25 MeV. Treatment energy is measurable in MeV or 1 million electron-volts. One electron-volt is the energy gained by an electron that accelerates through a potential difference of 1 volt.

The cobalt sources are arranged in a cylindrical array via a collimator helmet to focus the beams to converge at a single target. Overlapping targets can generate "hot spots" within the treatment volume.[5] The cylindrical configuration limits its use to intracranial and some upper cervical spine level pathologies, and it is used mainly for the delivery of a single fraction.[5] This system uses an automated internal collimation system (with sizes of 4, 8, or 16 mm) to replace the helmet.[44] Cobalt-60 will decay to nickel, with a half-life of 5.26 years. This means that the cobalt sources need to be replaced after a period or time to avoid prohibitively long treatment times.

Linear accelerator radiosurgery

Linear accelerator SRS was introduced clinically in the early 1980s.[5] This technique utilizes a linear-accelerator-based device to generate high-energy x-rays, which are produced by a magnetron that accelerates electrons against a tungsten metal target. The electrons are produced from an electron gun by thermionic emission. X-rays are emitted as the electrons decelerate after striking a metal target. The photon output is a continuous spectrum of x-rays, with peaks at specific energies depending on the atoms in the target. The number of x-ray particles produced increases as the kinetic energy of the electron increases. A 6 MeV electron beam is often the choice of energy for SRS.[5] The photon output can take any energy from 0 to 6 MeV. Linear accelerator-produced energy is denoted as MV or megavolts, not MeV. This means that photon beams from linear accelerator radiosurgery have a continuous "bremsstrahlung" or "deceleration radiation" spectrum of energy that is fixed compared to radioisotope sources that decay over time. A collimator focuses these x-rays.[6] The accelerator is mounted on a gantry that can rotate with intersecting arcs of radiation focused at the isocenter. The accelerator can be used for extracranial targets and as fractionated treatments. 

Another SRS/SBRT delivery system is intensity-modulated radiation therapy (IMRT), which utilizes a linear accelerator-based application to vary the shape and intensity of the beam, allowing for a nonuniform dose distribution with steep dose gradients between target volumes and normal brain tissue.[53] This system delivers helical tomotherapy plans using a 6 MV unflattened x-ray beam and a binary multileaf collimator.[54] Many small, elongated, and off-axis subfields create the intensity modulation.[53] This method is useful for extracranial and spinal SRS delivery.[46]

Protons are positively charged particles found in the nucleus of an atom and used for particulate radiosurgery. A synchrotron or cyclotron-based device can generate proton beams by ionizing hydrogen. The proton beam can have an energy range of 20 to 190 MeV, depending on the device.[44] When charged particles move through the material, they ionize atoms and deposit energy. Protons interact with matter by Coulomb collisions between electrons and nuclei (a binary elastic collision of charged particles interacting through their own electric field), by bremsstrahlung radiation loss, and by nuclear reactions.[55] As proton beams travel through tissue, they lose energy, and this energy loss is inversely proportional to the square of the velocity, meaning that the dose distribution pattern shows a slowly rising dose, followed by a sharp increase called the Bragg peak near the end of the range.[55] Most of the energy gets deposited at a discrete band of spatial depth, just before the particle stops (unmodulated Bragg peak). Several superimposed, intersecting beams produce a modulated Bragg peak, with sufficient energy for larger treatment volumes.[5] This can provide a moderate entrance dose, a uniformly high dose within the target tissue, and no doses beyond the target.[55] Proton beams can be precisely focused to control the depth of penetration while depositing most of the energy at the target.[6] Due to both the considerable cost of the equipment and the housing space considerations, proton SRS is an infrequent option.

To date, no randomized controlled trial exists to compare the various devices, so physician expertise and machine availability generally guide clinical practice. Comparative studies are inconclusive, although they commonly demonstrate equivalence.[40][56]

Dosimetry

Dosimetry is a vital but difficult component of SRS/SBRT. With SRS/SBRT, small fields are used that are typically 0.3 x 0.3 cm up to 4 x 4 cm in size. By definition, a small field is when at least 1 of 3 conditions are satisfied: (1) loss of lateral charged particle equilibrium on the beam axis; (2) partial occlusion of the photon source by the collimator; and (3) the size of the detector is similar or larger than the beam dimensions.[54][57] Challenges due to these small fields include lack of charged particle equilibrium, partial blocking of the beam source, changes in stopping power ratios, and the availability of detectors with comparable sizes to the field dimensions.[57][58] With lateral electronic disequilibrium, there is a lack of lower energy electrons, so that the average energy spectrum at the central axis will increase, and the stopping power ratio of water to air will decrease.[57] This results in an overlap between the field penumbra and the detector volume. The detector itself produces a perturbation that is hard to quantify, and a correction factor is necessary.[58]

The use of the IAEA/AAPM (International Atomic Energy Agency/American Association of Physicists in Medicine) protocol for reference and relative dosimetry based on Alfonso et al (2008) is recommended.[54] This uses a correction factor dependent on beam energy, type of detector, type of machine, and focal spot size.[54] Reference dosimetry is based on a 10 x 10 cm field. For SRS machines, where the conventional 10 x 10 cm reference field cannot be established, specific reference conditions related to the machine-specific reference fields are used.[54][58] Calculations are indirect, and significant deviations in the dosimetry result.[54][58] Monte Carlo simulation is extensively used to improve the accuracy of small-field dosimetry under nonequilibrium radiation conditions.[57][58]

The dose is in gray (Gy), which is the absorption of 1 joule of radiation energy per kilogram of matter. The SRS dose is biologically equivalent to 5 to 6 weeks of daily conventional radiation therapy.[6] The dose is deliverable in either a single session or 2 to 5 sessions of fractionated therapy over several days known as fractionated SBRT (or FSR).[5] The advent of nonfixed SRS immobilization systems, onboard imaging to verify accuracy, and patient monitoring systems during radiation allows reproducible, precise patient positions, making fractionation possible.[22]

SBRT can be used in eloquent areas, for larger lesions, and for lesions close to critical structures, eg, the optic chiasm, ventral cochlear nucleus, and the brain stem, which have a lower tolerance to radiation therapy.[5] The result of SBRT is that the biologically effective dose is increased, with decreased toxicity.[22][59] By using fractions, the total treatment dose can be kept below the radiation tolerance of critical structures, while still achieving therapeutic tumor control.[5] For example, an optic apparatus maximum dose associated with a clinically reasonable risk of radiation-induced necrosis of the optic nerve is 10 Gy in 1 fraction, 20 Gy in 3 fractions, and 25 Gy in 5 fractions.[60] The Radiation Therapy Oncology Group Trial, which looked at patients receiving SRS for brain metastases following WBRT or recurrent gliomas postradiation, developed dose limits for SRS of 24 Gy for lesions <2 cm, 18 Gy for lesions 2 to 3 cm, and 15 Gy for tumors 3 to 4 cm.[50] This meant that tumors with larger volumes received lower doses, resulting in poorer rates of local tumor control.[22] FSR may provide an improved balance of tumor control and toxicity in these instances. Meta-analysis has shown that for large brain metastases, SBRT can reduce the rate of radionecrosis while maintaining or improving the rates of 1-year local control versus single-fraction treatment.[22][59]

Deciding on using SRS or SBRT for brain metastases is multifactorial. Physical factors (eg, tumor size, margins, optimal dose), biological factors (histology of the metastases, use of systemic treatment agents), and clinical factors (life expectancy, comorbidities, concurrent treatments required) all play a role in this decision-making.[22] Currently, trials are ongoing to determine the role of FSR for brain metastases, glioblastomas, meningiomas, and vestibular schwannomas, with the primary outcome goal of determining the maximum tolerated dose for these lesions.[22]

Personnel

Healthcare team members who may be involved with SRS and SBRT include:

• Neurosurgeons
• Radiation oncologists
• Neuroradiologists
• Medical physicists
• Dosimetrist
• Radiation therapist
• Nurse specialists (oncology nurses)

Technique or Treatment

SRS/SBRT is a noninvasive, outpatient procedure that does not require a general anesthetic (see Image. Stereotaxic Surgery). The first step of the treatment procedure is to localize the treatment target. Focused gamma rays will be directed to the target using a specialized head frame that is physically attached to the skull using moderate sedation and a local anesthetic. The patient is immobilized during treatment by a head frame that is secured to the skull by sharp pins that pierce the scalp. A frameless head fixation device is also available. A fiducial reference box is secured to the head frame to provide target coordinates, and magnetic resonance imaging (MRI), computed tomography (CT), or angiography is performed to define the target in 3 dimensions. With the focused gamma ray treatment, the patient and the machine remain stationary throughout the procedure.

For linear accelerator-based SRS, either the patient or the gantry will move in space to change the radiation delivery point. In the past, a stereotactic frame was used to restrict head movement. However, newer frameless approaches have been developed, with improved patient comfort.[44] One of these technologies utilizes a small 6 MV linear accelerator mounted on a robotic arm with 6 degrees of freedom and 2 orthogonal x-ray cameras to create a dynamically manipulated, real-time therapy beam.[44] The treatment couch features electronic controls for 5 degrees of freedom (x, y, z, head tilt, and left-right rotation), as well as an additional manual rotation (clockwise or counterclockwise) based on the patient's position.[61]

An individualized treatment plan is developed to focus the radiation treatment as accurately as possible. A computerized treatment planning system is used and requires depth doses, tissue maximum ratio (the ratio of the dose at a given point in a phantom to the dose at the same point at the reference depth of maximum dose), off-axis factor, and collimator output factor for each stereotactic collimator.[57] Individual beams can be developed to conform to irregular shapes, minimizing radiation delivery and potential damage to normal brain tissue. Dose adjustment is according to the maximum dose that is deliverable to surrounding structures. Multiple lesions can be treated sequentially or simultaneously, depending on the type of device used.

Once quality assurance is guaranteed and an independent output check has been performed, the patient is positioned on the treatment couch, and the radiation treatment is delivered.[62] The treatment time varies from 30 minutes to 3 hours. On occasion, intravenous dexamethasone is administered to prevent complications caused by swelling from the radiation treatment that can be tapered posttreatment. Most patients are discharged within an hour after treatment and can resume normal activities within a few days.

Complications

SRS/SBRT causes vascular endothelial damage and demyelination of the white matter, leading to tissue necrosis.[63] Acute effects, which occur within weeks of treatment, are often due to cerebral edema resulting from disruption of the blood-brain barrier. These effects can also be caused by white matter tract damage following an injury to vascular endothelial cells, axonal demyelination, and coagulation necrosis, which can be permanent and may include progressive memory loss.[4] Symptoms include headache, nausea, and vomiting.[4] Subacute effects are due to diffuse demyelination and resolve within 6 months of treatment. Symptoms include somnolence and fatigue.[4] Late effects that occur 6 months after treatment are the result of damage to white matter tracts following an injury to vascular endothelial cells, axonal demyelination, and coagulation necrosis, which can be permanent and may include progressive memory loss.[4]

SRS has fewer adverse effects than WBRT.[64] An acute adverse effect commonly experienced after treatment is headache, with less frequent complications that include pin-site infections, seizures, and short-term exacerbations of neurological symptoms, affecting less than 5% of patients.[6][65] Late effects that occur in <5% of patients include radiation necrosis, brain edema, and new or worsening neurological deficits.[65][6] Systematic reviews suggest that the development of brain edema depends on the size of the tumor treatment margin, maximum treatment dose, tumor size/volume, lesion location (particularly parasagittal, parafalcine, or convexity), first surgical resection, and preexisting cerebral edema.[66] SRS can cause delayed cranial neuropathies, especially of the optic nerve and visual pathways.[67] Factors associated with the injury include the type of nerve involved, the maximum dose, and the length of the irradiated nerve.[67]

SBRT is emerging as a valuable treatment option. This technique has a lower rate of radiation necrosis than SRS and a reduced risk of radiation-induced optic nerve neuropathy.[59][60]

Clinical Significance

SRS is an invaluable treatment option for brain and spine metastases, primary brain tumors, vascular malformations, and functional disorders. As the techniques continue to develop, the clinical indications for treatment are expanding. 

SRS is a superior treatment option to WBRT, which is associated with significant neurological complications, including neurocognitive impairment, which limits its use.[4] The advantage of SRS over surgery is its minimally invasive nature. The procedure is performed as an outpatient and does not require general anesthesia; multiple lesions can be treated in the same session. Lesions located in deep, surgically inaccessible areas or those in eloquent areas can be safely and successfully treated.

Enhancing Healthcare Team Outcomes

The successful delivery of SRS/SBRT depends on a coordinated interprofessional team approach involving physicians, advanced practitioners, nurses, pharmacists, radiation therapists, medical physicists, and dosimetrists. Each professional contributes specialized expertise throughout the care continuum, from pretreatment assessment to postprocedural follow-up. Physicians—particularly neurosurgeons and radiation oncologists—jointly evaluate clinical indications, determine treatment plans, and ensure alignment with surgical or systemic therapies. Advanced practitioners support clinical assessments and ongoing monitoring, often serving as critical liaisons between the patient and specialists. Neuroradiologists assist in precise imaging and target delineation, while dosimetrists and medical physicists collaborate to design and verify highly conformal treatment plans, ensuring both safety and efficacy. Effective team communication, real-time collaboration, and shared decision-making are essential for accurate execution and optimal patient-centered outcomes.

Radiation oncology nurses play a pivotal role in facilitating procedural and supportive care. Their responsibilities extend beyond technical assistance to include patient education, emotional support, and coordination of interprofessional efforts. Before treatment, nurses conduct comprehensive assessments, provide patients and their families with vital information, and serve as key communicators. During treatment, they assist with setup and immobilization, monitor patient status, and ensure comfort and safety throughout the session. Postprocedure, they manage discharge planning and address any patient concerns or complications.[68] This level of coordination requires seamless communication across all involved professionals, reinforcing safety, improving outcomes, and ensuring a high standard of care delivery. The success of SRS/SBRT rests not only on advanced technology but also on the cohesion and performance of the interprofessional team.

Nursing, Allied Health, and Interprofessional Team Monitoring

• Monitor the vital signs 
• Assess post-procedural pain and communicate the pain score to the clinician
• Watch for any immediate adverse events after the procedure and get back to the clinician
• Communicate discharge orders to the patient and make sure they have the necessary contact information
• Communicate with the interprofessional team if any untoward changes in vital signs or any other issues develop