# Stereotactic Radiosurgery: Principles, Platforms, and Quality Assurance

## Introduction

Stereotactic radiosurgery (SRS) is a technique that delivers a high dose of radiation in a single fraction, or in a few fractions when administered as fractionated stereotactic radiation therapy (FSRT), to a precisely defined intracranial target. The defining features of SRS include submillimeter accuracy, steep dose gradients, and an ablative radiobiologic effect. This approach has become a standard treatment modality for brain metastases, benign intracranial tumors, arteriovenous malformations, and functional disorders such as trigeminal neuralgia.

## Fundamental Principles

### Stereotactic Localization

SRS relies on a stereotactic coordinate system to define the target location in three-dimensional space. In frame-based SRS, a rigid head frame is fixed to the skull using pins, providing a fiducial reference system for precise targeting. Frameless SRS, on the other hand, depends on image guidance technologies such as cone-beam CT (CBCT), optical tracking, or intrafraction monitoring to achieve similar accuracy. Both frame-based and frameless approaches are capable of submillimeter targeting precision.

### Radiobiologic Considerations

The high single-fraction doses used in SRS, typically ranging from 15 to 24 Gy, induce both direct DNA damage and vascular endothelial injury. The traditional linear-quadratic model may underestimate the extent of cell kill at these very high doses per fraction. The ablative mechanism of SRS differs from that of conventionally fractionated radiation therapy, involving vascular damage, immune system activation, and ceramide-mediated apoptosis. The steep dose gradients characteristic of SRS serve to protect the surrounding normal brain tissue from excessive radiation exposure.

### Dose Fall-Off and Conformity

A hallmark of SRS is the rapid dose fall-off, where the radiation dose decreases by approximately 50% within 2 to 3 millimeters from the edge of the target. The conformity index (CI) is a metric used to assess how well the prescribed dose conforms to the target volume; it is defined as the ratio of the prescription isodose volume to the target volume, with an ideal value of 1.0. The gradient index (GI) measures the steepness of dose fall-off and is calculated as the ratio of the half-prescription isodose volume to the prescription isodose volume, with lower values indicating steeper gradients. Selectivity refers to the fraction of the prescription isodose volume that overlaps with the target.

![Diagram illustrating stereotactic coordinate system, dose gradient, and conformity metrics for SRS planning](images/srs-principles-diagram.jpg)

## SRS Delivery Platforms

### Gamma Knife (Leksell)

The Gamma Knife system utilizes 192 or 201 cobalt-60 sources arranged in a hemispherical configuration. These sources are collimated to converge at a single isocenter, with collimator sizes of 4, 8, or 16 millimeters. Multiple isocenters, or "shots," can be combined to conform the dose to irregularly shaped targets. The traditional Gamma Knife uses a frame-based system (Perfexion), but the newer Icon model allows frameless treatment with CBCT image guidance. Gamma Knife remains the gold standard for single-fraction intracranial SRS.

### Linear Accelerator (LINAC)-Based SRS

LINAC-based SRS employs a standard medical linear accelerator that has been modified for stereotactic applications. Collimation is achieved using either a multi-leaf collimator (MLC) or cone-based systems. Delivery techniques include dynamic conformal arcs, volumetric modulated arc therapy (VMAT), or multiple non-coplanar arcs. Immobilization is frameless, typically using a thermoplastic mask combined with image guidance. This platform is versatile, capable of treating both intracranial and extracranial targets.

### CyberKnife

The CyberKnife system features a compact linear accelerator mounted on a robotic arm with six degrees of freedom, allowing flexible beam delivery angles. It uses real-time image guidance through orthogonal kilovoltage imaging and is frameless, tracking the skull position or implanted fiducials during treatment. Its non-isocentric delivery enables highly conformal dose distributions. CyberKnife is widely used for both intracranial SRS and extracranial stereotactic body radiation therapy (SBRT).

| Platform | Source / Energy | Collimation | Immobilization | Key Features |
|---|---|---|---|---|
| Gamma Knife (Perfexion/Icon) | 192 Co-60 sources | 4, 8, 16 mm collimators | Frame-based or frameless (Icon) | Gold standard for single-fraction intracranial SRS |
| LINAC-based SRS | 6 MV (± FFF) | MLC or cones | Frameless (mask + IGRT) | Versatile; treats intracranial and extracranial |
| CyberKnife | 6 MV compact linac | Circular collimators or MLC | Frameless; robotic tracking | Non-isocentric; real-time image guidance |
| Proton SRS | Proton beam | Spot scanning | Frame or frameless | Bragg peak advantage; limited availability |

### Proton SRS

Proton SRS utilizes proton beams characterized by the Bragg peak, which allows for precise dose deposition with potential for superior sparing of normal tissue. Spot-scanning proton SRS is employed for small intracranial targets. However, proton SRS is limited in availability and remains under investigation in ongoing comparative studies.

## Clinical Applications

### Brain Metastases

SRS is well established for treating patients with one to four brain metastases, as demonstrated in the RTOG 9508 trial. Emerging data support its use for patients with five to fifteen or more metastases. The typical dose ranges from 18 to 24 Gy in a single fraction, adjusted according to lesion size based on RTOG 90-05 guidelines: lesions smaller than 2 cm receive 24 Gy, those between 2.1 and 3 cm receive 18 Gy, and lesions between 3.1 and 4 cm receive 15 Gy. Post-operative cavity SRS is also common, with doses of 12 to 18 Gy delivered either as a single fraction or fractionated.

### Meningioma

For small meningiomas less than 3 cm in size, SRS doses typically range from 12 to 15 Gy in a single fraction, achieving local control rates exceeding 90% at 10 years. Fractionated stereotactic radiation therapy (FSRT) delivering 25 to 30 Gy in five fractions is preferred for lesions located near the optic apparatus to reduce the risk of toxicity.

### Vestibular Schwannoma

SRS for vestibular schwannomas involves delivering 12 to 13 Gy in a single fraction to the tumor margin. Hearing preservation rates range from 50 to 70% at five years, while facial nerve preservation exceeds 95%. SRS serves as a non-surgical alternative for small to medium-sized tumors.

### Arteriovenous Malformations

SRS targets the AVM nidus when it is smaller than 3 cm, achieving obliteration rates of 70 to 80% at three years. Doses typically range from 16 to 25 Gy to the nidus margin, with higher doses correlating with improved obliteration. There is a latency period of one to three years before obliteration occurs, during which the risk of hemorrhage persists.

### Trigeminal Neuralgia

In trigeminal neuralgia, SRS targets the trigeminal nerve root entry zone with a maximum dose of 70 to 90 Gy. Pain relief is achieved in 70 to 90% of patients, typically within weeks to months after treatment. Facial numbness occurs in 10 to 30% of cases. SRS offers a non-invasive alternative to microvascular decompression surgery.

![Comparison of dose distributions across Gamma Knife, LINAC-based VMAT, and CyberKnife platforms for a single brain metastasis](images/srs-platform-comparison.jpg)

## Quality Assurance

### Machine-Specific QA

Machine-specific quality assurance includes the Winston-Lutz test, which verifies the coincidence of the radiation and mechanical isocenter within a tolerance of less than 1 millimeter. End-to-end testing involves treating a phantom through the entire workflow, including imaging, planning, and delivery, to ensure system accuracy. Multi-leaf collimator positional accuracy must be maintained within 0.5 millimeters, and output calibration follows the TG-51 protocol with corrections for small fields.

### Patient-Specific QA

Patient-specific quality assurance involves pre-treatment plan verification using ion chambers, film, or detector arrays. Independent dose calculations, or monitor unit (MU) checks, are performed by medical physicists. Image guidance verification occurs at the time of treatment, and intrafraction monitoring is essential for frameless treatments to maintain accuracy during delivery.

### Small-Field Dosimetry

SRS employs very small radiation fields, typically between 5 and 30 millimeters, where standard dosimetry assumptions may not hold. Guidelines such as TG-155 and TRS-483 provide protocols for small-field dosimetry. Appropriate detectors include micro-ionization chambers, diodes, radiochromic film, and diamond detectors. Output factors for small cones or MLC apertures must be carefully measured due to lateral electronic disequilibrium, which becomes significant below 1 centimeter field size.

### Safety Considerations

Safety protocols require redundant verification of patient identity, target laterality, and prescribed dose. A time-out procedure is performed before treatment delivery to confirm all parameters. Independent review of contours and treatment plans by a second physician is standard practice. The ASTRO/ACR Practice Parameters outline minimum standards for SRS programs to ensure safe and effective treatment.

![Quality assurance workflow diagram for SRS including Winston-Lutz test, end-to-end testing, and patient-specific verification](images/srs-qa-workflow.jpg)

## Key Clinical Pearls

SRS delivers ablative radiation doses with submillimeter accuracy, and the steep dose gradient is critical for protecting normal brain tissue. The Gamma Knife remains the gold standard for single-fraction intracranial SRS, while LINAC-based and CyberKnife systems provide comparable accuracy with greater versatility. For brain metastases, dosing follows the RTOG 90-05 size-based guidelines: 24 Gy for lesions under 2 cm, 18 Gy for those between 2 and 3 cm, and 15 Gy for lesions between 3 and 4 cm. Rigorous quality assurance, including the Winston-Lutz test, end-to-end testing, and small-field dosimetry, is essential for safe SRS delivery. Frameless SRS with intrafraction monitoring has largely replaced frame-based approaches for multi-fraction treatments, enhancing patient comfort without compromising precision.

## References

1. Shaw E, Scott C, Souhami L, et al. Single dose radiosurgical treatment of recurrent previously irradiated primary brain tumors and brain metastases: final report of RTOG protocol 90-05. *Int J Radiat Oncol Biol Phys*. 2000;47(2):291-298.  
2. Halvorsen PH, Cirino E, Das IJ, et al. AAPM-RSS Medical Physics Practice Guideline 9.a. for SRS-SBRT. *J Appl Clin Med Phys*. 2017;18(5):10-21.  
3. Solberg TD, Balter JM, Benedict SH, et al. Quality and safety considerations in stereotactic radiosurgery and stereotactic body radiation therapy: executive summary. *Pract Radiat Oncol*. 2012;2(1):2-9.  
4. Palmans H, Andreo P, Huq MS, et al. Dosimetry of small static fields used in external beam radiotherapy: an IAEA-AAPM international code of practice for reference and relative dose determination. TRS-483. *IAEA*. 2017.