3.9 SRS, SBRT & Stereotactic Radiosurgery Techniques

Definitions: SRS, SRT, and SBRT (SABR)

Stereotactic radiation therapy delivers ablative biological radiation doses using narrow, highly conformal radiation beams coupled with rigid stereotactic target localization and sub-millimeter positioning accuracy.

+-----------------------------------+        +-----------------------------------+
|  Stereotactic Radiosurgery (SRS)  |        | Stereotactic Body RT (SBRT/SABR)  |
| Intracranial Targets (Brain/Spine)|        | Extracranial Targets (Lung/Liver) |
| 1 Single Fraction (15 – 24 Gy)    |        | 1 – 5 Fractions (48 – 60 Gy Total)|
+-----------------------------------+        +-----------------------------------+

1. Stereotactic Radiosurgery (SRS)

Single-fraction high-dose ablative radiation delivered to intracranial targets (e.g., brain metastases, acoustic neuromas, arteriovenous malformations [AVM], trigeminal neuralgia). Prescribed dose ranges from 15 Gy to 24 Gy in 1 single fraction.

2. Fractionated Stereotactic Radiotherapy (SRT)

Intracranial stereotactic treatment delivered in 2 to 5 hypofractionated doses (e.g., 24–30 Gy in 3 fractions) for lesions located immediately adjacent to sensitive structures like the optic chiasm or brainstem to minimize normal tissue toxicity.

3. Stereotactic Body Radiation Therapy (SBRT / SABR)

Extracranial stereotactic treatment delivered in 1 to 5 hypofractionated fractions to thoracic, abdominal, pelvic, or spinal targets (e.g., stage I non-small cell lung cancer, liver oligometastases, localized prostate cancer, spine lesions). Typical dose fractionation schemes include:

• 50 Gy in 4 or 5 fractions (10–12.5 Gy per fraction).
• 48 Gy in 4 fractions or 60 Gy in 3 fractions (lung SBRT).
• 16–24 Gy in 1 single fraction (spine stereotactic radiosurgery).

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Dedicated Stereotactic Treatment Platforms

     [ Leksell Gamma Knife ]                 [ CyberKnife Robotic System ]
  192 Cobalt-60 Fixed Sources               6 MV Compact Linac on KUKA Robot Arm
  Tungsten Collimator Helmets               Real-Time X-Ray Target Tracking
  Intracranial Single-Fx Only               Full Body (Intracranial & Extracranial)

1. Leksell Gamma Knife (Elekta)

Utilizes 192 Cobalt-60 ($^{60}\text{Co}$) radioactive sources arranged in a hemispherical vault. Beams converge through primary tungsten collimators and secondary helmets (4 mm, 8 mm, 16 mm apertures) onto a single focal isocenter. Rigid invasively pinned stereotactic skull frames achieve sub-millimeter mechanical spatial accuracy (< 0.5 mm). Exclusively used for single-fraction intracranial radiosurgery.

2. CyberKnife (Accuray)

Mounts a compact 6 MV X-band linear accelerator onto a KUKA robotic arm with 6 degrees of freedom. Tracks target movement in real time during treatment using twin ceiling-mounted X-ray imaging cameras that detect internal fiducial markers or skeletal landmarks (SynchroNY tracking system). Requires no rigid invasive head frame, allowing frameless full-body intracranial and extracranial treatments.

3. Dedicated C-Arm Linear Accelerators

Standard C-arm linear accelerators upgraded for stereotactic delivery, equipped with High-Definition Multi-Leaf Collimators (HD-MLCs, 2.5 mm leaf width), high dose-rate Flattening Filter Free (FFF) beam modes (up to 1400–2400 MU/min), Surface Guided Radiation Therapy (SGRT), and cone-beam CT (CBCT) capable of 6D robotic couch corrections (pitch, roll, yaw).

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Physics of Steep Dose Falloff and Small-Field Dosimetry

Steep Isodose Falloff Metrics

Stereotactic plans require an extremely steep dose gradient outside the target volume to protect adjacent normal tissues.

1. RTOG Conformity Index (CI): $\text{CI}_{\text{RTOG}} = \frac{V_{\text{RI}}}{\text{TV}}$ Where $V_{\text{RI}}$ is the volume enclosed by the prescription isodose line, and $\text{TV}$ is the target volume (PTV). Ideal value is 1.0 to 1.2.

2. Gradient Index (GI): $\text{GI} = \frac{V_{50\%}}{V_{100\%}}$ Where $V_{50%}$ is the volume receiving 50% of the prescription dose, and $V_{100%}$ is the prescription volume. A low Gradient Index ($\text{GI} < 3.0$) confirms rapid dose falloff into surrounding normal brain or tissue.

Small-Field Dosimetry Challenges

Fields smaller than $3 \times 3\text{ cm}^2$ introduce severe physical measurement challenges:

1. Loss of Lateral Charged Particle Equilibrium (CPE): Secondary electron range exceeds beamlet width, causing central axis dose collapse.
2. Source Occlusion: Collimator jaws partially block the primary focal spot (1–2 mm), underestimating dose.
3. Detector Volume Averaging: Standard 0.6 cc Farmer ionization chambers are larger than the small radiation field, causing severe underestimation of peak central dose. Micro-chambers (0.01 cc), synthetic diamond detectors, or unshielded diodes are mandatory.

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Target Volumes & Motion Management: ITV Concepts

Extracranial targets in the chest and abdomen move dynamically during patient respiration, requiring specialized volume expansion and motion management.

  [ Gross Tumor Volume (GTV) ]
               |
               v
  [ Clinical Target Volume (CTV) ]  --->  + 4D-CT Respiratory Motion Envelope
               |                                     |
               v                                     v
  [ Internal Target Volume (ITV) ] <-----------------+
               |
               v  + Setup Uncertainty Margin (2 – 5 mm)
  [ Planning Target Volume (PTV) ]

1. 4D-CT Simulation

Acquires CT images correlated with a respiratory belt or surface tracking signal. Images are sorted into 10 respiratory phase bins (0% peak inhalation to 50% peak exhalation).

2. Internal Target Volume (ITV)

Defined by enveloping the Clinical Target Volume (CTV) across all breathing phases of the 4D-CT scan:

$\text{ITV} = \text{CTV} + \text{Internal Margin (Respiration Motion)}$

3. Planning Target Volume (PTV)

Expands upon the ITV to account for mechanical daily positioning and alignment uncertainties:

$\text{PTV} = \text{ITV} + \text{Setup Margin (2 – 5 mm)}$

4. Respiratory Motion Management Strategies

• Active Breathing Control (ABC) / Deep Inspiration Breath-Hold (DIBH): Temporarily holds patient breathing at a reproducible lung volume to freeze target motion during beam delivery.
• Respiratory Gating: Delivers radiation only when the tumor moves within a pre-defined window of the breathing cycle (e.g., 40% to 60% end-exhalation phase).
• Real-Time Target Tracking: Continuously repositions the radiation beam (or robotic arm) in sync with moving internal markers (e.g., CyberKnife SynchroNY).