Stereotactic Radiosurgery — Procedure Guide, Recovery & Risks | MyMedicPlus

Stereotactic radiosurgery (SRS) is a non-invasive procedure that delivers a single large radiation dose (12-24 Gy) precisely to an intracranial target using convergent beams, exploiting steep dose fall-off to spare surrounding tissue. Platforms include Gamma Knife (201 cobalt-60 sources), CyberKnife (robotic LINAC), and dedicated LINAC systems (HyperArc, Trilogy). Despite the name, SRS involves no surgical incision — it is a radiation treatment that achieves biological effects equivalent to surgical excision for small, well-defined intracranial lesions. Sub-millimetre accuracy is maintained throughout the session using rigid frame or frameless thermoplastic mask immobilisation.

The Gamma Knife (Leksell Gamma Knife) uses 192 cobalt-60 sources arranged in a hemisphere to focus all beams at a single isocentre, delivering a precise spherical dose distribution. The CyberKnife uses a robotic arm with a compact LINAC to deliver beams from hundreds of non-coplanar angles, conforming to irregular targets. Linear accelerator (LINAC)-based SRS uses dynamic arc delivery (RapidArc, VMAT) on standard radiotherapy machines modified for cranial SRS. All systems achieve dose fall-off steep enough to protect adjacent critical structures including the optic chiasm, brain stem, and cochlea. SRS achieves 85–95% local control for brain metastases at 12 months while preserving neurocognitive function — superior to whole-brain radiotherapy.

Primary indications include brain metastases (1-4 lesions, each below 3 cm), acoustic neuroma (vestibular schwannoma), meningioma, trigeminal neuralgia (70-90 Gy to the nerve root), cerebral arteriovenous malformations (AVM), and selected pituitary adenomas not cured by surgery or medication. SRS is preferred over conventional neurosurgery when tumours are in eloquent or deep brain locations, when the patient is medically unfit for open surgery, or for multiple metastases across different brain regions. Fractionated SRS (3-5 fractions) extends eligibility to larger lesions (3-4 cm) or targets adjacent to the optic nerves or brainstem.

Brain metastases: SRS is preferred over whole-brain radiotherapy (WBRT) for patients with 1–4 metastases less than 3 cm, preserving neurocognition (WBRT causes significant memory loss in 40% of long-term survivors). Recent trials support SRS for 5–10 metastases in selected patients. Acoustic neuroma: small-to-medium sized tumours (less than 2.5 cm) are treated with SRS as an alternative to surgery, with 95% tumour control at 5 years and hearing preservation in 50–60%. Trigeminal neuralgia: 70–90 Gy delivered to the trigeminal nerve root entry zone achieves pain-free status without medication in 60–75% of patients at 12 months.

A stereotactic head frame is applied under local anaesthesia (Gamma Knife) or a frameless thermoplastic mask is fitted (CyberKnife, LINAC). High-resolution MRI and CT are acquired on the day of treatment and co-registered for target delineation. The radiation oncologist and neurosurgeon plan the dose prescription, specifying the tumour target volume and critical structure dose constraints. For Gamma Knife, 201 cobalt sources converge simultaneously on a single isocenter. For CyberKnife, the robotic arm circles the head delivering beams along hundreds of non-coplanar trajectories. Treatment delivery takes 20-90 minutes, and the patient is awake throughout. The entire process from frame application to discharge takes 4-6 hours.

Treatment planning requires detailed dose-volume histogram (DVH) analysis to confirm target coverage (typically V12Gy — the volume receiving 12 Gy or more — is limited to 10–14 cc for brain parenchyma) and to keep critical structures within tolerance. Conformality index (ratio of prescription isodose volume to target volume) and gradient index (rate of dose fall-off) are planning quality metrics. A single isodose line prescription (typically 12–20 Gy to the 50% isodose for Gamma Knife, or 15–24 Gy to the 80–90% isodose for LINAC) ensures the target receives the prescribed dose while the surrounding brain receives far less.

Brain metastasis local control is 85-95% at 12 months with SRS, equivalent to whole-brain radiotherapy (WBRT) but preserving neurocognition. AVM obliteration is achieved in 70-80% at 3 years for lesions below 3 cm. Acoustic neuroma tumour control exceeds 95% at 5 years with hearing preservation in 50-70% of patients. Trigeminal neuralgia pain relief is achieved in 70-80% of patients within 6 months. SRS is an outpatient procedure requiring no general anaesthesia, hospital admission, or recovery period, making it accessible to patients with significant comorbidities who cannot tolerate open surgery.

For meningiomas, SRS achieves 10-year control rates of 85–95% for WHO grade 1 tumours, with the advantages over surgery of no anaesthetic risk, no surgical morbidity, and same-day outpatient delivery. Cerebral AVM obliteration after SRS occurs over a 2–3 year latency period: 70–80% of lesions less than 3 cm are obliterated at 3 years, eliminating the lifetime 2–4% annual haemorrhage risk. For melanoma brain metastases, SRS combined with immunotherapy (pembrolizumab) achieves intracranial control rates of 75–80% at 12 months, significantly superior to prior outcomes with chemotherapy.

Radiation necrosis occurs in 5-10% of patients at 1-2 years and may mimic recurrence on MRI; it is managed with dexamethasone, bevacizumab, or surgical resection. Transient brain oedema is common in larger lesions. Cranial nerve injury risk for skull base tumours is 2-5%. Alopecia at entry points occurs with some Gamma Knife configurations. Re-treatment is limited by cumulative dose constraints to critical structures. The 2-3 year latency period before AVM obliteration is confirmed represents an ongoing haemorrhage risk during this interval.

For acoustic neuroma, 3–5% risk of post-treatment facial nerve weakness and 3–5% risk of worsening hearing beyond what the tumour itself would cause. Trigeminal radiosurgery: facial numbness or dysaesthesia in 10–20% of patients at 12 months (the numbness sometimes coincides with pain relief). AVM: a 4-year risk of haemorrhage persists during the latency period before obliteration, representing the most significant limitation of SRS vs upfront resection for accessible AVMs. Pituitary adenoma radiosurgery: new pituitary hormone deficiency develops in 20–30% of patients at 5 years, requiring annual hormone surveillance and appropriate replacement therapy.

SRS is an outpatient procedure; patients go home the same day. Mild headache, scalp tenderness, and fatigue are common for 24-48 hours and managed with paracetamol and a short course of dexamethasone if significant oedema is anticipated. No activity restriction is imposed post-treatment. AVM obliteration is confirmed at 2-3 years by MRI or digital subtraction angiography. Tumour response is assessed by MRI at 3 months and 6-monthly thereafter. Patients with trigeminal neuralgia may notice pain relief beginning at 4-8 weeks, with maximum benefit at 3-6 months.

SRS is an outpatient procedure requiring no anaesthesia and no hospital admission. Patients return home the same day within 2–4 hours for Gamma Knife (which requires frame application under local anaesthesia) or after 1–2 hours for frameless mask-based systems. Mild headache, scalp tenderness, and fatigue are common for 24–48 hours and managed with paracetamol and a short course of dexamethasone (4–8 mg daily for 3 days). Alopecia at the beam entry points is rare with modern systems. Follow-up MRI is performed at 6–12 weeks and then at 6-month intervals to assess treatment response and detect late complications early.