BEAIN METSASTASIS-TARGET VOLUME DELINEATION

Whole brain RT versus SRS

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Clinical decision-making

  • The choice of whole brain radiation therapy (WBRT) versus stereotactic radiosurgery (SRS) depends on:
    • Number of brain metastases.
    • Aggregate tumor volume and location.
    • Performance status and overall prognosis.
    • Availability and results of prognostic tools (e.g. molecular graded prognostic assessment).
  • These decisions are complex and typically individualized; the detailed discussion is beyond the scope of the manual from which this content is derived.

Relative advantages

  • SRS
    • Offers better preservation of neurocognitive function.
    • Associated with improved quality of life in appropriately selected patients.
  • WBRT
    • Improves distant intracranial control.
    • Improves overall control rates of intracranial metastases, particularly when there are numerous lesions.
In practice, many patients are treated with SRS alone for a limited number/volume of metastases, while WBRT (with or without hippocampal avoidance and neuroprotective agents) is considered for diffuse disease or when SRS is not feasible.

WBRT: Target volume, planning and general principles

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Planning CT and immobilization

  • Non-contrast treatment-planning CT scan:
    • From vertex to upper cervical spine.
    • Axial slice thickness ≤ 2.5 mm.
  • Patient positioned supine, head in neutral position.
  • Immobilization with a thermoplastic mask; field of view ≈ 600 mm.

Neurocognitive preservation strategies

  • Use of memantine during and after WBRT.
  • Hippocampal avoidance WBRT (HA-WBRT) when feasible:
    • Requires accurate hippocampal contouring (see “WBRT fields” section).
    • Helps preserve memory and neurocognitive function.

Beam arrangement and technique

  • 3D-CRT WBRT
    • Opposed lateral photon beams, usually 6 MV.
    • Delivered with a multileaf collimator (MLC) block.
  • Hippocampal avoidance WBRT
    • Inverse-planned IMRT (or VMAT) using a planning CT fused to a gadolinium contrast-enhanced MRI.
    • MRI acquisition: three-dimensional spoiled gradient sequence, axial slice thickness ~1.25–1.5 mm to define hippocampi.

Setup verification

  • Orthogonal MV films weekly for 3D-CRT WBRT.
  • Daily kV or other IGRT methods are typically reserved for IMRT/VMAT-based WBRT, especially HA-WBRT.
For WBRT, target volumes and margins depend on the clinical scenario (diffuse metastases, leptomeningeal disease, lymphoma/leukemia, PCI, scalp-sparing, hippocampal avoidance). Details are summarized in the “WBRT fields” and “WBRT dose and fractionation” sections.

SRS target volume – general principles

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Clinical scenarios for SRS

  • Intact brain metastases treated with:
    • Single-fraction SRS, or
    • Fractionated SRS (typically 2–5 fractions).
  • Post-resection cavity SRS (after gross total resection of brain metastasis).
  • Dose fractionation depends on:
    • Target size or volume.
    • Distance from critical structures (see SRS OAR constraints table).

Equipment

  • Frame-based or frameless cobalt-based Leksell Gamma Knife®.
  • LINAC-based systems capable of high conformality and image guidance.

Imaging for target delineation

  • Preferred: Volumetric contrast-enhanced T1-weighted MRI.
    • Slice thickness 1–2 mm.
    • Used for GTV and CTV definition.
  • Alternative: Contrast-enhanced CT if MRI is contraindicated or not tolerated.
  • For LINAC-based SRS:
    • Thin-slice CT is acquired and co-registered with the MRI.
    • Fusion accuracy is crucial for tight PTV margins.

Image guidance

  • For LINAC-based SRS, daily high-quality image guidance is required (e.g. CBCT, stereoscopic kV, surface guidance) due to small margins and steep dose gradients.
Detailed SRS target volume construction for unresected lesions and post-resection cavities is provided in the “SRS target volume – methods” section.

SRS target volume – specific methods

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Suggested SRS target volume delineation (two approaches)

Target GTV CTV
Unresected brain metastases
  • Contrast-enhancing lesion on T1-weighted MRI.
  • GTV + 0 mm (no additional margin) in most protocols.
Postoperative gross total resection cavity (Method 1)
  • n/a – cavity is delineated directly on postcontrast MRI.
  • Uniform 2 mm expansion margin around the resection cavity borders visualized on postcontrast MRI.
Postoperative gross total resection cavity (Method 2)
  • n/a – use composite of enhancing tissues and cavity on postoperative MRI.
  • Includes the entire contrast-enhancing region, surgical cavity, and any enhancing surgical tract on postoperative MRI.
  • 5–10 mm margin along the bone flap beyond the initial region of preoperative tumor contact if the tumor was in contact with the dura.
  • 1–5 mm margin along the venous sinus if the initial tumor was not in contact with the dura.
  • 1–5 mm margin along the venous sinus if the initial tumor was in contact with a venous sinus (adjust per institutional protocol and critical structure constraints).
Institutions may adopt either method for resection cavity SRS. Choice depends on local practice, imaging quality, and proximity to critical structures. PTV margins are typically small (e.g. 1–2 mm) and technique-specific.

WBRT fields and target volumes

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Suggested WBRT fields by clinical scenario

Clinical scenario Conventional WBRT Leptomeningeal disease Lymphoma / leukemia Scalp sparing HA-WBRT / PCI
Indications
  • Diffuse brain metastases (numerous or “too many to count”).
  • Prophylactic cranial irradiation (PCI) for SCLC (when HA-WBRT not employed).
  • Leptomeningeal disease.
  • CNS prophylaxis for acute lymphoblastic leukemia (ALL).
  • CNS leukemia (high-risk patients).
  • Cosmetic outcome is a priority (technique may result in a “reverse Mohawk” appearance).
  • Diffuse brain metastases when hippocampal sparing is feasible (no lesion within 5 mm of hippocampus).
  • PCI for SCLC with hippocampal avoidance when appropriate.
Technique / fields
  • 3D-CRT with lateral opposed beams, slightly rotated off-axis (RAO/LAO) to avoid divergence into the lenses.
  • Fields usually similar to conventional WBRT but with additional coverage of cribriform plate and temporal lobes (see field edges below).
  • WBRT fields as per leptomeningeal disease, with attention to cribriform plate and posterior orbital regions.
  • MLC designed to spare scalp: beam aperture conformed to skull; scalp-sparing technique may create a “reverse Mohawk” region of hair preservation.
  • Inverse-planned IMRT/VMAT for hippocampal avoidance WBRT.
Target volumes & margins
  • CTV: entire cranial contents.
  • CTV: entire cranial contents including:
    • Optic nerves.
    • Retro-orbital region.
    • Lamina cribrosa / cribriform plate.
  • CTV: entire cranial contents including:
    • Optic nerves.
    • Retro-orbital region.
    • Retina.
    • ± Whole globe if ocular involvement is present.
  • CTV: entire cranial contents; scalp is excluded using MLC shaping.
  • Brain metastases:
    • CTV: whole brain parenchyma down to the foramen magnum.
    • PTV: CTV minus hippocampi (hippocampi expanded by 5 mm are avoided).
  • Normal structure constraints for HA-WBRT:
    • Hippocampi D100% ≲ 9 Gy.
    • Hippocampi Dmax ≤ 16 Gy.
Field edges (Conventional WBRT)
  • Superior (Sup): Approximately 2 cm flash above the skull.
  • Posterior (Post): 2 cm flash; ± posterior neck MLC block to protect soft tissues when appropriate.
  • Inferior (Inf): Bottom of C1 vertebral body.
  • Anterior (Ant): MLC block from 2 cm flash to anterior aspect of C1, blocking parotids and lenses.
  • Ensure coverage of temporal lobes and cribriform plate, with additional 8–10 mm margin for penumbra and daily setup uncertainty.
  • MLC edge set along the outer table of the calvarium to reduce scalp dose while maintaining intracranial coverage.
  • Field edges determined by IMRT/VMAT optimization; see hippocampal and optic apparatus constraints below.
Additional coverage notes
  • Cover temporal lobes and cribriform plate thoroughly; avoid under-dosage in these regions due to propensity for leptomeningeal involvement.
  • Cover posterior one-third of the globes if no ocular involvement on slit-lamp exam.
  • If ocular involvement is present, include entire bilateral globes.
  • HA-WBRT constraints for WBRT:
    • Optic nerves and chiasm Dmax ≤ 30 Gy (brain metastases WBRT).
  • PCI for SCLC with hippocampal avoidance:
    • Hippocampi D100% ≤ 7.5 Gy.
    • Hippocampi Dmax ≤ 13.5 Gy.
    • Optic nerves and chiasm Dmax ≤ 25 Gy.
Field definitions may be adapted to local protocols and planning systems. The key concepts are full coverage of at-risk CNS compartments and adherence to hippocampal and optic apparatus constraints when scalp-sparing or hippocampal-avoidance techniques are used.

WBRT dose and fractionation

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Suggested WBRT dose–fractionation schedules

Clinical scenario Dose and fractionation
WBRT for brain metastases / leptomeningeal disease (WBRT, LMD)
  • 30 Gy in 10 fractions (most common regimen).
  • 37.5 Gy in 15 fractions (RTOG regimen).
  • 30 Gy in 12 fractions (alternative schedule).
  • 20 Gy in 5 fractions (short course; generally for poor-prognosis patients).
WBRT reirradiation
  • 20–25 Gy in 10 fractions.
  • Time interval between courses should be at least 4–6 months.
PCI for SCLC
  • 25 Gy in 10 fractions (most common regimen).
CNS prophylaxis for ALL
  • 12 Gy in 8 fractions.
CNS leukemia (high-risk)
  • ≥ 18 Gy in 9–10 fractions (dose individualised based on intensity of systemic therapy).
Dose selection should incorporate prior cranial irradiation, patient age, systemic therapy, and expected prognosis. When hippocampal avoidance and memantine are used, WBRT is typically delivered with 30 Gy in 10 fractions.

SRS dose and organ at risk constraints

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SRS dose schemes and OAR limits (Alliance A071801-based)

Parameter 1 fraction 3 fractions 5 fractions
PTV dose – postoperative cavity
  • 20 Gy (for volumes < 4.2 cm³).
  • 18 Gy (≥ 4.2 to < 8.0 cm³).
  • 17 Gy (≥ 8.0 to < 14.4 cm³).
  • 16 Gy (≥ 14.4 to < 20 cm³).
  • 14 Gy (≥ 20 to < 30 cm³).
  • 27 Gy (for appropriately selected volumes; see trial protocol for exact cut-offs).
  • 30 Gy (for larger volumes within protocol limits).
PTV dose – unresected metastases
  • 24 Gy (lesion ≤ 1 cm).
  • 22 Gy (> 1.0 to ≤ 2.0 cm).
  • 20 Gy (> 2.0 to ≤ 3.0 cm).
  • 18 Gy (> 3.0 to ≤ 4.0 cm).
  • 27 Gy (typical for fractionated SRS to unresected lesions).
  • 30 Gy.
Brainstem constraint
  • V12 Gy < 1 cm³.
  • Maximum point dose ≈ 23.1 Gy.
  • Maximum point dose ≈ 28 Gy.
Optic apparatus constraint
(optic nerves, chiasm)
  • Maximum dose 9 Gy.
  • Maximum dose 17.4 Gy.
  • V18.3 Gy < 0.2 cm³.
  • Maximum dose 23 Gy.
  • V20 Gy < 0.2 cm³.
These constraints assume modern image-guided SRS/SRT techniques. For lesions abutting critical structures, further dose reduction or alternative fractionation may be required.