How do you delineate the resection bed, especially in cases of brain metastases, for radiation therapy planning using imaging modalities such as Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) scans?

Delineation of the Resection Bed for Brain Metastases

For radiation therapy planning after brain metastasis resection, delineate the resection cavity using postoperative MRI with gadolinium contrast obtained within 24-72 hours of surgery, combining T1-weighted postgadolinium sequences with additional sequences including T2 FLAIR, diffusion-weighted imaging (DWI), and susceptibility-weighted imaging (SWI) to accurately define the surgical bed margins and detect residual tumor. 1, 2

Optimal Imaging Timing and Modality

Early postoperative MRI is critical for accurate resection bed delineation:

• Obtain MRI within 24-72 hours after surgical resection to minimize confounding postoperative changes while accurately assessing extent of resection and defining the cavity for radiation planning 1, 2
• MRI with intravenous gadolinium contrast is the gold standard imaging modality, providing superior soft tissue contrast compared to CT for visualizing tumor margins and relationship to critical neural structures 1, 3
• The 60.8% of physicians who perform early postoperative MRI within ≤72 hours represent current best practice, as this timing optimally balances assessment of residual disease before significant postoperative inflammation develops 2

Essential MRI Sequences for Resection Bed Contouring

Use a standardized multimodal MRI protocol that includes:

• High-resolution 3D T1-weighted postgadolinium sequences (such as MPRAGE, SPACE, CUBE, or VISTA) with 1 mm isotropic voxels for anatomic detail and cavity margin definition 1
• T2 FLAIR sequences to identify vasogenic edema surrounding the resection cavity and detect leptomeningeal enhancement that may indicate pachymeningeal seeding 1
• Diffusion-weighted imaging (DWI) to distinguish residual hypercellular tumor (which restricts diffusion) from postoperative fluid collections 1, 4
• Susceptibility-weighted imaging (SWI) to detect hemorrhage and blood products that commonly occur postoperatively and may obscure cavity margins 1

Specific Contouring Guidelines

Define the clinical target volume (CTV) using these principles:

• Delineate the entire resection cavity as visualized on T1-weighted postgadolinium images, including any rim enhancement which represents disrupted blood-brain barrier at the surgical margin 1, 4
• Include areas of residual enhancement that may represent residual tumor, as multimodal MRI detects residual disease with 82% sensitivity compared to only 61% for single-observer gadolinium-enhanced imaging alone 4
• Incorporate T2 FLAIR hyperintensity extending beyond the enhancing cavity rim, as this represents vasogenic edema and potential microscopic tumor infiltration 1
• Add a 5-10 mm margin around the resection cavity to create the CTV, accounting for microscopic disease extension beyond visible margins 1

Advanced Imaging for Challenging Cases

When standard imaging is insufficient for margin delineation:

• Use perfusion-weighted imaging (PWI) to assess blood flow and blood volume maps, which confer the greatest improvements in margin detection for late-stage metastases, particularly after prior radiation therapy 4
• Consider dynamic contrast-enhanced MRI to evaluate tumor vascularity and distinguish viable tumor from postoperative changes 1, 3
• Employ MR spectroscopy to provide metabolic information that can differentiate residual tumor from postoperative inflammation or necrosis 1, 3
• Amino acid PET imaging (preferred over FDG-PET) can be added to MRI to distinguish recurrent tumor from treatment-related changes when conventional imaging is equivocal 3

Critical Pitfalls to Avoid

Common errors in resection bed delineation:

• Do not rely solely on single-modality gadolinium-enhanced T1 imaging, as independent observers consistently underestimate true tumor volume by approximately 35% (85 μL vs 131 μL histologic measurement) using this approach alone 4
• Do not delay postoperative imaging beyond 72 hours, as progressive postoperative inflammation and blood-brain barrier disruption make it increasingly difficult to distinguish residual tumor from reactive changes 2
• Do not use CT alone for resection bed delineation unless MRI is contraindicated, as CT has inferior soft tissue contrast and lower sensitivity for detecting small residual tumor deposits 1, 3
• Do not ignore blood flow maps in previously irradiated patients, as these provide the most accurate margin detection in this challenging population 4

Integration with Treatment Planning

Translate imaging findings into radiation therapy planning:

• Combine MRI with planning CT for stereotactic radiosurgery treatment planning, using rigid registration to transfer the delineated cavity from MRI to the CT coordinate system 1
• Deliver adjuvant stereotactic radiation within 4 weeks of surgery to the defined resection cavity, as this timing is supported by randomized data showing improved local control compared to observation 1
• Recognize that cavity recurrence rates remain 28-40% at one year even with optimal adjuvant stereotactic radiation, underscoring the critical importance of accurate initial cavity delineation 1