How do I perform a volumetric study for brachytherapy?

How to Perform a Volumetric Study for Brachytherapy

Perform volumetric studies for brachytherapy using CT-based or MRI-based 3D imaging with the patient in treatment position, contour the clinical target volume (CTV) and organs at risk (OARs), and generate dose-volume histograms (DVHs) to assess dose distribution using specific volumetric parameters rather than point doses alone.

Imaging Acquisition

Obtain cross-sectional imaging (CT or MRI) with the brachytherapy applicator in place:

• For gynecologic brachytherapy: MRI is preferred for superior soft tissue delineation of the CTV 1. Use vaginal gel during pre-brachytherapy MRI to optimally delineate vaginal canal anatomy 2
• CT-based planning: Provides adequate volumetric dose assessment and is widely available 3
• Image the patient in the exact treatment position with applicator inserted to ensure accurate spatial relationships

Target Volume and OAR Contouring

Define structures according to established protocols:

For Target Volumes:

• Contour the gross tumor volume (GTV), high-risk CTV (HR-CTV), and intermediate-risk CTV (IR-CTV) 1
• Delineate the upper 1-2 cm of vaginal canal when relevant 2

For Organs at Risk:

• Contour bladder, rectum, sigmoid colon, and urethra
• Contour organ walls for volumes >2 cm³ 1
• Ensure contiguous volumes throughout the region of interest

Dose Calculation and Grid Resolution

Calculate 3D dose volumes using appropriate parameters:

• Use TG-43 formalism as the calculation basis 4, 5
• Grid resolution: Use 1.0 × 1.0 mm² or finer (down to 0.5 × 0.5 mm²) for accurate dose assessment 4
• Consider applicator volume in calculations, as excluding it can underestimate D90 by 2-5% 4
• Account for air gaps, which occur in approximately 81% of applications 6

DVH Parameter Extraction

Extract specific volumetric parameters rather than point doses:

For Target Volumes:

• D90: Minimum dose to 90% of volume (primary parameter) 1
• D100: Minimum dose to 100% of volume
• V100, V150, V200: Volumes receiving 100%, 150%, 200% of prescribed dose 1

For Organs at Risk:

• D2cc: Minimum dose to the most irradiated 2 cm³ (primary parameter) 1
• D1cc and D0.1cc: Additional volumetric parameters 1
• Optional: D5cc and D10cc for comprehensive assessment

Critical Dosimetric Considerations

Point doses significantly underestimate OAR exposure:

• ICRU bladder point doses are 1.6 times lower than D2cc bladder doses 3
• ICRU rectal point doses are 1.13 times lower than D2cc rectal doses 3
• Therefore, volumetric assessment is essential and point doses should not be used as the sole metric 3, 1

Biological Dose Conversion

Convert physical doses to biologically equivalent doses:

• Use the linear-quadratic model to calculate EQD2 (equivalent dose in 2-Gy fractions) 1
• Apply EQD2 calculations for both brachytherapy and external beam components
• Account for different dose rates in the calculation 1
• This allows systematic comparison within and between patients and institutions 1

Quality Assurance

Validate your volumetric calculations:

• Perform independent dose verification using secondary calculation systems 4
• Gamma analysis should achieve >98% passing rate at 3%/3mm criteria 4
• DVH variance should be <5% at any radial distance and <2% average variance compared to TG-43 calculations 5

Common Pitfalls to Avoid

• Do not rely on 2D orthogonal images alone - they provide only point dose estimates and miss critical volumetric information 3
• Do not ignore applicator volume in dose calculations, as this leads to systematic underestimation of target coverage 4
• Do not assume identical OAR location across fractions without verification 1
• Recognize that treated length correlates with increased OAR doses, particularly for rectum and urethra 6