What are the IBS-GEC (International Brachytherapy School - Groupe Européen de Curiethérapie) ESTRO (European Society for Radiotherapy and Oncology) ABS (American Brachytherapy Society) recommendations for CT-based contouring in image-guided adaptive brachytherapy for cervical cancer?

CT-Based Contouring Recommendations for Image-Guided Adaptive Brachytherapy in Cervical Cancer

The IBS-GEC ESTRO-ABS joint recommendations provide a systematic framework for CT-based target delineation in cervical cancer brachytherapy that categorizes tumor response into 4 remission patterns across 8 defined clinico-radiological environments, enabling accurate contouring when MRI is unavailable. 1

Minimum Requirements for CT-Based IGABT

The essential prerequisites for implementing CT-based adaptive brachytherapy include:

• Clinical examination and documentation at both diagnosis and at the time of brachytherapy 1
• Baseline imaging with either CT or MRI at diagnosis to establish initial tumor extent 1
• CT imaging with applicator in situ at minimum for each brachytherapy fraction 1

Core Conceptual Framework

The recommendations are structured around four fundamental assessment components:

1. GTV Assessment Strategy

• Evaluate gross tumor volume (GTV) at two critical timepoints: diagnosis and at brachytherapy 1
• Document tumor dimensions, location, and relationship to adjacent structures at both assessments 1

2. Response Categorization System

The framework categorizes clinical response to external beam radiotherapy into 4 distinct remission patterns that guide subsequent contouring decisions 1. This classification system accounts for varying degrees of tumor regression and residual disease patterns.

3. Clinico-Radiological Environments

Eight specific environments are defined that combine clinical findings with imaging characteristics 1. Each environment requires tailored contouring approaches to minimize uncertainties specific to that clinical scenario.

4. Target Definition on CT at Brachytherapy

Target volumes are delineated on CT imaging obtained with the applicator in place, incorporating information from clinical examination and baseline imaging 1

Target Volume Delineation Priorities

High-Risk CTV (HR-CTV) as Primary Volume

HR-CTV demonstrates the lowest delineation uncertainties with volumetric conformity indices of 0.72-0.76 and inter-delineation distances of 3.6-3.8 mm, making it the most robust volume for dose prescription and optimization 2. This superior reproducibility compared to GTV (conformity index 0.58-0.59) and IR-CTV (conformity index 0.68-0.77) establishes HR-CTV as the preferred target for treatment planning 2.

Areas of Greatest Uncertainty

Delineation variation is most prominent in:

• Caudal and cranial margins across all target volumes 2
• Posterolateral aspects specifically for intermediate-risk CTV 2

These regions require particular attention during contouring to minimize systematic errors.

Integration with MRI When Available

Hybrid MRI/CT Workflow

When MRI is available for the first fraction only, a practical workflow involves:

• MRI-based target delineation for the initial brachytherapy fraction 3
• Applicator-based rigid registration to co-register the MRI dataset to subsequent CT scans 3
• Automatic transfer of target volumes to CT-based planning for subsequent fractions 3

This approach achieves excellent registration quality with median Dice indices of 0.879 for tandem overlap, demonstrating reliable geometric correspondence 3. The method delivers clinically appropriate doses with median HR-CTV D90 of 7.2 Gy per fraction 3.

MRI Superiority for Soft Tissue Assessment

While CT-based approaches are valid, MRI remains the gold standard with 86% agreement with surgical pathology for tumor size and local extent 4. MRI demonstrates superior diagnostic performance with:

• 71-88% sensitivity and 86-95% specificity for assessing invasion extent 4
• 71% sensitivity and 91% specificity for parametrial invasion, significantly better than CT 4
• Enhanced performance with diffusion-weighted imaging (DWI), increasing parametrial assessment to 82% sensitivity and 97% specificity 4

Implementation Strategies to Minimize Uncertainty

The recommendations emphasize three critical strategies:

1. Adequate imaging quality with proper CT technique and applicator visualization 2
2. Structured training programs for radiation oncologists in CT-based contouring 2
3. Adherence to standardized contouring recommendations based on the ICRU Report 89 framework 1

Clinical Context and Limitations

When CT-Based Approach is Appropriate

CT-based IGABT serves as a practical alternative when MRI availability, logistics, or financial constraints limit access to MRI-guided brachytherapy 1. The systematic approach aims to achieve benchmarks approaching MRI-based gold standard outcomes 1.

Critical Caveat for Dose Prescription

Traditional point A dosing must not be abandoned when implementing image-guided approaches 5. Current 3D image-guided techniques should optimize tumor coverage and reduce organ-at-risk doses while ensuring total point A doses reach at least 80 Gy for small tumors and ≥85 Gy for larger tumors (combining external beam and brachytherapy) 5.

Future Directions

Ongoing evaluation focuses on:

• Feasibility and reproducibility studies of CT-based contouring across institutions 1
• Clinical outcomes research comparing CT-based versus MRI-based IGABT 1
• Automated segmentation tools using deep learning models to improve consistency and efficiency 6

The adaptive radiotherapy paradigm continues to evolve with multi-parametric dose prescription protocols being explored in prospective trials 7.