What is the difference between Involved Field Radiation Therapy (IFRT), Involved Site Radiation Therapy (ISRT), and Involved Node Radiation Therapy (INRT) in radiation therapy?

Radiation Field Concepts in Lymphoma Treatment: IFRT vs ISRT vs INRT

Involved Site Radiation Therapy (ISRT) is the current standard of care for lymphoma radiation treatment, representing an evolution from the larger Involved Field Radiation Therapy (IFRT) fields while being more practical than the highly conformal Involved Node Radiation Therapy (INRT). 1

Key Differences Between Radiation Field Concepts

Involved Field Radiation Therapy (IFRT)

• Definition: Traditional approach that treats entire nodal regions/stations containing the involved nodes
• Target Volume: Includes the entire anatomic lymph node region (e.g., entire neck, entire mediastinum)
• Historical Context: Previously standard approach before modern imaging techniques
• Limitations: Exposes larger volumes of normal tissue to radiation, increasing risk of late toxicity

Involved Site Radiation Therapy (ISRT)

• Definition: Current standard approach that targets originally involved nodal sites and possible extranodal extensions
• Target Volume: Smaller than IFRT but slightly larger than INRT to account for uncertainties
• Key Advantage: Balances disease control with reduced normal tissue exposure
• Clinical Implementation: Recommended by the International Lymphoma Radiation Oncology Group (ILROG) and NCCN guidelines 1, 2
• Practical Benefit: Can be implemented even when optimal pre-chemotherapy imaging is not available

Involved Node Radiation Therapy (INRT)

• Definition: Most conformal approach targeting only the initially involved lymph nodes
• Target Volume: Smallest field, limited strictly to involved nodes identified on pre-treatment imaging
• Requirements: Demands optimal pre-chemotherapy imaging and precise co-registration with planning CT
• Limitations: Often impractical in routine clinical practice due to stringent imaging requirements 2

Clinical Implementation Considerations

Imaging Requirements

• IFRT: Based on anatomical boundaries of nodal regions
• ISRT: Requires contrast-enhanced CT, PET/CT, MRI, or combination for accurate target delineation 1
• INRT: Requires optimal pre-treatment imaging and precise image co-registration, which is often challenging in routine practice 2

Treatment Planning Concepts

• Gross Tumor Volume (GTV): Visible tumor on imaging
• Clinical Target Volume (CTV): GTV plus areas of potential microscopic disease
• Planning Target Volume (PTV): CTV plus margins for setup uncertainties
• Internal Target Volume (ITV): Accounts for organ motion during treatment

Dose Guidelines

• Early-stage Hodgkin lymphoma: 20-30 Gy with ISRT following ABVD chemotherapy 1
• Bulky disease: 30-36 Gy when combined with chemotherapy 1
• RT alone (uncommon except for nodular lymphocyte-predominant Hodgkin lymphoma): 30-36 Gy for involved regions 1

Benefits of Modern Field Reduction

Organ Sparing

• Significant dose reduction to organs at risk (OARs) including lungs, heart, breasts, thyroid, and other critical structures 3
• INRT consistently provides lower doses to all OARs compared to IFRT 3
• ISRT provides intermediate sparing between IFRT and INRT 2

Clinical Outcomes

• Modern field reduction maintains excellent disease control while reducing toxicity 1, 2
• Reduced risk of late complications including secondary malignancies and cardiovascular disease

Advanced Techniques to Further Reduce Toxicity

• Intensity-modulated radiation therapy (IMRT)
• Image-guided radiation therapy (IGRT)
• Respiratory gating or deep inspiration breath hold
• 4-dimensional imaging for motion management
• Proton therapy for selected cases 1, 4

Common Pitfalls and Caveats

1. Geographic Miss: Inadequate coverage of initially involved sites can lead to local recurrence
2. Excessive Normal Tissue Sparing: Overly aggressive field reduction without adequate margins may compromise disease control
3. Imaging Limitations: Poor quality pre-treatment imaging can compromise accurate target delineation
4. Patient Positioning: Changes in patient position between diagnostic imaging and treatment planning can lead to inaccurate target delineation
5. Anatomical Changes: Body habitus changes after chemotherapy can make precise co-registration challenging

The evolution from IFRT to ISRT to INRT represents a continuous effort to minimize radiation exposure to normal tissues while maintaining excellent disease control. ISRT has emerged as the practical standard that balances these goals in the modern era of combined modality therapy.