Radiation Dose in Total Neoadjuvant Therapy (TNT) Protocol
For TNT protocols in locally advanced rectal cancer, the standard radiation dose is 45.0-50.4 Gy delivered in 25-28 fractions at 1.8-2.0 Gy per fraction for long-course chemoradiotherapy, or 25 Gy in 5 fractions (5 × 5 Gy) for short-course radiotherapy. 1
Long-Course Chemoradiotherapy Dosing
The most commonly used approach delivers:
- Pelvic dose: 45.0-50.4 Gy over 25-28 fractions
- Single fraction dose: 1.8-2.0 Gy 1
For resectable tumors requiring additional boost after the initial 45 Gy:
- Preoperative boost: 5.4 Gy in 3 fractions (total 50.4 Gy)
- Postoperative boost: 5.4-9.0 Gy in 3-5 fractions 1
Short-Course Radiotherapy Dosing
An alternative TNT approach uses:
- 25 Gy delivered in 5 fractions (5 × 5 Gy) 1, 2
- This is followed by chemotherapy before surgery
- Surgery typically occurs 6-8 weeks after completion of preoperative treatment 2
Dose Escalation Strategies
For unresectable tumors or when technically feasible:
- Radiation dose can be escalated to 54-56 Gy 1
- Recent research suggests dose-escalated IMRT (54 Gy or 57.5 Gy) may increase clinical complete response rates 3, 4
- MRI-guided radiotherapy with simultaneous integrated boost of 5 × 6 Gy to the gross tumor volume (while delivering 5 × 5 Gy to mesorectum) shows promise 5
Critical Technical Considerations
Radiation field coverage must include:
- Tumor (or tumor bed) with 2-5 cm safety margin
- Presacral lymph nodes
- Internal iliac and obturator lymph nodes
- External iliac lymph nodes for T4 tumors invading anterior structures 1
Small intestine dose constraints:
- Maximum dose limited to 50 Gy
- Volume V15 <120 mL (based on small bowel loops)
- Volume V45 <195 mL (based on entire abdominal cavity) 1
Common Pitfall
The key distinction in TNT is that all chemotherapy is delivered preoperatively (either as induction before or consolidation after radiotherapy), unlike traditional approaches where chemotherapy was split between neoadjuvant and adjuvant phases. The radiation dose itself remains consistent with established standards, but the integration with systemic therapy timing is what defines TNT 6, 2.