What are the dose constraints for organs at risk in abdominal Stereotactic Body Radiation Therapy (SBRT)?

Dose Constraints for Organs at Risk in Abdominal SBRT

For abdominal SBRT, the most critical dose constraints are: duodenum/stomach/bowel V36Gy ≤0.5cc, liver mean dose <15-20 Gy (depending on baseline function), and spinal cord maximum dose ≤20 Gy in 3 fractions or equivalent, with adaptive MR-guidance enabling safe treatment of tumors abutting gastrointestinal organs that would otherwise be contraindicated. 1, 2

Gastrointestinal Organs at Risk

Small Bowel, Duodenum, and Stomach

• The critical volume constraint is V36Gy ≤0.5cc for all gastrointestinal structures when delivering 50 Gy in 5 fractions, which represents the most commonly used abdominal SBRT regimen 1, 2
• The duodenum receives the highest doses when treating pancreatic head, uncinate process, or neck tumors, with mean pre-adapted volumes of 3.4cc receiving 36 Gy, requiring online adaptation to reduce this to 0.33cc 2
• The stomach more frequently exceeds dose constraints when treating pancreatic body or tail tumors 2
• Tumors directly abutting or invading gastrointestinal organs require adaptive MR-guidance to safely deliver ablative doses, as 99% of fractions required online adaptation to meet OAR constraints 2
• Grade 3-4 gastrointestinal toxicity occurred in 4 of 6 patients with dose-limiting toxicities when NRG-BR001 constraints were exceeded, including colitis from high bowel dose 1

Colon and Rectum

• The same V36Gy ≤0.5cc constraint applies to colon when treating abdominal-pelvic metastases 1
• For anal cancer (non-SBRT context), intensity-modulated techniques with strict dose constraints to bowel are recommended, though specific SBRT constraints for rectal tumors are not well-established in the provided evidence 3

Liver

Dose-Volume Constraints

• Mean liver dose should be kept at 15-20 Gy depending on baseline liver function, with stricter constraints for compromised hepatic reserve 4, 5
• SBRT dosing for liver lesions typically ranges from 30-50 Gy in 3-5 fractions, depending on ability to meet normal organ constraints and underlying liver function 4
• Sufficient uninvolved liver volume must remain after treatment, with dose constraints strictly adhered to 4

Patient Selection Based on Liver Function

• Child-Pugh A patients represent the safest population for liver SBRT, with most safety and efficacy data available for this group 4, 5
• Child-Pugh B patients can be treated with dose modifications and strict constraint adherence 5
• Child-Pugh C cirrhosis is an absolute contraindication to SBRT, as safety has not been established in this population with very poor prognosis 4, 5

Spinal Cord

• Maximum spinal cord dose should be limited to 20 Gy in 3 fractions or 13 Gy in a single fraction to maintain myelopathy risk below 1% 3, 6
• For 5-fraction regimens, the equivalent constraint would be approximately 25-30 Gy maximum dose 6
• Long-term data on spinal cord tolerance with SBRT remain insufficient, requiring conservative approach 3

Esophagus

• Mean esophageal dose (MED) should be kept below 28 Gy to maintain grade 3+ esophagitis risk below 15% in conventional fractionation 3
• For SBRT fractionation, equivalent constraints would need biological effective dose conversion, though specific abdominal SBRT esophageal constraints are not well-defined in the provided evidence 3

Chest Wall and Ribs

• Dose to chest wall should be limited to <30 Gy delivered in 3-5 fractions on a volume <30 mL 3
• Risk of rib fractures is approximately 5% when dose to 2 mL volume is limited to 27 Gy in 3 fractions (3 × 9 Gy) 3

Kidneys

• While specific SBRT constraints for kidneys are not detailed in the provided evidence, renal cell carcinoma metastases show excellent 2-year local control rates of 90% with SBRT, suggesting kidneys can tolerate ablative doses when appropriately planned 6, 5

Practical Implementation Strategies

Planning Optimization

• PTV optimization structures should exclude a 5mm expansion of gastrointestinal OAR contours (PTV_opt) to enable adequate target coverage while respecting OAR constraints 2
• Mean PTV_opt receiving prescription dose should be ≥93% when using this approach 2
• Planning target volume contraction may be necessary when GTV/ITV exceeds 65 cm³ to limit OAR dose 1

Adaptive Techniques

• Online adaptive MR-guidance is essential for tumors abutting gastrointestinal organs, with 77% of fractions requiring normalization and 99% requiring some form of adaptation 2
• Predicted and adapted critical volumes (V36Gy) are statistically significantly different (p<0.001), demonstrating necessity of adaptation 2
• Hydrodissection techniques can enable safe treatment of lesions abutting critical structures including diaphragm 4

Treatment Delivery

• Four-dimensional CT-based planning with daily cone-beam CT or stereoscopic kV imaging for pretreatment image guidance is standard 7
• Full body immobilization is required 7
• SBRT should be delivered in 1-5 consecutive daily fractions in a single week 7

Common Pitfalls to Avoid

• Never use conventional low-dose palliative radiation (8 Gy in 1 fraction) for abdominal tumors in patients with reasonable life expectancy, as this achieves suboptimal local control rates below 50% at 1 year for bulky tumors 6, 5
• Do not exceed NRG-BR001 dose constraints, as 52% of patients had at least one constraint exceeded, and 4 of 6 dose-limiting toxicities were directly attributable to planned OAR dose violations 1
• Avoid treating patients with Child-Pugh C cirrhosis with liver SBRT 4, 5
• Do not treat tumors where PTV overlaps trachea or main bronchi (ultracentral location) due to unacceptable toxicity risk 6
• Ensure adequate follow-up beyond 2 years when treating central structures, as severe bronchial stenosis and fistula may occur late when large bronchi receive >80 Gy equivalent 3

Clinical Outcomes with These Constraints

• Local control at 6 and 12 months was 98% and 87% respectively when using these dose constraints for abdominopelvic tumors 7
• Treatment was well-tolerated acutely without grade ≥3 toxicity in the acute setting 7
• Five grade 3 late toxicities occurred in 47 patients, with one death from duodenal perforation at 11 months, emphasizing importance of strict GI constraints 7
• Complete or partial response was achieved in 60% of evaluable lesions 7