What are the recommended organs-at-risk dose constraints for stereotactic body radiotherapy of a segment‑6 liver lesion involving a portal‑vein branch, delivered in 3–5 fractions?

Organs-at-Risk Dose Constraints for Liver SBRT (3-5 Fractions)

For stereotactic body radiotherapy of a segment-6 liver lesion involving a portal vein branch in 3-5 fractions, adhere to the following dose constraints: central hepatobiliary tract (cHBT) V40 <21 cm³ for 5 fractions or V33.8 <21 cm³ for 3 fractions, mean dose to cHBT <14 Gy (BED10), and ensure adequate uninvolved liver volume with strict adherence to normal tissue constraints given the portal vein involvement. 1

Critical Hepatobiliary Tract Constraints

The most important constraint for your case involves the central hepatobiliary tract (cHBT), defined as a 15-mm expansion around the portal vein from the splenic confluence to the first bifurcation. Given your lesion involves a portal vein branch, these constraints are paramount:

For 5-Fraction Regimens (typical dose 30-50 Gy):

• V40 Gy <21 cm³ (corresponds to BED10 of 72 Gy)
• V37.7 Gy <24 cm³ (corresponds to BED10 of 66 Gy)
• Mean dose to cHBT <14 Gy (BED10)

For 3-Fraction Regimens:

• V33.8 Gy <21 cm³ (corresponds to BED10 of 72 Gy)
• V32 Gy <24 cm³ (corresponds to BED10 of 66 Gy)
• Mean dose to cHBT <14 Gy (BED10)

These constraints are derived from multivariate analysis showing that exceeding these thresholds significantly increases the risk of grade 3+ hepatobiliary toxicity (relative risk 9.2-11.6) 1. This is particularly critical in your case given the portal vein involvement.

Liver Parenchyma Constraints

Adequate uninvolved liver must be preserved, especially given the portal vein involvement which increases toxicity risk:

• Sufficient uninvolved liver volume must remain to maintain function 2
• Most safety data exists for Child-Pugh A patients; Child-Pugh B requires dose modifications and stricter constraint adherence 2
• No established safety data for Child-Pugh C patients 2

The NCCN guidelines emphasize that SBRT dosing of 30-50 Gy in 3-5 fractions depends critically on the ability to meet normal organ constraints and underlying liver function 2.

Additional Organ-at-Risk Constraints

Major Vessels (Aorta/IVC):

For 5 fractions:

• D0.5cc <52.5 Gy (RTOG 0813 standard, 1.2% risk of grade 3-5 toxicity) 3

For 3 fractions:

• D0.5cc <45 Gy (2.3% risk of grade 3-5 toxicity) 3

Small Bowel/Duodenum:

For 3 fractions:

• D5cc <21 Gy (6.5% risk, reasonably safe)
• D5cc <16.2 Gy (2.5% risk, lower-risk option) 4

For 5 fractions, proportionally higher doses are acceptable 5.

Critical Caveats for Portal Vein Involvement

Your case requires special attention because the lesion involves a portal vein branch:

1. Increased toxicity risk: Portal vein involvement increases the risk of hepatobiliary complications, making strict adherence to cHBT constraints essential 1

2. Tumor-lumen distance: Ideally maintain >5 mm distance between high-dose region and vessel lumen 6

3. Liver function monitoring: Patients with portal vein involvement require careful baseline liver function assessment and close monitoring 6

4. Consider fractionation carefully: The 5-fraction regimen may be safer than 3 fractions given the vascular involvement, allowing better normal tissue sparing while maintaining efficacy 2

Planning Approach

Use advanced IMRT or VMAT techniques with image-guided delivery to achieve these constraints 7. The UK consensus recommends standardized implementation of these constraints across SBRT programs 5.

If constraints cannot be met: Consider alternative fractionation schemes, dose reduction, or alternative locoregional therapies such as transarterial radioembolization, which may be safer with portal vein involvement 8, 9.

The evidence strongly supports that exceeding the cHBT dose constraints significantly increases severe toxicity risk, with the VBED10 72 Gy threshold being the most robust predictor across multiple analyses 1.