D mean vs D95 in VMAT Planning
In VMAT planning, D mean is often accepted over D95 because VMAT inherently creates dose heterogeneity within the target volume through its rotational delivery technique, making mean dose a more clinically relevant metric for assessing overall target coverage and biological effect, particularly when dose escalation strategies are employed. 1, 2
Technical Rationale for D mean Preference
VMAT's rotational arc delivery fundamentally differs from static field techniques, creating a characteristic dose distribution pattern:
- VMAT produces superior high-dose conformity but increases low-dose bath to surrounding tissues, which affects the dose gradient within the target volume itself 1
- The continuous modulation during arc rotation creates intentional dose heterogeneity that can be leveraged for dose escalation to inner target volumes while maintaining acceptable maximum doses 2
- D95 may underrepresent the biological effectiveness of VMAT plans because it only captures the coldest regions, missing the therapeutic benefit of higher doses delivered to the majority of the target 2, 3
Dose Escalation Considerations
VMAT planning specifically enables internal dose scaling strategies that make D mean more clinically meaningful:
- Auxiliary inner volumes can be used to intentionally escalate dose within the target, with studies showing D mean increases of approximately 2 Gy while maintaining acceptable D95 values 2
- The median dose (D50) and mean dose better reflect the biological dose delivered to the tumor when dose painting or simultaneous integrated boost techniques are employed 2, 3
- For dose escalation protocols reaching 60 Gy or higher, VMAT demonstrates advantages in normal tissue sparing (particularly V30 to normal liver) while maintaining adequate target coverage, making mean dose a better indicator of therapeutic intent 3
Clinical Context and Plan Evaluation
The International Commission on Radiation Units and Measurements (ICRU) Report 83 provides standardized dose reporting parameters including D98, D95, D50, D2, and homogeneity index for IMRT techniques including VMAT 1:
- D95 remains important for ensuring minimum dose coverage, but should be evaluated alongside D mean and D50 for comprehensive plan assessment 1, 4
- Homogeneity index (HI) values for VMAT typically range from 1.09-1.11, indicating acceptable dose uniformity despite the preference for mean dose reporting 4
- Target coverage metrics show D95 values of 69.79-71.7 Gy for 76 Gy prescriptions, demonstrating that adequate minimum dose is maintained even when mean dose is the primary optimization parameter 4
Practical Planning Implications
When optimizing VMAT plans, the planning approach should prioritize:
- Mean dose optimization for the planning target volume (PTV) while constraining maximum dose (D2 or D0.03cc) to prevent hotspots 1, 2
- D95 as a constraint rather than primary objective, ensuring minimum coverage thresholds are met (typically ≥95% of prescription dose) 2, 5
- Conformity index (CI) and normal tissue sparing as co-primary objectives alongside mean dose, since VMAT's efficiency advantage (55% reduction in beam-on time, 290 MU vs 642 MU for IMRT) allows for more complex optimization 4, 5
Common Pitfalls to Avoid
Critical considerations when accepting D mean over D95:
- Do not ignore D95 entirely—it must still meet minimum thresholds (typically 95-97% of PTV receiving 95% of prescription dose) to ensure adequate coverage 5
- Monitor maximum dose (D2) carefully—VMAT's dose escalation capability can create unacceptable hotspots if not properly constrained 2, 4
- Verify normal tissue constraints are met—the increased low-dose bath characteristic of VMAT requires careful evaluation of V20, V30 parameters for organs at risk 3, 4
- Consider the clinical scenario—for advanced or metastatic disease where local control is paramount, mean dose better reflects the biological effectiveness needed for tumor control 6, 7