1198 - Clinically Significant Target Dose Overestimation in Small, Low-Density Lung SBRT: Monte Carlo-Validated Characterization of AAA and Acuros XB Across Modern Eclipse Versions

Purpose/Objective(s):

Ultra-small lung SBRT targets exist in a regime of extreme lateral electronic disequilibrium where analytic dose algorithms are known to struggle, yet this vulnerability remains insufficiently quantified in modern clinical planning systems. Prior Analytical Anisotropic Algorithm (AAA) and Acuros XB (AXB) comparisons lack Monte Carlo (MC) validation in the ultra-small, low-density lung environment where prescription accuracy is most fragile. We conducted the first combined EGSnrc MC benchmark and cross-version Eclipse analysis (v16.1–v18.1) specifically interrogating 1–2 cm targets embedded in 0.2–0.5 g/cm³ lung to determine whether contemporary AAA calculations remain reliable under worst-case SBRT conditions.

Materials/Methods:

Small-field output factors (0.5×0.5–10×10 cm², 6X FFF) were computed using AAA and AXB (v16.1, v18.1) in a heterogeneous lung phantom at 10 cm depth (2.5 cm solid water + 7.5 cm cork with ? = 0.27 g/cm³) and benchmarked against EGSnrc MC.

A representative lung SBRT case was re-modeled with 36 tightly controlled plans incorporating spherical PTVs (1.0, 1.5, 2.0 cm), overwritten lung densities (0.2–0.5 g/cm³), fixed beam geometry, and 1 mm dose grid resolution. Both AXB and AAA used dose-to-water reporting mode, ensuring direct comparability. Primary endpoint: difference in PTV D95% (AAA vs AXB). Secondary endpoints: Dmax, D2cm, R50%, and GI.

Results:

AAA v16.1 underestimated MC small-field outputs for all fields =3×3 cm² (mean -7.8%, max -9.4%); v18.1 improved but remained inaccurate (mean -3.8%, max -5.4%). AXB demonstrated significantly improved agreement, with mean errors of 2.7% (v16.1) and 2.1% (v18.1).

Clinically, AAA introduced systematic overestimation of target dose, most pronounced for 1.0–1.5 cm PTVs in 0.2–0.3 g/cm³ lung.

For 1.0 cm PTVs:

• D95% overestimated by 6.7% ± 2.1%
• Dmax overestimated by up to 5.7%
• R50% (0.2 g/cm³): 11.08 (AAA) vs 7.04 (AXB) in v16.1; 7.88 vs 6.88 in v18.1
• GI (0.2 g/cm³): 12.98 (AAA) vs 8.07 (AXB) in v16.1; 8.22 vs 8.05 in v18.1

Errors diminished with increasing target size and lung density but persisted across Eclipse versions, demonstrating that the limitation is intrinsic to AAA, not version-specific.

Conclusion:

In the ultra-small (=1.5 cm), highly aerated lung regime, AAA produces systematic and clinically significant misrepresentation of target dose—specifically, D95% deviations exceeding accepted SBRT planning tolerances. These discrepancies remain in Eclipse v18.1 and reflect the algorithm’s fundamental breakdown under lateral electronic disequilibrium. For 1–1.5 cm targets in 0.2–0.3 g/cm³ lung, continued reliance on AAA risks concealed peripheral underdosage; Acuros XB (1 mm grid) should be regarded as the required standard for accurate, protocol-compliant lung SBRT dose calculation.