Dosimetric and radiobiological evaluation of a spatially fractionated core-boost strategy in lung metastases within 5 mm of the esophagus: Single-center plan-comparison study.

Abstract

For lung metastases lying within 5 mm of the esophagus, organ-at-risk dose limits often force the prescription below the ablative range, lowering biologically effective dose and tumor control. We evaluated whether a simultaneous integrated boost to the tumor core, rather than dose reduction, is dosimetrically and radiobiologically rational, and which inner-volume retraction (0.3 vs 0.5 cm) is preferable. Using deep-inspiration breath-hold planning computed tomography from 30 consecutive patients, 3 clinically deliverable 5-fraction volumetric modulated arc therapy plans were generated de novo with Monte Carlo calculation: Plan A, a fixed uniform 35 Gy reference (5 × 7 Gy) applied identically to all patients as a standardized comparator; Plan B, a 60 Gy boost to the gross tumor volume retracted 0.5 cm; and Plan C, a 60 Gy boost to the gross tumor volume retracted 0.3 cm, each with a 35 Gy shell. Tumor-control and organ-at-risk endpoints were compared (Friedman and Bonferroni-corrected Wilcoxon tests). The simultaneous integrated boost met the targeted dose-escalation goal: volume-weighted total biologically effective dose rose from 61.5 to 97.4 Gy (Plan A to Plan C; 58.5%), approaching the BED10 level associated with improved local control and modeled tumor control probability increased from 0.77 to 0.93 (15.8%; all p < 0.001). Plan C exceeded Plan B by 5.4% (p < 0.001), the gain scaling with boost-volume fraction and tumor volume. Esophageal maximum dose stayed below 35 Gy with normal-tissue complication probability rising only from 6.1% to 6.9%; mean lung dose increased from 3.60 to 5.46 Gy and the volume receiving 20 Gy to 5%, all within tolerance. High-dose spillage exceeded the 17.5 Gy threshold in 53% of Plan C plans. A 0.3 cm core boost improves modeled tumor control at an acceptable, sub-tolerance organ-at-risk cost, outperforming the 0.5 cm definition. Prospective clinical validation is warranted.