2449 - Effects of Liberalizing Lung V5 on Intermediate and High-Dose Exposure: Insights from Lung and Esophageal Cases Treated with Photons

Purpose/Objective(s): Balancing effective tumor dose delivery while minimizing cardiopulmonary toxicity remains a key challenge in thoracic radiotherapy. Lung V5 (volume receiving =5 Gy) is a traditionally used constraint to limit low-dose lung exposure, but its clinical significance remains uncertain as outcome data do not support its benefit. In this in silico study, we hypothesized that relaxing the lung V5 would improve organ-at-risk (OAR) sparing, particularly for cardiac and intermediate-dose lung metrics.

Materials/Methods: Five representative cases with centrally-located targets suitable for full-arc volumetric modulated arc therapy (VMAT) were selected: three esophageal cases (two distal tumors 5cm and 15cm in length and one upper tumor) treated to 45 Gy including elective nodal coverage and a simultaneous integrated boost to 50 Gy, and two lung cases (right-sided primary with N2 and left-sided primary with N3) treated to 60 Gy. Three optimization scenarios were deployed: V5-Standard (V5 <50% for esophagus, V5 <60% for lung; Plan A), V5-Intermediate (V5 <75% for both; Plan B), and V5-Agnostic (no V5 constraint; Plan C). Dose metrics were collected for heart (V15-V50, mean), lung (V5-V40, mean), and the esophagus (V60, mean, maximum) in lung cases only.

Results: Liberalizing the lung V5 constraint was associated with notable improvements in heart dose metrics. In the left-sided N3 lung case, heart V40 decreased from 25% to 17% and mean heart dose was reduced from 22.51 Gy to 17.12 Gy. Additionally, the lung V20 improved from 49% to 42%. In the right-sided N2 lung case, where heart exposure was lower, heart metrics were not improved; however, lung V20 still decreased from 21% to 14%. Esophageal metrics remained stable across lung plans. OAR sparing was also observed in esophageal cases. In the 15cm distal esophageal case, heart V30 decreased from 29% to 14%, while lung V20 decreased from 22% to 17%. In contrast, the upper esophageal case showed no notable changes in heart or lung dose metrics across planning scenarios.

Conclusion: Relaxing the lung V5 constraint during plan optimization improved heart and intermediate-dose lung metrics, in both esophagus and lung planning, with particular benefit in cases with higher baseline heart exposure. In esophageal cases, the impact appeared more pronounced in cases with greater craniocaudal tumor extent. These findings suggest that current V5 constraints may be overly restrictive and that strategic liberalization could lead to clinically meaningful reductions to OARs that may be broadly applicable across thoracic radiotherapy.