Chest wall dose reduction using noncoplanar volumetric modulated arc radiation therapy for lung stereotactic ablative radiation therapy.

Abstract

Stereotactic ablative radiation therapy (SABR) to lung tumors close to the chest wall can cause rib fractures or chest wall pain. We evaluated and propose a clinically practical solution of using noncoplanar volumetric modulated arc radiation therapy (VMAT) to reduce chest wall dose from lung SABR. Twenty lung SABR VMAT plans in which the chest wall volume receiving 30 Gy or higher (V30) exceeded 30 mL were replanned by noncoplanar VMAT with opposite 15° couch kicks. Dosimetric parameters including chest wall V30 and V40; lung V5, V10, V20, and mean dose; Paddick high-dose conformity index; intermediate-dose conformity index; and monitor units (MU) for each plan were used to evaluate the plan quality. The treatment time was also estimated by delivering the entire treatment. Two-sided paired t test was used to evaluate the difference of the dosimetric parameters between coplanar 1 arc (cVMAT₁), coplanar 2 arcs (cVMAT₂), and noncoplanar two arcs (nVMAT₂) plans; differences with P < .05 were considered statistically significant. V30 and V40 for chest wall were reduced on average by 20% ± 9% and 15% ± 11% (mean ± standard deviation) from cVMAT₂ plans to nVMAT₂ plans (P < .01 for both comparisons) and by 8% ± 7% and 16% ± 13% from cVMAT₁ plans to cVMAT₂ plans (P < .003 for both comparisons). The differences in lung mean dose were <0.2 Gy among cVMAT₁, cVMAT₂, and nVMAT₂. There were no significant differences in lung V5, V10, and V20. On average, the number of MU increased 14% for nVMAT₂ compared with cVMAT₂. The Paddick high-dose conformity indexes were 0.88 ± 0.03, 0.89 ± 0.04, and 0.91 ± 0.03, and intermediate-dose conformity indexes were 3.88 ± 0.49, 3.80 ± 0.44 and 3.51 ± 0.38 for cVMAT₁, cVMAT₂, and nVMAT₂, respectively. We found that noncoplanar VMAT plans are feasible, clinically practical to deliver, and significantly reduce V30 and V40 of chest wall without increasing lung dose.