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

Purpose: 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.

Methods and materials: 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.

Results: 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.

Conclusions: 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.