Dosimetric planning study of respiratory-gated volumetric modulated arc therapy for early-stage lung cancer with stereotactic body radiation therapy.

Abstract

To evaluate the dosimetric potential of respiratory-gated volumetric modulated arc therapy (VMAT) to reduce the dose to normal lung when treating early-stage non-small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT). Twenty patients with inoperable stage I/II NSCLC with motion greater than 5 mm were retrospectively planned with 4-dimensional computed tomography-based gated and nongated VMAT. Each plan was optimized using two 225° arcs with 10-MV flattening filter-free beams with maximum dose rate of 2400 MU/min. A host script was generated and used to optimize all 40 plans to minimize dosimetric bias. The main dosimetric parameters compared were percent volume of the lung receiving 20 Gy or more (V(20Gy)) and the absolute volume of lung minus the internal tumor volume receiving at least 50% of the prescription dose for normal lung (V(50%)). Other parameters considered were the maximum dose 2 cm from the planning target volume (D(2cm)), percent volume of the contralateral lung receiving 5 Gy or more (V(5Gy)), mean lung dose, maximum dose to normal structures, and monitor units. There was a significant decrease in both parameters for the normal lung with gated VMAT. V(20Gy), predictive for pneumonitis, decreased from (6.05 ± 2.06%) to (5.25 ± 1.75%) (P = .00009) and the absolute volume of lung minus the internal tumor volume receiving at least 50% of the prescription dose decreased from (158.17 ± 61.12 cm(3)) to (125.71 ± 49.46 cm(3)) (P = .00002). Also, there was a significant decrease in D(2cm), contralateral V(5Gy), mean lung dose, and monitor units. Respiratory-gated VMAT has the potential to reduce the dose to normal lung when treating early-stage NSCLC with SBRT for tumor motion greater than 5 mm.