21 VMAT vs. 3DCRT : quantitative dosimetric study for the lung cancer

Abstract

Introduction The dosimetric impact of VMAT in lung cancer compared with 3DCRT is well known and confirmed by several publications. The aim of this study is to quantify these results more accurately in our local clinical context, based on thirty-six patients treated with 3DCRT between 2015 and 2017. Patients and methods For each patient treated with 3DCRT for non-small cell pulmonary tumors with a prescription dose of 2 Gy/daily to 66 Gy, a second VMAT treatment plan was calculated with the Eclipse 13.6 treatment planning system (AAA 13.6.23, Varian®). Volumetric modulated arc therapy plans were created using either two complete arcs or two partial arcs, depending on the location of the irradiated volume. The Paddick conformity index and a homogeneity index ([D98% -D2%]/ Dmoy) were used to compare the doses delivered at the target volume (PTV). Concerning the organs at risk we compared the values of the usual dosimetric parameters used to validate a plan treatment. The volumes of the 5, 10 and 15 Gy isodose lines were compared to investigate the low doses delivered to the body by the two irradiation plans. Results The Paddick conformity index for the PTV is 52% greater for VMAT (0.87) than the 3DCRT (0.57) (p 0.001). The homogeneity of dose is better by 39% in VMAT than in 3DCRT, the homogeneity index being respectively 0.07 and 0.11 (p 0.001). For the spinal cord PRV, the average maximum dose is 45.6 Gy in 3DCRT against 19.3 Gy in VMAT (p 0.001). Heart volume receiving at least 35 Gy (V35) decreased from 15.6% in 3DCRT to 8.28% in VMAT (p 0.001). Esophageal V50 is also higher in 3DCRT than in VMAT, increasing from 14.03 to 25.45% (p = 0.002). The mean lung dose is 17.9 Gy in 3DCRT versus 15.5 Gy in VMAT, a decrease of 13% (p = 0.041); the V30 value is 23.6% in 3DCRT and 18.8% in VMAT, an improvement of 20% (p = 0.001). Nevertheless, there is no significant decrease in the V20 value, from 29% in 3DCRT to 24.84% in VMAT (p = 0.105). With regard to the low doses, it is found that the volumes receiving 5, 10 and 15 Gy, are not significantly different between the two irradiation techniques when the VMAT dosimetry is performed with partial arcs (p > 0.09). On the other hand, the volume receiving 5 Gy is significantly higher by 50% (p = 0.024) in VMAT compared to the 3DCRT for full arcs set up. Conclusions The conformity and homogeneity indices at the target volume are improved in VMAT compared to the 3DCRT. The doses received by the organs at risk are significantly reduced, in particular the maximum dose to the spinal cord PRV and the V35 value for the heart, excepting the V20 value in lung which is not significantly smaller although it is an essential parameter in a lung plan evaluation. All these benefits are possible without increasing low doses when using a two partial arcs planning strategy.