A method of determining the prescription isodose line for stereotactic radiotherapy VMAT plan / 中华放射医学与防护杂志

ABSTRACT

Objective:To establish a method of determining the prescription isodose line for steretactic radiotherapy (SRT) volumetric modulated arc radiotherapy (VMAT) plan.Methods:Eight patients with brain metastases treated with SRT were enrolled. The volume of planning target volume (PTV) ranged from 3.5 to 11.7 cm 3 (median 6.1 cm 3). Reference VMAT plans were designed for each patient with identical prescription dose. Then, the original PTV was contracted by a few millimeters to form a new target for optimization to get plans with different IDLs. The minimum margin which was needed to be contracted to achieve optimal IDL range for each PTV was also studied. Results:To achieve the optimal IDL range, 4 or 5 mm PTV contraction was needed for all patients, and the average IDL was (66.05±0.02)%. Compared with reference plans, the average gradient index (GI) of optimal IDL plans decreased by 20% from 4.05±0.39 to 3.37±0.24 ( Z=-2.521, P<0.05). The V40, V30, V5 and mean dose in normal brain tissue decreased by 11.5% ( Z=-1.973, P<0.05), 7.2% ( Z=-2.105, P<0.05), 12.8% ( Z=-2.521, P<0.05) and 8.1%, respectively ( Z=-2.382, P<0.05), and there was no statistically significant difference with V20, V10 and conformity index ( P>0.05). Conclusions:The optimization of IDL for SRT-VMAT plan can be achieved with the method of contracting PTV to form new target for planning. 4 or 5 mm is needed to be contracted to achieve the optimal IDL range, and to get lower GI and protect the normal brain tissue.