Selection of prescription isodose line for brain metastases treated with volumetric modulated arc radiotherapy

Purpose To exploit the optimal prescription isodose line (IDL) for brain metastases treated with volumetric modulated arc radiotherapy (VMAT) as there is no consensus on the selection of IDL with VMAT. Methods and materials Eighteen patients with 20 brain tumors, who were treated with VMAT, were enrolled in this study. For each tumor of every patient, five plans were designed with IDL ranging from 50% to 90% in 10% increments. Different IDLs were obtained through adjusting the constraint parameters during planning optimization. Prescription dose (10 x 5 Gy) were identical for all plans, and the plans were compared in terms of gradient index (GI), conformity Index (CI), V26 Gy/V-PTV, and V32 Gy/V-PTV in normal brain tissue, which correlate to radiation necrosis. Results IDL with lowest GI has a median value of 60.0% (ranging from 50% to 80%). Except for one tumor with volume larger than 10 cc, the IDL with lowest GI varies from 50% to 70%, which depends on the shape of PTV, location, and whether the target volume is adjacent to crucial OAR. Moreover, there is no significant difference for CI with varying IDL plans. The average V26 Gy/V-PTV and V32 Gy/V-PTV in normal brain tissue 60% IDL plans are 27.3%, 31.7% lower than 90% IDL plans separately (P < 0.05). However, by further decreasing IDL from 60% to 50%, the average V26 Gy/V-PTV and V32 Gy/V-PTV may increase comparing with 60% IDL plans (P > 0.05). Furthermore, a lower IDL is found to result in higher mean dose to the target volume (P < 0.05). Conclusions Plans using VMAT with PTV smaller than 10 cc tend to be optimal with IDL around 60-70% for lower GI, smaller V26 Gy/V-PTV, V32 Gy/V-PTV in normal brain tissue, and higher mean dose in tumor comparing with high IDL plans which have potential benefit in reducing risk of radiation necrosis and increasing the local control. However, IDL lower than 60% is not recommended for the disadvantage of increasing V26 Gy/V-PTV and V32 Gy/V-PTV in normal brain tissue.