Purpose: LINAC-based stereotactic body radiotherapy (SBRT) has gained popularity in treating early stage lung cancer. This study aims to compare dosimetric outcomes of different SBRT
methods: one-ISO plan (one-I) and two-ISO plan (two-I).
Methods: There are two early stage lung tumors within the same lung lobe at the same level. PTV volumes are 10.0cc and 17.4cc. The distance between these two tumors is 3.5 cm. 48Gy in 4fx were prescribed to both two tumors. Partial arcs VMAT plans were implemented for both the one-I and two-I at Varian Truebeam machine with 6X FFF beam. VALOR dose constraints were used for plan evaluation.
Results: For both SBRT plans, the PTV coverages were the same as 95% PTV volumes covered by 100% prescribed dose. For all organ-at-risk listed in the trial, the Dmax of one-I is 4.6% lower than that of two-I. The volume dose of one-I is 3.46% lower than that of two-I. CI of one-I is 2.73% lower than that of two-I and the D2cm of one-I is 3.3% lower than that of two-I. R50 of one-I is 16.67% lower than that of two-I. MU of one-I is 46.58% lower than that of two-I.
Conclusion: This study showed that one-isocenter plans, compared with two-isocenter plans, provided better normal lung sparing and incurred much less MUs for shorter beam-on time and less radiation leakage to patient.