SU‐F‐T‐627: A Conformity Index Study of SBRT Plans for Lung Cancer Patient

Abstract

Purpose:To evaluate the conformity index for SBRT lung plan in Eclipse (version 11) treatment planning system.Methods:In the Eclipse treatment planning system the conformity index was defined as VRI/TV, where VRI is prescription isodose surface, and TV is the target volume. In 1997, Van't Riet et al introduced TVRI/TV × TVRI/VRI as a conformal index, where TVRI is the target volume closed by the prescription isodose surface. We chose a lung cancer patient who was assigned to be treated with SBRT. SBRT immobilization device was used and non‐gated CT/4DCT was performed. The tumor movement was less than 1 cm based on 4DCT. A MIP image set was generated and used to delineate the internal target volume (ITV). A 5 mm margin was added on the ITV to generate the PTV. The prescription was 5000 cGy in 5 fractions and 100% prescription dose covered 95% of the PTV. A VMAT plan (plan I) was made to achieve the physician's goal. Then a high dose structure was generated by VRI subtracting the TV, a low dose structure was generated by TV subtracting VRI, and medium dose structure was named for the overlapping area between VRI and TV. The other VMAT plan (plan II) was made by adding constraints to high/low/medium dose structures, respectively. The conformity index was determined and compared between plan I and plan II.Results:For plan I, the Eclipse's conformal index was 1.06; the Van't Riet's conformal index was 0.82. For plan II, the Eclipse's conformal index was 1.02; the Van't Riet's conformal index was 0.85. The conformal index was improved in plan II.Conclusion:The high/low dose/medium dose structures may be useful to make more conformal plans to treat lung area using SBRT based on the conformal index values.