Room: AAPM ePoster Library
The purpose of this study is to retrospectively investigate the treatment plan quality based on ICRU 83 homogeneity, conformity, ICRU 91 Paddick conformity and gradient indices (Paddick). We applied these metrics to evaluate volumetric modulated arc therapy (VMAT) plans for lung stereotactic body radiation therapy (SBRT).
Thirty patients previously treated with lung SBRT were selected for this study. Plans were calculated in RayStation and normalized to at least 95% of the PTV receiving 100% of the prescribed dose. For all plans, prescription doses ranges were from 50 Gy to 60 Gy in 5 fractions. All lung SBRT cases were treated with 6MV-FFF or 10MV-FFF energies using Edge (Varian Medical System, Palo Alto, CA) with 2.5 mm HDMLC. PTV volumes ranged between 1.35 and 73.35 cm3 and their geographic locations were distributed in both lungs.
The mean target volume (PTV) was 22.09 (1.46-157.39) cm3. At our clinic, we typically prescribe 50 or 60 Gy in 5 fractions for lung SBRT tumors. The PTV coverage in all patients was met in =95% of the prescribed dose. For this study, the plan quality metrics were: homogeneity (0.29 ± 0.08), conformity (1.15 ± 0.16), gradient (4.85 ± 0.95) and Paddick conformity (0.8 ± 0.08), when averaged over the 50 patients.
The obtained results are in good agreement with similar published data. Evaluation of our data demonstrates GI becomes worse for the small PTV volumes of less than 10 cm3, therefore dose fall-off deteriorates. The correlation between PI and CI with target volume shows these indices improve with increasing the volume. Upon further evaluation, it was determined these two indices become worse for DIBH patient plans. Moreover, smaller lesions tend to have larger HI. The provided data is valuable tools to ensure the plan quality in lung SBRT.