Planning Target Volume Density Impact On Treatment Planning For Lung Stereotactic Body Radiation Therapy

Background To evaluate the impact of the planning target volume (PTV) density on treatment planning for lung Stereotactic Body Radiation Therapy (SBRT). Material and methods The PTV coverage was analyzed in two groups of 40 lung SBRT patients. One group had PTV density <0.5 g/cm(3), while the other group had PTV density >0.5 g/cm(3). The treatments were planned in Pinnacle 9.10, using the collapsed cone convolution (CCC) algorithm. The prescribed dose was 60 Gy to the PTV in 4-8 fractions. Respecting constraint for the PTV coverage (D98% > 95%), we compared changes in the isodose line prescription, the number of monitor units (MU), maximum dose (D-max), irradiated volume covered with 30 Gy (V-30Gy), and the optimization planning volume (OPV). Results For the same median values of the PTV coverage (98.3%), the differences are presented with median values between lower and higher density than 0.5 g/cm(3). The isodose line prescription was 83 vs. 90% (p < 0.0001), the MUs were 2294 vs. 1655 MU (p < 0.0001), D-max was 74.26 vs. 68.09 Gy (p < 0.0001), V-30Gy was 117.03 vs. 104.81 cc (p = 0.04), and OPV was 28.48 vs. 39.35 cc (p < 0.001). By overriding the ITV density to 0.8 g/cm(3), the isodose line prescription decreases. The D-max and MUs decrease by 7%, V-30Gy by 34%, and OPV by 70%. Conclusion To obtain similar PTV coverage for PTV which is <0.5 g/cm(3), a larger margin irradiating a large OPV was used. More MUs and a higher maximum dose were delivered. For the PTV density of <= 0.36 g/cm(3,) overriding is recommended to reduce the dose and irradiated volume.