38. Use of Flattening Filter Free Beams for Stereotactic Body Radiotherapy Treatments: Impact of the Size of the Planning Target Volume

Introduction Flattening filter free (FFF) beams substitude flattened filter (FF) beams for stereotactic body radiotherapy (SBRT). There are several treatment modalities available for SBRT: conventional static fields, dynamic conformal arc (DCA) or Volumetric Modulated Arc Therapy (VMAT). We compared SBRT plans in DCA FF, DCA FFF, VMAT FF, and VMAT FFF for different volumes of planning target volume (PTV). The aim was to obtain a consensus of the employment of FFF beams and of the DCA or VMAT technique depending of the size of PTV. Methods Three environments were studied with several concentrics PTV: 11 in an elliptical water equivalent phantom, 6 in lung, 7 in liver. For each PTV, 8 treatments plans were generated: DCA and VMAT in 6 MV and 10 MV FF and FFF. Homogeneity (HI), conformity (CI), gradient (GI) index, healthy tissue mean dose, organ at risk (OAR) dose, number of monitor unit (MU) and beam on time were evaluated. The ratio FFF/FF and VMAT/DCA of these parameters were performed and Spearman test was executed in order to establish a correlation with the size of the PTV. Results Dosimetrics index were improved in FFF but not significantly (median CI: DCA 6 FF = 1.19; DCA 6 FFF = 1.17; VMAT 6 FF = 1.04; VMAT 6 FFF = 1.04). Healthy tissue mean dose was significantly damaged in DCA 10 FFF with the incease of PTV size (phantom: p = 0.0001, rho = 0.91; V20 lung: p = 0.000, rho = 1.00; D700cc liver: p = 0.000, rho = 1.00). The results obtained in liver showed a better dose gradient with VMAT than DCA for extensives PTV (GI: p < 0.0001, rho = −1.00). VMAT associated with FFF allowed better OAR sparing (V20 lung: p = 0.0188, rho = −0.89). Beam on time was significantly shorter with FFF energies (55% in 6 MV, 75% in 10 MV) and DCA technique (55% in 6 MV and 70% in 10 MV). Number of MU was greater in FFF and VMAT technique. Conclusion No significant difference between FFF and FF beams were noted in our SBRT plans. It seems that FFF beams provide a better OAR sparing except for DCA with large PTV volume particularly in 10 FFF. DCA allows shorter beam on time than VMAT and remains immune to the MLC interplay effect.