Room: Exhibit Hall
Purpose: In this study, we compared lung SBRT VMAT plans made with 6 MV FFF and 10 MV FFF which have significantly higher dose rates (1400 MU/min and 2400 MU/min respectively) than the traditionally used 6 MV beams.
Methods: Ten cases of patients previously treated with 50 Gy in 5 fractions were planned using two half VMAT arcs using auto-planning (SmartArc). Test plans were made for each case using 6 MV FFF (6FFF) and 10 MV FFF (10FFF) beams. The same auto-planning template (objectives and settings) was used for both plans to reduce planner bias. 10FFF was optimized using the auto-planning objectives and settings that were used for 6FFF. The template utilized varied across patients and determined iteratively by the planner until the RTOG 0813 objectives were achieved. All 10FFF plans met planning constraints using 6FFF template except for 1 case in which heart constraint was violated. This case was replanned using a different template until a clinically acceptable plan was achieved.
Results: Respectively for 6FFF and 10FFF, the mean values were: cord maximum dose (Dmax) 3.1 Gy and 3.1 Gy; esophagus Dmax 6.0 Gy and 6.3 Gy; heart mean dose 14.8 Gy and 15.3 Gy; heart V32Gy (volume receiving 32 Gy) 10 cc and 10.4 cc; proximal bronchus Dmax 15.4 Gy and 15.5 Gy; proximal bronchus V18Gy 3.7 cc and 4.2 cc; trachea Dmax 7.5 Gy and 7.7 Gy; Lung V20 2.8% and 2.9%; conformity index 1.02 and 1.02; gradient index 3.53 and 3.67; treatment time 2.0 and 1.2 minutes. The only statistically significant differences between 6 FFF and 10FFF plans were lung V20 values and treatment time.
Conclusion: Both energies provided similar plan qualities but overall, 6 MV FFF achieved better dosimetric endpoints for normal tissues even though not statistically significant. 10 MV FFF was more efficient.