Purpose: Linacs with unmatched beam characteristics create operational challenges. The feasibility of transferring patients between unmatched Linacs was investigated for a limited number of treatment fractions.
Methods: Twenty 4-field 3DCRT and 20 VMAT prostate plans were evaluated on unmatched Elekta Infinity and Versa Linacs using Pinnacle TPS. With MUs held constant, the mean dose to four OARs (rectum, right femur, left femur, and bladder), PTV, and prescription point were evaluated. Differences in mean dose were assessed with paired t-tests (p<0.05). Relative differences in mean dose were calculated. The number of interchangeable fractions allowable before surpassing a 5% difference in dose to the prescription point was determined as recommended by ICRU-50. VMAT plans were delivered on both Linacs for QA with ArcCheck. Measured dose distributions were compared with distributions calculated on the Infinity. Gamma analysis passing rates were compared between machines using 2.0%/2.0mm criteria.
Results: The mean dose to the PTV and prescription point was greater for all 3DCRT plans calculated on the Versa and reduced for 17 and 16 out of 20 VMAT plans, respectively. The averaged mean dose difference for the rectum, right femur, left femur, bladder, and PTV were 0.9%, 1.3%, 5.5%, 0.6%, and 0.9% among 3DCRT plans, and 2.9%, 0.9%, 1.4%, 2.8%, and 0.6% among VMAT plans, respectively. Mean dose significantly differed between Linacs for all OARs, PTV, and prescription point. The relative difference to the prescription point ranged from 0.75%-1.35% (mean: 0.96%) among 3DCRT plans and 0.06%-1.40% (mean: 0.43%) among VMAT plans resulting in an average of 5 (range: 4-7) and 11 (range: 4-90) allowable interchangeable fractions between Linacs, respectively. Among VMAT plans, gamma passing rates differed by 0.0%-3.0% (mean: 0.18%).
Conclusion: Despite small differences in calculated dose, transferring patients between unmatched machines for a limited number of fractions (<5) is possible to improve clinical flexibility.