Compatibility of IMRT Plans for the Matched Beam Linear Accelerators

Purpose: To investigate the compatibility of IMRT plans for closely matched beam linacs in terms of patient‐specific quality assurance results. We are interested to know if there is a systematic passing rate difference between IMRT QA results when planned and delivered on the same machine and when planned and delivered on different machines. Method and Materials: An Elekta Precise (L5) and Elekta Synergy (L6) linacs with closely matched 6 MV beam data were commissioned in Pinnacle3 (version 7.6c) for planning of IMRT. A total of 56 fields were planned on L5 and 40 fields were planned on L6. Planar dose distributions of all the fields were measured with a MapCHECK 2D diode array. Comparison between the planned and measured dose distributions were done using both the 3%/3mm and 2%/2mm criteria in the absolute dose comparison mode. Results: When planned and delivered on L5, the average passing rates were 98.3±1.4 % with 3mm/3% criteria and 93.2 ± 3.1 % with 2mm/2% criteria. The same plan delivered on L6 had the average passing rates of 97.0 ± 2.5 % with 3mm/3% criteria and 88.6 ± 5.0 % with 2mm/2% criteria. For the plans planned and delivered on L6, the average passing rates were 96.2 ± 3.5 % with 3mm/3% criteria and 88.5 ± 6.2 % with 2mm/2% criteria. When the same plans were delivered on L5, the average passing rates were 97.9 ± 1.8 % with 3mm/3% criteria and 92.1 ±4.0 % with 2mm/2% criteria. The average passing rates were about 98.1% on L5 and 96.6% on L6 with 3%/3mm criteria regardless of which beam data were used for the planning. Conclusion: No systematic differences were observed in the currently adopted QA criteria. More planning studies are needed to investigate the differences in dose distribution and DVH for typical clinical cases.