Dosimetric Validation of Treatment Plan Interchangeability between Factory Beam-matched Varian TrueBeam and Clinac...

The purpose of this study was to assess the dosimetric consistency and clinical interchangeability of treatment plans between factory beam-matched Varian TrueBeam SVC and Clinac iX linear accelerators across various anatomical locations.

Eighty clinical plans were analyzed, including 40 intensity modulated radiotherapy (IMRT) plans (20 for cervix and 20 for head and neck) and 40 volumetric modulated arc therapy (VMAT) plans (20 for breast and 20 for urinary bladder). Both linacs were independently commissioned per AAPM-106 protocols. Plans were recalculated on the alternate machine and compared statistically. Patient-specific QA was performed using ArcCHECK with 3%/2 mm gamma criteria. Transferable fractions were estimated using ICRU-50 (±5%) and AAPM TG-40 (±2%) guidelines to determine fractions safely deliverable without re-planning.

Beam commissioning showed excellent agreement within ±1% for all dosimetric parameters. Head and neck IMRT had minimal differences (D95%: -0.26%, P < 0.001) and the highest transferability (median 39%, 10-14 fractions). Breast VMAT achieved consistent organ-at-risk sparing with moderate transferability (median 19%, 2-4 fractions). Urinary bladder VMAT displayed acceptable transferability (median 29%, 7-11 fractions). Cervical IMRT showed systematic dose increases on TrueBeam (planning target volume D95%: +1.57%, P = 0.012) with limited transferability (median 14%, 1-2 fractions). All plans exceeded gamma passing rates of 97.3%, with VMAT demonstrating superior consistency over IMRT (99.1% vs. 97.3-98.6%).

Linear accelerators that are beam-matched in factories provide safe interchangeability of treatment plans for the majority of clinical scenarios, with VMAT showing improved consistency over IMRT due to continuous arc delivery averaging out minor machine-specific variations in multileaf collimator positioning and dose rate.