A dosimetric study of photon energy and flattening filter mode combinations in Stereotactic Central Ablative Radiotherapy.

BACKGROUND: In Stereotactic Central Ablative Radiotherapy (SCART), the volume of tumor receiving the full prescribed ablative dose decreases rapidly as the prescription dose increases, posing a critical challenge. To address this issue, this study investigates the dosimetric impact of combining various photon energies and flattening filter modes available on modern linear accelerators (linacs), aiming to provide more options for clinical application. METHODS: Fifty-one cases of abdominal large-volume tumors treated with conventional radiotherapy at the Second People's Hospital of Changzhou from January 2021 to August 2023 were retrospectively selected. Plans were created using the Monaco 5.11 treatment planning system. All plans employed volumetric modulated arc therapy (VMAT) with two coplanar 360° arcs (clockwise + counterclockwise). The two arcs used different combinations of photon energies/flattening filter modes: 6 MV & 6 MV, 6 MV & 10 MV, 6 MV & 6 MV flattening-filter-free (FFF), 10 MV & 10 MV, 6 MV-FFF & 6 MV-FFF, and 6 MV-FFF & 10 MV, while keeping all other planning parameters consistent. RESULTS: Compared to the standard 6 MV & 6 MV combination, the 6 MV & 10 MV combination reduced the dose to normal tissues without a statistically significant change in ablation volume. The 6 MV & 6 MV-FFF combination yielded significantly larger ablation volumes, albeit with a statistically significant increase in normal tissue dose, which remained within clinically acceptable limits. The 6 MV-FFF & 6 MV-FFF combination also yielded a significantly larger ablation volume, but the increase in normal tissue dose was not statistically significant. The 10 MV & 10 MV and 6 MV-FFF & 10 MV combinations reduced normal tissue dose while increasing ablation volume, though the increase was less than that achieved with the 6 MV & 6 MV-FFF and 6 MV-FFF & 6 MV-FFF combinations. CONCLUSION: The choice of energy/mode combination in SCART can be tailored to clinical priorities. For paramount organ-at-risk sparing, 6 MV & 10 MV is recommended. For maximizing ablation volume, 6 MV-FFF & 6 MV-FFF is recommended, followed by 6 MV & 6 MV-FFF. For a balanced approach, the 6 MV-FFF & 6 MV-FFF combination demonstrated the best overall performance, followed by the 6 MV-FFF & 10 MV combination.