Influence of non-coplanar arcs in stereotactic ablative radiotherapy of Liver: A clinician's plan acceptance based comparative analysis between Ring gantry and C- Arm linear accelerators.

This study aims to evaluate the dosimetric compatibility of coplanar-only arc plans from a ring gantry linear accelerator with coplanar and non-coplanar arc-based plans from C-arm linear accelerators for liver stereotactic ablative radiotherapy (SABR). Halcyon (HAL) is a ring gantry, while Novalis Tx (NTx) and TrueBeam STx (TB) are C-arm linear accelerators. A total of 40 liver stereotactic ablative radiotherapy (SABR) patients, with either primary disease or metastasis, were planned using these three competing linear accelerators. HAL plans used only coplanar arcs, while NTx and TB plans incorporated both coplanar and non-coplanar arcs. All plans were dosimetrically and statistically evaluated using Greenhouse-Geisser statistical analysis (one-way ANOVA) among the three systems (HAL vs NTx vs TB), and paired sample t-tests (HAL vs NTx, HAL vs TB, and NTx vs TB). Both statistical analyses were considered significant at p ≤ 0.05. Target dose coverage to PTVD98 % were 95.4 ± 7.1 %, 95.8 ± 7.7 % and 95.6 ± 7.1 % for HAL, NTx and TB; and all variations were statistically insignificant p = 0.495, 0.333, 0.563 and 0.147. Dosimetric parameters like D15cc Heart (%), D5cc_Duodenum (%), mean dose (Liver-GTV) and liver volume ≤ 1500 cGy shows a statistically significant dose variation for both Anova and t-test. However absolute variation is very less , mean dose difference (liver-GTV) for (HAL-NTx)=37.5 ± 76.5 cGy and (HAL-TB)= 50.5 ± 88.8 cGy. Similarly for liver volume < 1500 cGy (HAL-NTx) and (HAL-TB) were -20.7 ± 39.4 cc and -21.2 ± 40.3 cc which is small compared to mean liver volume of 1293.0 ± 327.4cc. C-arm and ring gantry plans were the definitive choices for 5 (12.5 %) and 2 (5 %) patients, respectively, while the remaining 33 patients were independent of the arc arrangement. All three systems achieved the required dose-volume parameters. Coplanar plans demonstrated no significant disadvantages compared to mixed-arc plans. Although some statistically significant differences were observed, the variations were too small to be clinically relevant.