Examination of Conversion Method of Dose Distribution of Lung Cancer IMRT Using Fluence Reversible Calculation Function in O-Ring Type Linac and C Type Linac.

Purpose/Objective(s) An O-ring type Linear Accelerator (O-Linac) has a dual-layer multi-leaf collimator (MLC) with flattening filter-free (FFF). Generally, when Linac fails, the converted irradiation between C-arm typed Linac (C-Linac) has the same mechanical structure, so it is possible to perform converted irradiation without re-calculating the dose distribution by the radiation therapy planning system (TPS). The O-Linac differs from C-Linac in the process of forming the dose distribution. Therefore, if O-type Linac becomes unusable due to a breakdown or inspection, it will be necessary to formulate a treatment plan from scratch. It is of great benefit to patients to clarify the consistency of treatment plans between these lilacs and to establish a simpler converted plan creation method. In this study, we investigate a method for more easily performing converted irradiation from O-Linac to C-Linac. Materials/Methods Thirty patients with lung cancer planned with VMAT using O-Linac are included. The O-lilac's dose distribution is converted into energy fluence by the function of TPS. The alternative Linac MLC is then optimized to achieve energy fluence. The time required to convert one plan is about 20-30 minutes. The HI, CI, and PTV (D95%, D2%) of the converted plan were compared with the original plan. For OARs, the DVH of lung, esophagus, heart, and spinal Cord were evaluated. The shapes of the iso-dose curves for each 10% in each plan were compared using the dice similarity coefficient (DSC). Results The statistical results of PTV and OARs are reported in Table. There was no significant difference for PTV (P > 0.05). The lung V5 (%) and Dmean (Gy) of the converted plan were slightly higher than those of the original plan. The lung V10 (%) and V20 (%), heart V50 (Gy), esophageal V60 (Gy), and spinal cord Dmax (Gy) of the converted plans were slightly lower than the original plans. However, these differences were not statistically significant (P > 0.05). The mean DSCs of the 30% to 100% iso-dose curves of the prescribed dose are above 0.8, and the DSCs on the iso-dose curves of ≥ 105% and ≤ 20% were less than 0.8. Conclusion Due to the structural differences of lilacs, it is considered that the dose-volume and the generation position were different in the dose range of ≥ 105% and ≤ 20%, and as a result, DSCs have decreased. However, no statistically significant difference in DVH has been identified for both treatment plans. From this result, it is considered possible to convert the dose distribution planned by O-Linac into C-lilac's dose distribution. It was proposed a simple replanning method.