The Feasibility of Fast, Four-Dimensional Computed Tomography-Based O-Ring Linac Plans for Stereotactic Body Radiotherapy in Patients with Poor Performance Status

Abstract

As patients with poor performance status cannot endure long treatment times, it can lead to unplanned and prolonged radiation treatment or discontinuation of treatment. Moreover, prolonged radiation treatment appears to negatively affect survival for patients. We aimed to verify the feasibility of fast 4DCT-based O-ring linear accelerator (LINAC) treatment in lung 4D stereotactic body radiotherapy (SBRT). This study included data of 38 patients who received lung 4D-SBRT. Gating20-70% plans were based on 4DCT data obtained at phase values ranging from 20-70% using a C-arm LINAC. Non-GatingFB plans were retrospectively established based on 4DCT data obtained at phase values of 0-90% using an O-ring LINAC. The prescribed dose of 4,800 cGy was delivered to PTV in four fractions. The conformity index (CI), homogeneity index (HI), and gradient measurement (GM) of the PTV were analyzed to compare dosimetric data between Gating20-70% and Non-GatingFB plans. Organs at risks (OARs) were analyzed in accordance with the RTOG 0915 protocol. Treatment delivery time and total monitor units (MU) were analyzed to compare the efficiency of treatment delivery. Statistical comparisons were performed using the Wilcoxon signed-rank test (p<0.05). For the PTV, there was no significant difference in the CI or HI between Gating20-70% and Non-GatingFB plans (CI: 1.337±0.137, 1.335±0.174, HI: 0.939±0.015, 0.939±0.016, all at p>0.05). However, there was a significant difference in GM between Gating20-70% and Non-GatingFB plan (1.528±0.206 cm, 1.381±0.222 cm, p<0.001). For OARs, all plans met the criteria for dose constraint. There was a significant difference between Gating20-70% and Non-GatingFB plans except in the spinal cord. In particular, D1000cc and D1500cc values for the ipsilateral lung for Gating20-70% plans were 31.6% and 59.9% lower than those for Non-GatingFB plans (D1000cc: 50.780±132.061 cGy, 34.755±102.480 cGy, p<0.001, D1500cc: 6.641±14.598 cGy, 2.666±7.926 cGy, p<0.001). Treatment delivery time was 92% longer for Gating20-70% plans than for Non-GatingFB plans (5.0±0.4 min, 9.6±3.3 min, p = 0.043). The total MU value for Gating20-70% plans was 9.6% higher than that for Non-GatingFB plans (3665.8±230.7, 4016±236.9, p = 0.039). When compared to OARs both plans were acceptable per RTOG-0915 protocol, but significant differences were between Non-GatingFB and Gating20-70% plans. However, treatment delivery time of Non-GatingFB plans decreased by 47.9% than Gating20-70% plans. We verified the feasibility of fast 4DCT-based O-ring LINAC treatment for patients who cannot endure long treatment times due to poor performance status in lung 4D-SBRT. In further study, we will analyze dosimetric differences and treatment delivery efficiency considering tumor motion according to tumor location.