Fiducial marker matching versus vertebral body matching: Dosimetric impact of patient positioning in carbon ion radiotherapy for primary hepatic cancer

Purpose: The aim of this study was to compare the dose-volume parameters of fiducial marker matching (MM) with vertebral body matching (VM) in patient positioning for carbon ion radiotherapy for primary hepatic cancer.

Materials and methods: Twenty patients with primary hepatic cancer were retrospectively studied to assess changes in reproducibility of tumor position and dose distribution on two CT scans. One was for treatment planning and another was for dose confirmation, acquired the day before the first treatment day. The coverage of the clinical target volume (CTV) (D₉₈) and normal liver volume excluding the CTV which received 20Gy relative biological effectiveness (RBE) (V₂₀) were used as evaluation parameters. Additionally, the correlation of tumor movement and D₉₈ was calculated in VM and MM. The prescription dose was 60.0Gy (RBE) delivered in four fractions (15Gy/fx).

Results: The median (range) D₉₈ for VM and MM was 57.9 (20.8-59.9) and 59.9 (57.2-60.3) Gy (RBE), respectively. The median (range) V₂₀ for VM and MM was 17.9 (4.8-44.4) and 16.2 (4.7-44.9) Gy (RBE), respectively. The D₉₈ for MM was significantly larger than that for VM (p=0.001), although V₂₀ showed no significant difference (p>0.05). Twelve patients were clinically acceptable (D₉₈>57Gy (RBE)) with VM, while all patients were clinically acceptable with MM. Marker movement correlated with a decrease of D₉₈ for VM (R=-0.814).

Conclusion: Compared with VM, MM was clinically acceptable in all patients. This suggests that MM is more robust than VM.