Purpose: To determine the daily initial set-up errors and the patient position reproducibility pre-irradiation with 4D-DIBH breathing control radiotherapy technique the whole-body Vac-lock immobilisation device.

Materials and methods: A total of 53 patients were assigned to the Thoracic Radiotherapy (Esophageal cancer: 31 patients; lung cancer: 15 patients; thymic tumor: 5 patients; Rib metastasis: 1 patient) at Vinmec Times City International Hospital from June 2023 to September 2024. Patients were treated by VMAT technique with 4D – DIBH (Deep Inhale Breath-Hold) using Whole Body Vac-lock immobilization devices (QFIX, PA, USA) and (CIVCO, Orange City, IA, USA). Determined the set – up error and pre-treatment repoducibility by collecting daily kV and CBCT images before treatment. A total 1027 of 3D-CBCT image serries were collected and analyzed offline on the ARIA system (Varian Medical System, CA, USA). These data were used to determined the setup margin (SM), systematic margin, and random margin errors in three directions: anterior-posterior (AP); left-right (LR); Cranial-Caudal (CC), and stored on the ARIA system (Varian Medical System, CA, USA).

Results: Innitial set – up error of 4D-DIBH radiotherapy for the thoracic tumors were: 2.4mm  ± 0.9mm (in the AP direction), 3.2mm  ± 1.3mm (in the Cranial-Caudal direction), and 1.9mm ± 1.1mm (in the LR direction). Average set-up error for the 4D-DIBH radiotherapy group of patients with esophageal cancer: 2.4 mm ± 0.9mm, 3.4 mm ± 1.2mm, and 1.8mm  ± 1.1mm in the AP, CC, and LR directions, respectively; average setting error for the 4D-DIBH radiotherapy group of patients with lung cancer: 2.7 mm ± 0.9mm (AP direction), 2.9 mm ± 1.3mm (CC direction), and 1.8mm  ± 0.9mm (LR direction). Average setting error for the 4D-DIBH radiotherapy group of patients with thymic cancer and ribs metastases: 1.8 mm ± 0.6mm, 3.0 mm ± 2.3mm, 3.1mm ± 1.4mm in the AP, CC, LR directions respectively; Set – up error (SM) for 4D-DIBH radiotherapy using Whole – Body Vac-lock is calculated according to the van Herk formula for the Thoracic region: 3.7mm (AP) 4.8mm (CC); 4.0mm (LR); esophageal cancer: 3.9mm (AP) 4.8mm (CC); 3.6mm (LR) ); lung cancer: 3.5mm (AP) 4.8mm (CC); 4.1mm (LR) ); thymic cancer, bone metastasis: 2.9mm (AP) 4.7mm (CC); 4.9mm (LR).

Conclusion: 4D-DIBH radiotherapy for the thoracic cancer using Body Vac-lock ensures well position repositioning, reduces initial set – up error to deliver accurate radiation dose to the tumors, and limits side effects on organ at risk (OAR). The results of the study and set – up error (SM) calculation is the base – evidence to help Radiation oncolosist determine the appropriate PTV – Margin for treatment in each location.

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