Mid-ventilation based PTV margins in Stereotactic Body Radiotherapy (SBRT): a clinical evaluation

Large tumor motion leads to large treatment volumes with an Internal Target Volume (ITV) based approach, whereas mid-ventilation (MidV) based Planning Target Volumes (PTV) margins typically lead to smaller treatment volumes. The purpose of this study was to evaluate the MidV approach on clinical outcome data of Stereotactic Body Radiotherapy (SBRT) in NSCLC.

297 patients with 314 peripheral tumors treated from 2006 to 2012 were retrospectively analyzed. In all patients a 4D-CT was acquired and the MidV-CT-scan was selected. Tumor amplitudes were determined in left-right (LR), cranio-caudal (CC) and anterior-posterior (AP) direction, to calculate patient specific PTV margins.

The median LR, CC and AP tumor amplitudes were 2mm (0-16 mm), 4mm (0-39 mm) and 3mm (0-18 mm), respectively, yielding a median CTV-to-PTV margin of 8mm. An ITV+5mm based PTV margin would have been bigger in 47% of the patients. After a median follow up of 22 months, local recurrence occurred in six patients (2%). Two year LC and OS were 98% and 67%, respectively.

Using the MidV approach combined with online image guidance an excellent LC of 98% was established with SBRT. This provides clinical support that incorporating respiratory motion into the PTV margin is a safe approach.