Positioning Accuracy For Frameless Lung Sbrt Using 4-Dimensional Cone Beam Ct Guidance

To quantify the localization accuracy and intrafraction stability of lung cancer patients treated with frameless stereotactic body radiotherapy (SBRT) using 4-dimensional cone beam CT (4D-CBCT) guidance and to calculate the Internal Target Volume (ITV) margins to account for the setup errors. Nine patients with a single peripheral lung tumor, smaller than 30 cc, were included in this study. These patients were positioned in a flat and reproducible position: supine with both arms above their head. For each patient a 4D-CT was acquired in a Philips Brilliance CT. The ITV was delineated according to six breathing phases. In total, 35 fractions were delivered to these patients, who were treated with SBRT without a body frame. Two risk-adapted fractionation schemes were used: 3 fractions of 18 Gy and 5 fractions of 10 Gy. For each fraction, three 4D-CBCT scans were acquired: before treatment to measure and correct the mean tumor position (initial set-up), after correction to validate the correction applied, and after treatment to estimate the intrafraction stability. These scans were volume registered with the localization CT, using a soft-tissue match, in the Elekta XVI software. Corrections were performed by a robotic patient positioning platform (Elekta HexaPOD RT system). Treatment was delivered using 6 MV photons generated from Elekta Synergy Beam Modulator Linac. Patient positioning data from all scans were recorded to determine systematic (Ʃ) and random (σ) set- up errors for initial set-up, after correction and after treatment imaging, in the left-right (X), craniocaudal (Y) and anteroposterior (Z) directions. The ITV to PTV margin (M) was also calculated for after correction and after treatment imaging, using the Van Herk formula: M=2.5 Ʃ +0.7 σ A summary of patient set-up errors and ITV to PTV margins, in the three orthogonal directions, is shown in Table I.Abstract 3691; Table ISummary of set-up errors and ITV marginsDirectionsCBCT initial set-upCBCT aftercorrectionCBCT aftertreatmentƩ(mm)σ(mm)Ʃ(mm)σ(mm)M(mm)Ʃ(mm)σ(mm)M(mm)X2.12.20.71.02.51.11.53.8Y5.43.61.31.24.01.51.44.6Z2.62.70.30.81.41.01.23.3 Open table in a new tab Intrafraction stability was 1.1, 0.6 and 1.0 mm (systematic) and 1.2, 0.9 and 1.1 mm (random) for X, Y and Z directions, respectively. Calculated margins do not account for target delineation and organ motion uncertainties and are consistent with our current 7 mm margins in all directions. Frameless SBRT can be safely administrated using 4D-CBCT. This was demonstrated with small intrafraction movements after initial set-up correction using imaging guidance. Our data suggest that lung SBRT should not be delivered without image guidance to correct initial set-up uncertainties owing to the small size of the lesions treated and the large dose delivered each fraction. ITV margins can safely be kept small, allowing patients to benefit of advantages of SBRT.