<b>Assessment of geometric verification to determine the setup uncertainty margin in lung cancer patients.</b>

AimThe aim of the study was to evaluate the results of geometric verification by determining systematic and random errors to establish the setup margins (SM) for patient positioning uncertainty in a group of lung cancer patients. Material and Method The study included 20 patients treated with the Volumetric Modulated Arc Therapy (VMAT) technique with respiratory gating. The prescribed dose was 60 Gy in 30 fractions. The Planned Target Volume (PTV) averaged 247.62 cm³ (range 62.10–562.10 cm³). The PTV volume included the anatomical volume, accounting for tumor positions due to respiratory motion (Internal Target Volume, ITV), which was defined based on 4DCT (Four-Dimensional Computed Tomography) imaging at selected respiratory phases covering inhalation ±20% of the respiratory cycle, along with a setup margin (SM) of 0.5 cm. Respiratory motion was monitored using the Real-time Position Management (RPM) System. Patients were positioned with their arms above their heads using dedicated immobilization to stabilize the patient’s position. During each fraction, the patient's position was verified based on Cone Beam CT (CBCT) in four dimensions (4D CBCT). Based on the collected shift results in the cranio-caudal (Lng), anterior-posterior (Ver), and left-right (Lat) axes, the displacement vector (V) was calculated, as well as the systematic (∑) and random (σ) errors. Using these errors and applying the van Herk formula (PTV margin = 2.5∑ + 0.7σ), the setup margin (SM) was determined [van Herk M, Remeijer P, Rasch C, Lebesque JV. The probability of correct target dosage: dose-population histograms for deriving treatment margins in radiotherapy. Int J Radiat Oncol Biol Phys. 2000;47:1121-35]. Results600 geometric shift results were collected, with the mean values and standard deviations in the respective axes (Lng, Ver, Lat) being 0.192 ± 0.241 cm, 0.047 ± 0.234 cm, and 0.020 ± 0.239 cm, respectively. The displacement vector (V) was 0.425 ± 0.169 cm. The systematic error (∑) in each axis was 0.134 cm, 0.139 cm, and 0.136 cm, while the random error (σ) was 0.236 cm, 0.220 cm, and 0.233 cm. Based on these values, the SM margin was calculated to be 0.499 cm, 0.500 cm, and 0.502 cm, respectively. ConclusionsBased on the obtained results of geometric verification, it was concluded that for the applied patient positioning method for lung cancer patients treated with VMAT and respiratory gating, the adopted setup margin (SM) of 0.5 cm is consistent with the margin determined from the analyzed results of geometric verification.