Evaluation of Daily Patient Position Accuracy and Calculation of Setup Errors with Skin Markers and Surface-Guided Radiotherapy (SGRT) in Breast Cancer

Abstract

This study aims to compare the inter-fractional patient positioning errors between the use of skin markers and SGRT in patients receiving breast radiotherapy. It also includes calculating the inter- and intra-fractional PTV margins required for free breathing (FB) and deep inspiration breath-hold (DIBH) techniques due to setup uncertainties. The treatment data of a total of 2209 fractions from 80 patients who received whole breast or chest wall radiotherapy at an institution between March 2021 and December 2022 were retrospectively analyzed. In the first group of patients, the skin markers placed in the Computed Tomography (CT) scan with the FB technique were aligned and positioned with the room lasers in the Elekta Versa HD, and plan-specific shifts were made. The optical surface scanning procedure was applied to the second group of patients with the Sentinel™ system for FB or DIBH techniques in simulation. Patients were positioned for treatment by matching reference surface images with real-time images from the Catalyst™ HD surface imaging system. Positions of all patients were verified with kilovoltage or cone-beam CT imaging systems, and 3D shift vectors were recorded daily. To compare both setup methods, PTV margins including mean positioning shifts (MPS), the vector length of the shifts and setup errors were calculated from van Herk's formulas. The MPS of all patients using skin markers were 0.20±0.85 cm, 0.32±1.55 cm, and 0.18±0.44 cm in the vertical, longitudinal and lateral directions, respectively. Then, these values given above were found to be 0.12±0.15 cm, 0.06±0.17 cm, 0.07±0.23 cm for FB using Catalyst™ HD, and 0.34±0.30 cm, 0.29±0.26 cm, 0.23±0.30 cm for DIBH. In different setup techniques, no statistically significant difference was found between the 3D shift vectors in any direction with Wilcoxon signed-rank test (p>0.05). The mean standard deviation values in all directions were calculated as 1.22 cm and 0.61 cm, respectively, in patients using skin markers and SGRT. This study indicates a significant difference between the positioning shifts of patients positioned with skin markers and SGRT. The surface imaging system is an advantageous non-invasive method that increases setup accuracy and reproducibility in breast radiotherapy.