Assessment of setup accuracy in patients receiving postmastectomy radiotherapy using electronic portal imaging

Purpose The aim of this study was to investigate the setup accuracy for patients undergoing postmastectomy radiotherapy using electronic portal imaging. Materials and methods Ten patients undergoing radiotherapy via tangent (TG), supraclavicular-axillary (SA), and internal mammary (IM) fields were included. To explore the setup accuracy, distances between chosen landmarks were taken as reference parameters (RPs). The difference between measured RPs on simulation films and electronic portal images (EPIs) was calculated as the setup error. Results A total of 30 simulation films and 120 EPIs were evaluated. In the SA field, calculated RPs were lung length (LL), clavicle-field center perpendicular distance, and clavicle-field center transverse distance. The mean of the standard deviations (SDs) of the random errors (σ) for these parameters were 4.7, 7.3, and 7.6; and the SDs of the systematic errors (Σ) were 6.8, 4.4, and 13.5, respectively. In the TG fields, the calculated RPs were the central lung distance (CLD), maximum lung distance (MLD), and central soft-tissue distance (CSTD). In the medial TG field, the σ values for these parameters were 3.4, 3.6, and 4.1, respectively; and the σ values were 6.6, 2.6, and 3.4, respectively. In the lateral TG field, Σ values for the calculated RPs were 2.4, 3.2, and 3.3l, respectively; and the Σ values were 5.6, 3.6, and 4.8, respectively. Conclusion CLD, MLD, and CSTD in TG fields and LL in SA fields are easily identifiable and are helpful for detecting setup errors using EPIs in patients undergoing postmastectomy radiotherapy.