An exploratory study of individualized CTV-PTV expansion margin in thoracic esophageal cancer radiotherapy

There is no consensus on the preferable CTV-PTV expansion margin for EC. We aimed to explore whether an individualized CTV-PTV expansion margin based on the first five setup error was a feasible strategy. One hundred EC patients were included. We obtained the CBCT registration data and analyzed the setup errors. The setup errors of daily CBCTs and the first five fractions CBCTs were compared. The expansion margins based on daily CBCTs and the first five fraction CBCTs were calculated. The dose distribution to the target volume based on the different expansion margins were compared. The average setup errors of daily CBCTs were 0.24 ± 0.21 cm, 0.40 ± 0.31 cm, and 0.21 ± 0.18 cm in the Lat, Lng, and Vrt directions, respectively. The average setup errors of the first five fraction CBCTs were 0.23 ± 0.20 cm, 0.43 ± 0.33 cm, and 0.19 ± 0.17 cm in the Lat, Lng, and Vrt directions, respectively. The setup margins were 0.74 cm in Lat direction, 1.02 cm in Lng direction and 0.66 cm in Vrt direction for daily CBCTs and 0.75 cm in Lat direction, 1.02 cm in Lng direction and 0.64 cm in Vrt direction for the first five fraction CBCTs. The individualized CTV-PTV expansion margin based on the first five fraction CBCTs showed better coverage of the setup errors, with the coverage rate of 96.62%. The dose distribution to the target volume using the individualized expansion margins and traditional uniform expansion margins showed no difference. Individualized CTV-PTV expansion margins based on the first five fraction CBCTs may be an alternate strategy.