A simulation of the effects of set-up error and changes in breast volume on conventional and intensity-modulated treatments in breast radiotherapy

The effect of interfractional patient movement on dosimetry has been investigated for breast radiotherapy. Errors in patient set-up and changes in breast volume were simulated individually to determine how each contributes to the total dosimetric error. Two treatment techniques were investigated: a conventional treatment and an intensity-modulated treatment delivered using compensators. Six patients were investigated and anterior-posterior (AP) and superior-inferior (SI) displacements were simulated by displacing the isocentre in both directions by 2, 5 and 10 mm. A model of the breast was developed from the six patients to simulate changes in breast volume. In this model, the breast was described as a set of semi-ellipses. The volume of the breast was changed by varying the magnitude of the semi-major and semi-minor axes. Anisotropic changes in breast volume were also investigated. The dosimetric error was evaluated for each dose plan by calculating the volume outside the 95-105% dose range resulting from the simulations. A number of parameters describing the size and shape of the breast were also investigated to determine whether a susceptibility of outline sets to interfractional patient movement could be predicted. A parameter describing the increase in the breast volume outside the 95-105% dose range was calculated for AP and SI set-up errors of +/-3 mm and changes in breast volume of 5%. Results revealed that set-up errors were likely to cause larger increases in the volume outside the 95-105% dose range than changes in breast volume for typical set-up errors and changes in breast volume that occur during treatment. The magnitude of the set-up errors and changes in breast volume were observed to be dependent on the shape outline set. Statistically significant linear correlation was observed between the increase in the volume outside the 95-105% dose range and several of the evaluation parameters.