[Cone beam CT-derived adaptive radiotherapy for setup error assessment and correction in whole breast intensity modulated radiotherapy].

OBJECTIVE: To quantify the setup error (SE) in breast cancer patients treated with intensity modulated radiotherapy (IMRT) based on cone beam CT (CBCT), and to explore the feasibility of using several CBCT scans to presume and correct SE in the treatment for breast cancer patients.

METHODS: Eighteen breast cancer patients after breast conserving surgery who underwent whole breast IMRT were included in this study. Three dimensional interfraction motion before and after on-line CBCT-based corrections were quantified. The on-line CBCT-based corrections were performed using automated greyscale match. The system SE (Σ) and random error (σ) were calculated for each patient based on the consecutive multiple online scanning based on CBCT (≥5). The trends in magnitudes of Σ and σwere assessed during the treatment.

RESULTS: The magnitude variation of Σ was less than 1 mm before and after on-line CBCT-based corrections. As the CBCT scanning times increase (before 10 times), the Σ in anteroposterior (AP) direction was increased significantly, and σin three dimensional directions was also increased after 7 times of CBCT scanning. After on-line CBCT-based corrections, the Σ showed a steady trend by variation near zero for the first 20 times irradiation; but after 20 times, the Σ in AP and superoinferior (SI) directions was increased slightly (less than 0.5 mm), and σdecreased in three-dimensional directions. There were no significant differences for Σ, σand setup margin (SM) before and after on-line CBCT-based corrections in all three directions (P>0.05).

CONCLUSIONS: For breast cancer patients who underwent IMRT after breast conserving surgery, the setup error is relatively stable during the whole irradiation. The first 5 CBCT scans are suitable to presume and correct SE, and also can be used as the right time for adaptive radiotherapy planning revision.