Use of electronic portal images to evaluate setup error and intra-fraction motion during free-breathing breast IMRT treatment.

Before delivering of intensity-modulated radiotherapy, kilo-voltage image-guidance radiotherapy is widely used in setup error correction and monitoring intra-fraction motion effectively. Accordingly, this study proposes and tests an image integration technique for observing intra-fraction motion during beam delivery, with the wider objective of reducing both image-guidance time and the dose delivered to normal breast tissue. The study sample comprised 33 female patients with breast cancer, and 241 sets of portal images acquired using a VARIAN aSi-1000 electronic portal imaging device. Motion amplitudes and vectors were collected and calculated separately by two senior therapists. The setup error in 3 axes was computed for every fraction, with average shifting for lateral, longitudinal and vertical direction was -0.3-mm ± 0.5, -0.1-mm ± 0.5 and -0.6-mm ± 1.6, with the average vector of setup error being 2.9-mm ± 1.4. The average intra-fraction motion for vertical direction was (A: -0.1-mm ± 1.0; B: -0.0 ± 1.1), for longitudinal was (A: -0.4-mm ± 1.7; B: 2.0 ± 1.1), and for lateral direction was (A: 0.3-mm ± 1.3; B: 0.2 ± 1.8). The average intra-fraction vector was 2.9-mm ± 1.3 for therapist A, and 3.4-mm ± 1.8 for therapist B. Offline Review commercial software was utilized for setup error and motion analysis, and data analysis and reliability testing were conducted with statistical package of the social sciences. Pearson correlations between the two therapists was moderate (0.59, p << 0.01), and the Cohen's kappa value for inter rater agreement between different evaluators was fair in the anterior-posterior direction (0.25, p << 0.01), with slight agreement in other two directions and vectors. The study presented efficient and dose reduction method to evaluate setup error and intra-fraction motion during breast intensity-modulated radiotherapy treatment.