Influence of the collimator rotation and the position of the isocenter on quality controls of treatment plans in dynamic arctherapy

• Published in: Physica medica 2013-06, Vol.29, p.e36-e36
• Main Authors: Boutevin, F, Feuillade, J, Gautier, M, Diagne, M, Dejean, C
• Format: Article
• Language: English
• Subjects: Radiology
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2013.08.112

Purpose To study influence of the collimator rotation and of isocenter position on the results of the treatment plans quality controls for dynamic arctherapy (RapidArc, Varian). Material and method 167 treatments plans have been studied (85 for 18 MV and 82 for 6 MV). Calculated dose in an octagonal phantom (Octavius) is compared to measured dose by a matrix of ionization chambers (2DArray PTW), thanks to Gamma index (3% in dose and 3 mm in distance). The results will be discussed accordance to the collimator rotation (10° or 30°) and the isocenter position relative to the field symmetry center. The isocentre position has been distributed in 4 areas; a ‘‘centered” area (10% of the field size around the symmetry center), an eccentric area in X or in Y (>10% of the field size relative to the symmetry center in the X or Y direction) and an eccentric area in both X and Y. Results At 6 MV, the average percentage of points that is not respecting tolerances (γ > 1) is 5.56 ± 4.34%. For a collimator rotation of 10°, the average is 4.67 ± 3.62% and at 30° it is 6.64 ± 5.06%. For the isocentre position, the average is 3.91 ± 2.90% in the ‘‘centered” area, 6.00 ± 4.55% in the eccentric area in X, 5.83 ± 4.92% in the eccentric area in Y and 7.20 ± 4.78% in the eccentric area in both X and Y. At 18 MV, the average is 6.21 ± 6.27%. For a rotation of 10°, the average is 6.43 ± 6.65%, while at 30° it is 4.36 ± 4.42%. For the isocentre position, the average is 5.34 ± 6.69% in the ‘‘centered” area, 9.01 ± 5.39% in the eccentric area in X, 6.12 ± 5.83% in the eccentric area in Y and 6.98 ± 6.98% in the eccentric area in both X and Y. Conclusion Concerning the collimator rotation, the results being opposed according to the energy, they require further investigation. In our measurement conditions, the main influence is the isocenter position relative to the symmetry center of the jaws, more it's eccentric in X and in Y, more results deteriorate.