Effect of collimator and couch angle change on breast IMRT dose distributions

Intensity modulated tangential photon beams for breast cancer treatment can improve the dose uniformity significantly throughout the whole breast and reduce the dose to the lung and the heart comparing with the conventional technique. Before the first treatment, patient setup may require a change on the collimator angle and/or the couch angle based on the chest wall coverage according to the port films. The objective of this work is to investigate the effects of the collimator and the couch angle change on the dose distribution for breast cancer treatment using intensity modulated tangential photon beams, and thus to determine the clinical acceptable range of the angle change for routine treatment. Ten breast cases treated with intensity modulated tangential photon beams were analyzed in this study. Patient-specific CT data and the RTP files obtained from our home-grown Monte Carlo based breast IMRT treatment planning system were used for IMRT dose re-calculation with collimator or couch angle changes. The isodose distributions and DVHs were compared with the original plans and the effects of the collimator and couch angle change to breast IMRT dose distributions were evaluated. Our results show that a 4-degree change in the collimator angle or the couch angle did not affect the dose distribution significantly and it is acceptable in the clinic for patient treatment.