Effect of respiration on the radiation dose distribution within target volume in radiotherapy

Objective: To evaluate the influence of respiration on the radiation dose distribution within target volume in radiotherapy with film dosimetry. Methods: Radiation of 50MU was delivered in a square, round, ellipse, dumb bell, or female shaped field to the films within a moving or static Respiration Motion Phantom respectively, the dose distributions for the two motion status were measured and compared. In order to further verify the impact of quantity of respiration movement, a slab phantom was used on different shifting value: 0 cm, 0.5 cm, 1.0 cm, 1.5 cm, and 2.0 cm respectively. A square, round, or ellipse-shaped filed was used for irradiation and the distributions in different status were measured and compared with film dosimetry. Iso-dose line comparison, NAT (Normalized Agreement Tests) and γ comparison were used for the comparison of dose distributions. F_s, was used as an index to evaluate the differences of the areas that surrounded by iso-dose lines in different situations (F_S90, F_S50, F_S25 delegates the ratio of the areas that surrounded by 90%, 50%, 25% iso-dose line in different situation respectively). Results: (1) In horizontal movement situation compared with in static situation, the F_S90 became smaller and the F_S25 became larger. The more the displacement became larger, the more the F_S90 and the F_S25 deviate remarkable. In vertical movement situation, F_s changed significantly in square field and dumb bell shaped field while changed a little in the others. (2) γ and NAT comparison: In the horizontal movement situation, compared with in the static phantom, the P_γ was <;;60% and the P_NAT was <;;75% in every radiation field. In vertical movement situation, the P_γ was less than 85% for all the square, round, dumb bell and female shaped fields. In the slab phantom verification, the Pγ and the P_NAT became smaller as the movement became larger. Conclusions: The respiration can impact on the dose distribution within the target volume in radiotherapy, leading to a smaller area of higher dose level and an expanded area of lower dose level. The influence will become more significant with larger movement of the target.