Fig. 1. Single ring model

Fig. 2. Dose distribution of y=0 direction in different planes

Fig. 3. Dose distribution of different planes: (a) z=3 cm; (b) z=9 cm

Fig. 4. Relationship between the dose and distance

Fig. 5. Relationship between the dose and distance

Fig. 6. Dose distribution with different inner radius of z =8 cm plane: (a) R₁=4 cm; (b) R₁=7 cm; (c) R₁=9.5 cm

Fig. 7. Relationship between the dose uniformity and ring radius

Fig. 8. Fivering model

Fig. 9. Dose distribution of different planes of z=(a) 3 cm, (b) 7 cm, (c) 9 cm, and (d) 11 cm

Fig. 10. Dose distribution in the diagonal direction at different distances of z=(a) 3 cm, (b) 7 cm, (c) 9 cm, and (d) 11 cm

Fig. 11. Dose distribution in the axis direction at different distances of z=(a) 3cm, (b) 7cm, (c) 9 cm, and (d) 11 cm (color online)

Fig. 12. Relationship between the dose uniformity and distance