Abstract: Background and purpose: The radiation should move at the same direction in accordance with the position of the couch during radiotherapy due to the different location of gross target volume. However, under the condition of the long distance movement of the couch, collisions between the gantry and the couch (or the patients) may happen. To avoid the collisions between the gantry and the couch, the motion area confliction between the gantry and the couch was analyzed. Methods: The distance from isocenter to the surface center of linear accelerator head, and the width and the thickness of treatment bed were measured when the treatment bed was empty, and gantry and collimator were at zero position. Forty sets of bed movement data were collected. The sum of squares of X-and Y-coordinates of reference point was calculated and compared with squares of rotation radius. Then, it was anticipated whether the linear accelerator head will collide with treatment bed. Finally, practical verification was made. Results: Whether linear accelerator head collides with treatment bed depends on the difference between the sum of squares of X-and Y-coordinates and the squares of rotation radius. No collisions will happen when the sum of squares of X-and Y-coordinates is less than that of the squares of rotation radius. Conclusion: It is feasible to avoid collisions of linear accelerator head with treatment bed by comparing the sum of squares of X-and Y-coordinates of reference point with squares of rotation radius.

Key words: Standard equation of circle, Radiotherapy, Collision, Gantry, Couch