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 conlfiction 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 ofX-andY-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 veriifcation was made.Results:Whether linear accelerator head collides with treatment bed depends on the difference between the sum of squares ofX-andY-coordinates and the squares of rotation radius. No collisions will happen when the sum of squares ofX-andY-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 ofX-andY-coordinates of reference point with squares of rotation radius.

OBJECTIVE： To screen the hepatoprotective active fractions from Ixeris chinensis and study its chemical constituents. METHODS：The petroleum ether，ethyl acetate，n-butanol and residual water fractions from 70％ ethanol extract of I.chinensis were extracted by systematic solvent method. Human hepatocytes HL-7702 were induced by acetaminophen to induce liver injury model. MTT method was used to detect the protective effect of the above fractions（40 μg/mL，by the dosage of crude drug）on injured cells，and the active fractions were screened. The active fractions were separated and purified by silica gel column and Sephadex column chromatography. The structure of the compounds were identified by physical and chemical properties and spectral data （hydrogen spectrum，carbon spectrum）. RESULTS：After treated with different fractions of I. chinensis，the cell survival rate of each administration group was increased significantly，compared with model group（P＜0.01），and the n-butanol and water fractions had the strongest activity （the cell survival rates were 49.3％ and 52.2％ ，respectively）. Six compoundswere isolated from n-butanol fraction and identified as sonchifolignan A（Ⅰ），apigenin-7-O-β-D-glucopyranoside methyl ester（Ⅱ），luteolin-7-O-β-D-glucuronopyranoside methyl ester （Ⅲ），luteolin-7-O-β-D-glucopyranoside （Ⅳ），apigenin-7-O-β-Dglucopyranoside（Ⅴ）and luteolin（Ⅵ）. CONCLUSIONS：The n-butanol fraction is regarded as an effective position for protecting liver，and flavonoids are the main active omponents.KEYWORDS Ixeris chinensis；Hepatoprotective activi