Objective:To investigate the influence of different prescribed doses on organs at risk through comparing the dosimetry of 3-dimension conformal radiotherapy(3D-CRT) in preoperative rectum cancer.Methods:With the application of the treatment planning system(PINNACLE3)

25 patients with preoperative rectum cancer had 3D-CRT plan which had prescribed doses of 46 Gy and 50 Gy

and the dose characteristic of target volume and the exposure doses and volume of small bowel

bladder and bilateral femoral were compared.Results:(1)The two prescribed doses showed PTV average dose evenly

which could satisfy 95% of prescribed doses for covered PTV.For 46 Gy

CI

HIGTV

HICTV and HIPTV were close to those of 50 Gy and these differences were of no significance.(2)For 46 Gy

V50 of small bowel and bladder was smaller than that of 50 Gy.These differences were significant.Conclusion:For radiotherapy of different prescribed doses before rectum cancer operation

a higher dose has greater influence on irradiated volume of small bowel and bladder.