Abstract: ObjectiveTo compare the dosage characteristics between threedimensional conformal radiotherapy (3DCRT) plan and simplified inverse dynamic intensity modulated radiotherapy (IMRT) in patients with earlystage breast cancer after breastconserving surgery. Methods3DCRT and IMRT treament plans were designed for 14 female patients with earlystage breast cancer after breastconserving surgery, 4 of whom were left breast cancer cases. A dose of 50 Gy in 25 fractions to the whole ipsilateral breast was delivered using 6 MV photons for 3DCRT or IMRT. For 3DCRT plans, tangential field irradiation was adopted. While for IMRT, reverse dynamic intensity modulated technology was done through two pairs of tangentiallikely fields, and 10 Gy was boosted to the tumor bed concomitantly in 25 fractions. The conformity index (CI), heterogeneity index (HI), dose and volume of organs at risk were evaluated by dose volume histograms (DVH). ResultsCompared with 3DCRT plans for ipsilateral lung, the high dose volumes were reduced and the low dose volumes were increased in IMRT plans. The same phenomenon was also observed for the heart of the patient with left breast cancer. The crosspoint doses of 3DCRT DVH and IMRT DVH for lung or heart were (25.16±9.11) Gy, (28.63±10.41) Gy respectively. There was no difference between the two plans in the V10 of contralateral breast ［IMRT(4.13±5.17)%∶3DCRT(1.99±2.43)%, t=2.11, P＞0.05］, but the D30 and mean of IMRT plan were higher than that of 3DCRT ［(2.23±1.77) Gy∶(1.20±0.46) Gy, t=2.58, P＜0.05; (2.35±1.59) Gy∶(1.54±0.88) Gy, t=3.15, P＜0.01］. The difference in HI between the two plans was not observed ［IMRT(1.25±0.10)∶3DCRT(1.23±0.11), t=1.25, P＞0.05］. While the CI of IMRT plans were improved compared with 3DCRT ［(0.75±0.07)∶(0.62±0.09), t=5.68, P＜0.000 1］. ConclusionCompared with 3DCRT plan in patients with earlystage breast cancer after breastconserving surgery, the main advantages of four fields simplified inverse dynamic IMRT are concomitant tumor boosting, decreasing the high dose volumes of ipsilateral lung, and improving the CI of planning target volume at the same time, but the HI is not improved. The IMRT plan is a simple, rational and feasible design scheme.

Key words: Breast neoplasms, Intensitymodulated radiotherapy, Radiotherapy dosage, Breast conserving surgery