Background and purpose: Breast-conserving surgery (BCS) plus post-operative radiation therapy has become the standard of care for early stage breast cancer. This study aimed to design whole breast and simultaneous tumor bed boost radiation therapy plans by adopting 3 different types of intensity-modulated radiation therapy (IMRT) techniques

and to compare the dosimetric parameters involving target volumes

organs at risk (OARs) and other aspects of the three IMRT plans. Methods: Fifty female patients with early stage breast cancer (25 left and 25 right

respectively) treated with BCS during the year of 2018 in Department of Radiation Oncology

Shanghai Jiao Tong University Medical School Affiliated Ninth People’s Hospital Huangpu Branch

were randomly selected. Adjuvant whole breast and simultaneous tumor bed boost radiation therapy plans were designed by adopting forward-planned IMRT

reverse-planned IMRT and volumetric modulated arc therapy (VMAT) techniques. Conformity index (CI) and homogeneity index (HI) of target volumes

dosimetric parameters of OARs

including ipsilateral lung (V

5

V

20

V

30

)

heart (D

mean

V

25

for left breast cancer

V

15

)

contralateral breast (D

2

D

mean

)

single fraction minute (MU) and beam-out time of the three IMRT plans were compared

respectively. Further analysis was made to identify the effect of breast volume on IMRT plans. Results: CI of forward-planned IMRT

reverse-planned IMRT and VMAT for whole breast target volume were 0.69±0.09

0.86±0.06

0.79±0.07

respectively (pairwise comparison P0.001). CI for tumor bed target volume were 0.71±0.15

0.79±0.15 and 0.80±0.12

respectively (pairwise comparison

P=0007

P0.001

P=0.624

respectively). HI of the three plans for whole breast target volume were 0.17±0.03

0.13±0.03 and 0.18±0.03

respectively (pairwise comparison

P0.001). HI for tumor bed target volume were 0.17±0.05

0.07±0.01 and 0.10±0.02

respectively (pairwise comparison

P0.001). Ipsilateral lung V 5 was 56.08±7.24

46.08±5.48 and 57.82±6.64

respectively (pairwise comparison

P0.001

P=0.079

P0.001

respectively). V

20

was 27.96±2.57

20.28±2.13 and 23.44±2.71

respectively (pairwise comparison

P0.001

P=0.025

P0.001

respectively). V

30

was 22.34±2.20

15.40±2.37 and 16.42±2.82

respectively (pairwise comparison

P0.001

P=0.006

P0.012

respectively). Heart D

mean

for left breast cancer was 775.48±113.23

584.20±223.04 and 634.24±174.38

respectively (pairwise comparison

P0.001

P0.001

P=0.045

respectively). Heart D

mean

for right breast cancer was 209.32±84.60

125.56±41.65 and 200.80±49.74

respectively (pairwise comparison

P0.001

P=0.524

P0.001

respectively). Heart V

25

for left breast cancer was 8.20±1.73

5.02±1.38 and 6.65±1.56

respectively (pairwise comparison

P0.001

P0.001

P=0.037

respectively). Heart V

15

for right breast cancer was 0 for all

and no further comparison was made. Contralateral breast D mean was 288.05±105.14

108.25±56.47 and 123.59±73.79

respectively (pairwise comparison

P0.001

P0.001

P=0.023

respectively). Single fraction MU was 285.74±17.73

1463.94±227.74 and 445.50±98.22

respectively (pairwise comparison

P0.001). Beam-out t

ime was 205.12±20.68

343.26±37.59 and 138.06±13.53

respectively (pairwise comparison

P0.001). A total of 50 patients were divided into small/large breast volume groups according to the mean breast volume (764.89 mL). Better dosimetric parameters of target volumes

fewer single fraction MUs and shorter beam-out time were found in small volume group. No statistical difference in OARs’ dosimetric parameters was found in the two groups. Conclusion: Reverse-planned IMRT and VMAT were better than forward-planned IMRT in the aspects of target volumes’ dosimetric parameters and protection of OARs. Reverse-planned IMRT was slightly superior to VMAT

and was recommended for younger patients with better tolerability and longer life expectancy. VMAT had both fewer single fraction MUs and shorter beam-out time

and was preferable for patients with older age and larger breast volume.