Abstract:

Objective To explore the dosimetry difference between volumetric modulated arc therapy （VMAT） and tomo direct （TD） in tumor bed simultaneous push radiotherapy after left breast-conserving surgery, and to provide more dosimetry reference for clinic. Methods A total of 22 patients with left breast cancer who underwent simultaneous quantitative radiotherapy after breast-conserving surgery were selected from the Department of Radiation Oncology, Yunnan Cancer Hospital from December 2018 to June 2020. The localized CT images and target organs at risk and other structural data were collected. Two radiotherapy plans, VMAT and TD, were designed for the same patient, and the dosimetry differences of target areas and organs at risk were compared and analyzed between the two groups.Results In terms of target dosimetry, there were statistically significant differences in the D_2% [（59.99±0.19） Gy vs. （59.55±0.51） Gy, t=4.09, P<0.001], D_98% [（57.19±0.08） Gy vs. （57.46±0.22） Gy, t=-5.10, P<0.001], conformal index （CI） （0.76±0.05 vs. 0.58±0.13, t=8.19, P<0.001） and homogeneity index （HI） （0.05±0.00 vs. 0.04±0.01, t=4.89, P<0.001） of the planning gross tumor volume （PGTV） between VMAT and TD plans. However, there was no statistically significant difference in the D_50% [（58.73±0.10） Gy vs. （58.73±0.24） Gy, t=-0.03, P=0.974]. There were statistically significant differences in the D_50% [（52.21±0.33） Gy vs. （53.00±0.72） Gy, t=-4.81, P<0.001], D_98% [（48.44±0.43） Gy vs. （49.09±0.21） Gy, t=-6.80, P<0.001], CI （0.83±0.06 vs. 0.67±0.06, t=10.52, P<0.001） and HI （0.20±0.01 vs. 0.19±0.01, t=8.75, P<0.001） of the planned target volume （PTV） between the two plans. However, there was no statistically significant difference in the D_2% [（59.01±0.45） Gy vs. （59.00±0.48） Gy, t=0.22, P=0.830]. In terms of organs at risk, there were statistically significant differences in the V₂₀ [（18.81±2.86）% vs. （22.03±1.91）%, t=-5.36, P<0.001] and D_mean [（11.66±1.32） Gy vs. （12.85±1.46） Gy, t=-4.10, P=0.007] of left lung, V₅ [（5.70±2.90）% vs. （0.30±0.13）%, t=16.44, P<0.001] and D_mean [（2.45±0.29） Gy vs. （0.43±0.14） Gy, t=9.09, P<0.001] of right lung, D_mean [（3.22±0.72） Gy vs. （1.69±0.80） Gy, t=5.41, P<0.001] of right breast, D_2% [（5.37±1.97） Gy vs. （0.46±0.09） Gy, t=11.75, P<0.001] of cord between VMAT and TD plans. There were no significant differences in the V₅ of left lung [（53.00±5.99）% vs. （50.00±7.69）%, t=1.91, P=0.061], V₅ of right breast [（11.51±4.60）% vs. （8.06±3.49）%, t=1.59, P=0.120], V₃₀ [（1.49±0.69）% vs. （1.51±0.71）%, t=-0.06, P=0.952] and D_mean [（3.99±0.97） Gy vs. （3.90±1.03） Gy,t=0.56, P=0.581] of heart between the two plans. Conclusion TD and VMAT can meet the clinical dosimetry requirements for patients with left breast cancer after breast-conserving surgery. However, the two techniques have their own characteristics. VMAT has better conformity and TD has better uniformity. TD is significantly better than VMAT in protecting the right lung, right breast and spinal cord of healthy organs at risk. VMAT is better in protecting the left lung. Both VMAT and TD basically achieve the same protection for heart.

Key words: Unilateral breast neoplasms, Breast-conserving surgery, Volumetric modulated arc therapy, Tomo direct, Dosimetry