A dosimetric comparison study : tangential forward intensity modulated radiothearpy versus volumetric modulated arc radiotherapy and the effects of boost PTV locations for simutaneous integrated boost technique of left breast cancer

Abstract

Background and objectives: Simultaneous integrated boost technique (SIB)is believed to be as safe and effective radiotherapy technique as conventional electron boost on tumour bed for treating Ca breast. With the use of SIB technique, hypo-fractionated dose scheme is therefore introduced to shorten the overall treatment times. The objectives of the present study are to evaluate the dosimetric effects on target volumes and organs at risk (OARs) of SIB technique by comparing (1) two planning techniques: tangential forward intensity modulated radiotherapy (tangential FIMRT) and volumetric modulated arc radiotherapy (VMAT) and (2) four differenttumour locations: upper-inner quadrant (UIQ), upper-outer quadrant (UOQ), lower-inner quadrant (LIQ) and lower-outer quadrant (LOQ).

Materials and methods: A total of thirty-two patients of stage I or II left breast cancer were selected where there were eight patients from each group of four quadrants. The CT image set was collected and each patient’s CT image set was planned with both tangential FIMRT and VMAT techniques. All the data was collected from the dose-volume histogram. In the first part of this study, thirty-two patients of tangential FIMRT and VMAT plans were compared. The conformity number (CN) and homogeneity index (HI) were examined for evaluation of planning target volumes (PTV) and planning boost volumes (PBV). The sparing effect of left and right lung, heart, left anterior descending artery (LAD) and contralateral breast tissue were explored. In the second part of this study, the dosimetric effect on OARs using tangential FIMRT technique for tumor located in different quadrants, with 8 patients in each quadrant group, were compared. All the data underwent statistical analysis.

Results: Significant results were found in both parts of the study. In the first part, the average CN value and HI of PTV were 0.68 and 0.32 for tangential FIMRT while they were 0.81 and 0.29 for VMAT (p-value<0.05). The average CN value and HI of PBV were 0.58 and 0.07 for tangential FIMRT while they were 0.82 and 0.06 for VMAT (p-value < 0.05). V20 and Dmean of left lung, V20 of right lung, V10 andV15 of heart, Dmax, Dmin and Dmean of contralateral breast tissue were significantly higher in VMAT plans. In the second part, tumors at UIQ showed significantly higher Dmax(average Dmax: 23.19Gy) to LAD when comparing with tumours in the two outer quadrants groups (average Dmax of UOQ: 7.01Gy; LOQ: 6.16Gy) and the Dmean(average: 14.7Gy) was proven to be the highest among four groups (p-value<0.005).Besides, the tumors at LIQ (average V10: 7.8%, Dmean.4.77Gy) displayed higher cardiac dose of V10 and Dmean compared with tumors in UOQ (average V10: 2.32%, Dmean.2.62Gy) (p-value<0.005). The Dmax of contralateral breast tissue was higher in LIQ tumors (average: 16.6Gy) than LOQ tumors (average: 3.77Gy) (p-value<0.005). Conclusion: VMAT showed higher quality of plan with better conformity and homogeneity. However, tangential FIMRT was effective in reducing the dose to OARs except Dmax and Dmean of LAD and the V5 of heart. Additionally, this study showed that tumour locations would have effect on dose to OARs especially on the heart dose, LAD dose and contralateral breast tissue dose.