Planning comparisons and dosimetric studies of different field IMRT or VMAT in breast cancer have been evaluated in a large number of studies and there’s always been a debate on employing which technique in the radiation practice. This study compares different arcs of VMAT and fields of IMRT in radiotherapy planning, and evaluates the plans with the quality score table which focused on heart dose and coronary area in left sided breast cancer radiotherapy. However, the advantage of suitable radiotherapy plan for the patients with relative smaller breast has not been fully clarified.
Patients with early stage left breast cancer could survive for a long time and adapt to receive techniques that may reduce the incidence of acute and late toxicity induced by radiotherapy. The cardiovascular complications induced by radiation- as a main radiotherapy-related late toxicity event progresses over time, and may manifest decades after the initial exposure . The coronary artery injury was considered to be the most serious radiation-related complication in the heart. Darby SC et al. reported that exposure of the heart to ionizing radiation during radiotherapy for breast cancer increases the subsequent rate of ischemic heart disease linearly with the mean dose to the heart by 7.4 % per gray, with no apparent threshold . Other studies further suggested that 1 Gy irradiation added to the mean heart dose could increase the cardiotoxic risk by 4 % . Several studies had observed substantial radiation-induced heart disease when the heart receives more than 40 Gy and that the reduction of the V40 was pertinent in reducing heart toxicities [22, 23]. In the present study, when calculated the scores from the sections of heart and coronary arteries, we found that both the 1-arc VMAT and 2-F IMRT have the highest scores of 4 points. The former showed advantage on V20 and V40 for heart and coronary arteries, and the latter showed favorable results on Dmean, V5 and V20 for heart and V5 for coronary arteries. This meant the 2-F IMRT and 1-arc VMAT plan showed a statistically significant improvement for heart dose for left-sided breast irradiation.
Nowadays, the developed radiation techniques could also be used to spare the cardiac area sparing in breast radiation practice. It was reported that radiation delivering in Deep Inspiration Breath Hold (DIBH) conditions could reduce the dose to heart for left-sided breast cancer patients . Some other studies demonstrated that whole breast irradiation with prone position seems to be beneficial for 85 % of the patients regarding heart irradiation . Further studies found that IMRT with prone position is superior to supine treatment for right-sided breast cancer patients and left-sided breast cancer patients with larger breasts  and benefited most from prone position with DIBH for heart sparing by radiation dose . But for patients of smaller breast volume in left side, some studies argued that the prone position might result in worse cardiac dosimetry than supine position [28, 29].
Moreover, when a dose comparison of heart minus coronary artery calculated, the observed V20 and V40 for the rest of heart, in all plans were very small and closed, suggesting that the dose volume for the coronary arteries can be used to predict the dose volume of the high dose for the entire heart.
For other OARs, it has been reported that the doses to the ipsilateral lung have been shown to be responsible for radiation pneumonia in breast cancer radiotherapy . Dosimetric parameters of mean lung dose and V20 showed a significant correlation with the development of radiation-induced pneumonitis in radiotherapy for breast cancer [31, 32]. In our study, we found that for Dmean and V5 to ipsilateral lung, the best results were from 2-F IMRT and 2-arc VMAT plans, and the V20 to the ipsilateral lung was uneventful in all plans. In addition, the 2-F IMRT, 4-F IMRT and 2-arc VMAT plans were associated with the most favorable dose deposition in the liver, esophagus, spinal cord, contralateral lung compared with 1-arc VMAT.
Except for acute and late radiation damage induced by high dose radiation, the low dose irradiation raises the concern of radiation-induced secondary malignancy . The delivery of low-dose irradiation to healthy tissue, especially to the contralateral breast, has been estimated to double the risk of subsequent malignancy , and this risk increases with increasing dose . Based on our study, it was demonstrated that 2-F IMRT and 2-arc VMAT resulted in a reduction of the mean dose to healthy tissue and ID as compared with that in other plans. And the 2-F IMRT plan also showed advantage on Dmean in contralateral breast.
So from the overall consideration we suggest to choose 2-F IMRT with the highest scores which was suitable for the protection of heart and coronary artery in left-sided breast cancer radiotherapy. We also found that 2-arc VMAT technique with the second highest scores could improve the homogeneity and conformity in PTV and sparing of some OARs in some dosimetric indications. So if the doctor has not concerns on the heart and coronary artery, the 2-arc VMAT technique may also be a good choice. The 1-arc VMAT plan with fewer composite scores has apparent advantages on Dmax and high doses regions to heart and coronary artery, which might also be a selective plan for sparing heart dose in practice. The 4-F IMRT plans did not show special advantages when compared with other plans in our research.