A comparative dosimetric study for treating left-sided breast cancer for small breast size using five different radiotherapy techniques: conventional tangential field, filed-in-filed, Tangential-IMRT, Multi-beam IMRT and VMAT

Many studies on comparison of dose distribution for breast cancer radiotherapy techniques have been reported [1‐5]. In these studies, the comparative irradiation techniques mainly include [2‐6]: 1) conventional tangential wedge fields (TW), 2) field-in-field (FIF), 3) tangential fields inverse intensity-modulated radiation therapy (T-IMRT), 4) multi-field IMRT (M-IMRT) and 5) irregular surface compensator (ISC). Recently, a new technique known as volumetric modulated arc therapy (VMAT) has been introduced. Compared to the traditional forward planning, the inverse-planned modulated irradiation therapy may benefit in better target dose homogeneity index (HI) and PRV-OARs dose reduction [5, 6].

The technologies mentioned above have been implemented in many institutions in China [7, 8]. Although the T-IMRT is reported having better target dose homogeneity and sparing normal tissue such as the heart and the ipsilateral lung, there are still some aspects of concern. Firstly, the planning target volume (PTV) of Chinese patients, which maximum and mean volume of 589.77cc and 427.2 cc reported by Huang [9] are obviously smaller than the Caucasians one with the maximum and the mean volume of 2170 cc and 994 cc as reported by Popescu [10]. This may lead to different results in using various irradiating techniques. Secondly, for irradiation of the left breast, cardiac dose is one of the most important issues. The most serious radiation induced complication of the heart is coronary artery injury [11]. Currently reported literature mainly [12‐15] focused on the volumetric dose of the heart, but few studies concentrated on the coronary artery region specifically. Xu et al [15] conjectured the cardiac dose might be associated with the breast volume for whole left breast irradiation. In their report IMRT treatment could significantly reduce cardiac dose for those clinical target volume (CTV) larger than 500 cc compared with conventional tangential techniques. In our study, we specifically compared the coronary artery dose of various radiation treatment techniques for the Chinese patient which having relative smaller PTV. Moreover, Popescu CC et al [16] reported that VMAT was able to improve dosimetry and reduce treatment time compared to conventional intensity modulated radio- therapy for loco-regional radiotherapy of left-sided breast cancer and internal mammary nodes. Whether VMAT offers dose benefits for whole left breast irradiation is another issue of our interest.

In this study, it is aimed to give some advice about the individual irradiation therapy to the patients after left conservative surgery whose planning target volume was relative smaller based on the dose comparison of five radiation methods and the irradiated dose analysis of planning target volume and OARs.

Twenty patients with left-sided breast cancer were randomly selected for this treatment planning study. They have undergone breast-conserving surgery.

There have been many reports about the choice of radiation treatment technique for breast cancer after conserving surgery. Even in Rongsriyam’s report [21], T-IMRT should be the best treatment. However, Bhanagar A.K et al [22] found that the size of primary breast significantly affect the scatter dose to the contra-lateral breast. The PTV size of Chinese patients was smaller compared to those of the Caucasians. The mean and maximum size of PTV in our study was nearly one fourth to one third of those reported by Popescu CC. et al [10]. With respect to the dose parameters of PTV in this study, T-IMRT plan had obvious advantages on the HI than TW plan and VMAT plan, but it was not superior to the FIF plan and 7-IMRT plan when strict limitation was applied to the CI. In addition, T-IMRT plan had worse CI than 7-IMRT. This was different from the reported results of Jagsi et al [23], which might be the influence of the PTV size between European and Chinese. The multi-field plan and VMAT plan reduced the high dose-volume of PRV-OARs but increased the low dose-volume, and the VMAT plan even could not meet the constraint of PTV95%≥95%. To better conclude the most superior technique from the multi-parameter results of our study, we use the following score table to help making the evaluation. In the score table, it is scored to point 1 if the difference showed significant advance between the compared parameters, otherwise scored to 0. Thus, the best treatment technique goes to the one having highest score in the Table 7.

From the summary of scoring, T-IMRT has the most point of 26 which is almost 2 time of each all other technique. Although the PTV size was much smaller in this study, the score table led to similar result with the reported study of western cases. Caudell JJ.et al [24] reported that electronic compensation (CE) technique produced superior dose distribution in both CTV and normal tissue compared with conventional T-IMRT. One can expect that the dose distribution could be even better if CE technique was integrated.

The application of IMRT offers the potential for improved local-regional control without increase heart toxicity in those requiring local-regional treatments [25]. 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 [26]. Most of the literature analyzed the irradiated dose of heart, but they did not specify the dosimetric parameters of coronary artery when comparing the dose difference of treatment plans for the left-side breast cancer after conserving surgery. There were some studies suggested that coronary heart disease after postoperative radiotherapy for breast cancer was one of the radiation-related complications [27, 28]. In this study, the dose-volume (V_5Gy~V_40Gy) of heart and coronary artery were detailed and their relationships were described with the quadratic polynomial. Figure 3 shows respectively the dose volume correlation of heart and coronary artery of the five plans. The correlation coefficients were 0.993 (TW), 0.996 (FIF), 0.9972 (T-IMRT), 0.995 (7-IMRT) and 0.986 (VMAT). This could be helpful to use the dose-volume of heart to predict the dose-volume of coronary artery. For example, if the V_30Gy of heart is 10% in T-IMRT, the V_30Gy of coronary artery can be calculated as 76.41% with the relation coefficient in Figure 3c). Dose-volume evaluation of coronary artery can be included in the constraint of heart dose. This was similar to the findings of other reports [29], which suggested to predict the dose of CA but not figured out the relationship in between.

Various reports have shown that the incidence of second cancer risk would increase with the increasing of the irradiated dose of contralateral breast [3]. Although 7-IMRT and VMAT were planned with same dose constraint for contralateral breast as the other techniques, the V₂~V₅ of contralateral breast were significant higher than the other three plans.

There are some reports about the application of VMAT in the clinical treatment of intact breast treatment with or without nodal involvement as well as for partial breast treatment [16]. In our study, the average MU of VMAT plan (363.7±45.3) was significantly smaller (P<0.05) than that of 7-IMRT (513.4±83.3MU). VMAT technique was superior in the irradiation MUs compared to 7-IMRT. Also, VMAT plan had apparent advantages in reducing the volume of high dose and drawbacks in increasing the volume of lower dose.

In our study, Comparing to the three tangential treatments VMAT reduced the normal tissue volume receiving high dose but significantly increased the volume of low dose. Especially, the average PTV95% of the 20 cases was worse than 95% when planned with same optimization objective of other treatment techniques. Considering the risk of tumor recurrence and the relative high dose in lung and contralateral breast, we do not suggest to choose VMAT for left-sided breast cancer radiation therapy.

According to the data of our study, for the breast cancer patient whose PTV is rather smaller than western population, the size of primary breast do not significantly increase the dose of contra-lateral breast as reported by Bhanagar [22], and T-IMRT is still an adequate technique for the Chinese patients who undergo conserving breast surgery. For planning for left-breast irradiation, the volumetric dose of the heart which is more easy to be contoured can be used to predict the volumetric dose of coronary artery, if the relationship in between is well fitted. VMAT plan had a few advantage in improving the HI of PTV but may decrease the PTV dose coverage and increase the dose irradiate to lung and contra-lateral breast. The T-IMRT plan may be clinically.

Written information consent was obtained from the patient for publication of this report and any accompanying images.