Dosimetric study of tangential and multi-fields applied in the dynamic intensity modulated radiotherapy plans for breast cancer patients after breast-conserving surgery

Objective: To compare the dosimetric differences of target and organs at risk (OAR) induced by the tangential and non-coplanar multi-fields used in the dynamic intensity modulated radiotherapy (IMRT) plans for breast cancer patients after breast-conserving surgery. Methods: Forty patients with early-stage left breast cancer after breast-conserving surgery were included. Based on the CT anatomy of the same patient, dynamic IMRT treatment plans using two tangential fields, 3, 4 and 5 non-coplanar fields were designed respectively utilizing the same optimization objects. The plans were compared by means of target dose distribution, the doses and irradiated volumes of heat, left lung, right breast, and total monitor units (MUs). Results: The conformal index (CI) and homogeneity index (HI) of planning target volume (PTV) in the plans using 4 and 5 fields were better than that using 2 fields (P < 0.05). Maximum doses (D_max) in PTV were significantly lower in the plans of 4 and 5 fields than of 2 fields (P < 0.05). Yet the plans of 4 and 5 fields generated significant higher minimum doses (D_min) in PTV than that of 2 fields (P < 0.05). No significant difference was observed between plans of 2 or 3 fields. Across the four plans, the differences of right breast V₅ (relative volume acquired at least 5 Gy of dose), heart V₃₀, heart mean dose (D_mean), left lung V₂₀, V₅ and D_mean were not significant. Yet the disparities of total MUs were statistically significant (F = 25.63, P < 0.05). The least MUs were used by the 2 fields and the most MUs were observed in the 5-field plans. Conclusions: Comparing with IMRT plans of 2 fields, using 4 or 5 non-coplanar fields can improve the target dose distribution without increasing OAR doses. As a tradeoff, more MUs are needed for multi-fields plans. (authors)