Optimization of tangential fields arrangement in the breast cancer 3D conformal radiation therapy

Introduction: The incidence of breast cancer increases with the rate of 1-2% at the world. Radiation therapy is one of the available choices for breast cancer treatment. The single isocentre half-beam block technique is considered as a standard technique to avoid hot and cold spots within the PTV. The major advantage of half beam technique is that the both contralateral breast and lung are completely spared. Also, using this technique, we don’t have any field matching problem. But, Lateral and medial movement of isocenter in conjunction of supraclavicular and tangential beams affect the divergence of tangential fields. Therefore, opposite tangential field can be parallel either in posterior border or beam central axis. The aim of this study is to compare the dosimetric parameters of three plans for tangential beams arrangement and select the optimal one.     Materials and Methods: In this study, we selected 6 patients with breast cancer that underwent breast conserving surgery and considered for radiotherapy. CT simulation of patients were done based on standard protocols and imported to Isogray treatment planning system. Contouring of target volume and organs at risk (OARs) was done by radiotherapist. The tangential fields were considered in three different geometries of parallel in the posterior border with divergence in back jaws, parallel in central axis with divergence in back jaws and Parallel in posterior border without divergence of back jaws (half beam). Dosimetric parameters of each plan for OARs and target volumes extracted from DVHs data and compared statistically using ANOVA one-way repeated measurement test.   Results: There was no statistical difference between dosimetric parameters (D95, V95, Dmax, and Dmean) of the target volume in three mentioned plans (P-value = 0.93). Although, there was no statistical difference between mean dose of the ipsilateral lung, the difference of V10 and V20 parameters for this organ is significant (P=0.01). Furthermore, there was no statistical difference in dosimetric parameters of OARs such as contralateral breast, lung, heart, and spinal cord.   Conclusion: This study showed that back jaw divergence and arrangement of parallel opposite field can affect the ipsilateral lung dose. The V20 and V10 of ipsilateral lung are two dose constraints parameters in quantitative treatment planning evaluation of breast treatment. Quantitative variations of these parameters with changing the tangential field’s arrangement was significant which depends on the isocenter position and back jaw divergence. The values of these parameters were the highest when tangential fields were arranged as parallel in posterior border with divergence in the back jaw.