Chest Wall Separation and the Effect of Collimator Direction in Postoperative Radiotherapy in Breast Cancer Patients

Purpose: Motorized or dynamic wedge is routinely used nowadays to facilitate the work and reduce time on the treatment machine. However, this will necessitate keeping the Multileaf collimator (MLC) direction parallel to the chest wall. The aim of our work is to study the effect of Multileaf collimator in the perpendicular (┴) or parallel(//) direction on: target volume coverage, dose homogeneity, and dose to risk structures in the setting of postoperative radiotherapy in breast cancer patients treated conservatively. Methods and Materials: Twenty five breast cancer patients treated by breast conservative surgery referred for postoperative radiotherapy were included in this study. For each patient two different plans were done; one with the MLC direction parallel to the chest wall (using the dynamic or motorized wedge) and a second plan with the MLC direction perpendicular (using a physical static wedge). Parameters that might affect planning outcome were studied included: PTV volume, Mid Lung distance (MLD), Maximum heart distance (maximal thickness of heart irradiated), Volume of the lung irradiated with 20Gy (V20), dose to the contralateral breast, Conformity index (CI) and Homogeneity Index (HI). Results:The Mean PTV volume for the whole group is 1066cc (range = 243-2063cc). Conformity indices for the vertical MLC direction was better 1.39 vs 1.43 (p=0.03). Homogeneity indices were identical in both MLC directions. The use of perpendicular MLC direction resulted in significantly better mean V20 for the left lungs, compared to plans with the MLC in parallel position, 12.9Gy versus 14.78Gy (p=0.001). Maximum dose and volume of the contralateral breast receiving 5Gy was more with the use of dynamic wedges 7.49 vs 4.8 Gy, 2.5 Vs 1.9Gy however this difference was not statistically significant. Conclusion: Use of MLC direction perpendicular to the chest wall significantly improved lung dose compared to opposite direction with some compromise to conformity Index. Patients with large mid-beam separation and less convex chest wall may get more benefit without target volume compromise.