Whole breast irradiation for small-sized breasts after conserving surgery: Is the field-in-field technique optimal?

Background: To determine the optimal whole breast irradiation technique in patients with small-sized breasts, tangential and field-in-field IMRT (FIF) techniques were compared. Methods: Sixteen patients with ≤3 cm breast height and ≤350 cc volume were included. Seven patients had 4D CTs performed. The planning target volumes (PTV), editing 5 and 2 mm from the surface on the whole breast, were delineated and called PTV(5) and PTV(2), respectively. Dose-volume histograms of tangential techniques with open beam (OT) and wedge filter (WT), conventional FIF (cFIF), and modified FIF (mFIF) blocking out the lung were produced. Various dose-volume parameters, the dose heterogeneity index (DHtrI), dose homogeneity index (DHmI), and PTV dose improvement (PDI) were calculated. Results: OT compared with WT showed a significantly favorable V₉₀ of the heart and lung, and PTV(5)-dose distribution. Comparing OT and cFIF, OT showed significant improvement in the V₉₅ of PTV(2), whereas cFIF showed significant improvement in the V₉₅, DHtrI, DHmI, and PDI of the PTV(5). In comparing cFIF and mFIF, mFIF showed improved dose distributions of the heart and lung, while cFIF presented the better V₉₅, DHtrI, DHmI, and PDI of the PTV(5). Respiratory influences on the absolute dose were mostly within 1 %. The ratio of free breathing and each respiratory phase was similar among OT, cFIF, and mFIF. Conclusions: cFIF has favorable dose conformity and is suggested to be an optimal method for small-sized breasts. However, OT for dose coverage close to the skin and mFIF for normal tissue may also be potential alternatives. Respiratory effects are minimal.