Field-in-field tangent technique improves radiation dose homogeneity in breast conservation

Purpose: Post lumpectomy radiotherapy is the standard for women with early stage breast cancer who desire breast conservation. Tradi-tionally, coplanar tangent beams and physical tissue compensators (eg., wedges) were used to radiate the intact breast. The resultant plans often contains 'hot spots' in excess of 115% of our 50 Gy minimum prescribed breast dose (PD) and a variable dose gradient. A field-in-field (F1F), 3-D dose-compensation technique using multileaf collimated (MLC) fields was developed to achieve a more uniform dose throughout the target volume. This study compares the dose distributions in the bronst when using both of these techniques. Methods: Eight-seven women with Stage I or II breast cancer were treated with post-lumpectomy radiotherapy using t!<e FIF technique. Each patient also had an optimized wedge plan in the event of MLC failure. The lung, skin, and breast mound encompassed in the tangents were contoured. The target volume was defined as the tissue more than 0.5 cm from the skin surface, the posterior tangent field boundaries, and the chest wall-lung interface. Dose volume histograms were used to analyze the dose distribution and CTbased heterogeneity corrections weie used. Results: The mean maximum dose delivered was 53 Gy in the FIF plans compared to 55 Gy in the wedge plans (P < 0.0001). On average, 87.5% of the radiation dose in FIF plans was delivered through an open field.For the FIF plans, only 2.2% of the treated volume received 105% PD compared to 19.5% in the wedge plans (P < 0.0001). A correlation between the patient separation and the maximum dose in breast was noted in both plans (P < 0.001). Conclusion: The FIF treatment plans improved the dose homogeneity and gradient in the target volume compared to traditional wedge plans. Clinically, significant 'hot spots' were reduced which may potentially translate into less adverse radiation sequelae.