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Despite the potential benefit for IMN-RT to improve disease free survival (DFS), distant DFS, locoregional DFS, and breast cancer survival as shown by recent large trials, inclusion of the IMN nodes in RT fields for breast cancer continues to be debated, fueled by concerns regarding cardiac and lung toxicity. In the post-mastectomy setting, the authors’ institution uses a three-field technique with narrowed tangents matched to a medial AP electron field, similar to the technique used in major trials evaluating IMN-RT. However, the primary intent of this medial electron field is to decrease the volume of lung irradiated and not to intentionally treat the IMN.
The purpose of this study is to conduct a dosimetric analysis of the incidental radiation dose to the IMNs by the current radiotherapy technique utilized by the authors’ institution.
This retrospective study uses 3D conformal treatment plans generated by the Philips Pinnacle treatment planning system. Fifty post-mastectomy patients were included in the study (25 left and 25 right). Based on the authors’ institutional postmastectomy radiotherapy protocol, all plans utilized a three-field technique for the chest wall, matched superiorly to the supraclavicular field. The three-field technique for the chest wall is composed of narrowed opposed tangents matched medially to an AP electron field (9 MeV). All plans were prescribed to a total dose of 50 Gy in 28 fractions to the involved chest wall and the supraclavicular area. The IMN was not intentionally treated in the original plans. Two radiation oncologists contoured for the IMN-PTV following the ESTRO contouring guidelines for breast cancer. The internal thoracic vessels from the 1st to the 3rd interspaces were contoured with a 5mm medial, lateral and anterior margin.
The mean dose to the IMN-PTV was 42.93Gy (21.77-56.51, SD 6.79). Percent volume of the IMN-PTV receiving 50Gy, 47.5Gy, 45Gy, and 40Gy are 39.36% (3-92, SD 24.4), 49.26% (5-93, SD 23.61), 55.22% (6-96, SD 23.05), and 66.29% (8-98, SD 20.98), respectively. Right-sided plans showed a non-significant trend for an increased mean dose and an increased percent volume of IMN-PTV receiving 50Gy, 47.5Gy, 45Gy, and 40Gy compared to left-sided plans (Table 1). Only 6% (4/50) of all plans had at least 90% of their IMN-PTV’s receiving 45Gy. Radiation dose to OARs were below recommended dose limits with an average lung V20 of 6.74% (1.40-10.85, SD 1.81) and an average heart mean dose of 2.5Gy (0.45 – 7.84, SD 1.58).
The results of the study suggest that the use of a three-field technique may yield increased radiotherapeutic levels to the IMN PTV’s compared to standard tangential fields. However, the incidental IMN PTV dose even with this technique still does not attain the recommended radiation dose levels of at least 45 Gy and at least 90-95% of the IMN PTV receiving the target radiation dose. It is therefore recommended to contour the IMN PTV for intentional treatment if the IMNs are deemed high risk for involvement and IMN RT is indicated.