A Dosimetric Analysis of Incidental Radiation to the Internal Mammary Nodes with a Three-field Chest Wall Technique

Objectives: The authors’ aim was to conduct a dosimetric analysis of the incidental radiation dose to the internal mammary node (IMN) region using a three-field chest wall technique (TFCWT). Methods: This retrospective study utilized 3D-conformal radiotherapy plans of 50 post-mastectomy patients (25 left-sided and 25 right-sided). All plans used the TFCWT, composed of narrowed tangents matched medially to an AP electron field, and prescribed a total dose of 50 Gy in 28 fractions. The IMNs were not intentionally treated in all included plans. Results: The mean dose to the IMN-planning target volume (IMN-PTV) was 45.1 Gy (26.4 - 55.6, SD 6.5). Minimum doses received by 95% and 90% of the IMN-PTV were 29.3 Gy (8 - 49, SD 10.0) and 34.0 Gy (10.0 - 52.0, SD 8.6), respectively. The percent volume of IMN-PTV receiving 100%, 95%, 90%, and 80% were 47.4% (3 - 94, SD 21.6), 55.6% (6 - 97, SD 22.4), 61.92% (7 - 98, SD 22.2), and 72.61% (18-100, SD 20.2), respectively. The average ipsilateral lung V20 Gy (with supraclavicular fields) was 25.0% (16 - 29, SD 3.4), and the average heart mean dose was 2.5 Gy (0.5 - 7.9, SD 1.58). Conclusion: Although the results suggest increased IMN radiation doses with the TFCWT when compared historically to standard tangents, the incidental doses are comparatively less than that traditionally prescribed to the IMNs in high-risk patients. It is unknown whether this incidental IMN dose confers any clinical benefit.