Abstract
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.
Highlights
Regional nodal irradiation (RNI) as part of radiotherapy (RT) for breast cancer has been a topic of much debate in recent years
The mean dose of the internal mammary node (IMN)-PTV was 45.1 Gy (26.4 55.6, standard deviation (SD) 6.5), which corresponded to 90% of the Entire group (N=50)
The mean dose of our plans translated to 90% of the prescription dose, the mean dose received by (2) 95% (D95) (29.3 Gy) and D90 (34.0 Gy) were only 58.6% and 68.0% of the prescription dose, respectively
Summary
Regional nodal irradiation (RNI) as part of radiotherapy (RT) for breast cancer has been a topic of much debate in recent years. Results from previous dosimetric studies have shown that wide tangents or supplementary AP fields may cover the target prescription for the IMNs. there was some increased radiation dose to the organs at risk (OAR) with these techniques, for left-sided breast cancer. DIBH techniques have been shown to safely deliver target prescription doses to the IMNs even for left-sided breast cancer.[13,14] DIBH has been known to be quite resource intensive and may still not be widely available in developing countries.[15]
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