- •Significant changes were observed in the breast surface during the course of breast cancer radiotherapy.
- •Because of deformations an additional outer margin is required which is important to consider when using IMRT or VMAT.
- •An additional margin of 8 mm outside the breast surface is required to take account of the changes in the breast surface.
- •The breast deformations were not fully predictable, although a moderate correlation was found with body mass index.
The aim of this retrospective study was to investigate and quantify the extent of breast deformation during the course of breast cancer (BC) radiotherapy (RT). The magnitude of breast deformation determines the additional outer margin needed for treatment planning to deliver a full dose to the target volume. This is especially important when using inverse planning techniques.
A total of 93 BC patients treated with RT and with daily CBCT image guidance were selected for this study. Patients underwent either only breast-conserving surgery (BCS) (n = 5), BCS with sentinel node biopsy (n = 57) or BCS with radical axillary node dissection (n = 31). The treatment area included the whole breast and chest wall (54%) or also the axillary lymph nodes (46%). 3D-registration was conducted between 1731 CBCT images and the respective planning CT images to assess the difference in breast surface.
The largest maximum breast surface expansion (MBSE) was 15 mm; the average was 2.4 ± 2.1 mm. In 294 fractions (17%), the MBSE was ≥5 mm. An outer margin of 8 mm would have been required to cover the whole breast in 95% of the treated fractions. There was a statistically significant correlation between the MBSE and body mass index (r = 0.38, p = 0.001).
Significant changes in the breast surface occur during the course of BC RT which should be considered in treatment planning. An additional margin outside the breast surface of at least 8 mm is required to take into account the anatomical changes occurring during BC RT.
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Published online: August 05, 2019
Accepted: July 25, 2019
Received in revised form: July 18, 2019
Received: March 11, 2019
© 2019 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.