Breast deformation during the course of radiotherapy: The need for an additional outer margin.
PURPOSE
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.


METHODS
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.


RESULTS
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).


CONCLUSIONS
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.
['Seppälä|J|J|', 'Vuolukka|K|K|', 'Virén|T|T|', 'Heikkilä|J|J|', 'Honkanen|J T J|JTJ|', 'Pandey|A|A|', 'Al-Gburi|A|A|', 'Shah|M|M|', 'Sefa|S|S|', 'Koivumäki|T|T|']
D000328:Adult D000368:Aged D000369:Aged, 80 and over D001940:Breast D001943:Breast Neoplasms D054893:Cone-Beam Computed Tomography D005260:Female D006801:Humans D007091:Image Processing, Computer-Assisted D008875:Middle Aged D011879:Radiotherapy Dosage D011880:Radiotherapy Planning, Computer-Assisted D050397:Radiotherapy, Intensity-Modulated D012189:Retrospective Studies
Breast deformation Outer margin Breast radiotherapy VMAT