Association between underestimation of tumour size by imaging and incomplete excision in breast‐conserving surgery for breast cancer

Background Completeness of excision is the most important factor influencing local recurrence after breast‐conserving surgery ( BCS ). The aim of this case–control study was to determine factors influencing incomplete excision in patients undergoing BCS . Methods Women with invasive breast cancer treated by BCS between 1 June 2008 and 31 December 2009 were identified from a prospectively collected database in the Edinburgh Breast Unit. The maximum size of the tumour, measured microscopically, was compared with the size estimated before operation by mammography and ultrasound imaging. A multivariable analysis was performed to investigate factors associated with incomplete excision. Results The cohort comprised 311 women, of whom 193 (62·1 per cent) had a complete ( CE group) and 118 (40·7 per cent) an incomplete ( IE group) excision. Mammography underestimated tumour size in 75·0 per cent of the IE group compared with 40·7 per cent of the CE group ( P  < 0·001). Ultrasound imaging underestimated tumour size in 82·5 per cent of the IE group compared with 56·5 per cent of the CE group ( P  < 0·001). The risk of an incomplete excision was greater when mammography or ultrasonography underestimated pathological size: odds ratio ( OR ) 4·38 (95 per cent c.i. 2·59 to 7·41; P  < 0·001) for mammography, and OR 3·64 (2·03 to 6·54; P  < 0·001) for ultrasound imaging. For every 1‐mm underestimation of size by mammography and ultrasonography, the relative odds of incomplete excision rose by 10 and 14 per cent respectively. Conclusion Underestimation of tumour size by current imaging techniques is a major factor associated with incomplete excision in women undergoing BCS .