Summary of findings 2. Any O‐BCS compared to mastectomy for women with primary breast cancer.

Any O‐BCS compared to mastectomy for women with primary breast cancer
Patient or population: women with primary breast cancer Setting: mixed multicentre/single‐centre studies with initial inpatient procedure and outpatient follow‐up Intervention: any O‐BCS Comparison: mastectomy
Outcomes	Anticipated absolute effects* (95% CI)		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Risk with Mx	Risk with any O‐BCS
Local recurrence‐free survival (up to 5 years)	Study population		HR 0.55 (0.34 to 0.91)	4713 (2 observational studies)	⊕⊝⊝⊝ Very lowa,b	Estimates of risk with BCS were calculated using an average of non‐adjusted baseline control rates from included studies.
	161 per 1000	92 per 1000 (58 to 148)
Cumulative local recurrence rate				(0 studies)	‐	No studies evaluated local recurrence as cumulative rate
Disease‐free survival	Study population		RR 0.58 (0.41 to 0.82)	1193 (1 observational study)	⊕⊝⊝⊝ Very lowc,d	Dichotomous data used as no studies reported time‐to‐event data
	139 per 1000	81 per 1000 (57 to 114)
Re‐excision rates				(0 studies)		Re‐excisions are not often needed for mastectomy, therefore this outcome is not relevant for this comparison.
Complications	Study population		RR 0.75 (0.67 to 0.83)	4839 (4 observational studies)	⊕⊝⊝⊝ Very lowc,e	O‐BCS may reduce complications compared to mastectomy but the evidence is very uncertain.
	312 per 1000	234 per 1000 (209 to 259)
Recall rates				(0 studies)		Recall biopsies are not often needed for mastectomy, therefore this outcome is not relevant for this comparison.
Patient‐reported outcome measures	There are insufficient data to make any conclusions		‐	(1 observational study)	‐	There are insufficient data to make any conclusions
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). BCS: breast‐conserving surgery; CI: confidence interval; HR: hazard ratio;Mx: mastectomy; O‐BCS: oncoplastic breast‐conserving surgery; RR: risk ratio.
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate; the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited; the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate; the true effect is likely to be substantially different from the estimate of effect.

^aDowngraded by one level due to study limitation: moderate risk of bias due to confounding.
^bDowngraded by two levels due to heterogeneity: I² = 81%, P = 0.02.
^cDowngraded by two levels due to study limitation: serious risk of bias due to confounding.
^dDowngraded by one level due to imprecision: optimal size not met.
^eDowngraded by two levels due to heterogeneity: I² = 61%, P < 0.0001.