Sentinel node biopsy (SNB) using superparamagnetic iron oxide (SPIO) nanoparticles is a novel method in breast cancer. Several studies have verified the non-inferiority of SPIO compared with the standard use of radioisotope ^99mTc with or without blue dye. The aim of the MONOS study presented here was to evaluate the use of SPIO as a sole tracer and the efficacy of tracer injection in the preoperative setting. This prospective cohort study was carried out in two hospitals, one using ^99mTc and the other SPIO. ^99mTc was injected in the morning of the day of surgery or the day before. SPIO was either injected before surgery in the outpatient clinic or 1 h before the operation. A total of 338 consecutive patients with breast cancer underwent 343 procedures; SPIO nanoparticles were used in 184 procedures and ^99mTc-labelled tracer in 159. Detection rates for SPIO and ^99mTc were 95·6 and 96·9 per cent respectively (P = 0·537). All nodes with SPIO uptake were coloured brown. Fewer nodes were retrieved with SPIO (mean 1·35 versus 1·89), regardless of whether blue dye was used (P < 0·001). Preoperative SPIO injection (58·7 per cent of procedures), a median of 16 (range 2–27) days before the procedure, was associated with a better tracer-specific detection rate (95·3 versus 86 per cent; P = 0·031) and retrieval of more nodes (mean 1·43 versus 1·03; P < 0·001) than perioperative administration. Skin staining was present in 39·9 per cent of patients, and was related to breast-conserving surgery and periareolar injection. The use of SPIO alone is a safe alternative, with results comparable to those of the standard dual technique using ^99mTc and blue dye. The efficacy of injection in the preoperative setting simplifies logistics and improves performance. Skin staining can be prevented by a deeper peritumoral injection.

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