Local recurrence rates in the conserved breast after MRI targeted radiotherapy.

Abstract #5143 Background: To optimise the effects of whole breast irradiation after breast conservation surgery, precise identification and delineation of clinical target volumes is essential. If the site of the “post-operative cavity” (POCx) is inaccurately located, any target miss of residual cancer cells may lead to underdosing and thus impact on local recurrence rates and survival. Studies with surgical clips have shown rates of geographical miss of the POCx to be worryingly high for radiotherapy boost doses (up to 87%). Of even more concern is that the whole breast tangential fields frequently miss the tumour bed even though the fields appear adequate based on external landmarks (in approximately a quarter of cases). MRI has subtle breast tissue characterisation that may demonstrate post-operative detail more clearly than CT and more accurately than surgical clips.
 Methods and Materials: Simple opposed tangential fields were defined by surface anatomy in the conventional manner in 221 consecutive patients. Following MR imaging, fields were modified by a single radiation oncologist (EW) to encompass the POCx with a 10mm margin. Patients were treated to a dose of 45Gy in 20 fractions or 50Gy in 25 fractions, with or without an electron boost to the tumour bed (usually 12.5Gy in 5 fractions). Genetic analysis, using genome wide single nucleotide polymorphism (SNP) arrays and loss of heterozygosity (LOH), was performed on all local relapses to distinguish true recurrences (TRs) from new primaries (NPs)
 Results: This cohort was at relatively high risk, with only 9.8% classified as low risk by St Gallen criteria. 43.4% were grade 3 and 19.9% had surgical margins Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5143.