Precise correlation between MRI and histopathology – Exploring treatment margins for MRI-guided localized breast cancer therapy

BackgroundMagnetic resonance imaging (MRI) is more often considered to guide, evaluate or select patients for partial breast irradiation (PBI) or minimally invasive therapy. Safe treatment margins around the MRI-visible lesion (MRI-GTV) are needed to account for surrounding subclinical occult disease.PurposeTo precisely compare MRI findings with histopathology, and to obtain detailed knowledge about type, rate, quantity and distance of occult disease around the MRI-GTV.Methods and materialsPatients undergoing MRI and breast-conserving therapy were prospectively included. The wide local excision specimens were subjected to detailed microscopic examination. The size of the invasive (index) tumor was compared with the MRI-GTV. The gross tumor volume (GTV) was defined as the pre-treatment visible lesion. Subclinical tumor foci were reconstructed at various distances to the MRI-GTV.ResultsSixty-two patients (64 breasts) were included. The mean size difference between MRI-GTV and the index tumor was 1.3 mm. Subclinical disease occurred in 52% and 25% of the specimens at distances ⩾10 mm and ⩾20 mm, respectively, from the MRI-GTV.ConclusionsFor MRI-guided minimally invasive therapy, typical treatment margins of 10 mm around the MRI-GTV may include occult disease in 52% of patients. When surgery achieves a 10 mm tumor-free margin around the MRI-GTV, radiotherapy to the tumor bed may require clinical target volume margins >10 mm in up to one-fourth of the patients.