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

Background: Magnetic 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. Purpose: To 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 materials: Patients 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. Results: Sixty-two patients (64 breasts) were include. 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. Conclusions: For 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 achives a 10 mm tumor-free margin around the MRI-GTV, radiotherapy to the tumor bed may require clinical target volume margins >10mm in up to one-fourth of the patients. (C) Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 97 (2010) 225-232