Cytological features of myxomatous fibroadenoma of the breast

Fibroadenoma (FA) is a benign tumor that must be differentiated from carcinomas. FAs often exhibit myxedematous changes (myxomatous FA, M-FA). We previously reported on the clinical significance of M-FA. M-FA and (mucinous) carcinoma share clinical findings, rapid growth and a relatively large size, a high-depth/width (D/W) ratio, a relatively round shape, and posterior echo enhancement with internal hyperechogenicity on ultrasonography (US). Next, a biopsy is required for differential diagnosis. In this study, we evaluated the diagnostic significance of the cytological findings of M-FA with US findings. Among 13 FAs that were diagnosed by cytology, we compared (i) a group of six mucinous carcinomas with acellular mucin and a D/W ratio ≥ 0.7 (a suspicious factor for malignancy) with a group with a D/W ratio of <0.7, and (ii) the frequency of metachromasia on Giemsa stain between M-FAs and non-M-FAs among eight FA cases confirmed by histology. (i) FA lesions (7 of 13) showed metachromasia with Giemsa staining significantly more frequently than did mucinous carcinoma (0/6) (Fisher's exact test, P < 0.044). FA lesions with a D/W ratio ≥ 0.7 (6/7) showed metachromasia significantly more frequently than did FA with a D/W ratio <0.7 (1/6) (Fisher's exact test, P < 0.029). Among eight FA cases confirmed by histology, M-FA cases (6/6) demonstrated metachromasia significantly more frequently than non-M-FA cases (0/2) (P < 0.036). M-FA cytologically exhibits marked metachromasia on Giemsa staining. Combining cytological examination and understanding the clinical features of M-FA may allow us to choose cytological examination as a first-line diagnostic method for tumor-forming lesions.