Myositis Ossificans of the Breast Mimicking Breast Cancer: A Case Report

Abstract

Myositis ossificans (MO) is an uncommon tumor characterized by a rapidly growing mass following a history of local trauma. Few cases of MO affecting the breast have been reported, and some were misdiagnosed as primary osteosarcoma of the breast or metaplastic breast carcinoma. The following case report presents a patient with a growing breast lump whose core biopsy result was suspicious for breast cancer. MO was diagnosed after analysis of the mastectomy specimen. This case highlights the importance of MO as a differential diagnosis of a growing soft-tissue mass after trauma to avoid unnecessary overtreatment. Keywords: Myositis Ossificans, Osteosarcoma, Breast Cancer, Mastectomy, Heterotopic Ossification © RSNA, 2023.

Keywords: Breast Cancer; Heterotopic Ossification; Mastectomy; Myositis Ossificans; Osteosarcoma.

Figure 1:

A 74-year-old female patient with a palpable lump in the left breast. (A) Mammogram (mediolateral oblique view) shows a high-density calcified mass with spiculated margins in the posterior depth of the left breast. (B) Breast US image shows an irregular hypoechoic mass measuring 53 × 53 × 31 mm with intense posterior shadowing in the lower inner quadrant of the left breast. (C, D) Chest CT images ([C] soft-tissue window settings; [D] bone window settings) confirm a high-attenuating calcified mass in the left breast. (E–H) Breast MR images show an irregular mass measuring 55 × 50 × 30 mm in the same location in contact with the pectoral muscle, presenting low signal intensity on (E) T1-weighted and (F) T2-weighted images with perilesional edema, heterogeneous contrast enhancement ([G] axial; [H] sagittal), and a type III (washout) kinetic curve.

Figure 2:

(A, B) Core biopsy material. High-grade malignant neoplasm with extensive osteosarcomatous differentiation at (A) 4× and (B) 20× magnification. (Hematoxylin-eosin stain.)

Figure 3:

Mastectomy specimen. (A, B) Immunohistochemical stains show cellular stroma with new bone formation, mild cellular atypia, and high mitotic activity ([A] 4× and [B] 20× magnification, hematoxylin-eosin–stain). The immunohistochemical study was (C) negative for cytokeratins and (D) diffusely positive for special AT-rich sequence-binding protein 2.