Inny Busmanis, Kankanamage Malinda Amesh Karasinghe
Department of Anatomical Pathology, Singapore General Hospital   

A 59 year old Chinese female presented with pain and redness due to an ulcerated breast mass.  Breast ultrasound (US) examination showed a hypoechoic solid cystic mass, associated with increased vascularity, likely carcinoma.  Bone scan was negative.  The diagnosis of an ‘atypical spindle cell proliferation’ followed a core biopsy.   Left mastectomy specimen showed a 13cm. tumour with well circumscribed margins and variegated white, yellow, and pink appearance.   Histologically, the tumour comprised cellular atypical spindle cells, significant amounts of malignant osteoid surrounded by spindle cells admixed with osteoclast type multinucleated giant cells, and interspersed cartilage.  No glandular elements were identified.    The spindle cells were immunonegative with CD34, S100, ER, PR, STAT6, βcatenin, myogenin and MyoD1.  All keratins (CK5/6, 34βE12, EMA, MNF-116) were completely negative.  A miniscule population of isolated cells were positive with CK14.  p53 was wild type.  SMA was positive, and desmin focally positive.                                                                                                                                  
The favoured diagnosis in this scenario is a rare primary osteosarcoma of breast.  Other considerations discussed are metaplastic carcinoma with heterologous mesenchymal differentiation, metastasis of sarcoma from an occult primary site, or a dedifferentiated liposarcoma with osseous transformation.   Malignant phyllodes appears excluded due to lack of appropriate glandular elements, and sarcomatoid carcinoma due to lack of definitive keratin expression.