A 42-year-old woman presented with a 3-week history of a painless, mobile, firm, 2.5-cm lump in the lower outer quadrant of the left breast without axillary lymphadenopathy. No evidence of nipple retraction or discharge was observed. The physical exam of the contralateral breast was unremarkable.
She was already known, having been treated 2 years previously for a left invasive ductal carcinoma (lower outer quadrant, triple negative, grade 3, and Ki-67 70%) without nodal involvement (pT1bN0M0, stage IA). At that time, she underwent lumpectomy and sentinel lymph node biopsy followed by anthracycline/taxane-based adjuvant chemotherapy and radiotherapy (40 Gy in 15 fractions prescribed to ipsilateral whole breast with a 10-Gy boost in four fractions to the tumor bed).
At first, the new symptomatic swelling presenting in the same quadrant 2 years after primary treatment was highly suspicious of local recurrence. The mammography and ultrasonography revealed an irregular, bulky mass with a lobulated border in the lateral part of the left breast ( Figures 1A-C ). On MRI, there was a 3-cm mass in the lateral part of the left breast with a high signal intensity at the periphery of the tumor ( Figure 1D ). The diagnosis and treatment timeline are demonstrated in  Supplementary Figure S1 .
The initial core needle biopsy suggested a spindle-cell malignant tumor with osteoid matrix and necrosis, and therefore, an excisional biopsy was then performed. On gross examination, the specimen was 50 mm x 40 mm x 30 mm and contained a medium-to-firm texture nodular lesion measuring 20 mm in maximum dimension and surrounded by fibro-fatty tissue.
Microscopically, the lesion was composed of abundant pleomorphic, spindle, and oval cells with infiltrative growth patterns. The tumor cells revealed eosinophilic cytoplasm, prominent nucleoli, and a high mitotic index. Osteoid matrix and necrosis were frequently seen at the periphery of the tumor ( Figures 2A-C ). No evidence of infiltrating ductal carcinoma or ductal carcinoma in situ was observed. The following immunohistochemistry (IHC) results were obtained: Cytokeratin AE1/AE3 (AE1/AE3) (-), cluster of differentiation 56 (CD56) (focal+), special AT-rich sequence-binding protein (SATB) (+), murine double minute2 (MDM2) (+), smooth muscle actin (SMA) (partial+), Ki-67 (80%), cytokeratin 7 (CK7) (-), estrogen receptor (ER) (-), progesterone receptor (PR) (-), human epidermal growth factor receptor 2 (Her2) (0), cluster of differentiation 34 (CD34) (-), and S-100 (-) ( Figures 2D, E ). The negativity of AE1/AE3, an epithelial marker, reconfirmed the lack of an epithelial component. On the other hand, SATB was proven to be involved in the process of osteoblastic differentiation, which also authenticated our pathological prognosis as PBOS.
To validate the pathological diagnosis of PBOS and explore the molecular connections between the PBOS and the previous invasive ductal carcinoma (IDC), a commercially targeted NGS was performed on both primary and secondary tumor slices for somatic mutations and peripheral blood samples for germline gene variants. A total of 421 gene variants related to target therapy, immune therapy, chemotherapy response, and genetic predisposition among breast cancer patients were included in the genomic panel. No germline variations were found for this patient. Several copy number variant (CNV) events were identified in the PBOS sample, including CN gains of FGFR1 and CN loss of CDKN2A and TSC2. Regarding somatic mutations, PIK3CA p.H1047R, PTEN p.V275G, TP53 p.T81Nfs*64, and TSC2 p.C728Lfs*34 were detected, with the highest variant allele frequency (VAF) of 59.22% happening in the PIK3CA mutation. Interestingly, somatic PIK3CA p.H1047R, PTEN p.V275G, and TP53 p.T81Nfs*64 were repeatedly detected in both PBOS and IDC tumor samples. On the other hand, somatic EGFR p.E709K was uniquely found in the IDC sample.
A written and signed informed consent was obtained from the patients and presented as supplementary material.
A CT of the chest, abdomen, and pelvis did not identify any metastases. An 18F-FDG PET/CT scanning was undertaken, and no evidence of a distant lesion or primary osteosarcoma arising from bone was detected, indicating that the breast lesion was primary osteosarcoma.
As per our institute routine, the patient was discussed in a multidisciplinary team (MDT), and a skin-sparing mastectomy followed by immediate breast reconstruction with a deep inferior epigastric perforator (DIEP) flap was achieved for her. According to the MDT's opinion, axillary lymph node sampling was not performed. No residual lesion was identified histologically. No adjuvant treatment was recommended. The patient is under regular follow-up right now. The latest follow-up was done on 25 November 2022, and the patient is still alive.