*5.2.2 Histology*

The gold standard in the diagnosis of osteosarcoma is tissue histology, from an incisional or open biopsy.

The diagnosis of osteosarcoma is based on recognition of osteoid production by tumor cells [13, 19]. Besides the production of osteoid and immature bone, histological features are the presence of neoplastic cells showing anaplasia with epithelioid, plasmacytoid, or spindle aspects and the growth with a permeative pattern, filling the marrow space surrounding and eroding preexisting trabeculae.

Of note, it is important to highlight that osteoid and immature bone can generate confusion in the differential diagnosis of low-grade carcinomas and other conditions of fibro-osseous lesions that may contain osteoid, such as ossifying fibromas, especially in pediatric patients.

This allows solving sampling errors, histologic heterogeneity, and necrosis that can be often found in a sarcoma sample. Also, it often grants a reliable mitotic count and estimation of the percentage of necrosis, thus permitting accurate grading.

Grossly the tumors are gritty, tan-white, and sometimes myxoid. They destroy the underlying bone with or without soft tissue extension [1].

Histologically, osteosarcomas are matrix-producing tumors that contain neoplastic osteoblasts that produce bone. These osteoblasts are highly pleomorphic and/or may be spindled, epitheloid, plasmacytoid, round, or a mixture of all the above (**Figure 6**). Approximately, half of all osteosarcomas present as high-grade lesions [1].

On small biopsies, it can sometimes be difficult to distinguish osteosarcoma from a fibro-osseous lesion. In those instances, the presence of an infiltrative growth pattern can be helpful as it is seen in osteosarcoma, but not in benign lesions [13]. According to the WHO 2017, immunohistochemistry such as Ki67, Mdm2, and cdk-4 is useful in diagnostic confirmation for inconclusive cases.

### **Figure 6.**

*Histology of patient shown in* **Figure 1***, a 35-year-old female with a maxillary chondroblastic osteosarcoma. Courtesy of Dr. Cristobal Araya.*
