case_images: PMC8607481_01
case_images: Radiological findings (a and b): X-rays showing sunburst focal perisostal
  reaction. (c and d): MRI showing a cortico-medullary diaphyso-metaphysal
  lesion (white star), with pericortical extension and soft tissue
  invasion.
case_images: 10.1177_2632010X211057555-fig1.jpg
case_images: PMC8607481_01_10.1177_2632010X211057555-fig1.jpg
case_images: fig1-2632010X211057555
case_images: Plain radiographs revealed a sunburst periosteal reaction (Figure 1). MRI was performed 2 weeks later, it demonstrated a locally advanced corticomedullary lesion in the distal femur with extension to soft tissue consistent with an osteosarcoma, which has rapidly increased in size (Figure 1).
case_images: Histological findings of the first biopsy: a lobular tumor, with peripheral
  hypercellularity. Cells are mildly pleomorphic, spindled to stellate-shaped.
  Nuclei are elongated, with diffuse chromatin, inconspicuous nucleoli, and
  low mitotic activity. (a): HE x 40B: HE x 100, (b) HE x 400, and (c)
  HE x 400.
case_images: 10.1177_2632010X211057555-fig2.jpg
case_images: PMC8607481_01_10.1177_2632010X211057555-fig2.jpg
case_images: fig2-2632010X211057555
case_images: Nuclei were elongated, with diffuse chromatin, moderate hyperchromasia, inconspicuous nucleoli and low mitotic activity (1/10HPF (40x)) (Figure 2).
case_images: Histological findings of the second biopsy: a hypercellular lobular tumor
  (a). Cells are mildly pleomorphic, spindled to stellate-shaped. Nuclei are
  elongated, hyperchromatic, with diffuse chromatin (b), focal osteoid (c),
  and mitotic figures (d). (a) HE x 40 Lobules show hypo and hypercellular
  areas with peripheral condensation. (b) HE x 100 mildy pleomorphic cells
  with nuclear atypia (white arrow). (c) HE x 400 focal osteoid (white arrow).
  (d) HE x 400 mitotic figure (white arrow).
case_images: 10.1177_2632010X211057555-fig3.jpg
case_images: PMC8607481_01_10.1177_2632010X211057555-fig3.jpg
case_images: fig3-2632010X211057555
case_images: Morphologically, the tumor showed similar features, but atypias were moderate to severe and mitotic count was estimated: 15 mitoses/10 HPF (Figure 3).