Which of the following is not true about osteosarcoma?

Sunburst appearance is due to inability of periosteum to lay down new layer and instead the Sharpy's fibres (a matrix of connective tissue consisting of bundles of strong predominantly type I collagenfibres connecting periosteum to bone) stretch perpendicular to the periosteum. Lung metastasis is most common(15%) cases. CT chest is mandatory to rule out lung metastasis before sta of the treatment. It occurs in metaphyseal region with most common site being where the growth of long bones occur i.e. around knee(distal femur and proximal tbia) proximal femur and humerous. Secondary osteosarcoma occurs in previous bone lesions like Paget's disease, previous irradiation, bone infarcts,chronic osteomyelitis. This explaining its bimodal distribution with second peak in older age groups.