Surface osteosarcoma - case report

Clinical History

The patient was referred to our Hospital with a 6-month history of a painless swelling on the right thigh, associated with march disturbance. Upon physical examination, a firm mass was palpable on the anterior surface of the thigh. The patient denies traumatic events, weight loss or other associated signs/symptoms.

Imaging Findings

Antero-posterior and lateral plain radiography revealed an excentric lesion (7cm) on the distal antero-lateral femoral diaphysis, with spiculated periosteal reaction (fig.1).
On T1 –weighted MRI (fig.2) and FS T2–weighted MRI (fig.3) the distal antero-lateral femoral diaphysis showed a cortical lesion, measuring 6, 8x4, 6x2cm. This lesion presented peripheral high signal intensity on FS T2–weighted MRI. Central smaller foci of lower intensity were observed on both sequences. There was no intramedullary invasion. There were no skip lesions.
On CT scan (fig.4) there was calcified matrix extending into a low attenuation heterogeneous mass. There was an image suggestive of spiculated periosteal reaction. There was no evidence of intramedullary invasion.
The histological examination showed a low-grade neoplastic proliferation area, consistent with a superficial osteosarcoma (OS), according to clinical and radiological features. Histopathologic findings of surgical specimen (14 cm of distal femur) have confirmed to be a low-grade parosteal osteosarcoma.

Discussion

Surface OS constitute 4-10% of all osteosarcomas and have been subdivided in parosteal, periosteal and high grade surface OS.
Parosteal OS is the most common type of surface OS, accounting for 5% of all OS cases. It is seen in patients in their 3rd to 4th decades, with a female predominance. The tumor usually occurs in the metaphyses of long bones, characteristically affecting the posterior aspect of the distal femur. These lesions have an excellent prognosis. The classic appearance is a lobulated and exophytic mass adjacent to the bone, with central dense ossification, sharply delineated from the peripheral soft tissues, and cortical thickening. There is often a cleavage plane separating the tumor and adjacent normal cortex (string sign) that corresponds histologically to the periosteum interposed. The lesion arises on the outer fibrous layer of the periosteum, virtually never provoking a periosteal reaction. At MRI the ossified tumour is predominantly low in signal intensity on both T1- and T2-weighted images, similar to the appearance of the cortex. When the lesion is predominantly high in T2 signal intensity, the tumor is more likely to be of high grade.[1][2][3]
Periosteal OS has a peak incidence during the 2nd decade of life. These tumors show a strong predilection to arise in the diaphysis. These lesions are intermediate to high-grade and prognosis is usually worse than parosteal OS. Common radiological findings include a broad-based soft tissue mass, cortical thickening and extrinsic scalloping of the cortex, with periosteal reaction, often extended perpendicularly from the inner cortex. Hyperintensity on T2-weighted MRI and low attenuation on CT image are consistent with its cartilaginous component. In fact, histopathologically, the tumour is predominantly chondroid matrix with moderate cellularity, and reveals a large amount of cartilage matrix undergoing calcification, with areas of moderately differentiated osteoblastic osteosarcoma (osteoid formation). Invasion into the medullary canal is considered rare and needs to be distinguished from areas thought to represent reactive changes. This point is very important in directing the extent of surgical resection [1][2][3].
High-grade surface OS is rare and commonly involves metaphyses and diaphysis of long bones, especially femur. The prognosis is similar to that of conventional OS. High-grade surface OS presents dense ossification, bone destruction and periosteal reaction in the majority of cases. This tumour is often difficult to distinguish from periosteal OS. However, it usually surrounds a much higher percentage of bone circumference and is more likely to invade the medullary canal [1][2][3].

Differential Diagnosis List

Final Diagnosis

Parosteal Osteosarcoma

URL:	https://www.eurorad.org/case/9579
DOI:	10.1594/EURORAD/CASE.9579
ISSN:	1563-4086