The \'disappearing\' osteochondroma

Clinical History

A 12-year-old girl came to our centre for occasional knee pain related to physical activity.
There was no history of trauma, night pain or fever.
On physical examination a firm, fixed mass, mildly tender to palpation was located at the medial aspect of the right thigh.

Imaging Findings

On plain radiograph (Fig. 1), a solitary sessile exostosis was identified at the medial aspect of the distal femoral metaphysis.
An MR examination (Fig. 2) revealed the cortical and medullary continuity of the lesion with the original bone.
Treatment options were discussed and surgical excision was recommended, however, the family chose to observe the lesion.
At three-year follow-up, plain X-rays and MR (Fig. 3) showed disappearance of the lesion.

Discussion

Osteochondroma represents the most common bone tumour and constitutes 20%-50% of all benign bone tumours. [1]
It most commonly occurs in the metaphyseal region of the long bones. [2]
Most lesions appear in children and adolescents, and tend to be painless, slow-growing masses that may go unnoticed. Additional symptomatic complications include osseous deformity, fracture, vascular compromise, neurologic sequelae, overlying bursa formation, and malignant transformation [3].
The radiographic appearance of this tumour is often diagnostic and reflects its pathologic characteristics, a lesion composed of cortical and medullary bone with an overlying hyaline cartilage cap. The presence of this hyaline cartilage cap thickness is an important criterion in determining malignant transformation of the lesion. Additional imaging modalities frequently employed in the evaluation of this lesion include bone scintigraphy, US, CT and MR [4].
There are few cases of spontaneous regression of solitary osteochondromas reported in the literature and many theories have been proposed. One of them says that the active resorption and metaphyseal remodelling caused by altered vascular supply due to fractures may lead to cessation of the cartilage cap and subsequent tumour regression [5]. It is also described that osteochondroma resorption occurs as a result of an accompanying pseudoaneurysm [6]. Other authors have proposed that osteochondroma growth ceases before somatic growth ceases, followed by incorporation of the lesion into the cortex by appositional growth of the adjacent bone [7]. This last theory relates to our case. We attributed the spontaneous disappearance to a physiological mechanism, probably the osteochondroma growth ceased because our patient was near to her somatic growth cessation period.

Spontaneous regression of others benign bone tumours or tumour-like lesions have been also reported, for example eosinophilic granuloma, fibrous dysplasia, fibrous cortical defect, non-ossifying fibroma, osteoid osteoma and bone island [8]. In inflammatory-related lesions such as eosinophilic granuloma, cessation of inflammation may be the mechanism of growth arrest. Necrosis within the tumour and/or local changes in hormonal control, plus remodelling of the sclerotic area may lead to tumour regression.

Treatment of osteochondroma is individualized, with small asymptomatic or minimally symptomatic lesions requiring only supportive management and close follow-up. Larger symptomatic lesions may be resected, although surgical resection of benign osteochondromas is not without complications [9]. As the patient had already developed clinical manifestations, surgical approach was recommended. However, the possibility of regression of solitary osteochondromas should be taken into account when considering surgical excision [10].

Differential Diagnosis List

Final Diagnosis

Evanescent osteochondroma

URL:	https://www.eurorad.org/case/12698
DOI:	10.1594/EURORAD/CASE.12698
ISSN:	1563-4086