Teaching Case

A 15-year-old boy presented to the paediatrician with precocious puberty, hyperthyroidism, hyperprolactinaemia and large, asymmetric head.
Inspection of the skin showed “café-au-lait” spots located mainly on the back (Fig. 1) and left buttock.

Conventional radiography (CR) showed multiple bone lesions with a ground glass appearance in both femurs (Fig. 2, 3), left humerus (Fig. 4), right tibia and right talus (Fig. 5, 6). Bone scintigraphy showed multifocal tracer uptake in the axial and appendicular skeleton suggesting active osteoblastic activity (Fig. 7).
Magnetic Resonance Imaging (MRI) of the brain showed asymmetric expansion of the calvaria and facial bones with significant bone thickening. Lesions showed predominantly low, slightly inhomogeneous signal intensity on all pulse sequences (Fig. 8, 9, 10). The brain was normal (Fig. 8, 9). The orbits were small. (Fig. 10). However, there was no compression of the optic nerve (Fig. 10).

Based on the combination of clinical and imaging findings, the diagnosis of McCune-Albright syndrome (MAS) was made.
MAS is a genetic disorder characterized by a typical triad of symptoms: polyostotic fibrous dysplasia (FD), skin pigmentation (café-au-lait spots) and endocrinological disorders [1]. Gonadal dysfunction is considered to be the most common endocrine defect in females, resulting in precocious puberty and vaginal bleeding. However, Cushing syndrome, pituitary adenomas and hyperthyroidism are also reported [2]. MAS develops from activation of a post-zygotic, sporadic or somatic mutation of the GNAS1 gene with different clinical manifestations [3]. Affected children usually present with skeletal deformities and short stature, resulting from premature closure of the epiphyses.
FD is a tumour-like condition of the bone. Histologically it corresponds to immature bone with delicate trabeculae and interspersed fibrous matrix [4]. This makes the bone more prone to microfractures, repetitive remodelling and subsequent deformation [5]. FD associated with MAS usually affects multiple bones and leads more frequently to complications such as fractures, skeletal deformities or limb-length discrepancies. It affects most commonly the femora, tibiae, pelvis, skull and ribs. Imaging features of FD may vary from either completely radiolucent lesions, sclerotic delineated radiolucent lesions or lesions with a ground-glass matrix appearance. In case of disease progression, bowing of the proximal femora (coxa vara) may occur due to multiple fractures and repetitive remodelling, resulting in the so-called shepherd’s crook deformity. [4, 6].
Conventional radiography (CR) is usually sufficient to make a reliable diagnosis.
Bone scintigraphy may reveal increased osteoblastic activity in affected bones. It is particularly useful to assess multifocality.
Computed tomography (CT) is usually not mandatory, but may be used to evaluate the precise extent of FD and potential compression of adjacent structures in complex anatomical structures such as the facial bones, skull base and the axial skeleton. However, due to high radiation dose, the use of CT should be considered with caution.
Particularly in the paediatric population, MRI is the preferred technique for evaluation of the lesion extent in the skull bones. Bone expansion may cause compression of adjacent structures, such as the optic nerve, resulting in visual disturbances. Osteotomy of the involved bones should be considered in this scenario.
Malignant transformation of bone lesions is rare.
Paget's disease may appear similar on imaging but is seen in older patients. Association of FD and intramuscular myxoma is known as Mazabraud syndrome.