Teaching Case

A 63-year-old female patient with stage 1a adenocarcinoma of the lung status post wedge resection presented for follow-up PET/CT scan. She complained of pain while walking and used a walker to ambulate. On examination, she was afebrile with severe pain on internal and external rotation of the hip.

FDG PET/CT scan demonstrated a left femoral intertrochanteric mildly active fibro-osseous lesion. A posterior intensely hypermetabolic component was concerning for collision metastasis or malignant transformation of primary bone tumour (Fig. 1).

MRI demonstrated a heterogeneous lesion, low SI on FSET1Wi, heterogeneous high SI on T2 with posterior marked enhancement upon administration of gadolinium contrast. The anterior section measured 3.0x2.8x5.4 cm, and the posterior section measured 1.7x1.6x1.4 cm (Fig. 2).

CT-guided biopsy of the concerning lesion was performed. On CT, the lesion demonstrated calcified periphery with a polylobulated, ground glass appearance in the posterior section (Fig. 3). Pathology results were fibro-osseous tissue without evidence of malignancy.

Follow-up FDG PET/CT scan demonstrated a slightly less active, hypermetabolic nodule associated with a mildly hypermetabolic, fibrotic lesion in the left femoral intertrochanteric region (Fig. 4). It was determined to represent active fibrous dysplasia after considering pathology results. Continued follow-up was advised.

Fibrous dysplasia is a rare disease in which bone marrow is being replaced by large fibrous stroma. It is not associated with neoplastic change. The lesions can occur in single or multiple bones and most commonly present in the proximal femur, tibia, ribs and skull. Clinically, patients can present with a broad spectrum of symptoms, dependent upon the location and severity of their disease. Most often, patients present asymptomatically, but specific complaints of localized bony pain and difficulty ambulating may arise. Some patients also complain of nervous symptoms such as tingling, radiating, or shooting pain [1, 2].

Radiographically, fibrous dysplasia is recognized as ground glass in appearance with cortical bone thinning in a scalloped, undulating pattern due to endosteal erosion. Usually, there is no break in the cortices and lesions are well circumscribed. CT will demonstrate ground-glass opacities with a homogeneously sclerotic appearance. MRI demonstrates marked variability of lesions, often resembling a neoplasm or more aggressive lesion [3]. However, the appearance of fibrous dysplasia on MRI has not been widely published. Generally, T1-weighted images demonstrate isointense lesions with areas of hypointensity, while T2-weighted images depict heterogeneously hyperintense lesions. Lesions demonstrate cortex scalloping and heterogeneous enhancement following gadolinium administration [4]. Fibrous dysplasia demonstrates variable activity (from inactive to intense) on FDG PET/CT, that can change with time, particularly if there is a transformation to bone-forming primary malignancy [1, 5].

Treatment usually consists of scheduled follow-up with expectant management, since patients are more susceptible to pathological fractures due to their weakened skeleton.

Interestingly, this case demonstrates fibrous dysplasia in an elderly woman with increased uptake on a PET/CT scan. Over 75% of patients with fibrous dysplasia present before the age of 30 with the highest presentation occurring in teenage years [6]. This case reiterates the importance of tissue sampling following a potentially false-positive PET/CT scan. Once histological assessment determined the lesion to be a fibrous dysplasia, she can be managed expectantly with no increased risk of metastatic neoplasms [7]. Her prognosis after a T1aN0M0 lung cancer is indicative of a 50% 5-year survival rate, providing her curative wedge resection. However, a presumed metastatic lesion would have drastically changed her treatment and prognostic outlook with a 5-year survival rate of merely 2% [8]. Therefore, ensuring neoplastic transformation of a lesion before beginning treatment is essential to maintaining optimal patient care.