DTC is one of the most curable cancers [
12]. DTCs are characterized by a slowly progressive course, and have a 10-year survival rate of 80–95 % [
2]. However, the occurrence of distant metastases reduces the overall 10-year survival rate to 40 % [
13]. Previous studies have reported that 25 % of metastases were to the bone, 49 % to the lung and 15 % to both. Bone metastases have been reported in 2–13 % of patients with DTC, being significantly more frequent in patients with follicular cancer (7–28 %) than in those with papillary cancer (1.4–7 %) [
13‐
15]. The patient described here had follicular cancer. Patients with thyroid cancer and bone metastases have a poor prognosis, with 10-year survival rates ranging from 0–34 % [
16]. Complete resection of bone metastases of DTC has been associated with a significant improvement in survival [
3,
17].
Most bone metastases of DTC occur in regions of high blood flow, including the vertebrae (42-52 %), femur (9–20 %), skull (2–16 %) and pelvis (5–13 %) [
4,
5]. Anatomically, bone metastasis to the femoral head is very rare, with direct involvement of the subchondral region reported in only one previous patient [
10]. Diagnosis is therefore very difficult, with these lesions appearing similar on imaging modalities to osteonecrosis and osteoarthrosis.
Bone metastases of thyroid cancer may be accompanied by pain, but often are clinically silent, making them difficult to detect. Imaging modalities, including whole body bone MRI, CT, bone scan,
131 I whole-body scan (WBS) and FDG-PET, should be performed in patients with suspected bone metastases of DTC. Whole body MRI provides detailed images of both bone and bone marrow. CT can evaluate the extent of metastatic lesions and is especially useful for sites that are difficult to evaluate. If whole body MRI or CT detects a bone metastasis, a directed MRI or CT scan should be employed to specifically define the lesion of interest and aid in the planning of surgery or radiotherapy or the use of novel modalities to treat destructive osseous metastases. Bone scans with
99mTc diphosphonate or methylene diphosphonate (MDP) are most frequently used for localization and staging.
131 I-WBS is more specific and sensitive than a bone scan but only for well differentiated thyroid cancer [
4]. These scans have low sensitivity in detecting distant metastases from thyroid cancer and in localizing bone metastases. In contrast post-treatment
131 I-WBS may be highly sensitive (61–65 %) in detecting osseous lesions, as in the patient described here. Because
131 I-WBS can detect some, but not all, bone metastases, further testing is needed [
16]. For example, FDG-PET shows preferential tracer uptake by malignant cells with a high turnover rate due to increased glucose metabolism. FDG-PET is useful in patients with metastatic poorly differentiated thyroid cancer, in those with high thyroglobulin levels and in patients negative on
131 I-WBS [
18]. X-rays and CT scans in the patient described here showed femoral head collapse, whereas MRI showed that the focal lesion of the femoral head had the shape of a band, with low intensity on T1WI and high intensity on STIR. Furthermore, bone scans showed no other metastatic lesions. The differential diagnosis included osteoarthritis of the hip and idiopathic necrosis of the femoral head. Three years later, a metastasis in the right rib was detected, with
131 I-WBS showing the metastasis to the left femoral head. Because the overall survival of patients with bone metastasis is improved by complete resection, early diagnosis and treatment is important. Bone metastasis of DTC should be considered in the differential diagnosis of patients with a previous history of DTC and femoral head collapse.