Two cases of flow void sign of temporal bone metastasis from follicular thyroid carcinoma

Abstract

This report describes CT and MRI findings of temporal bone metastasis from follicular thyroid carcinoma in two cases. Both of these had large, osteolytic, hypervascular masses of the temporal bone, accompanied by internal scattered bone fragments and extraosseous mass formation on unenhanced and contrast-enhanced CT images. In the first case, several dilated and tortuous vessels within the markedly hypervascular mass were observed on the arterial phase of dynamic contrast-enhanced CT images. Compared with the signal intensity of the cerebellum, temporal bone masses showed slightly hypo- to slightly hyperintense on T1-weighted images and slightly hypo- to moderately hyperintense on T2-weighted images. Both cases had flow voids in abnormally dilated vessels within the mass on T1- and T2-weighted images. Thyroid follicular carcinoma rarely metastasizes the temporal bone and presents with an osteolytic hypervascular mass with flow void sign.

Introduction

Follicular thyroid carcinoma (FTC) is the second most common subtype of thyroid cancer following papillary thyroid carcinoma (PTC), accounting for approximately 10%–15% of all thyroid cancers. ¹ FTC typically occurs in the age group of 40–60 years and is commonly diagnosed in elderly women. ¹ FTC tends to show blood vessel invasion and often metastasizes by hematogenous spread to distant sites, especially the bone and lungs. ² Even if distant metastasis occurs, long-term survival rates have been reported, with 10 years disease-specific survival rates of 31%–43%. ² Metastases generally involve red marrow areas of the axial skeleton and flat bones; therefore, the common sites of bone metastases in patients with thyroid cancer include the vertebrae, pelvis, ribs, and femur. ³ Regarding the skull metastasis from FTC, the calvarium, particularly at the midline of the frontal and parieto–occipital regions, is the most common site of involvement,^1,4 and FTCs rarely metastasize to the skull base, such as the clivus, sella turcica, petrous apex, and petrous ridge. ⁵ Herein, CT and MRI findings, including flow void sign, describe the large temporal bone metastasis from FTC in two cases.

Case report

Case 1

A 50 year-old woman presented with dizziness and left otalgia. Her physical examination revealed left facial nerve palsy, and an otoscopic examination detected a mass in the left external auditory canal. Unenhanced CT images (Figure 1(a)) showed a large osteolytic mass of the left temporal bone with internal scattered bone fragments, measuring 65 mm in diameter. The arterial phase of contrast-enhanced CT images obtained 30 s after a contrast material administration (Figure 1(b)) demonstrated that several dilated and tortuous vessels were found within the markedly hypervascular mass. The washout was observed on delayed-phase images (Figure 1(c)). The mass compressed the left internal carotid artery; therefore, moderate stenosis of the left internal carotid artery was observed. The left internal carotid vein was obstructed by tumor invasion. The invasion into the left jugular fossa and middle ear cavity was observed; however, the center of the mass was inferior aspect of the left temporal and occipital bone. Compared with cerebellar signal intensity, the temporal bone masses were slightly hyperintense on T1-weighted images (Figure 1(d)) and moderately hyperintense on T2-weighted images (Figure 1(e)). Many flow voids in abnormally dilated vessels within the mass were observed on T1- and T2-weighted images. Contrast-enhanced MRI images also showed early strong enhancement and delayed washout. Extraosseous mass formation compressed the adjacent cerebellum; however, no obvious invasion or edema was found within the compressed cerebellum. A whole-body CT scan revealed a 30 mm calcified lesion in the right thyroid gland and multiple lung metastases. The histological diagnosis of the radical thyroidectomy and biopsy specimen of the temporal bone tumor was FTC (Figure 1(f)). Other bone metastasis except for temporal bone metastasis was not revealed by whole-body CT and ¹⁸F-fluorodeoxyglucose PET/CT imaging. The patient received radioiodine therapy just one time after external beam irradiation for the left temporal bone metastasis. The left temporal bone metastasis was slightly decreased in size, and left facial nerve palsy was improved.

Figure 1.

A 58 year-old woman with left temporal bone metastasis from follicular thyroid carcinoma. (a) Unenhanced CT image shows a large osteolytic mass of the left temporal bone (arrow) with internal scattered bone fragments (arrowheads). (b) Arterial-phase image of contrast-enhanced CT obtained 30 s after a contrast material administration shows dilated and tortuous vessels (arrowheads) within the markedly hypervascular mass (arrow). (c) Delayed-phase image of contrast-enhanced CT obtained 120 s after a contrast material administration shows washout (arrow). (d) T1-weighted image shows a slightly hyperintense mass (arrow) with several flow voids (arrowheads). (e) T2-weighted image shows a moderately hyperintense mass (arrow) with several flow voids (arrowheads). (f) A biopsy specimen (hematoxylin-eosin staining, magnification ×400) obtained from the left external auditory canal shows colloids (arrowheads) surrounded by tumor cells with nuclear atypia.

Case 2

A 60 year-old woman monitored for a left thyroid nodule for 8 years presented with right facial nerve palsy. A physical examination revealed a right-sided deviation of the tongue and a right anterior chest wall mass. Unenhanced CT images (Figure 2(a)) showed a large osteolytic mass of the right temporal bone, with internal scattered bone fragments, measuring 63 mm in diameter. Contrast-enhanced CT images obtained 45 s after a contrast material administration (Figure 2(b)) showed heterogeneously moderate enhancement with internal unenhanced areas, suggesting necrosis or cystic degeneration. The right internal carotid artery was intact; however, the right internal carotid vein was obstructed by tumor invasion. The invasion into the right jugular fossa was observed, whereas the right middle ear cavity was intact. The center of the mass was inferior aspect of the right temporal and occipital bone. On T1- and T2-weighted images (Figures 2(c) and (d)), the temporal bone mass showed slightly hypointense relative to the cerebellum, with flow voids in abnormally dilated vessels. The extraosseous mass formation also compressed the adjacent cerebellum without invasion. A whole-body CT scan revealed a 38 mm calcified lesion in the left thyroid gland with multiple lung metastases and multiple bone metastases. The anterior chest wall mass was caused by an osteolytic lesion of the rib with extraosseous mass formation. The histological diagnosis of radical thyroidectomy and biopsy specimen of the rib bone tumor was follicular carcinoma; therefore, the right temporal bone mass was clinically diagnosed with bone metastasis from FTC. After surgical resection of rib metastases, the patient repeatedly received radioiodine therapy five times. The right temporal bone metastasis was slightly decreased in size, but right facial nerve palsy was unchanged.

Figure 2.

A 63 year-old woman with right temporal bone metastasis from follicular thyroid carcinoma. (a) Unenhanced CT image shows a large osteolytic mass of the right temporal bone (arrow) with internal scattered bone fragments (arrowheads). (b) Contrast-enhanced CT image obtained 45 s after a contrast material administration shows heterogeneously moderate enhancement (arrow) with internal unenhanced areas (arrowhead). (c) T1-weighted image shows a slightly hypointense mass (arrow) with a flow void (arrowhead). (d) T2-weighted image shows a slightly hypointense mass (arrow) with a flow void (arrowhead).

Discussion

The prevalence of bone metastases in patients with advanced metastatic disease has been estimated to be 65%–75% for breast cancer, 65%–75% for prostate cancer, 60% for thyroid cancer, 30%–40% for lung cancer, 40% for bladder cancer, and 20%–25% for renal cell carcinoma (RCC). ⁶ The median survival time from bone metastasis diagnosis is 6-7 months for lung cancer, 12–53 months for prostate cancer, 19–25 months for breast cancer, and 48 months for thyroid cancer. ⁶ Therefore, patients with bone metastasis from thyroid cancer have a better prognosis than those with lung or breast cancer, despite its relatively high frequency. ⁶

A previous study reported that 317 patients with differentiated thyroid cancer had 616 bone metastases and the most common metastatic site was the spine (34.6%), followed by the pelvis (25.5%), sternum and ribs (18.3%), extremities (10.2%), shoulder girdle (5.4%), and craniomaxillofacial region (5.4%). ⁷ Compared to PTC, FTC is more prone to spread hematogenously, especially to the lungs and bones. ⁷ The prevalence of bone metastases in FTC (7%–28%) is at least twice higher than in PTC (1.4%–7%); therefore, bone metastasis screening is essential, particularly in patients with FTC. ³

Among the 175 patients with calvarial and skull base metastases, the most common primary malignancy was breast cancer (54.9%), followed by lung cancer (14.3%), prostate cancer (6.3%), malignant lymphoma (5.1%), and other malignancies (19.4%). ⁸ A previous study revealed that thyroid cancer was observed in only one of 175 (0.6%) patients. ⁸ PTC accounts for approximately 80% of all thyroid cancers. However, Nagamine et al. ⁹ reported 12 patients with skull metastasis from thyroid cancer, and nine of them were histologically diagnosed with FTC (75%). Therefore, the predominant histological subtypes in patients with skull metastasis from thyroid cancers is FTC. ⁴ Until now, only two case reports have described temporal bone metastases from FTC.^5,10 The locations of temporal bone metastases from FTC were the apex of the petrous part in one case ¹⁰ and the mastoid part in another case. ⁵

Skull metastases can show an osteolytic, sclerotic, or mixed pattern depending on primary malignancies. ¹¹ Patients with bone metastases from thyroid cancer predominantly showed osteolytic lesions, with secondary bone formation in response to bone destruction, typically accompanied by extraosseous soft-tissue mass formation extending into adjacent structures.^1,4,5 As in previously reported cases, our presented cases showed large osteolytic masses, with internal scattered bone fragments and extraosseous mass formation.

As FTC is usually a hypervascular tumor, bone metastases from thyroid cancer showed a higher degree of homogeneous or heterogeneous enhancement on contrast-enhanced CT and MRI.^1,4,5 In case 1, the nature of hypervascularity revealed a marked enhancement on the arterial phase of dynamic contrast-enhanced CT images and washout on delayed-phase images. Furthermore, several dilated and tortuous vessels within the tumors were observed on arterial-phase CT images. Both cases had flow voids in abnormally dilated vessels within the mass on T1- and T2-weighted images. Previous studies reported that a flow void sign was observed in 77%–94% of patients with bone metastases from RCC and was a helpful imaging feature for diagnosing bone metastasis from RCC.^12,13 Although no bone metastases from thyroid cancer with flow void sign have been reported previously, flow void sign can be observed in patients with bone metastasis from thyroid cancer.

The patient (case 1) underwent multiphase contrast-enhanced CT/MRI. Multiphase contrast-enhanced CT/MRI can reveal tumor vascularity and may be useful for differentiating temporal bone tumors. In addition, if hypervascularity of the temporal bone tumor is suspected, severe bleeding due to surgical biopsy or resection can be avoided by preoperative endovascular embolization. Although multiphase contrast-enhanced CT/MRI is not necessary to routinely perform for temporal bone tumors, it is worth performing when developed tumor blood vessels and flow void sign are observed.

The primary differential diagnosis of hypervascular temporal bone tumors includes bone metastasis (especially from RCC), paraganglioma, endolymphatic sac tumor, meningioma, and solitary fibrous tumor (SFT). The radiographic feature of bone metastases from RCC is an osteolytic lesion with heavy or fine septa; however, MRI signal intensities of bone metastases from RCC are greatly varied and nonspecific. ¹² Hypervascular bone metastasis can increase from RCC, thyroid cancer, breast cancer, gastric cancer, melanoma, plasmacytoma, and sarcoma. Although the flow void sign is a helpful imaging feature for diagnosing bone metastasis from RCC,^12,13 our presented cases indicate that it can be observed in bone metastasis from thyroid cancer.

Paraganglioma is the most common middle ear or jugular foramen tumor. A salt-and-pepper appearance on T1-weighted images is an MRI characteristic of large paragangliomas, and hypointense “pepper” represents high-velocity flow voids of feeding arterial branches in the tumor. The location of the middle ear or jugular foramen is the key to differentiating bone metastases.

An endolymphatic sac tumor is a locally aggressive neoplasm occurring from the endolymphatic sac of the petrous portion of the temporal bone and is a hypervascular tumor that may erode the adjacent bony and vascular structures. The tumor diameter is relatively small (1.0–5.7 cm),^14,15 and the vascular flow void sign was observed in larger lesions (≥1.5 cm). ¹⁵

As meningioma and SFT, usually originating from the dura mater, is an important differential diagnosis of intracranial hypervascular tumors with flow void sign. ¹⁶ Adjacent hyperostosis is a well-known imaging finding of meningioma, whereas bone destruction caused by SFTs would be attributed to a long-standing pressure effect.

In conclusion, temporal bone metastases of FTC showed osteolytic hypervascular masses, with internal scattered bone fragments and extraosseous mass formation. On T1-and T2-weighted images, flow voids in abnormally dilated vessels can be observed within FTC bone metastasis. Although the differential diagnosis of hypervascular temporal bone tumors is challenging, radiologists should know the characteristic imaging features of bone metastasis from FTCs, such as flow void sign.

ORCID iD

Hiroki Kato https://orcid.org/0000-0001-5926-1895