Case report

A 77-year-old man visited the authors’ clinic with the chief complaint of a painless anterior neck mass for several months. He denied other symptoms such as dysphagia, odynophagia and hoarseness. He had a history of hypertension and hepatitis B. His family history was unremarkable.

On physical examination, a 2 × 2 cm, soft, non-tender mass was palpable just above the thyroid cartilage, and it moved with the laryngeal box during deglutition. A sagittal T1-weighted magnetic resonance image (MRI) showed a well-circumscribed, homogeneous, cystic mass attached to the hyoid bone ( Fig. 1 ). He underwent the Sistrunk operation, and a well-defined cystic lesion was extirpated together with the body of the hyoid bone. Postoperative recovery was uneventful except for dysuria, which occurred 1 day after the surgery and was relieved by tamsulosin.

Sagittal T1-weighted MRI image illustrating an infrahyoid, homogeneous, cystic mass lesion attached to the hyoid bone (white arrow).

Pathological analysis revealed that the cystic wall was lined with columnar and squamous epithelium with focal thyroid tissue ( Fig. 2 ). Two foci of epithelial nests and some compact acinus-like structures, which consisted of medium-sized cells with prominent nucleoli, were found in the hyoid bone ( Fig. 3 ). Immunohistochemically, the nests were strongly positive for cytokeratin and PSA ( Fig. 4 ), and negative for thyroid transcription factor-1 (TTF-1) and thyroglobulin. Prostate cancer with hyoid bone metastasis was confirmed by these immunohistochemical stains. The patient received combined hormone therapy and chemotherapy without complications for 3 years.

Low power view of the tumour with haematoxylin and eosin stain showing the cystic wall consisting of columnar and squamous epithelium with focal thyroid tissue.

Some medium-sized cells with prominent nucleoli were found in the hyoid bone, constructing compact acinus-like structures.

Immunohistochemical stain strongly positive for PSA.

Discussion

The authors present a unique case of occult prostate cancer with metastasis to the hyoid bone, incidentally found after resection of the thyroglossal duct cyst. Little information was reported in the literature regarding the origin of the tumour and how it involves the hyoid bone. Primary tumours arising from the hyoid bone include chondrosarcomas and giant cell tumours . Possible differential diagnosis also encompasses local or distant spread from other primary sites. According to the report by T imon et al. , the most frequent primary malignant lesion involving hyoid bone was from the larynx, followed by the vallecula and pyriform sinus. In the present case, theses potential sites were free of neoplasm, based on physical examination and MRI. Neoplasms arising from other possible local structures, such as thyroid or parathyroid glands and the thyroglossal duct cyst should also be considered.

The thyroglossal duct cyst is the most common congenital cyst in the neck and it can be located anywhere in the developmental pathway from the foramen caecum to the suprasternal area . Most patients present with a soft cystic midline mass intimately associated with the hyoid bone when they are less than 10 years old. It has been reported by M urphy & B udd that only about 0.6% of such lesions occur in patients over 60 years old. Thyroglossal duct cyst associated with carcinoma is uncommon and the incidence is approximately 1%. D ucic discovered a remarkable rate of malignant transformation in elderly patients and suggested early surgical intervention, but most cases are diagnosed incidentally after surgical excision. Regarding histology, thyroid papillary carcinoma is the most common type (80%), followed by mixed papillary/follicular carcinoma (8%) and squamous cell carcinoma (6%) . The histological finding in this case was not compatible with these cell types. Negative immunohistochemical staining for TTF-1 and thyroglobulin also excluded a possible origin from the thyroid gland.

Metastasis from an occult primary carcinoma has been reported to occur in 3–4% of all cancer patients. Skeletal metastases are the first lesions to be detected in approximately 10–15% of these patients . The most probable origin of occult primary cancer is lung cancer, followed by prostate, breast and liver cancers . The negative immunohistochemical staining for TTF-1 and thyroglobulin helped the authors to rule out the lung and thyroid as possible origins. Dysuria occurred in the postoperative period and the patient’s old age suggested prostate cancer. As the immunohistochemical staining revealed strong reactivity to PSA, the greatly increasing level of serum PSA (1805 ng/ml) further supported the diagnosis of prostate cancer with metastasis to hyoid bone. No literature has mentioned the incidence of distant metastasis to the hyoid bone. To date, the current case was the first case of prostate cancer with metastasis to hyoid bone.

Bone metastasis is a hallmark of advanced prostate cancer, and the tumour metastasizes via the venous plexus and readily spreads to the axial skeleton, especially to the pelvis and lumbar spine. Clinical symptoms of bone metastasis include new onset bone pain, pathological fracture and spinal cord compression. Bone scanning is a very sensitive diagnostic image modality for prostatic metastasis, and MRI may be helpful in some situations, such as clinically suspected cases with negative bone scans. The serum PSA level is also a good predictor because it is usually elevated and reaches a level higher than 20 ng/ml when bone metastasis is detected . Current treatments of advanced prostate cancer include individual or combined surgery, hormone therapy, chemotherapy, radiotherapy and some anti-resorptive medication.

The clinical and pathological importance of the hyoid bone is usually ignored and rarely mentioned. This case demonstrates a unique presentation of prostate cancer with bony metastasis, and meticulous clinical–pathological co-operation can help to confirm the diagnosis. The authors think that, although it is rare, prostate cancer with bone metastasis should be taken into consideration in the differential diagnosis of tumours involving the hyoid bone.