Teaching Case

A 60-year-old woman who had undergone nephrectomy for renal cell carcinoma 6 years earlier attended a rapid access breast clinic and was diagnosed with multifocal papillary intracystic carcinoma of the breast. She also reported feeling a neck lump of a few months onset.

Ultrasound reported the neck lump as being part of a benign thyroid goitre (Fig. 1 - 3) based on the presence of multiple bilateral thyroid nodules, cystic changes, the absence of microcalcification, a "wider than tall" appearance of the largest nodule measuring 37mm x 23mm on the transverse sonogram (Fig. 1), minimal internal vascularity and no invasion of adjacent soft tissues. There was also no lymphadenopathy in the neck and no prior neck irradiation. However, in view of the history and the firm feel of the lump, core biopsy of the mass in the right thyroid lobe was performed. This demonstrated a metastatic deposit from renal cell carcinoma.

A staging CT scan (Fig. 4 & 5) showed a heterogeneous thyroid containing multiple bilateral nodules and no further metastatic deposits. Correlation of the current thyroid lesion with the original renal primary (Fig. 6) showed them to differ in radiological appearance.

Ultrasound features suggestive of malignancy include microcalcification, marked hypoechogenicity greater than the strap muscles of the neck, an irregular margin, longitudinal dimension greater than cross-sectional, marked intrinsic vascularity, direct invasion of adjacent soft tissue and metastases to one or more lymph nodes [1]. None of these characteristics were seen in this thyroid lesion and therefore this case is unusual as the sonographic appearances favoured benign multinodular goitre as the diagnosis but histology showed it to be a renal cell carcinoma metastasis.

Most thyroid cancers are primary tumours. Metastases within the thyroid are rare accounting for only 3% of thyroid malignancies with possible sources including lung, breast, kidney, oesophagus and skin [2, 3]. Clear cell renal carcinomas account for 12-34% of all secondary thyroid tumours [2], making them the commonest cause of thyroid metastases [3]. As in our case, renal cell thyroid metastases usually manifest as a metachronous lesion several years after nephrectomy [2] with the longest reported lag time being 26 years after the primary renal tumour was diagnosed [3]. Autopsy series report a surprisingly high incidence of metastatic disease in the thyroid of up to 24% [3, 4] particularly from aggressive lung, breast and melanoma primaries. In the living, renal carcinoma is the commonest source of thyroid metastases [4].

There are no specific imaging appearances of thyroid metastases - the initial presentation can be uninodular or multinodular with a case series reporting metastatic thyroid disease presenting as a solitary mass in 83% (30/36) and multifocal in 8% (3/36) of cases [4]. Primary and metastatic tumour cannot be distinguished on imaging as both generally appear as cold nodules on isotope studies and as a homogeneous, hypoechoic mass on ultrasound. CT and MRI scans are also unhelpful unless there is widespread metastatic disease, in which case suspicions may be raised.

This case highlights the importance of having a higher index of suspicion than usual in investigating thyroid changes in patients with a background history of malignancy, even if the primary was several years earlier. Multinodular changes in the thyroid are very common in the general population, especially in females but a lower threshold for needle sampling should be applied in patients with a previous carcinoma as the disease may still be curable or a significant improvement in disease-free survival achieved with thyroidectomy if no further metastases are demonstrated.