Discussion:
Differentiated thyroid cancer carries an excellent prognosis and the development of metastasis is a rare event in everyday head and neck oncology practice. In spite of the evolution of numerous protocols to guide practice, metastases from DTC are treated heterogeneously across treating centres. In our series, metastases were identified in 32 of 381 (8.4%) patients with DTC. The presentation of metastases from DTC is insidious in many cases. In our series 19% (N=6) of patients underwent treatment for conditions in the spine without being aware of primary thyroid pathology. The fact that metastatic workup is not recommended by all established guidelines and that thyroidectomy being a common ENT procedure performed even in low resource non-specialty institutions results in many patients to undergo limited surgeries like hemithyroidectomy and who are subsequently diagnosed with metastases on referral to specialized services after the initial surgery. We had 5 such patients in our series (16%) The incidence of bone metastasis was 4.5% (N=17) which is in concordance with published literature. Distant metastases are associated with poor prognosis in differentiated thyroid cancers. The most common site of bone metastasis in differentiated thyroid cancers are vertebrae, ribs and hips. Tumour cell adhesive molecules bind the tumour cells to marrow stromal cells and bone matrix allowing them to grow and produce angiogenic and bone resorbing factors.
Metastasis of DTC have been treated with curative intent1717.In a study done by Isabelle et al, out of 43 patients who had undergone pulmonary metastasectomy predominantly for follicular carcinoma, showed an encouraging 10 year disease free survival of 84% in patients who underwent complete resection. This significantly declined to 62% on incomplete resection (p=0.013)10.
In our study, the most common thyroid cancer histopathology was papillary thyroid cancer followed by follicular cancer and poorly differentiated thyroid cancers respectively.
There is uniformity across protocols on the recommendation of radioactive iodine in patients with metastatic DTC in terms of a survival advantage. In a study done on 228 patients, 71 received iodine therapy with a cumulative dose of at least 600 mCi, This study reiterated that there seemed to be a better prognosis with greater cumulative doses of RAI but the disease specific mortality was 60% over a 10 year follow up period when treated with higher cumulative doses11, In our study, all except one received radioactive iodine. The median dose of 400mCi is in accordance with study published by Pitoia et al which showed that the patient treated with a mean effective cumulative RAI dose of 457.3 ± 29.7 mCi I131 (300-600 mCi) had a good response to treatment18.
It is imperative that young patients with metastatic DTC be warned of the adverse effects of high dose radioiodine, most importantly xerostomia, pulmonary fibrosis and infertility. The option of cryopreservation of semen or oocytes may be offered to potential recipients of high dose radioactive iodine19.
The role of conventional EBRT has been well established in the literature to allay pain, to improve neurological deficits and or to prevent pathological fractures. Four patients in our series received EBRT, all for the alleviation of pain. Studies show that radiation doses vary from low risk microscopic disease (54 Gy), High risk (59.5 Gy) to gross disease (63-70 Gy) to achieve local control rates20. However, EBRT has been found to show a higher rate of local progression and pain relapse with long term follow up21. In our research, only four patients received EBRT to the metastatic site with a maximum of 54 Gy and a minimum of 30 Gy.