Minimally invasive follicular thyroid carcinoma: Completion thyroidectomy or not?

Well, the answer is ‘it all depends’. Minimally invasive follicular carcinoma, as the term is currently used by many pathologists in Australia, covers a wide spectrum, from tumours with only minimal capsular invasion, which are essentially benign in nature, to those with vascular invasion, which have the potential to metastasize. Before any decision is made as to appropriate management, it is essential to have the slides reviewed by a pathologist who is experienced in interpreting endocrine pathology, and to have a discussion about what is meant exactly by the term ‘minimally invasive follicular carcinoma’ in the particular case concerned. One of the problems, and a source of confusion for many general surgeons, arises from the classification of follicular carcinoma that is still widely used by many pathologists. The Armed Forces Institute of Pathology classification, 1 which follows the World Health Organization guidelines, 2 classifies follicular carcinoma as either ‘widely invasive’ or ‘minimally invasive or encapsulated’. Widely invasive follicular carcinoma demonstrates extensive areas of invasion at the gross and microscopic levels, with evidence of the tumour extending beyond the tumour capsule and diffusely infiltrating the affected lobe or the entire gland. Up to 80% of patients with such tumours develop metastases and 20% will die of their disease. 3 A diagnosis of minimally invasive follicular carcinoma can be made either on the basis of capsular invasion alone, vascular invasion alone, or both capsular and vascular invasion, and is associated with long-term recurrence rates of <5%. 4 Some, but not all, pathologists consider only complete penetration of the capsule as indicative of capsular invasion. A number of authors have recommended that all patients with minimally invasive follicular carcinoma can be managed conservatively (i.e. by hemithyroidectomy alone) on the basis of the excellent survival data for this group of patients overall, and would argue that no further therapy is necessary. 4 However, studies by other authors have pointed out that not all minimally invasive follicular carcinoma behaves in a benign manner, with the important differentiation being the presence of vascular invasion. One of the earliest studies to report this important distinction was from the Mayo Clinic in 1992, where it was shown that follicular carcinoma with capsular invasion alone does indeed behave as a benign tumour, with a zero cause-specific mortality and distant metastases at 10 years follow up, regardless of primary treatment. 5 In contrast, minimally invasive follicular carcinoma with vascular invasion had a 28% cause-specific mortality at 10 years ( P = 0.019). The presence of vascular invasion is clearly an indicator of a poorer prognosis, as would be expected given that follicular carcinoma metastases via the bloodstream. Some authors now believe that lesions with vascular invasion should be distinguished from the minimally invasive follicular carcinomas that show capsular invasion only, because of the greater probability of recurrence and metastasis with angioinvasion in the former. 6 Baloch and LiVolsi have now recommended that only those follicular lesions with capsular invasion alone should be termed minimally invasive follicular carcinoma. 7 Clearly, a follicular lesion with this pathology behaves in a relatively benign manner and so completion thyroidectomy and radioiodine ablation are not warranted. However, any follicular neoplasm that demonstrates vascular invasion, but is not a widely invasive follicular carcinoma (i.e. it is encapsulated), should now be termed an ‘angioinvasive grossly encapsulated follicular carcinoma’. 7 We would recommend completion thyroidectomy and radioiodine ablation for such patients, because of the possibility of metastatic disease, as well as to allow appropriate follow up with serial iodine scans and measurement of thyroglobulin levels. The reporting of thyroid cancer pathology in this way provides much clearer guidelines to surgeons and endocrinologists for the management of patients with follicular thyroid carcinoma. In the meantime, the answer to the question ‘what should I do’ remains ‘it all depends ... it depends, that is, on what your pathologist really means by the term “minimally invasive follicular thyroid cancer”’.