Postoperative Radiotherapy Indications for Medullary Thyroid Carcinoma
Postoperative external beam radiotherapy should be considered for medullary thyroid carcinoma patients with gross extrathyroidal extension (T4a or T4b) with positive margins after resection, or moderate-to-high-volume disease in central or lateral neck lymph nodes with extranodal soft tissue extension. 1
Primary Indications for Adjuvant Radiotherapy
The evidence supports radiotherapy in specific high-risk scenarios:
- Gross extrathyroidal extension (T4a or T4b) with positive surgical margins after resection of all gross disease 1
- Moderate-to-high-volume lymph node disease in central or lateral neck compartments with extranodal soft tissue extension 1
- Incomplete surgical resection where residual microscopic disease is suspected 2
The National Comprehensive Cancer Network guidelines acknowledge that external-beam radiation therapy has not been adequately studied as adjuvant therapy in medullary carcinoma, though slight improvements in local disease-free survival have been reported for selected high-risk patients. 1
Technical Approach When Radiotherapy Is Used
When external-beam radiotherapy is administered:
- 40 Gy in 20 fractions to cervical, supraclavicular, and upper mediastinal lymph nodes over 4 weeks 1
- Subsequent booster doses of 10 Gy in 5 fractions to the thyroid bed 1
- Modern conformal techniques (IMRT or 3D-CRT) should be employed to minimize toxicity 3
Evidence Supporting Radiotherapy in High-Risk Disease
Research data demonstrates meaningful benefit in appropriately selected patients:
- 87% locoregional relapse-free survival at 5 years in stage IVa-c disease treated with postoperative radiotherapy, with only 9% chronic symptomatic morbidity 3
- 86% local/regional relapse-free rate at 10 years in high-risk patients (microscopic residual disease, extraglandular invasion, or lymph node involvement) who received postoperative radiation, compared to 52% without radiation (p=0.049) 2
- Effective locoregional control in patients with T4 disease, extrathyroidal extension, N1 disease, extranodal extension, and residual disease after surgery 4
Important Caveats and Contraindications
Radiotherapy is rarely recommended in children with medullary thyroid carcinoma, even when high-risk features are present. 1
Radioiodine therapy has no role in medullary thyroid carcinoma, as these C-cell derived tumors do not concentrate iodine. 5
Palliative Radiotherapy Indications
External-beam radiotherapy can be given to:
- Palliate painful or progressing bone metastases 1
- Control symptomatic metastases threatening vital structures (bronchial obstruction, spinal cord compression) 6
Postoperative Risk Assessment
The decision for adjuvant radiotherapy requires assessment 2-3 months postoperatively:
- Measure basal serum calcitonin and CEA levels to establish baseline tumor markers 1, 5
- Patients with calcitonin >1000 pg/mL likely have distant metastases and require systemic staging rather than local radiotherapy 1
- High-risk pathologic features (extraglandular invasion, gross residual disease, vascular invasion, lymph node involvement) predict for local recurrence and support radiotherapy consideration 2
The strongest evidence supports reserving radiotherapy for patients with locally advanced disease at high risk for locoregional recurrence, particularly those with positive margins or extranodal extension, where surgery alone provides inadequate local control. 3, 4, 2