What are the current radiotherapy guidelines for thyroid malignancies, including indications, recommended dose and fractionation, and treatment planning considerations?

Thyroid Radiotherapy Guidelines

External beam radiotherapy (EBRT) for thyroid cancer is indicated primarily when complete surgical excision is not possible or when there is no significant radioiodine uptake in the tumor, with specific dose and fractionation schedules varying by histologic type and clinical scenario. 1

Indications for External Beam Radiotherapy

Differentiated Thyroid Cancer (DTC)

• EBRT is reserved for locoregional disease when complete surgical excision is not achievable 1
• Use EBRT when tumors lack significant radioiodine uptake (radioiodine-refractory disease) 1
• Consider EBRT for bone metastases in combination with radioiodine therapy, though prognosis remains poor 1
• Brain metastases require surgical resection and/or EBRT as the only therapeutic options (radioiodine ineffective) 1

Anaplastic Thyroid Carcinoma (ATC)

• EBRT/IMRT can increase short-term survival and improve local control in select patients 1
• Use EBRT for palliation (e.g., to prevent asphyxiation from airway compromise) 1
• Hyperfractionated EBRT combined with radiosensitizing doses of doxorubicin increases local response rate to approximately 80%, with median survival of 1 year 1
• Consider EBRT for isolated skeletal metastases 1
• For solitary brain lesions, neurosurgical resection, radiation therapy, or both are recommended 1

Medullary Thyroid Cancer (MTC)

• Radiotherapy is used in the presence of local invasion when surgery cannot achieve complete resection 1

Dose and Fractionation Recommendations

Standard EBRT Technique

• Use anterior and posterior fields extending from tips of mastoid processes or hyoid down to carina, laterally including both sides of neck and supraclavicular fossae 2
• Shield the mandible and infraclavicular portions of both lungs, but no midline lead in phase one volume 2
• Mid-plane dose of 46 Gy in 23 daily fractions for phase one to avoid late spinal cord damage 2
• Determine maximum cord dose using lateral simulator film due to considerable variation in interplanar distance 2
• Phase two volume requires three-dimensional CT planning with conformal beam shaping using multileaf collimator to avoid further cord dose 2

ATC-Specific Dosing

• Weekly chemotherapy regimens are recommended when using concurrent chemoradiation (generally more toxic than chemotherapy alone) 1
• IMRT may be useful to reduce toxicity in ATC patients 1

Treatment Planning Considerations

Technical Approach

• CT-based treatment planning is highly recommended before initiating RT 1
• Use 3D conformal technique to reduce damage to surrounding normal tissue (heart, lungs, esophagus, spinal cord) 1
• IMRT may decrease dose to normal tissues but requires adherence to Advanced Technology Consortium/NCI and ASTRO/ACR guidelines 1
• Limit total heart dose to 30 Gy or less given younger patient age and typically long-term survival 1

Field Design

• For treatment confined to thyroid bed: use antero-oblique wedged pair of beams 2
• When volume includes both sides of neck: anterior and posterior fields are necessary, extending adequately to suprasternal notch level 2
• Extensive elective nodal radiation is NOT recommended for thymomas/thyroid tumors as they do not typically metastasize to regional lymph nodes 1

Critical Pitfalls and Caveats

Avoid These Common Errors

• Do not use iodinated contrast CT if radioiodine therapy is planned - requires 2-month delay between iodinated contrast and subsequent I-131 therapy 3
• Ensure maximum spinal cord dose is calculated - do not rely solely on mid-plane dosing given anatomical variation 2
• Do not attempt debulking surgery in ATC - surgical intent should be gross tumor resection only if achievable 3
• Initiate palliative care discussions early in ATC - tracheostomy is often morbid and temporary, may not align with patient wishes 1

Patient Selection

• EBRT in ATC should focus on patients with localized disease and good performance status who may benefit from multimodality therapy 3
• For DTC with distant metastases, prioritize radioiodine therapy for iodine-avid lesions before considering EBRT 1
• Chemotherapy is no longer indicated for DTC due to lack of effective results - enroll patients in tyrosine kinase inhibitor trials instead 1

Multimodality Integration

• Combination of hyperfractionated RT and doxorubicin-based regimens followed by surgery in responders shows improved local disease control in ATC 1
• Distant metastases become the leading cause of death despite improved local control with chemoradiation 1
• Addition of larger chemotherapy doses has NOT improved control of distant disease or survival 1