What is the role of external beam radiotherapy in medullary thyroid cancer?

Role of External Beam Radiotherapy in Medullary Thyroid Cancer

External beam radiotherapy (EBRT) should be considered for locally advanced medullary thyroid carcinoma (MTC) with high-risk features for locoregional recurrence, though its role remains controversial and should not be routinely recommended for all MTC patients.

Evidence Quality and Limitations

The evidence base for EBRT in MTC is notably weak—no randomized controlled trials exist 1. All available data comes from retrospective studies with conflicting results, making definitive recommendations challenging. The most recent NCCN guidelines (2025) 2 and ENDOCAN-TUTHYREF consensus 3 acknowledge MTC but provide limited specific guidance on EBRT, focusing instead on systemic therapies for metastatic disease.

When to Consider EBRT

Based on retrospective evidence, EBRT may reduce locoregional recurrence risk in specific high-risk scenarios 1:

• Locally advanced disease (T3/T4 tumors)
• Positive surgical margins (R1/R2 resection)
• Extensive nodal involvement (N1b disease)
• Gross residual disease that cannot be surgically resected

Critical Evidence Showing No Survival Benefit

The most recent and highest quality study (2025) found no survival benefit from adjuvant EBRT 4. In this single-institution analysis of 30 patients:

• No improvement in overall survival (92.9% vs 71.4%, p=0.202)
• No improvement in locoregional recurrence-free survival (100% vs 85.7%, p=0.157)
• No improvement in disease-free survival (64.3% vs 64.3%, p=0.91)
• The authors explicitly recommend caution before advising adjuvant EBRT

This finding is corroborated by a large SEER database analysis 5 of 2,046 patients showing EBRT was not associated with improved overall survival (HR 1.12,95% CI 0.76-1.65) or disease-specific survival (HR 1.66,95% CI 0.93-2.95), even in subset analyses by age and disease extent.

Contradictory Evidence

Earlier retrospective studies suggested potential benefit:

• One SEER analysis 6 showed univariate OS benefit in node-positive patients, but this disappeared in multivariate analysis controlling for age and tumor size
• Older reviews 7, 8 suggested EBRT "should be considered" for locally advanced disease, but these predate the negative 2025 data

Technical Considerations When EBRT Is Used

If EBRT is administered despite limited evidence 1, 9:

• Dose: Median 60 Gy in 30 fractions (2 Gy per fraction)
• Technique: Intensity-modulated radiotherapy (IMRT) preferred to minimize toxicity to surrounding structures
• Target: Region at risk for locoregional recurrence

Critical Pitfalls

1. Do not use EBRT routinely for all MTC patients—the 2025 data shows no benefit 4
2. Recognize the morbidity: EBRT to the neck carries significant toxicity including dysphagia, xerostomia, and rare but serious complications like radiation-associated sarcomas (reported 22 years post-treatment) 8
3. Age and tumor size are stronger prognostic factors than EBRT administration 6
4. Tumor stage (T2 vs advanced) significantly impacts outcomes more than EBRT 4

Practical Algorithm

For patients with completely resected MTC (R0, low-risk features):

• Do not recommend EBRT 4, 5

For patients with high-risk features (R1/R2 resection, T4 disease, extensive N1b):

• Discuss EBRT as an option for potential locoregional control benefit 1
• Emphasize lack of survival benefit and treatment toxicity 4, 5
• Consider observation with close surveillance as equally valid given negative recent data
• If EBRT chosen: 60 Gy with IMRT technique

For metastatic disease:

• Prioritize systemic therapy (kinase inhibitors) per NCCN guidelines 2
• EBRT reserved for palliative indications (symptomatic bone metastases, brain metastases) 9

Current Consensus Position

The weight of evidence, particularly the most recent 2025 study, does not support routine use of adjuvant EBRT in MTC 4. While older retrospective data suggested possible locoregional control benefits 1, 7, these have not translated to survival improvements in larger analyses 5, 6. Given the significant toxicity and lack of proven benefit, EBRT should be used selectively if at all, with informed patient discussion about the uncertain risk-benefit ratio.