437P - Long-term efficacy and toxicity outcome of adjuvant external beam radiotherapy for medullary thyroid cancer: A single institution cohort study

Background

Medullary thyroid cancer (MTC) is a relatively aggressive entity with high risk of locoregional recurrence and distant metastases. There is no role for adjuvant radioactive iodine and medical treatment, while adjuvant external beam radiotherapy (EBRT) remains controversial. Guidelines and a recent systematic review suggested EBRT for extensive disease, which includes nodal involvement, extrathyroidal extension and residual disease, but data remained scarce. Toxicity of EBRT was inconsistently reported.

Methods

All consecutive patients diagnosed with MTC who received curative intent treatment from 2000 to 2018 in a tertiary institution were included. All patients received total thyroidectomy, prophylactic central compartment dissection and as-required therapeutic lateral compartments dissection. Patients at high risk of relapse (R1-2 resection, cervical lymph node metastases, extrathyroidal extension) were offered adjuvant EBRT. Radiotherapy volume covered thyroid bed and level II - VI. The whole treated volume received 60Gy with additional boost to gross residual up to 66Gy. Relapse, survival and toxicity outcomes were reported.

Results

41 patients were included, median follow-up was 12 years. Median age was 48.8, 34 (82%) patients were of pT1/2 and19 (38%) were pN1. 4 (8%) underwent R1 resection and 2 (4%) had gross residual disease despite maximal resection. 19 (46%) completed EBRT. Despite at high risk, only 3 (15.7%) had locoregional relapse after EBRT compared to 2 (9%) in those not required. Higher incidence of distant metastases in EBRT group (5 compared to 2) may reflect more advanced underlying disease. Overall survival was similar (both not yet reached median, p=0.31). Acute side effects of EBRT were well tolerated, with all patients experiencing only G1/2 mucositis and dermatitis. None required hospitalisation due to side-effects nor artificial nutrition. No G3/4 long term side-effects were observed.

Conclusions

Adjuvant EBRT in advanced MTC results in encouraging locoregional control and low incidence of significant short and long-term side-effects.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.