Clinical and dosimetric predictors of primary hypothyroidism in nasopharyngeal carcinoma patients following intensity-modulated radiotherapy
Purpose/Objective
Advances in the treatment of nasopharyngeal carcinoma (NPC) have led to improved clinical outcomes in terms of tumor control and toxicities mainly through the widespread use of intensity-modulated radiotherapy (IMRT). However, hypothyroidism (HT) remains a frequent late complication observed in half of patients who have undergone cervical irradiation. The aim of our study was to identify clinical and dosimetric factors predictive of primary HT in patients irradiated for NPC with IMRT at the Salah Azaiez Institute in Tunisia.
Material/Methods
Data were collected retrospectively from hospitalization registers and medical records. We included a total of 50 patients treated for NPC with curative intent by IMRT between January 2017 and January 2019. Patients with thyroid dysfunction prior to treatment were excluded from the study. The mean dose (Dmean), the percentage of thyroid volume exposed to specific dose levels denoted V30-V50, and the absolute volume of thyroid tissue spared from particular dose levels, denoted VS45-VS60 were studied.
Results
The median follow-up was 60 months. Primary hypothyroidism had cumulative incidence rates of 45.2% and 58.1% at 3- and 5-year intervals, respectively, highlighting the progressive nature of thyroid dysfunction over time. Among the factors examined, young age, female sex, and reduced thyroid volume (less than 16 cc) emerged as the most influential predictors of susceptibility to primary hypothyroidism in univariate analysis. In the dosimetric study specific thresholds have been established. Patients were classified into high-risk and low-risk groups based on these thresholds, improving our ability to identify individuals at greater risk for primary hypothyroidism. The threshold values defined were 45 Gy for average dose. The volume receiving 40 Gy in more than 80 % of the total thyroid volume appear as a significant risk factor of HT in the univariate analalysis. In term of sparing volume, the 5 cm3 and 10 cm3 were identified factors for the VS 45Gy an VS 60Gy respectively.
Conclusion
Given the prevalence of primary hypothyroidism in our study, we recommend the use of IMRT treatment optimisation targets aimed at limiting Dmean < 45 Gy, V40 < 80%, VS45 ≥ 5 cm3 or VS60 ≥ 10 cm3 for the thyroid gland.
