[Thyroid function after hypofractionated adjuvant radiotherapy for localized breast cancer]

Radiation-Induced Hypothyroidism in Patients With Breast Cancer After Hypofractionated Radiation Therapy: A Prospective Cohort Study

PURPOSE

Our objective was to assess the incidence and risk factors of radiation-induced hypothyroidism (RHT) after adjuvant hypofractionated radiation therapy (RT) in patients with breast cancer.

METHODS AND MATERIALS

Eligible patients with breast cancer who were treated with hypofractionated RT were prospectively evaluated. Thyroid function tests were performed before and at regular times after RT. RHT was defined as twice elevated serum thyroid-stimulating hormone (TSH) with decreased or normal free thyroxin after RT. The patient, tumor, and treatment factors were evaluated for possible associations with the risk of RHT.

RESULTS

Five hundred patients were analyzed. All patients underwent chest wall/breast with or without regional nodal irradiation. Among them, 369 (73.8%) patients received supraclavicular nodal radiation (SCRT). Eighty-two (16.4%) patients had elevated TSH before RT. At a median follow-up of 21.9 months, 131 (26.2%) patients developed RHT, and 59 (11.8%) patients received thyroid hormone-replacement therapy. Patients with SCRT had a significantly increased 2-year cumulative incidence of RHT compared with patients without SCRT (31.5% and 11.4%, P<.001). The peak incidence of RHT occurred around 6 to 12 months after RT. Multivariate analysis revealed that elevated baseline TSH and increased thyroid mean dose (Dmean) were independent risk factors for developing RHT. After adjusted for baseline TSH, there was a nonlinear relationship between thyroid Dmean and the risk of RHT. Dmean >21 Gy was the threshold value for predicting RHT (hazard ratio, 2.2; P<.001).

CONCLUSIONS

The incidence of RHT was high in patients with breast cancer. Thyroid function test should be started no later than 6 months after RT. We recommend that the Dmean of the thyroid should be kept lower than 21 Gy for hypofractionated RT.

NTCP model for hypothyroidism after supraclavicular-directed radiation therapy for breast cancer

BACKGROUND AND PURPOSE

Hypothyroidism (HT) is a well-known complication of radiation (RT) that includes supraclavicular (SCV) fields. We analyzed breast cancer patients who received SCV-directed RT to evaluate predictors of HT and developed the first normal tissue complication probability (NTCP) model for HT specific to breast cancer patients.

MATERIALS AND METHODS

192 breast cancer patients received SCV-directed RT between 2007 and 2019 and met inclusion criteria. Individual dose-volume histograms were analyzed to determine thyroid volume within and outside specific isodose lines as well as minimum, mean, and maximum doses. Multivariable logistic regression was performed to assess potential clinical and treatment factors for the development of hypothyroidism. An NTCP model was created, and model validation was performed.

RESULTS

Thirty-seven patients (19.3%) developed HT following SCV-directed RT at a median 25 months (range: 2-83 months). Multivariable analysis revealed longer length of follow-up (p = 0.015) and larger thyroid volume receiving less than 20 Gy (CV20Gy[cc]; p = 0.045) were significant prognostic factors (p = 0.039). IMRT was not associated with an increased risk of hypothyroidism (p = 0.28) despite lower CV20Gy[cc] (p = 0.0002). On NTCP modeling, CV20Gy[cc] ≥ 8.5 cc was associated with a risk of HT < 15%. For smaller thyroids, mean dose and thyroid volume were found to be predictive of HT risk. Model validation demonstrated comparable performances between our model and other published models (AUC 0.69-0.72).

CONCLUSION

NTCP modeling within our patient cohort suggested that greater than 8.5 cc thyroid volume receiving less than 20 Gy may be a recommended dosimetric guideline to minimize HT risk in breast cancer patients receiving SCV-directed RT.