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Hipotiroidismo clínico después de la irradiación de ganglios linfáticos regionales con protones frente a fotones: un
Shane S Neibart1, Meredith Taylor1, Nicolas Depauw1
1Massachusetts General Hospital, Department of Radiation Oncology, Boston, MA.
Purpose:
Thyroid dysfunction is a common complication of breast regional nodal irradiation (RNI). Proton therapy has been posed as a means to spare normal tissue. We prospectively evaluated the incidence of hypothyroidism in a cohort of patients receiving RNI on a randomized trial comparing proton to photon modalities. A secondary objective was to evaluate the associations between thyroid dose-volume metrics and the risk of subsequent hypothyroidism.
Methods:
We conducted a single-institution prospective correlative study within the randomized phase III Radiotherapy Comparative Effectiveness (RadComp) trial. Thyroid function testing was obtained at baseline, annually to 3 years, and at 5 years. Clinical hypothyroidism was defined as elevated thyroid stimulating hormone above institutional range with recommendation for or initiation of thyroid replacement therapy. Cumulative incidence of hypothyroidism after proton and photon RNI were compared using fine-gray regression. Thyroid dose-volume metrics were also explored as potential predictors of hypothyroidism, including previously validated metrics, thyroid Dmean>21 Gy(RBE) and thyroid volume spared from 20 Gy(RBE) (VS20Gy(RBE)).
Results:
Ninety-two patients enrolled; seventy-one patients met criteria for analysis (proton 38, photon 33). Median follow-up was 66 months. The 3-year cumulative incidence of clinical hypothyroidism was 13%. By modality, incidence was 16% (proton) versus 9% (photon) (p=0.14). Stratifying by technique, the incidence of hypothyroidism was 0% for 3-dimensional conformal radiation therapy (3DCRT) and 16% for both intensity modulated radiation therapy (IMRT) and pencil beam scanning (p=0.26). Dmean>21 Gy(RBE) (HR=3.02, 95% CI 1.02-8.98) and VS20Gy(RBE)<2.2 cc (HR=8.80, 95% CI 1.54-50.30) were associated with hypothyroidism.
Conclusion:
After RNI, the 3-year incidence of clinical hypothyroidism was 13%, with no statistically significant difference between proton and photon modalities. Thyroid Dmean>21 Gy(RBE) and VS20Gy(RBE)< 2.2 cc were associated with higher risk of hypothyroidism. Inverse planned RNI with superior coverage of the supraclavicular fossa, may come at the expense of increased thyroid dose.
