Predictors of Hypothyroidism in Hodgkin Lymphoma Survivors After Intensity Modulated Versus 3-Dimensional Radiation Therapy

Abstract

To identify predictors of hypothyroidism after chemoradiation therapy for Hodgkin lymphoma (HL) and to compare outcomes after intensity modulated radiation therapy (IMRT) with those after 3-dimensional (3D) conformal radiation therapy (CRT). Ninety patients who underwent involved-site IMRT in 2009 through 2014 were evaluated for treatment-induced hypothyroidism, defined as elevated thyroid-stimulating hormone or decreased free thyroxine levels (or both). Receiver operating characteristic curve analysis identified individuals at low versus high risk based on dosimetric variables. Dosimetric cutoff points were verified with an external data set of 50 patients who underwent 3D-CRT. In the IMRT group, most patients (75 [83%]) had stage II HL, and the median prescribed dose was 30.6Gy; in the 3D-CRT group, 32 patients (64%) had stage II HL, and the median prescribed dose was 32.0Gy. No differences were found in the proportions of patients with bilateral (P=.982) or unilateral (P=.074) neck involvement between the 2 groups.Hypothyroidism rates were marginally higher in the IMRT group, with estimated 3-year rates of freedom from hypothyroidism of 56.1% in the 3D-CRT group and 40% in the IMRT group (P=.057). Univariate analysis showed that smaller thyroid volume and higher thyroid dose were associated with hypothyroidism in both groups (P<.05). In the IMRT group, the percentage of the thyroid gland volume receiving ≥25Gy (V25) and the absolute volume of the thyroid gland spared from 25Gy (VS25Gy) were the strongest predictors of hypothyroidism (P=.001 and P<.001, respectively).Cutoff points of 63.5% (V25) and 2.2mL (VS25Gy) classified patients as high risk (80%-82%) or low risk (37%-44%) (P<.001). Use of a thyroid avoidance structure reduced the incidence of hypothyroidism (P<.05) in the IMRT group. The percentage of the thyroid receiving 25Gy and the volume of the thyroid spared from 25Gy predicted the risk of hypothyroidism after either IMRT or 3D-CRT for HL. IMRT may confer a higher risk than 3D-CRT unless a treatment avoidance structure is used during planning.