OBJECTIVE The thyroid is not routinely considered an organ at risk in supraclavicular (SC) nodal radiation therapy (RT) for breast cancer. We compared the dosimetric impact of the following 2 RT planning techniques on the thyroid: (1) conventional single anterior field to encompass the SC nodal volume defined clinically; and (2) 3-dimensional conformal radiation therapy (3DCRT) planning to encompass the computed tomography (CT)-contoured SC nodal volume. METHODS The thyroid, SC nodal volumes, and organs at risk were contoured on the planning CT of 20 patients who received 50 Gy in 2-Gy daily fractions to the breast or chest wall, and SC nodes. Comparisons of dosimetric parameters between the techniques were performed: thyroid, mean and maximum dose, V5, V30, and V50 (percentage of thyroid receiving ≥5 Gy, ≥30 Gy, and ≥50 Gy, respectively); SC nodal volume, homogeneity index (HI, percentage volume receiving 95%-107% of prescribed dose); and maximum doses of spinal cord and brachial plexus. Anatomic characteristics that influenced the dose distributions were investigated. RESULTS The 3DCRT planning technique significantly increased all thyroid dosimetric measures (mean dose 17.2 Gy vs 26.7 Gy; maximum dose 48.5 Gy vs 51.9 Gy; V5 45.7% vs 64.9%; V30 33.7% vs 48%; and V50 0.6% vs 26.7%; P < .001). It improved HI for the SC nodal volumes (P < .001) but resulted in higher maximum doses to the spinal cord (6.1 Gy vs 30 Gy) and brachial plexus (43.2 Gy vs 51.4 Gy). The thyroid volume and depth of SC nodes did not influence the thyroid dose distribution. The depth of SC nodes impacted on the HI of SC nodal volumes in the conventional technique (P = .004). CONCLUSIONS The 3DCRT planning improved dosimetric coverage of the SC nodal volume but increased thyroid radiation doses. The potential adverse effects of incidental thyroid irradiation should be considered while improving dosimetric coverage in SC nodal irradiation for breast cancer.