Abstract
198 Background: The thyroid is not routinely considered an organ at risk in supraclavicular (SC) nodal radiotherapy (RT) for breast cancer. We aim to compare the dosimetric impact of two RT planning techniques on the thyroid: (1) conventional single anterior field to encompass the SC nodal volume defined using clinical and radiological landmarks; and (2) 3-dimensional (3D)-conformal planning to encompass the SC nodal volume contoured on planning computed tomography (CT) scans. Methods: The thyroid and SC nodal volumes were contoured on the planning CT datasets of 20 patients who had RT to the breast/chest wall and SC nodes to a dose of 50 Gy in 2-Gy daily fractions. Comparison of the following dosimetric parameters between the techniques was performed: mean and maximum thyroid dose; V5, V30, and V50 (percentage of thyroid receiving ≥5, ≥30, and ≥50 Gy respectively); percentage of SC nodal volume receiving 95-107% of the prescribed dose (homogeneity index, HI); and maximum spinal cord dose. Anatomical characteristics that influenced the dose distribution were investigated. Results: The 3D-conformal planning technique significantly increased all dosimetric parameters for the thyroid (p<0.001) (Table). It improved the mean HI of the SC nodal volume (16% vs. 93.2%; p<0.001) but resulted in a higher spinal cord dose (mean maximum dose 6.1 Gy vs. 30 Gy). The thyroid volume and depth of SC nodes did not influence the differences in the thyroid doses between the two techniques. The depth of SC nodes was significantly associated with the difference seen in the HI of SC nodal volumes between the techniques (p=0.004). Conclusions: The 3D-conformal planning technique improved the dosimetric coverage of the SC nodal volume but increased radiation doses to the thyroid compared to the single field technique. The longer term clinical consequences of radiation to thyroid should be considered in SC nodal irradiation for patients with breast cancer. [Table: see text]
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