Radiation exposure to the thyroid gland is unavoidable in breast cancer (BC) radiotherapy (RT). This study was conducted to evaluate the impact of using different supraclavicular (SCV) field angles on the percentage volume of the dose absorbed by the thyroid relative to the maximum dose absorbed by the spinal cord.
A total of 50 consecutive BC patients undergoing planning computed tomography (CT) were evaluated. The thyroid gland, spinal cord, and SCV nodal volumes were contoured on the planning CT of patients who received 50 Gy of radiation in 2 Gy daily fractions to either the breast or chest wall, and to the SCV or full ipsilateral regional nodes. Dosimetric comparisons of three different angles (0º, 10º, and 15º) of the anterior SCV field with a corner spinal cord shield was performed on three-dimensional conformal radiation therapy (3DCRT). We compared the percentage volumes of absorption by the thyroid at low (V15), intermediate (V30), and high (V50) doses (15 Gy, 30 Gy, and 50 Gy), respectively; it was also compared with the maximum dose absorbed by the spinal cord.
The mean thyroid volume of our population was 8.3 cc (1.8-20 cc). The mean percentage volumes of radiation absorbed by the thyroid were as follows: low dose (V15) = 26%, intermediate dose (V30) = 16.3%, and high dose (V50) = 2.8%. We found that smaller field angles were associated with lower mean percentages of volume absorption for radiation doses of all dose levels (low, intermediate, and high doses; this finding was statistically significant (P < 0.05). The mean maximum dose to the spinal cord is lower at larger angles (15º, 10º, and 0º; 9.6 Gy, 17.3 Gy, and 33.6 Gy, respectively), which was also statistically significant (P < 0.05); a maximum dose was observed at 0º for 47.9 Gy. We found that the mean contoured thyroid volume only influences the percentage volume absorbed by the thyroid at a high dose and smaller angle (≤ 10º); this finding was statistically significant (P < 0.05).
Use of a larger anterior SCV field angle to spare the spinal cord led to a significant increase in the percentage volume of radiation absorbed in the thyroid at all dose levels; this has the potential to increase the thyroid post-radiation toxicity.
We propose routine utilization of field angle (≤ 10º) and a medial spinal cord shield as an optimal strategy to minimize the doses absorbed by both the thyroid gland and spinal cord.