The effect of supraclavicular lymph node irradiation upon the thyroid gland in the post-operative breast carcinoma patients

The dosimetric impact of supraclavicular nodal irradiation on the thyroid gland in patients with breast cancer

PURPOSE

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 AND MATERIALS

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.

Estimation of absorbed dose to thyroid in patients treated with radiotherapy for various cancers

This study is aimed at the evaluation of the absorbed dose to the thyroid gland from scattered/primary radiations in patients who have undergone radiotherapy. In this study, the skin entrance dose (SED) on the anterior neck at the level of the thyroid gland was measured using thermoluminescent dosimeters for 57 patients during radiotherapy with (60)Co teletherapy unit. Phantom dosimetry was also performed to find the correlation between the SED and the dose at the depth of the thyroid. SED of patients treated for breast cancer was 6.77±3.49 Gy and that of those with Hodgkin's disease and Ca. lung irradiation were 21.29±13.52 Gy and 28.80±14.94 Gy, respectively. SED at the level of the thyroid gland was found to be highest for patients whose thyroid gland was within the radiation field, while lowest for patients with skull irradiation. Thyroid gland is recommended to be shielded or excluded from the field whenever possible.