Type of publication:
*Pettit L., *Welsh A., *Khanduri S.
Radiotherapy and Oncology, April 2016, vol./is. 119/(S558)
Purpose or Objective: Hypothyroidism is the most commonly reported long-term toxicity following radiotherapy to structures near to the thyroid gland. Emami suggested the thyroid gland tolerance as 45Gy (TD 5/5) although a much wider range of 10-80 Gy has been reported in the literature. When irradiating the supraclavicular fossa (SCF) in adjuvant radiotherapy for breast cancer, it is inevitable that the thyroid gland will receive a high dose of radiation due to its proximity to the target volume. Recently there has been a move to CT based delineation of the CTV and organs at risk (OAR) in patients requiring nodal radiotherapy for breast cancer compared with the previous bony land mark/field based techniques. Dose received by the thyroid gland and subsequent late toxicity has not yet been well studied in breast cancer. Material and Methods: Patients undergoing external beam radiotherapy to the breast or chest wall plus SCF between 01/04/15-01/10/15 were identified. Radiotherapy planning contrast enhanced CT scans were taken. External beam radiotherapy was planned with tangents using a field in field approach with a matched direct anterior field. A low weighted posterior field was added if deemed appropriate for adequate dose coverage. Angle corrections were used as appropriate. A dose of 40.05 Gy in 15 fractions prescribed at depth was employed. CTV's were contoured in accordance with the RTOG contouring atlas. The thyroid gland was prospectively delineated and D5% was recorded. Results: Seventeen patients undergoing adjuvant SCF radiotherapy were identified. T stage was as follows: T1:2 patients, T2:9 patients, T3:4 patients, T4a:1 patient,T4d:1 patient. N stage; N1:1 patient, N2:14 patients, N3:2 patients. Fourteen were hormone receptor positive, 3 hormone negative. Twelve were HER2 negative, 5 HER2 positive. Mean D5% thyroid was 37.9Gy (range 7-42.7 Gy). Excluding one patient with a previous hemi-thyroidectomy, the mean D5% thyroid was 39.8 Gy (range 16-42.7 Gy). An abnormality requiring referral to a surgeon for was discovered in one patient. Conclusion: Our departmental tolerance for the thyroid gland was set as 40Gy (for 2.67Gy per fraction). It is hard to achieve this without compromise of the CTV. The effect modern chemotherapy/targeted agents may have on this prior to receiving radiotherapy is inknown. Baseline TSH recording is desirable. Long-term follow up to detect clinical or biochemical thyroid dysfunction is needed to inform practice but would present challenges with capacity in busy oncology departments.
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