Cormac Small, Presenter: Nothing to Disclose

Thirion Pierre MD, Abstract Co-Author: Nothing to Disclose

Orla McArdle, Abstract Co-Author: Nothing to Disclose

The use of a thyroid block on the supraclavicular field is usually recommended for 3 field breast radiotherapy, however there is little published data on the real risk of radio-induced thyroid dysfunction following breast radiotherapy and the dosimetric effectiveness of the thyroid shielding. This planning study aims to evaluate the organ sparing provided by standard thyroid shielding

The planning study was conducted on 20 patients with nodes positive breast cancer receiving post-operative radiotherapy using a 3-fields technique (two tangential fields and a matched supraclavicular fossa field). All patients where virtually simulated using a GE Advantage Sim workstation and plan were obtained using Helax TMS Planning System. For each patient two added dose plans where produced 1) the 2 tangentials plus supraclavicular field with the thyroid block, 2) the 2 tangentials plus supraclavicular field without the thyroid block. The thyroid gland was delineated as a single structure and thyroid DVH was obtained for each plan. The median dose, the maximum dose (Dmax), and the volumes receiving 10 Gy, 30 Gy, and 50 Gy (V10, V30 and V50) expressed in % of the thyroid volume where compared for each plan. Non-parametric paired tests were applied to evaluate the difference.

The thyroid gland volume varied from 7 to 26 cc (median 13.5 cc) . The main dosimetric parameter are summarised table 1.

Based on this planning study, the addition of a thyroid block to the superclavicular fossa field seems to have a limited organ sparing effect, and marginally reduces the volume of thyroid receiving a significant dose level. Further study is required to evaluate the real risk of radio-induced thyroid dysfunction after breast radiotherapy and to clarify the need of the additional blocking

Small, C, Pierre, T, McArdle, O, Planning Study Assessing the Effectiveness of the Addition of the Thyroid Block in the 3 Field Radiotherapy Technique for Breast Cancer.  Radiological Society of North America 2007 Scientific Assembly and Annual Meeting, November 25 - November 30, 2007 ,Chicago IL. http://archive.rsna.org/2007/6001681.html