Can dynamic wedges reduce thyroid dose in breast radiotherapy compared to physical wedges?

Background: In breast radiotherapy, enhanced dynamic wedge (EDW) and physical wedges are used to improve the homogeneity of the dose. Scattered photons are the major factor in the off-field organs' unwanted dose. In breast radiotherapy, the thyroid is a critical off-field organ at risk for scattered photons. This study was performed to compare the unwanted dose and the secondary fatal cancer risk to the thyroid in breast radiotherapy between EDW and physical wedge. Material and Methods: The 6-MV Varian 2100 C/D linac was used to irradiate the breast of a thorax phantom under two opposite tangential fields. The unwanted dose that reached the thyroid was estimated using Eclipse Treatment Planning System and Gafchromic film dosimetry. Corresponding fatal secondary cancer risks were also assessed according to the NCRP report 116 recommendations. Results: The measured dose for thyroid using a physical wedge and enhanced dynamic wedge were measured as 2.1 and 0.735 cGy, which are approximately 1% and 0.37% of the prescribed dose to the breast (2 Gy), respectively. In the case of radiotherapy with the physical wedge, the lifetime risk of secondary fatal cancer attributed to the thyroid is 0.0480 and 0.0504 % using TPS and measured data, respectively. In the case of the dynamic wedge, the above values were reduced to 0.0168 and 0.0176 %, respectively. Conclusions: Using an enhanced dynamic wedge in breast radiotherapy reduced the thyroid dose by about 65% compared to the physical wedge technique. As a result, it was concluded that the application of EDW is safer than the physical wedge in breast radiotherapy.