e12088 Background: Complementary medicine (CM) is used widely in cancer patients, mostly in palliative treatment regimes or during chemotherapy. In contrast, the use of CM during curative Radiotherapy (RT) is less frequent.The aim of this study was to evaluate the influence of CM with regard to toxicity and recurrence during postoperative radiotherapy for breast cancer. Methods: Between 2010 and 2013 all patients intended for adjuvant radiotherapy for breast cancer received a questionnaire concerning the use of CM e.g. Vitamins, trace elements, enzymes, natural herbs etc. (yes / no) and if yes what kinds. Radiotherapy was applied with 50.4 Gy to the whole breast and a Boost of 10.8 Gy in 6.5 weeks.6 weeks after radiotherapy the side effects were analyzed. Follow up for progression rates was done yearly. Results: A total number of 879 patients with breast cancer completed the questionnaire, 641 (73%) did not take any CM and 238 (27%) took CM. There were almost 40 different products mentioned and the most common were magnesium (n=71) and selenium (n=38). The RT side effects (Dermatitis RTOG grade 0, I, II, III/IV) 6 weeks after completion of RT were as follow: in the non CM group 53% grade 0, 41% grade I, and 6% grade II, 0% grade III/IV. And in the CM group 54%, 39%, 9%, and 0% respectively. The corresponding side effects for the group taking magnesium were 35% 42%, 6% and 1% and for the selenium group 47%, 47%, 5% and 0%. There was no significant difference in side effects between the non CM and CM patients, respectively magnesium and selenium group . The sum of all registered disease progression (0 years – 5 years) were in the non CM group 6.4%, in the CM group 6.3%, in the magnesium group 5.6% and in the selenium group 7.9%, no significant difference could be detected between all groups. Conclusions: In this cohort, 27 % of patients used CM during adjuvant radiotherapy of breast cancer. The most commonly product is magnesium followed by selenium. A measurable influence of CM on RT side effects or disease progression could not be identified.