ESTRO 2024 - Abstract Book

S521

Clinical - Breast

ESTRO 2024

nodes (according to ESTRO guidelines) and ALTJ (according to (1)), were contoured and agreed by two clinical oncologists. The global accuracy of image registration was assessed using normalised cross correlation (NCC). D50% for each structure was calculated. Doses as a percentage of the prescribed whole breast dose were calculated to allow comparison between different dose/fractionation regimens and boost/no boost. Analysis was performed using R.

Results:

Global accuracy of image registration was excellent, NCC=0.97 (where a maximum value of 1 represents perfect alignment). 1094 patients were available for analysis. 569 (52.0%) received tumour bed boost , 570 (52.1%) were treated with 3D-CRT, 395 (36.1%) simple field-in-field IMRT and 129 (11.8%) complex IMRT. Mean age was 59 years. 553 (50.5%) had left-sided treatment. 931 patients were identified who had breast radiotherapy +/- boost, with no prescribed regional nodal irradiation. The D50% incidental dose to the IMN, L1 and ALTJ varied considerably both within and between treatment centres. Mean D50 to the IMN was significantly lower for left-sided patients compared with right-sided (14.8% vs 41.0%, p <0.001) (figure 1B). Left-sided treatment remained a significant predictor of lower IMN D50 on multivariate analysis (p<0.001), (table 1). Mean D50 was significantly higher for left-sided patients compared with right-sided in L1 (59.9% vs 48.9%, p<0.001) and ALTJ (23.3% vs 17.7%, p<0.001) (figure 1B). Left -sided treatment remained a significant predictor of higher L1 and ALTJ D50 on multivariate analysis (p<0.001 and p = 0.003 respectively), (table 1). Increasing BMI was a significant predictor of higher incidental L1 and ALTJ dose (p<0.001). 115 patients were prescribed supraclavicular radiotherapy, of which 43 (37.4%) were not prescribed IMN treatment. The incidental D50 to IMN was significantly lower in left-sided treatment, compared with right-sided treatment (27.2% vs 66.1%, p<0.001 (figure 2).