ESTRO 2020 Abstract book

S947 ESTRO 2020

robustness of these kind of delivery towards residual setup errors and eventual target deformations. Material and Methods Ten breast cancer patients were selected for this study (5 left and 5 right). Of these, 5 were treated with 50 Gy in 25 fractions (postmastectomy with implanted prosthesis), while 5 were treated after conservative surgery with a simultaneous integrated boost (50-57 Gy in 25 fractions). In 4 patients the target was the residual breast only, while in 3 cases it was the breast plus the AL3-4, and in 3 the breast plus the AL3-4 and the IMN. The CTV of the whole breast, AL3-4 and IMN were contoured separately following the 2015ESTRO consensus guidelines. The PTV was generated by a 5 mm expansion of the CTV and kept 3 mm away from the patient skin surface. Heart, left and right lung, contralateral breast, and spinal cord were contoured as organs at risk. Plans were produced using a standard template previously established in order to standardize plan quality, to reduce inter-operator variability, and to speed up the planning process. For each patient, the dose was recalculated on the daily MVCT (for a total of 250 MVCT) and the accumulated dose was obtained using the Plan Adaptive module of HT TPS. Differences between planned and accumulated doses were recorded and statistical significance was evaluated using paired two-sided Wilcoxon signed-rank test with a significance level of 0.05. Results In Table 1 average values and standard deviations, together with the p values, are reported for some selected parameters for both planned dose distributions (P) and recalculated (S). As can be observed, differences are generally small for PTVs and CTVs and not statistically significant except for a slight reduction in the CTV N D mean . Although not statistically significant, a reduction in PTV coverage (for all PTVs) is observed, but CTV coverage is always maintained above 95%.

Conclusion The survey highlights that, in the UK, whilst almost all of the responding centres had access to software capable of DIR, only two out of five were using it in routine clinical practice. The results gave useful insight into how centres commission and QA DIR, and will form the basis for developing national guidelines. References [1] Brock et al. Med. Phys. 2017. PO-1636 Robustness of breast treatments with Tomotherapy toward residual setup errors L. Marrazzo 1 , F. Maresca 2 , M. Icro 2,3 , C. Arilli 1 , S. Calusi 2 , M. Casati 1 , A. Compagnucci 1 , C. Talamonti 1,2 , I. Desideri 2 , L. Livi 2,3 , S. Pallotta 1,2 1 Azienda Ospedaliera Universitaria Careggi, Medical Radiation Physics, Firenze, Italy ; 2 University of Florence, Department of Experimental and Clinical Biomedical Sciences “Mario Serio”, Florence, Italy ; 3 Azienda Ospedaliera Universitaria Careggi, Radiation Oncology, Firenze, Italy Purpose or Objective Breast radiotherapy is usually performed with tangential fields (both static and intensity modulated). In particularly challenging anatomies or when axillae level 3-4 (AL3-4) and internal mammary nodes (IMN) have to be treated, the use of rotational techniques generally allows to obtain improved dose distributions. Helical Tomotherapy (HT) can be used for breast treatments, but it does not allow to explicitly account for a skin flash margin. Although daily MVCT is used to correct patient setup at each fraction, it is worthwhile to test the

The box plots for breast PTV and CTV V95% and PTV N and CTV N D mean are reported in Figure 1.