ESTRO 2021 Abstract Book

S935

ESTRO 2021

and two-stage breast reconstruction (tissue expander later replaced by a permanent implant) followed by adjuvant radiation therapy at a cancer center, from 2008 to 2019. Thirty three patients (11.46%) were stage T4b, T4c or T4d. Twenty five patients (8.7%) were treated using a skin bolus, either daily (2.1%) or on alternate days (91.2%). Data regarding patients characteristics, diagnosis, surgeries, complications, pathology, staging, systemic therapy, radiation therapy and outcomes were all recorded. Results Median follow-up was 61 months (range 18 to 115 months). Radiation treatment was delivered using 2D, 3D or IMRT technique (4.5%, 92%, 2.1%), 93% of patients were treated with conventional fractionation (median dose: 50 Gy) and 7% with hypofractionation (median dose: 42.56 Gy). Five-year overall survival, locoregional control, and metastasis-free rates were 97%, 97% and 83%, respectively. In the first stage of reconstruction, capsular contracture, local infection and expander loss occurred in 18%, 5.2%, and 4.2% of cases, respectively. In the subgroup analysis of the 266 patients who had the second reconstruction stage performed, capsular contracture, local infection and implant loss occurred in 4.9%, 10.2% and 12.8% of cases, respectively. Ninety- three percent of patients had some degree of radiation dermatitis: 49.3% grade I, 38.2 grade II and 5.9% grade III. In a subset analysis of the 8.7% of patients who were treated with a skin bolus, daily bolus significantly increased the risk of expander infection and loss (HR 10.3 [CI 95% 1.7 - 61.8] and HR 13.89 [CI 95% 2.24 - 85.98], respectively), while alternate days bolus showed a nonsignificant increase for expander infection and loss (HR 1.14 [CI 95% 0.14 - 9.295]) and HR 1.5 [CI 95% 0.19 - 12.87], respectively). No significant association was observed between the use of skin bolus and complications in the second stage of reconstruction. There was also no association between the use of skin bolus and local regional failure (p = 1,00). Unsurprisingly, bolus was associated with radiation dermatitis (p = 0.000). Conclusion Despite a small prevalence of bolus usage in this cohort, it does not seem to improve the already excellent locoregional control rates. However, it significantly increases complications. Randomized trials are needed for a better understanding of its role in the era of modern radiation techniques and advances in systemic treatment. PO-1125 Cardiac subvolume dosimetry in breast cancer patients receiving DIBH hypofractionated radiotherapy E. González Del Portillo 1 , O. Alonso Rodríguez 1 , J. Hernández Rodríguez 2 , E. Tenllado Baena 2 , Á. Fernández Lara 2 , C. Cigarral García 1 , L.A. Pérez-Romasanta 1 1 hospital Universitario De Salamanca, Radiation Oncology, Salamanca, Spain; 2 hospital Universitario De Salamanca, Radiophysics, Salamanca, Spain Purpose or Objective To determine doses to cardiac subvolumes in left-sided breast cancer patients receiving DIBH hypofractionated RT (40 Gy / 15 fractions) and to compare the results to free-breathing (FB) treatment plans. Materials and Methods Voluntary DIBH with a spirometer-based video-assisted system (SDX, DynR) and CT-scans were performed under FB and DIBH conditions on 41 left-sided-BC patients and segmented according Duane’s atlas including the left anterior descending (LAD) artery. IMRT plans were obtained in Eclipse-TPS keeping the same dosimetric goals on FB and DIBH plans. Target, lungs and heart volumes were measured. PTV dose distribution (D50%, D98%, and V95%) and OARs dose/volume parameters (heart and ventricles D mean , and V20; lung and left lung D mean , V20, V10, V5), cardiac subvolumes (D mean and V20), and LAD artery (V20, D max , D2%) were calculated. Statistical analysis used non-parametric tests. Results Both lungs and left-lung volumes increased substantially in DIBH conditions (1637.8 ml ± 555.3 and 783.5 ml ± 286.4, respectively). Heart volume slightly decreased in apnea (489.5 ml vs. 465.4; p = 0.04), but target volumes, CTV and PTV, were similar in FB or DIBH plans (p values 0.82 and 0.86, respectively). PTV dose coverage was similar disrespect of respiratory conditions (D50% = 41.8 ± 2.7 vs 41.1 ± 0.5, p = 0.60; V95% = 96.9% ± 0.9% vs 97% ± 0.8%, p = 0.88). Mean dose for the whole heart and left ventricle were significantly reduced in DIBH plans (1.9 ± 1.0 vs. 3.1 ± 1.4 [p = 5.4E-8], and 2.5 ± 1.5 vs 4.5 ± 2.4 [p = 4.1E-8] respectively). V10 and V20 for those structures were also diminished significantly (Table 1). All the left ventricle subvolumes, apical, anterior, septal, lateral, and inferior, received significant smaller mean doses with DIBH (Table 1). V20 values for heart subvolumes were significantly different only for those that received meaning doses (apical and anterior) (Table 1). DIBH plans showed significantly smaller doses (Dmax, D2%, and V20) to the LAD artery (Table 1). Table 1. Dosimetric parameters in cardiac subvolumes