ESTRO 36 Abstract Book

S641 ESTRO 36 2017 _______________________________________________________________________________________________

Gy photon OR electron boost to the tumor bed) were evaluated with special focus on documented skin toxicity during RT course. Acute skin erythema (AE) was visually assessed and recorded using the RTOG scoring system, before RT and every 5 fractions. In this study, grade 2-3 AE during RT was considered as the primary endpoint. A number of relevant clinical risk factors was prospectively recorded: age, skin phototype, smoking habits, use of drugs, neoadjuvant chemotherapy with anthracyclines and/or taxanes and/or trastuzumab, hormone therapy with tamoxifen or aromatase inhibitors, comorbidities and related drugs, T stage, location of breast surgery. Dosimetric feature were extracted from the skin dose- volume histogram for the whole treatment (DVH, absolute volume in cc), with skin defined as the difference between the body contour and a 5mm inner isotropic contour from the body. Dosimetric and clinical variables were included into multivariable logistic regression. Goodness-of-fit was evaluated through Hosmer-Lemeshow test (HL) and calibration plot. Results a total of 147 breast cancer patients (median age 55 years, range 34–77) were enrolled. Grade 1, 2 and 3 AE were 65/147 (44%), 52/147 (35%) and 24/147 (16%), respectively. At univariate analysis only the dose to 20 cc of breast and use of aromatase inhibitors vs tamoxifen resulted as predictive factors for toxicity (61.8% vs 17.9%, p>0.01, for aromatase inhibitors vs tamoxifen, respectively). ML resulted in a two variable model including the dose to 20 cc of skin (continuous variable, OR=1.09, 10 th -90 th percentile 1-1.19) and use of aromatase inhibitors (OR=1.7, 10 th -90 th percentile 1.1-2.7). Calibration was good (HL test p=0.35, calibration slope 1.08). Results for model and calibration are presented in the figure. Smoking also resulted to be a risk factor (OR=4) in a reduced population (87 pts), it was not directly inserted into ML model due to the high prevalence of missing values, but it deserves attention and further analysis

For all breast cancer patients, adjuvant radiotherapy (RT) reduces locoregional recurrence and for high risk patients, regional nodal irradiation (RNI) improves overall survival. However, there is limited data on the anatomical location of regional nodal recurrence (RNR) after adjuvant RT. Nodal radiotherapy fields have historically been defined using anatomical landmarks but with the advent of 3D radiotherapy planning nodal contouring atlases have been developed. Validation of these atlases is scarce. Our objective was to map the location of RNR in patients previously treated with adjuvant RNI, and assess whether the treating RT fields provided adequate coverage. We also assessed whether these areas of RNR were within the boundaries of the Radiation Therapy Oncology Group (RTOG) nodal atlas. Material and Methods Between 2005 and 2013, we identified 32 patients previously treated with definitive surgery and adjuvant RNI for breast cancer that developed RNR detected with 18-fluorodeoxyglucose positron emission tomography (FDG-PET) imaging, before salvage treatment for RNR. FDG-PET positive regional lymph nodes were contoured on each individual PET scan. Deformable registration was used to fuse the FDG-PET scan with the patient’s original RT simulation scan, onto which the RTOG atlas had been retrospectively contoured. Each nodal area of recurrence was categorized as: in-field, defined as ≥ 95% of the RNR volume receiving ≥ 95% prescribed dose; marginal, RNR receiving < 95% prescribed dose; and out of field, RNR not intentionally covered with the original RT plan. RTOG coverage was defined for each RNR as ‘inside’, ‘marginal’ or ‘outside’. Results Of the 32 patients, 12 (37%) had limited RNR and 20 (63%) had RNR in addition to distant metastatic di sease on FDG- PET imaging. 27 (84%) patients received full axillary RT, 3 (9%) supraclavicular fossa (SCF) only, and 14 (44%) internal mammary node (IMN) RT. Of the 87 nodal relapses, 17 (20%) were out of field. Of those intentionally treated, 10 (33%) patients developed SCF relapse, 18 (66%) axillary relapse and 5 (36%) IMN relapse. 15 (68%) of SCF, 20 (50%) axillary and 1 (14%) IMN nodes were in-field relapses. The RTOG atlas covered 13 (60%) SCF, 20 (50%) axilla and 0 (0%) of IMN nodal relapses.

Conclusion this analysis shows that moderate/severe acute skin erythema is related to skin DVH, particularly to the dose to 20cc of skin. In the frame of the here used skin definition, this approximately corresponds to an area of 6x6 cm^2. Use of aromatase inhibitors acts as a dose sensitizing factor for kind of toxicity. EP-1195 Regional nodal recurrences after adjuvant breast radiotherapy – are we covering the target? L.E. Beaton 1 , L. Nica 1 , K. Sek 2 , G. Ayers 1 , C. Speers 3 , S. Tyldesley 1 , A. Nichol 1 1 British Columbia Cancer Agency, Radiation Oncology, Vancouver, Canada 2 British Columbia Cancer Agency, Radiology, Vancouver, Canada 3 British Columbia Cancer Agency, Breast Outcomes Unit, Vancouver, Canada

Purpose or Objective