ESTRO 2021 Abstract Book

S435

ESTRO 2021

Conclusion Robustness towards anatomical changes must be evaluated separately from range and setup uncertainties. Small amounts of extra tissue deteriorate the target coverage, while shrinkage increases the normal tissue dose. Using this planning strategy, the CTVns are in general robust to 3 mm and the CTVp to 5 mm extra tissue. Increasing it further will have consequences for the heart and lung dose. At DCPT, the weekly CT scans are now replaced by an adaptive re-scanning strategy based purely on daily CBCT evaluations using the variable outer limit as tolerance combined with a 3 mm tolerance on shrinkage.

Poster highlights: Poster Highlights 21: Hyperthermia

PH-0550 Importance of high thermal dose in post-operative re-irradiation and hyperthermia in breast cancer C. Tello Valverde 1 , A. Bakker 1 , G. van Tienhoven 1 , M.W. Kolff 1 , H.P. Kok 1 , B.J. Slotman 1 , I.R. Konings 2 , A.L. Oei 3 , H.S. Oldenburg 4 , E.J. Rutgers 4 , C.R. Rasch 5 , H.D. van den Bongard 6 , H. Crezee 6 1 Amsterdam UMC, University of Amsterdam, Department of Radiation Oncology, Cancer Center Amsterdam, Amsterdam , The Netherlands; 2 Amsterdam UMC, University of Amsterdam, Department of Medical Oncology, Cancer Center Amsterdam, Amsterdam, The Netherlands; 3 Amsterdam UMC, University of Amsterdam, Laboratory for Experimental Oncology and Radiobiology, Center for Experimental and Molecular Medicine, Cancer Center Amsterdam, Amsterdam, The Netherlands; 4 Netherlands Cancer Institute, Department of Surgical Oncology, Amsterdam, The Netherlands; 5 LUMC, Department of Radiation Oncology, Amsterdam, The Netherlands; 6 Amsterdam UMC, University of Amsterdam, Department of Radiation Oncology, Cancer Center Amsterdam, Amsterdam, The Netherlands Purpose or Objective Randomized clinical studies showed better complete response rates for patients with unresectable locoregional (LR) recurrence in previously irradiated area treated with re-irradiation and hyperthermia (reRT- HT) than with reRT alone. Single-arm studies suggested that post-operative reRT-HT is effective for patients with resected LR recurrence. However, HT dose delivered during treatment was often poorly monitored and documented. We investigated the impact of HT dose on LR control, overall survival (OS) and toxicity in patients with LR recurrent breast cancer treated with post-operative reRT-HT guided by extensive invasive thermometry in the target region. Materials and Methods Observational study of 112 women with LR recurrent breast cancer, stages (y)pT0-4N0-3M0-1, treated with