ESTRO 2020 Abstract book

S359 ESTRO 2020

tissue characterization is required, as well as for Monte- Carlo based planning, which requires accurate elemental mass fractions. This work experimentally demonstrates the benefits of using more energies to characterize human tissue equivalent materials. OC-0586 Can MRI predict pathologic response after single dose neoadjuvant partial breast irradiation? J. Vasmel 1 , M. Groot Koerkamp 1 , R. Charaghvandi 2 , C. Vreuls 3 , P. Van Diest 3 , A. Witkamp 4 , R. Koelemij 5 , A. Doeksen 5 , T. Van Dalen 6 , E. Van der Wall 7 , J. Wijnen 8 , B. Van der Velden 8 , M. Moman 9 , W. Veldhuis 9 , M. Philippens 1 , S. Mandija 1 , J. Lagendijk 1 , H. Verkooijen 10 , A. Houweling 1 , D. Van den Bongard 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands ; 2 Radboud UMC, Radiotherapy, Nijmegen, The Netherlands ; 3 UMC Utrecht, Pathology, Utrecht, The Netherlands ; 4 UMC Utrecht, Surgery, Utrecht, The Netherlands ; 5 St. Antonius hospital, Surgery, Nieuwegein, The Netherlands ; 6 Diakonessenhuis, Surgery, Utrecht, The Netherlands ; 7 UMC Utrecht, Medical Oncology, Utrecht, The Netherlands ; 8 UMC Utrecht, Image Science Institute, Utrecht, The Netherlands ; 9 UMC Utrecht, Radiology, Utrecht, The Netherlands ; 10 UMC Utrecht, Imaging Division, Utrecht, The Netherlands Purpose or Objective To assess radiologic response on MRI using radiologist assessment and quantitative parameters after single ablative dose neoadjuvant partial breast irradiation (NA- PBI) in low-risk breast cancer patients in order to predict Low-risk breast cancer patients were treated with NA-PBI (n=36; 1x 20 Gy to the tumor) followed by breast conserving surgery (BCS) after an interval of 6 or 8 months. Pathologic response was assessed as pathologic complete response (pCR), near pCR (<10% tumor cells), partial response (10-50% tumor cells), stable disease (>50% tumor cells), or no response according to EUSOMA criteria. Patients underwent MRI (3T Ingenia, Philips, Best, the Netherlands) in prone position pre-radiotherapy (RT), 1 week, 2, 4, 6, and, if applicable, 8 months post-RT. For each patient at least 4 scans were assessable. Radiologic response was evaluated by radiologists and by quantitative parameters. Breast radiologists assessed dynamic contrast enhanced (DCE) and diffusion weighted (DW) MRI according to clinical practice in neoadjuvant systemic treatment. Radiologic complete response was defined as complete absence of pathologic contrast enhancement and complete absence of diffusion restriction. The positive predictive value (PPV, probability that radiologic complete response predicts pCR) and negative predictive value (NPV, the probability that no radiologic complete response predicts residual disease) was calculated for the final MRI before BCS. For quantitative analysis the tumor was manually delineated on the pre-RT DCE-MRI. The delineation was rigidly transformed to the post-RT MRIs (Fig 1). Intrascan motion of the tumor was corrected using rigid registration. Within the GTV we evaluated the relative tumor enhancement at 2 min post-contrast (wash- in) and the distribution of enhancement curve types describing the wash-out of contrast (type 1: low, 2: intermediate, 3: high probability of malignancy) for voxels with at least 50% relative enhancement. Quantitative parameters were analyzed using descriptive statistics. pathologic response. Material and Methods

Results Using radiologist assessment PPV was 67% and NPV was 76% at the final MRI before BCS. Using quantitative parameters wash-in decreased post-RT in all patients, irrespective of pathologic response (Fig 2). At baseline, 42% of the voxels within the GTV showed type 1, 14% type 2, 13% type 3 wash-out curves and 30% no curve (Fig 2). Post-RT an increase in percentage of voxels without a wash-out curve (53% and 52% at 6 and 8 months post-RT, respectively), and a decrease in percentage of voxels with type 3 curve was observed for all pathologic responses (both 3% at 6 months and 8 months post-RT) (Fig 2).