ESTRO 37 Abstract book

ESTRO 37

S552

1 Haaglanden Medical Center, Department of Medical Physics, Leidschendam, The Netherlands 2 Haaglanden Medical Center, Department of Radiotherapy, Leidschendam, The Netherlands 3 Haaglanden Medical Center, Department of Surgery, Leidschendam, The Netherlands 4 Leiden University Medical Center, Department of Radiation Oncology, Leiden, The Netherlands Purpose or Objective Comparison of the delivered dose to the prescribed IORT dose with in vivo dosimetry was performed. For intraoperative radiation therapy (IORT) using electrons in accelerated partial breast irradiation (APBI), this is especially relevant since a high dose is delivered in a single fraction. Material and Methods For 39 of elderly (60+) patients, diagnosed with breast cancer (tumour diameter < 3 cm) and treated with IORT in our institution, in vivo dosimetry was performed with GAFCHROMIC EBT3 films. APBI with a total dose of 23.3 Gy prescribed at 100% (21 Gy at 90%) was given during surgery according to the method described in the ELIOT study. All patients were irradiated with electron beams of 9 or 12 MeV generated with a dedicated IORT mobile accelerator (Mobetron 2000, INTRAOP, USA). A protection disk was used to shield the thoracic wall, the lung and the heart (if applicable). During the dose measurements with GAFCHROMIC EBT3 films, the first and the second films were placed before (position 1) and behind (position 2) the protection disk, respectively. The calibration measurements were performed with the electron beams of the Mobetron 2000 in a water tank at the depth of dose maximum for each energy. The green and red colour channels were used for the dose evaluation in positions 1 and 2, respectively. Applicator output factors were determined relative to a 10-cm circular applicator, and the measurements were performed at d max for each applicator and beam energy. According to the AAPM TG72 Report 1 recommendations, the applicator factors should be measured annually with a tolerance of 2-3%. Results The results of in vivo GAFCHROMIC film dosimetry for 39 patients are presented in Fig. 1. For the first 21 patients, the dose in breast tissue, measured with GAFCHROMIC films (mean value 23.89 Gy) was on average 2.4% (SD=2.6%, range -4.3% to 6.0%) higher than the prescribed dose of 23.33 Gy. After that applicator factors of the IORT accelerator were checked using an electron diode and a Roos ionization chamber. A small difference in comparison to those measured during the commissioning in 2016 was found. After the correction for applicator factors (see Fig.2), the dose averaged over 18 patients (mean value 23.27 Gy) agreed within -0.3% (SD=1.9%, range -3.2% to 3.2%) with the prescribed dose. The mean dose measured behind the protection disk was within 0.46 Gy.

using IBEX. Spearman correlation was used to reduce the 145 features to 7 (cutoff: 0.7). Features were: ‘5-7 Correlation’, ‘11-7 Correlation’, ‘9-7 Correlation’ from GLCM3 category,’90-7 Correlation’ from GLCM2.5 category, ‘Skewness’ from ‘GradientOrientHistogram’ Category, ‘Busyness’ from NID2.5, and ‘Orientation’ from ‘Shape’ category. These were integrated to build 3 types of models using: 1) only the baseline (BL) value of the radiomics features, 2) the ratio between the mid- treatment value of the feature to its value at BL and 3)a functional principal component analysis model leveraging the structure of the temporal trajectory in the evolution of the feature from BL to mid-treatment. Afterwards, logistic regression was run to investigate the predictive capacity of clinical attributes as well as the synergistic effect of adding clinical attributes & either one of the three radiomic-derived models. Results 39 predominately locally advanced OPC patients who received concurrent chemoradiotherapy were included. 24 patients (61.5%) achieved complete response (CR) on the primary tumor, as compared to 15 (38.5%) partial responses (PR). The corresponding areas under the curve (AUCs) and confidence intervals (C.I.) for the prediction of tumor response to (chemo)radiation ‘CR vs PR’, according to the 3 models, augmented by clinical prognosticators, were: 0.44 [95%C.I.:0.25-0.64], 0.64 [95%C.I.:0.42-0.8] and 0.71 [95%C.I.:0.52-0.87], respectively. Whereas, the clinical attributes only model yielded a lower AUC of 0.49 [95%C.I.:0.31-0.7]. This suggests the functional approach yields a superior predictive power in the evaluation of tumor response

compared to other radiomic models or even strictly clinical models. ( Figure 1 ) .

Conclusion Temporal radiomic trajectories from sequential intra- treatment CT scans can be integrated with traditional clinical attributes into a multi-faceted decision-making tool. Radiomics may convey additional clinical information from routine imaging studies in radiation oncology towards adaptive RT.

Poster: Physics track: Implementation of new technology, techniques, clinical protocols or trials (including QA and audit)

PO-0992 Electron IORT in vivo film dosimetry in breast cancer for alignment and dose verification A. Petoukhova 1 , J. Nijst-Brouwers 1 , K. Van Wingerden 1 , J. Van Egmond 1 , T. Stam 2 , A. Marinelli 3 , J. Van der Sijp 3 , M. Straver 3 , O. Guicherit 3 , P. Koper 2 , H. Struikmans 4

Fig. 1: Results of in vivo dosimetry with GAFCHROMIC EBT3 films in breast tissue for position 1 and position 2