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.

during IORT for 39 APBI patients. The prescribed dose is given as a dark blue line.

Fig. 2: Results of applicator output factor measurements of Mobetron 2000 during commissioning in 2016 (dataset)

and in 2017. Conclusion

Based on our results we recommend to use GAFCHROMIC film dosimetry as a standard tool for patient quality assurance during breast cancer IORT and to check applicator output factors a few times per year for the mostly used applicator. 1. A. Sam Beddar, Peter J. Biggs, Sha Chang, et al. Med. Phys. 2006;33:1476–89. PO-0993 Evaluation of MV imaging dose for the first clinical Halcyon system D. Mihailidis 1 , C. Ling 2 , L. Brady 1 , R. Scheuermann 1 , C. Kennedy 1 , L. Dong 1 , J. Metz 1 1 PENN-Perelman Center for Advanced Med, Radiation Oncology, Philadelphia, USA 2 Varian, Radiation Oncology, Palo Alto, USA Purpose or Objective Evaluate the dosimetric effect of MV imaging that is included in the Halcyon TM system workflow by assessing the dose outside the target region for various treatment sites. Halcyon is a single 6X-FFF straight-through linac with an in-line MV flat panel capable of either MVCBCT or orthogonal MV imaging, which is part of the workflow. A detailed dosimetric study of MVCBCT imaging have been performed for the first clinical Halcyon unit at UPenn. Material and Methods The Halcyon system includes a 43x43 cm 2 MV imaging panel at 154 cm source-to-imager distance. MVCBCT imaging is performed with 28 cm fixed width (projected at isocenter-100 cm) and variable field length (up to 26 cm) FOV, 200º arc, 15 sec rotation time, at 5 MU (low dose) and 10 MU (high dose) dose levels. In addition, it offers orthogonal portal images as another option. The user selects the imaging option at the treatment planning stage, and the planning process includes the imaging dose for the overall plan optimization. We selected a number of our first clinical Head&Neck (H&N) cases to quantify the dose delivered to structures outside the target volume by the high dose MVCBCT imaging (as the worst case scenario). We also expanded our study to pelvis and breast sites. Comparisons of Halcyon plans (including imaging) and plans without imaging were performed to assess the imaging dose to sensitive structures outside the PTV. Subtraction of the plan without imaging from the plan with imaging, both normalized to PTV V 100%Rx =95%, gave the imaging dose outside the target volume.

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