ESTRO 37 Abstract book

ESTRO 37

S553

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

=95%, gave the imaging dose outside the target

V 100%Rx

volume. Results

Optimized Halcyon plans which include the 10 MU MVCBCT compare well with optimized plans that do not include MV imaging. . Fig. 1 presents the dose clouds and DVH of the Halcyon and Tx-only optimized plans, showing very similar results for the PTV and OAR doses. This is likely because both the MVCBCT and the treatment fields use the same isocenter and therefore primarily deliver dose to the same regions, and because the Halcyon optimization process takes into account the MVCBCT imaging dose. Nevertheless, subtracting the Tx-only plan from the Halcyon plan, there is MV imaging dose outside the target (Fig. 2), which amounted to ~0.5 - ~2 Gy.

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

Fig.1: Halcyon (squares ) vs. Tx only ( triangles ), showing similar DVHs.

Fig. 2: MVCBCT dose outside the target region.

Conclusion This is the first assessment of the MVCBCT imaging dose outside the target volume of H&N cases for the first clinical Halcyon in our institution. Our results show that Halcyon’s optimization process fully accounts for the MV imaging dose in generating the treatment fields so as to satisfy the clinical prescription. Imaging dose evaluation should be performed for treatment sites where OARs are contralateral to the treatment field arrangement since