ESTRO 2020 Abstract book

S763 ESTRO 2020

intervention level for in-vivo dosimetry at 5%.

Conclusion The results of this study indicate that beam modelling parameters based on 50 th percentile community-generated data compare well against a well-benchmarked model. These parameters may potentially provide an easier starting point for the input parameters or pave the way for standardisation in beam modelling and thereby improving multicentre dose calculation accuracy which is of particular importance in clinical trials. References [1] Glenn et al, 2019. DOI: 10.3252/pso.eu.ESTRO38.2019 PO‐1349 Geometric and dosimetric critical issues in iort by mobile linacs S. Andreoli 1 , M. Pimpinella 2 , C. De Angelis 3 , L. Menegotti 4 1 ASST Papa Giovanni XXIII, UOC Fisica Sanitaria, Bergamo, Italy ; 2 ENEA-INMRI, Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, Rome, Italy ; 3 Istituto Superiore di Sanità, Servizio Grandi Strumentazioni e Core Facilities, Rome, Italy ; 4 APSS, UOC Fisica Sanitaria, Trento, Italy Purpose or Objective The electron IntraOperative Radiation Therapy (IORT) consists in delivering an high dose to the target during surgery. The aim of this work is to highlight geometric and dosimetric critical issues of IORT technique and to report practical solutions, based on the expertise gained from clinical practice with mobile linacs, for about one thousand patients. Material and Methods The main geometric issue concerns the matching between the treatment and the beam characterization set-ups. This matching is mandatory for a correct dosimetric evaluation. In our approach, a circular PMMA disc (2 cm larger than diameter of applicator) is put on the target surface so that the applicator, giving a lightly pressure on the surface, homogenizes the target volume and ensures a useful buildup effect. The thickness of the disc is optimized according to the target thickness and the beam energy. To ensure the stability of the treatment set-up and the alignment applicator-target-internal shield, a flat (or little sloped) applicator vertically oriented is used. Dosimetric issues mostly concern beam and detector characterization, choice of treatment energy, knowledge of backscattered dose inside the target, output reproducibility and implementation of in-vivo dosimetry. The use of a disc on the target surface ensures a useful buildup effect allowing to use the highest available beam energy. Regarding the output reproducibility, a conditioning modality is implemented and systematic treatment simulations performed to check the output with a timing similar to that of the clinical practice. Moreover, since the applicator shape strongly affects the beam characteristics attention is paid to check the applicator wholeness after the washing and sterilization process (fig 1: effects due to a longitudinal applicator warp caused by an erroneous process with high temperature). For in-vivo dosimetry, the detector correct positioning is ensured by firmly fixing the detector to the disc by a steril-strip. Results The disc on the target surface allows to avoid herniation of the target inside the applicator, which could cause a significant increase in the delivered dose (about 5% for 1 cm of homogeneous herniation) and an unsuitable dose distribution to the target. Moreover, using the highest available energy and optimizing the disc thickness allow a better dosimetric coverage of target (entrance dose at least 95%, dose increase in the pre-buildup region up to 6%). Sometimes a damaged applicator can be delivered after sterilization then a careful check excludes reuse of the damaged applicator. A micromosfet detector sandwiched between the applicator and the target surface permits to set the

Conclusion The described treatment set-up ensures a good coupling between applicator and target surface and permits to take advantage of all the dosimetric evaluations obtained in water and slab phantoms. Furthermore, the implementation of in-vivo dosimetry ensures an independent dose assessment. PO‐1350 In vivo dosimetry with XR‐RV3 radiochromic films in intraoperative radiotherapy of the breast S. Lozares 1 , A. Gandía 1 , J.A. Font 1 , D. Villa 1 , V. Alba 1 , S. Jiménez 1 , M. Hernández 1 1 Miguel Servet University Hospital, Física y Protección Radiológica, Zaragoza, Spain Purpose or Objective To carry out treatment verification and in vivo dose measurements in intraoperative breast radiotherapy patients, treated with the Axxent (Xoft Inc.) equipment with 50 kVp of energy with XR-RV3 radiochromic films model, specific for low energies and high doses. With this work we will get to know the in vivo dose in skin, the main risk organ in IORT treatments. Material and Methods 480 patients were treated with IORT with the Axxent device in our hospital from May 2015 to October 2019. Dosimetric measurements were performed in vivo with XR- RV3 radiochromic films in 10 patients to determine the skin dose in IORT with Axxent.Gafchromic XR-RV3 radiochromic films are specific for measurements in the low energy and high dose range, the films were calibrated following the 3- channel method (Radiochromic.com) to measure dose in vivo in patients with breast IORT. The tratment prescription is 20 Gy on the balloon surface following the TARGIT study. Four pieces of properly oriented films were placed on each patient to obtain skin doses at different points on each patient. The pieces were placed progressively from the area closest to the applicator (point 1) to the furthest (point 4) following the example in the image (Fig 1). In addition to obtaining the doses on the skin, a piece was placed on the balloon applicator to monitor the prescribed dose for each treatment and verify the correct administration of the treatment. The process of converting the result into dose was carried out with an Epson Expression 12000XL scanner. The films were scanned before and after irradiation with five consecutive scans in both cases, in the case of those irradiated, 24 hours after irradiation. The calibration process was carried out with known doses calculated after measurements with the Exradin A20 ionization chamber and using the TG-61 protocol, the calibration curve extends from 0 to 25 Gy ,and another calibration curve from 0 to 10 Gy was performed more suitable for skin doses.