ESTRO 2020 Abstract book

S787 ESTRO 2020

Handling of the films was done following recommendations from the AAPM TG-55 report. Films have been digitized with an Epson 10000XL flatbed scanner and analyzed with the FilmQA-Pro software using a “One-Scan” protocol that employs two reference films to re-scale the calibration curve. The green color channel was chosen to measure the high doses considered. Results Absolute dose measurements with Gafchromic EBT-3 films are in accordance with ionization chamber values. The irradiation at the breast applicators surface is homogeneous with a relative standard deviation less than 3%. Percentage depth dose measurements made in water improve with increasing applicator size. The dose at 10mm is 39.9%, 37.46%, 33.4%, and 26.25% of the dose at the applicator surface for diameters of 50, 45, 40 and 35mm, respectively. Depth dose measurements with the phantom in air showed the importance of the backscatter factor at the used beam quality. Conclusion Gafchromic EBT-3 films are reliable for dosimetry at 50kV with the Papillon+ system. A reproducible and accurate method has been developed that use Gafchromic EBT-3 films for dose measurements at commissioning and during quality assurance tests. This procedure is now used in the clinic for in-vivo dosimetry during partial breast intraoperative irradiation with the Papillon+ unit. PO‐1392 Are the universal limits recommended by TG218 suitable for VMAT QA regardless of technical solution? G. Della Gala 1 , S. Russo 2 , M. Esposito 2 , S. Pini 2 , A. Ghirelli 2 , H.O. Ghafour 3 , G. Zatelli 2 1 Università degli Studi di Firenze, Specialization School in Medical Physics, Firenze, Italy ; 2 Azienda USL Toscana Centro, Medical Physics Unit, Firenze, Italy ; 3 Zhianawa Cancer Center, Sulaymani directorate of health, Sulaymaniyha, Iraq Purpose or Objective Although pre-treatment patient-specific QA is the main tool to validate the dose delivery during VMAT treatment, there is still a lack of consensus on using universal tolerance limits recommended by AAPM TG-218. The aim of this work is to determine locally based Control Limits (CL) and Action Limits (AL) and to show how limits can change due to adopted technical solution and anatomical treatment site. Material and Methods QA measurements of VMAT plans were performed for three anatomical sites with detectors array PTW Octavius1500 (46 Head&Neck, 35 Lung, 32 Breast) and with detectors array PTW Octavius729 (87 Head&Neck, 61 Lung, 26 Breast) inserted in the same PTW Octavius4D phantom. 50 of the total sample of patients were validated with both detector arrays. Measurements were compared with TPS Elekta Monaco 5.1 computed doses via 3D absolute dose γ- analysis by using PTW VeriSoft software 6.1. γ passing rate and γ-mean were calculated under several criteria (3%/3mm, 3%/2mm, 2%/2mm). Both global (90% of maximum dose) and local normalizations were adopted. According to the AAPM Report TG-218, the procedure to set locally based AL (eq. 1, 2) and CL (eq. 4) has been implemented by using the Statistical Process Control (SPC) analysis. In eq. 1-4, β=6.0 (suggested value), x-mean and σ² are the process mean and variance respectively (i.e., the results of QA on an initial set of measurements), T is the process target (typically 100% in γ passing rate analysis; T is set equal to x-mean when a target is not defined), mR is the moving range (i.e., the absolute difference between two consecutive QA verifications in time sequence, eq. 4), n is the total number of measurements on which SPC analysis is computed. SPC analysis is represented in an i-chart, showing CL and AL, as well as Central line (i.e., x-mean).

SciMoCa and Monaco TPS showed a mean deviation of -0.1% ± 1.6% and +0.1% ± 1.6% respectively compared to the ionization chamber measurement (p<0.01). The largest deviation compared to measurements showed a very complex and high modulated stereotactic brain case with 5.7% for SciMoCa and 4.7% for the Monaco respectively. In 21 cases the difference between measurement and calculation for both systems (mean 4.1% ± 3%, respectively 3.4% ± 1.5%) exceeded ±2%. Comparing the calculated dose for this 21 cases the difference between SciMoCa and the TPS was 0.4% ± 0.9%. Conclusion The secondary dose calculation system and the treatment planning system show good agreement to the measurement results. High deviations in single cases can be connected to the size of the target volume (stereotactic fields) or to the point of measurement in steep dose gradients of VMAT plans. Further challenges in dosimetry such as secondary electron equilibrium as well as the effective point of measurement in thimble chambers need to be addressed. In Conclusion, relying on just one measurement point and the lack of a reasonable interpretation of deviations lead to arguments from Kry et. al [1] and show the advantages of a totally independent secondary dose calculation system. [1] Idependent recalculation outperforms traditional measurement-based IMRT QA methodes in detecting unacceptable plans. Stephen F. Kry, Mallory C. Glenn, Christine B. Peterson, Daniela Branco, Hunter Mehrens, Angela Steinmann, David S. Followill, Med Phys. 2019 Aug;46(8):3700-3708. doi: 10.1002/mp.13638. Epub 2019 Jun 23 PO‐1391 Dosimetric measurements with Gafchromic EBT‐3 films for the Papillon+ intraoperative therapy system C. Colnard-Hofverberg 1 , M. Gauthier 1 , J. Herault 1 1 centre Antoine Lacassagne, Medical Physics, Nice, France Purpose or Objective The Papillon+ system is a mobile kilovoltage X-ray unit for Contact Radiotherapy with a high dose-rate delivery. The first prototype was acquired by Centre Antoine Lacassagne, in Nice France. It is operated at 50kV for partial breast intraoperative irradiation using spherical applicators of diameters between 30 and 50mm. The dose- rate at the applicators surface ranges from 10 to 46Gy/min. Methods have been developed for commissioning and quality assurance prior to integration in the clinical practice. A feasibility study is presented here for using Gafchromic EBT-3 films as a dosimetry tool and to assess their utility for in-vivo dose verification during treatment with the Papillon+ unit. The procedure which has been developed for handling and analyzing the films is discussed. Material and Methods A Gafchromic EBT-3 films lot has been calibrated against an independent 50kV X-ray device to determine the dose calibration curve between 0 and of 30 Gy as the standard prescription dose for partial breast intraoperative therapy is 20 Gy. Absolute dosimetry was carried out with the calibrated film lot and compared with measurements using a PTW 0.02cm³ soft X-ray chamber type 23342 for validation. Homogeneity of the irradiation was estimated by using strips of films placed around the spherical applicators at the equator and two orthogonal meridians. Depth dose profiles for all applicator sizes have been acquired in a water tank with pieces of films parallel to the beam central axis. The same measurements were also made with a dedicated phantom in water and in air for comparison and to determine the effects of photon backscattering.