ESTRO 38 Abstract book

S119 ESTRO 38

and 5 cm. An intercomparison of output factors was done using different detectors like microdiamond, unshielded diode, micro chamber and also film dosimetry to set against A1SL chamber response. Once more the information conveyed in TRS 483 has been abundantly explored to reach a sustained way of performing relative dosimetry in HT. Results In agreement with values from literature, an average value of 0.9962±0.0040 has been obtained for the global correction factor of Exradin A1SL chamber to be applied in absolute dose determination for helical Tomotherapy. Concerning relative response, output factors obtained with different detectors were within agreement, given the level of uncertainty. Considering film output factors as free of corrections, the average value for A1SL output factors corrections was 1.000±0.007. Conclusions The results of the present work justify Accuray practice of not correcting the A1SL chamber reading for quality and geometry in absolute dose determination in Tomotherapy as the derived factor is compatible with unity, within the associated uncertainty. Also in relative measurements, the ratio of A1SL chamber readings do not need further correction to be taken as output factors for the considered small clinical field sizes when normalized to the machine specific reference field in HT. References: [1] Alfonso R, Andreo P, Capote R, Huq MS, Kilby W, Kjäll P, et al. A new formalism for reference dosimetry of small and nonstandard fields. Med Phys 2008; 35:5179–86. [2] Thomas SD, Mackenzie M, Rogers DWO, Fallone BG. A Monte Carlo derived TG-51 equivalent calibration for helical TomoTherapy. Med Phys 2005; 32:1346–53. [3] Sterpin E, Mackie TR, Vynckier S. Monte Carlo computed machine-specific correction factors for reference dosimetry of TomoTherapy static beam for several ion chambers. Med Phys 2012; 39:4066–72. [4] De Ost B, Schaeken B, Vynckier S, Sterpin E, Van den Weyngaert D. Reference dosimetry for helical tomotherapy: Practical implementation and a multicenter validation. Med Phys 2011;38:6020-6. [5] Gago-Arias A, Rodriguez-Romero R, Sanchez-Rubio P, Gonzalez-Castano DM, Gomez F, Nunez L, et al. Correction factors for A1SL ionization chamber dosimetry in TomoTherapy: Machine-specific, plan-class, and clinical fields, Med Phys 2012; 39:1964–70.

fields used in external photon beam radiotherapy: Summary of TRS-483, the IAEA-AAPM international Code of Practice for reference and relative dose determination,” Med. Phys. 2018; 45:e1123–e1145. [3] R. Alfonso, P. Andreo, R. Capote, M. Saiful Huq, W. Kilby, P. Kjäll, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich and S. Vatnitsky, “A new formalism for reference dosimetry of small and non-standard fields,” Med. Phys. 2008; 35:5179–5186. [4] G. Cranmer-Sargison, P. H. Charles, J. V. Trapp and D. I. Thwaites, “A methodological approach to reporting corrected small field relative outputs,” Radiother. Oncol. 2013; 109: 350–355. SP-0239 Following TRS 483: reference and relative dosimetry in Tomotherapy M.D.C. Lopes 1 , T. Santos 1,2 , T. Ventura 1 , M. Capela 1 1 Ipocfg- Epe, Medical Physics, Coimbra, Portugal ; 2 fct- University Of Coimbra, Physics, Coimbra, Portugal Abstract text Introduction and purpose: The joint IAEA and AAPM international code of practice (CoP) for small static fields dosimetry – TRS 483– was issued on December 2017, after almost a decade from the publication, in 2008, of a formalism that outlined the extension of the dosimetry based on absorbed dose to water to small and composite fields and to non-reference conditions [1]. Both publications rely on universally adopted codes of practice like IAEA TRS 398 and AAPM TG 51, building on the established reference dosimetry for conventional reference 10 cm × 10 cm field and extending the dosimetry down to small and non-reference fields by introducing the concept of machine specific reference ( msr ) field. This concept applies to those treatment units where the conventional reference field is not permitted. Helical Tomotherapy (HT) is one of such machines. Exradin A1SL ionization chamber, from Standard Imaging, is the chamber included in the standard dosimetry package for HT but it is not included in the list of reference dosimeters in TRS 483. The motivation for the present work was to obtain the global correction factor for A1SL chamber that would account both for quality and geometry following the recommendations of TRS 483, in the context of performing reference dosimetry in Tomotherapy and exploring the different ways and concepts embodied in the document. Also relative dosimetry with A1SL has been explored through the determination of output factors using free- correction film results and other detectors suitable for small field dosimetry. Methods and materials: According to TRS 483 the absorbed dose to water for the msr field ( f msr ) , in the beam of quality Q msr , at the reference depth in water is given by: where the chamber reading corrected for all charge collection influences (M), multiplied by the chamber calibration factor from certificate (N) has to be corrected both for beam quality and geometry by a correction factor . This global correction factor for A1SL was obtained through different approaches following TRS 483 recommendations – cross calibration with reference chamber, radiochromic film independent dosimetry and using the hypothetical reference field concept. The results were compared with published values following other approaches like Monte Carlo calculations [1,2] or alanine dosimetry [3,4].To underpinning small field relative dosimetry in HT, a set of small fields was configured through its pneumatically controlled binary multileaf collimator (MLC) for the three field slits – 1, 2.5 denoted by

Symposium: New advances in image reconstruction in CBCT

SP-0240 Breathing motion in cone-beam CT S. Rit 1 1 CREATIS CLB, Radiotherapy, Lyon, France

Abstract text Breathing motion during cone-beam CT (CBCT) acquisition blurs moving structures and causes streaks along x-rays adjacent to these structures. Correcting for these artifacts is required to improve image-guided radiotherapy and for adapting the radiotherapy of lung and upper abdominal cancers. Current clinical solutions use respiration-correlated reconstruction: a large number of projection images is sorted in respiratory phases based on a breathing signal and a 3D CBCT image of the respiratory phase is reconstructed separately from each subset of projections to yield a 4D CBCT image. More advanced breathing motion correction techniques, which make use of less projection images, can be categorized in motion-compensated and iterative 4D CBCT. The former is essentially a 3D imaging technique in which the motion is first estimated and then compensated for during CBCT reconstruction. The latter compensates the lack of