ESTRO 37 Abstract book

S1107

ESTRO 37

VMAT plans with 6 MV FF and 6 MV FFF were calculated using a common prescription dose of 60 Gy in 5 fractions with the same optimization parameters. These plans were recalculated in a QA plan using OCTAVIUS 4D phantom (PTW, Germany) for the verification. For the QA treatment plan deliver, we used a new platform (Modus Medical, courtesy of the Hospital Clínico Universitario de Santiago de Compostela) that allowed move OCTAVIUS 4D in craniocaudal direction whereas the device is turning synchronized with the gantry. The period and range of platform movement were the same those GTV parameters acquire in the simulation for each patient. The ionization chamber array used was Octavius SRS1000 (PTW, Germany) with 977 liquid-filled ionization chambers 2x2 mm 2 active area and 2 mm of resolution. Trying to combine the experimental uncertainty with the possible deviations due to the movement, it was decided to use the gamma criterion. All plans were delivered in a TrueBeam STx HD120 MLC (Varian, CA) with 6DoF couch. Results In the graphs, the value of the percentage of points that fulfill the gamma criterion <1 is represented for the treatments calculated with FF and FFF in function of the amplitude of the movement of the GTV. Figure 1 shows the results with tolerance values for dose of 3% of central PTV dose and distance to agreement of 3 mm. For this tolerance, 5 of the 8 patients analyzed meet the criterion in more than 95% of the points. These 5 patients have amplitude of less than 6 mm. The results shown in Figure 2 are the analysis for a tolerance for dose of 2% dose and distance to agreement of 2 mm. In this case, 4 of the 8 patients analyzed, meet the gamma criteria with more than 90% of the points.

the 6 MV VMAT FFF in the treatments to decrease the time of treatment. EP-2023 Commissioning of a surface imaging system for beam gating at an Elekta Linac V. Batista 1,2 , S. Albert 3,4 , B. Rhein 1,2 1 Heidelberg University Hospital, Clinic for Radiation Oncology and Radiation Therapy, Heidelberg, Germany 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany 3 Duale Hochschule Baden-Württemberg, Radiation Protection, Karlsruhe, Germany 4 Studsvik GmbH & Co.KG, Radiation Protection, Pforzheim, Germany Purpose or Objective Align RT is a clinical system used to detect the respiratory motion through the detection of breathing changes in the patient’s surface. Currently, the system is used for patients under deep-inspiration breath-hold. The aim of this work was to evaluate the system performance and limitations for respiratory beam-gating, either for volumetric modulated arc therapy (VMAT) or three- dimensional conformal radiotherapy (3DCRT). Material and Methods The measurements were carried out using the CIRS Dynamic Thorax Phantom at the Elekta VersaHD linear accelerator (Linac). The phantom provides a repeatable three-dimensional target motion inside a tissue equivalent phantom and a surrogate arm gives the external motion to be detected with AlignRT. Inserts for an ionization chamber and a radiographic film were used to measure the delivered dose in the static, no-gating and gating schemes. The consignment and validation of the respiratory gating was subdivided into four parts: 1) safety and precision of Align RT; 2) evaluation of the configuration parameters that affect the respiratory gating irradiation; 3) performance of the system for VMAT and 3DCRT plans; 4) definition of a patient verification quality assurance (QA) workflow. Results The combined use of AlignRT with Elekta Linacs showed to be feasible and led to an improvement of the delivered dose distribution (see figure 1 and 2). However, in order to achieve a robust, clinical significance and accurate treatment delivery some aspects need to be considered: a) stability of the system during long acquisition periods, b) gantry angles where the acquired signal is strongly affected by the limited coverage of some camera; c) for small breathing magnitudes the signal detection accuracy is reduced; d) for irregular breathing patterns the response time of the system affects the irradiation within the gating-window; e) the defined Region-of-interest should be large enough to result in a statistical relevant measure; f) for a VMAT irradiation the gantry speed should be reduced to include the system response time; and g) the relevance and definition of beam-gating for each patient need to be pre-treatment evaluated through a patientspecific QA.

Conclusion For amplitudes of movement of the lesion in the cranio- caudal direction less or equal to 6 mm, the verification with the system composed by by the experimental platform of movement and the 4D OCTAVIUS with the detector array 1000 SRS, gives the conclusion that the results for treatments of VMAT with flattering filter and without filter are equivalent, being able to make use of