Abstract Book

S1057

ESTRO 37

Purpose or Objective To evaluate effects of inaccurate OF measurements on Fractioned Stereotactic Brain Radiotherapy (FSBRT) VMAT plans. Dependence on TPS, Linear Accelerators and treatment targets (volume and shape) were evaluated. Material and Methods Six centres were enrolled for a multi-institutional study. FSBRT VMAT plans were elaborated using four TPS (RayStation, Pinnacle, Monaco and Eclipse) and 6 MV photon beams. These TPS were coupled with two Elekta Beam Modulator, one Elekta MLCI2, and three Varian TrueBeam linear accelerators. The multi-institutional study was drawn on different phases. The first phase was oriented to establish a baseline for each center (dependent on TPS-Linac coupling and planner’s experience). For the purpose a CT data set of one patient underwent VMAT treatment for a single brain metastases was selected and anonymized for data sharing. The planning target volume (PTV) and organs at risks (OARs) were previously contoured by a single radiation oncologist. Planning rules (PTV coverage, OARs sparing), beams geometry, energy and calculation grid (1mm) were univocally fixed. DICOM_RT dose files, extracted by each TPS were imported into PlanIQ software (SUN NUCLEAR corporation) and analyzed. A quantitative total scorecard based on clinical goals (Quality Assurance goals) was used to measure the performance of each treatment plan. The second phase was oriented to evaluate the impact of inaccurate OF dose measurements on intra-center calculated dose. OFs for the smallest fields sizes used, was changed for known increments from the OF baseline values, treatments units re-commissioned in the TPS and plans re-calculated. The PlanIQ analysis were compared between the first and the second phase. Finally the dependence of OF values on TPS-Linac coupling was evaluated. Results PlanIQ analysis was performed identifying plan quality metric components and setting its goals (eg. at least 95% of PTV volume with 100% of the prescription dose, PTV Conformality Index, OARs sparing in terms of max dose or percentage of volume). A cumulative raw score for each center was obtained to evaluate the performance reached (Fig.1).

The prescribed dose in all cases was 25 Gy delivered in 10 days. All patients underwent both a CT and a MRI in order to delineate the different organs. Contouring of the brain and the OAR was made following the RTOG indications. A 5 mm PRV was defined for the hippocampus. Finally, the PTV was created adding a 5 mm margin to the brain and subtracting the hippocampal PRV. For inverse planning optimization, Dose Volume Optimization of Varian Medical Systems was used. Each treatment consisted of a total of 11 fields with 4 couch angles: 0, 45, 90 and 315 degrees. Results Target coverage and dose received by the hippocampus in each treatment are shown in Table 2.

Comparing the previous results with the specifications given by reference clinical trials, it can be seen that all treatments satisfied the minimum goal of D95 ≥ 95%. Also, all the values of D 2 were below the most demanding limit of 107%. On the other hand, D 98 results didn’t comply with the optimal treatment specifications given in Table 1, although they were well above the acceptable treatment ones. Finally, doses received by hippocampus and the rest of OAR were under the limits proposed by reference clinical trials. Conclusion Our experience shows that it is possible to achieve a good PTV coverage while reducing the dose received by the hippocampus, compared to classical PCI treatments. Taking into account the good preliminary results obtained in clinical trials, the future of this planning technique looks promising. EP-1946 Effect of small field (SF) output factor (OF) measurements on FSBRT VMAT plans M.D. Falco 1 , S. Clemente 2 , E. Cagni 3 , V. Landoni 4 , S. Russo 5 , C. Talamonti 6 , A. Alparone 7 , A. Vinciguerra 1 , C. Fiandra 8 1 University "G. D’Annunzio”- SS. Annunziata Hospital, Radiation Oncology Department, Chieti, Italy 2 AOU "Federico II", Medical Physics Department, Napoli, Italy 3 AUSL-IRCCS, Medical Physics Department, Reggio Emilia, Italy 4 Istituto Nazionale Tumori Regina Elena, Medical Physics Department, Roma, Italy 5 Azienda USL Toscana Centro, Medical Physics Department, Firenze, Italy 6 AOU Careggi, Biomedical Experimental and Clinical Science "Mario Serio" Department, Firenze, Italy 7 Tecnologie Avanzate, Medical Physics Unit, Torino, Italy 8 Università di Torino, Radiation Oncology Department, Torino, Italy

Conclusion All plans passed our fixed QA goals, with slight differences depending on TPS, machine and planner ’s experience. For the second phase, a preliminary study is under way with the aim of defining increment values of SF OFs (datasets from AIFM national studies with different detectors was taken as a reference to establish OF incremental values depending on field size). Moreover the recruitment of centers is under review for the study purposes.