ESTRO 2021 Abstract Book

S1282

ESTRO 2021

The aim of this work was to evaluate this SL system as a pre-positioning system. The evaluation of the intra- fraction monitoring system is beyond the scope of this work.

Materials and Methods The ED system incorporated in a Varian TrueBeam STX accelerator was tested using the Brainlab cranial verification phantom with its mid-brain radiopaque sphere. A simulation scan (slice thickness of 1,25mm) of the phantom was obtained in a General Electric LightSpeed CT, using the Cranial 4Pi Immobilization system. A treatment plan with 0º, 90º, 180º and 270º gantry angles beams collimated by the 15 mm Brainlab circular cone was created in iPlan 4.5 treatment planning system. The isocenter of the plan was placed at the center of the radiopaque sphere. Repeatability of the surface positioning system was evaluated placing the phantom with the same immobilization as above, letting the system to perform the movements to bring the couch to the treatment position. The procedure was performed five times without moving the phantom between them. SL system accuracy was determined using a hidden target (HT) test acquiring integrated images with the linac electronic portal image system, once the phantom was positioned at the isocenter, according to the SL system. The HT test was performed three times. For each HT test the assembly of the phantom and its immobilization system was repeated. The displacements that stereoscopic KV x-ray and CBCT positioning systems calculate after the surface SL prepositioning of the phantom were registered to obtain the accuracy of the SL system independently. After applying the displacements calculated by each of the systems, a HT test was done to assess their suitability as reference systems. Results Results are shown in table 1. SL system repeatability was better than 0,3 mm in all axes. Results of the two independent methods used to measure the accuracy were consistent and showed an accuracy below 1,2mm in the lateral and longitudinal axes, while on the vertical was higher than 3 mm. The HT test for the CBCT and the stereoscopic kV x-Ray systems showed an accuracy below 0,3 mm in all three axes for both systems. Similar accuracy is referenced in the bibliography.

Conclusion The ED SL system is repeatable and accurate to pre-position the patient at the isocenter. Nevertheless, it is necessary internal information from CBCT or kV X-Ray images to achieve submillimeter accuracy.

PO-1557 Automated Dose Monitoring On Target And Critical Organs Doses For Head And Neck Radiotherapy N. Kucuk 1 , O. Goren 2 , M. Kamislioglu 3 , E. Kucukmorkoc 1 , E. Sanli 1 , H.B. Caglar 1 1 Anadolu Medical Center, Radiation Oncology, Kocaeli, Turkey; 2 Uskudar University, Molecular Biology, Istanbul, Turkey; 3 Bandirma Onyedi Eylul University Vocational School of Healty Services, Medical Imaging, Balikesir, Turkey Purpose or Objective This study aims to reveal dosimetric changes in target and critical organ doses caused by physical changes due to varying anatomy for various reasons in patients undergoing head and neck (H&N) radiotherapy and to examine the necessity of adaptive radiotherapy (ART) using automated dose monitoring software. Materials and Methods A total of 10 H&N patients treated with helical IMRT were selected for the study. Treatment plans were prepared according to our clinical treatment planning objectives created for all target and critical volumes (OARs) in H&N irradiation. Treatment planning software (PreciseART TM ) which has applications to perform offline ART analyses were used to evaluate inter-fraction anatomical and dosimetric changes. Fan beam daily megavoltage computed tomography (MVCT) images were used to re-calculate each fraction dose with the help of the sinogram and the CT-number-to-physical-density table associated with the MVCTs. An automated dose monitoring process was initiated after each fraction. Daily dose, accumulated dose, and projected dose (an estimation of the dose that will be delivered if the treatment is not re-planned) were evaluated, and it was examined whether the predefined metrics are met. ART was performed when a clinically significant deviation occurs in targets or OARs for all patients. Results When all patients were examined, the decision for ART was made in the 18th (9-27) fraction. No significant differences were observed in projected doses for all targets and OARs on the first day of treatment. When the expected and accumulated doses of PTV volumes receiving 70Gy (PTV70), 60Gy (PTV60) and 56Gy (PTV56) were examined on the day of ART decision was made, the maximum point dose difference was calculated as 4.7% (p=0.005), 11.4% (p=0.005) and 6.2% (p=0.046), respectively. While the expected and accumulated maximum dose differences in the right and left parotid mean doses were 15.1% (p=0.007) and 10.4%(p=0.028), respectively, on the day of the ART decision, the maximum difference in the spinal cord maximum point dose