ESTRO 2021 Abstract Book

S1440

ESTRO 2021

selected indications.

[1] Mazal A et al. Quality assurance in protontherapy. Bull.CancerRadiother. 1996. [2] PTCOG Aspects of Safety in Particle Therapy, may2016, www.ptcog.ch

PO-1714 Commissioning of an OSLD dosimetric system for level I postal audits for radiotherapy in Argentina A. Ruiz 1 , J. Irazoqui 2 , S. Bianchini 1 , D. Tolabin 1 1 Fundación Instituto de Tecnologías Nucleares para la Salud (INTECNUS), Servicio de Radioterapia, San Carlos de Bariloche, Argentina; 2 Fundación Instituto de Tecnologías Nucleares para la Salud (INTECNUS), Servicio de Radioterapia, San Carlos de Bariloche, Argentina Purpose or Objective The aim of this work was to characterize, commission and evaluate the applicability of an optically stimulated luminescence dosimetry (OSLD) system for external photon radiation therapy level I postal audits in Argentina. Materials and Methods The dosimeters used were the OSL nanoDots and the LANDAUER Inlight MicroStar reader. The irradiations were performed mainly on an Elekta Synergy linear accelerator. The dosimetric system characterization was based on the analysis of several parameters of interest, such as: reader stability, sensitivity, bleaching, depletion, fading, linearity, energy and wedge dependence. Appropriate correction factors were also determined for the necessary cases. The reader’s response was also characterized and a calibration coefficient was obtained to determine the absorbed dose to water from the measurements with OSLD. Finally, an audit system was implemented and evaluated in reference conditions and other than those of reference, with the collaboration of a series of Radiotherapy Departments across the country. Results The results obtained from the characterization of the dosimetric system indicate that the sensitivity of the detector batch has a coefficient variation of less than 1%; the bleaching to obtain a signal loss of 99% was determined to be of 24 hours; the signal depletion per reading was 0.05%; the signal fading has a logarithmic loss response, we determined a signal fading of 2% after 30 days; the response of the detectors was studied and found to be linear with respect of the absorbed dose in the range of 100 to 500 cGy. Regarding energy dependence, we verified that the sensitivity of the OSLD decreased for 15 MV compared to 6 MV; detectors response was also found to be affected by the usage of a wedge filter. All these results are consistent with the ones reported in the literature. The results of the postal audit carried out show that all the machines of the collaborating centers are within the pre-established acceptance limit of ±5% between the reported dose and the measurement with the OSLD, except for one linear accelerator, which counted with a flattening filter free (FFF) modality for 6 MV. The detectors response for this condition should be further analysed in the future. The combined typical uncertainty estimate of the dosimetric system with k=1 was ±2.44 %. Conclusion We conclude that the commissioned dosimetric system is suitable for level I postal audits of photon beams, under reference conditions and other than that of reference in external radiotherapy, being its applicability feasible for a nationwide scope. PO-1715 Accuracy of a dedicated single isocenter stereotactic radiosurgery system for multiple brain lesions T. Gevaert 1 , A. Gutierrez 1 , J. Smeulders 1 , M. Boussaer 1 , T. Everaert 1 , A. Bom 1 , C. Ferro Teixeira 1 , B. Engels 1 , M. De Ridder 1 1 UZ Brussel, Radiotherapy, Brussels, Belgium Purpose or Objective The purpose of this study was to determine the dosimetric effects of translational and rotational intrafractional errors during stereotactic radiosurgery (SRS) when treating multiple brain lesions with a single isocenter technique. Retrospective correction and lesion data were used to calculate the impact of intrafractional errors on the dose delivery and the influence of lesion parameters such as volume or lesion-to- isocenter distance. Materials and Methods Lesion data and intrafractional uncertainties were gathered from 31 SRS treatments for a total of 103 brain metastases with varying sizes, locations and lesion-to-isocenter distances. 6DOF intrafraction errors were identified using stereoscopic x-ray images. The effect of motion in dose distribution was calculated applying the intrafractional displacements to the RT Doses corresponding to each of the treatment arcs. The accumulated dose of the transformed RT Doses was used to evaluate dose distribution and target coverage. Results Mean centroid deviation errors were found below 1mm for most of the lesions. Out of 98 lesions, 25 do not fulfill the >95% coverage of the prescribed isodose condition when intrafractional motion is applied. Statistical tests showed poor correlation with lesion-to-isocenter distance, but significant (p<0.05) for lesion volume. For lesions larger than 1cc, only one missed >95% coverage. 96,4% of the targets that failed to achieve coverage were smaller than 1cc, amounting to 24,5% of the total number of metastases. Conclusion Lesion volume was identified as the main cause of coverage loss in the SRS treatments included in this study. A 0mm margin can be implemented for lesions larger than 1cc when an adequate patient immobilization and positioning strategy is implemented. Our findings suggest that a 1mm margin is only necessary for lesions smaller than 1cc.