ESTRO 2021 Abstract Book

S1344

ESTRO 2021

Materials and Methods Ten helical Tomotherapy plans from patients that underwent SBRT/SRS treatments were considered. The prescription dose varied between 7 to 21 Gy per fraction. For pre-treatment QA verification, the treatment plans were recalculated in the Tomotherapy phantom (Cheese phantom) and both EBT3 and EBT-XD films were simultaneously positioned in a coronal plane. Point dose measurements were also performed using an Exradin A1SL ionization chamber (IC). All films were scanned on a flatbed scanner Epson Expression 10000 XL, in transmission mode, with a 48 bits colour depth, a spatial resolution of 72 dpi, and all colour correction options disabled. A glass compression plate was used to ensure film flatness. The resulting RGB images were saved in TIFF format and processed using an in-house developed MATLAB R2010a routine. The calibration curves established for the red, green and blue scanner colour channels were rescaled by using two reference film strips (one unexposed and the other irradiated to a dose 10% over the maximum dose). Dose determination from film response was done using single channel (red, green for EBT3 and red for EBT-XD) and triple channel dosimetry. The agreement between planned and film measured dose distributions was assessed by performing global gamma analysis with a criteria of 3% global dose/2 mm, and 20% threshold in RIT113 software v5.1. The passing rate acceptance limit was 95%. The tolerance for ionization chamber measurements was ±3%. Results The IC measurements percent difference between planned and measured doses was 1.1±1.4 %, on average, for the considered plans. Results of gamma analysis using EBT3 films were consistently poorer than those for EBT- XD with both single channel and triple channel dosimetry. For EBT3 films, the average global gamma passing rates over the entire set of SBRT/SRS plans was: 85.1±15.2 % with triple channel dosimetry, 94.4±4.8 % for red channel and 95.3±4.2% with green channel. For EBT-XD films, the mean gamma passing rates were: 98.6±2.1 % with the triple channel method, and 97.5±3.6 % for red channel. Conclusion EBT-XD films show a much better agreement to planned dose distributions than EBT3, which justifies their clinical selection for pre-treatment QA of SBRT and SRS plans. The adopted irradiation setup eliminates the dependency on the plan delivery uncertainties. PO-1623 Use of OSLDs for dosimetric verification of Helical TomoTherapy dynamic field width treatment plans N. Prountzos 1 , E. Pantelis 2 , P. Papagiannis 1 , P. Karaiskos 1 , A. Moutsatsos 3 , E. Pappas 3 , N. Fotos 4 , S. Kanellopoulou 5 1 National and Kapodistrian University of Athens, Medical School, Medical Physics Laboratory, Athens, Greece; 2 National and Kapodistrian University of Athens, Medical School, Medical Physics Laboratory, Athens, Greece; 3 Iatropolis Clinic, Radiotherapy Department, Athens, Greece; 4 MediRay Inc, Dosimetry Laboratory, Athens, Greece; 5 MediRay Inc, Dosimetry Laboratory , Athens, Greece Purpose or Objective To evaluate the use of Optically Stimulated Luminescence Dosimeters (OSLDs) for the verification of Helical TomoTherapy (HT) (Accuray Inc, CA, USA) treatment plans using the dynamic jaw delivery feature. Materials and Methods A batch of nanoDot OSLDs (Landauer Inc, IL, USA) were used. The dosimeters were calibrated in terms of dose to water using a 6MV x-ray photon beam for doses up to 300cGy. Due to the helical treatment delivery fashion, radiation is incident to the dosimeters from variable directions. Therefore, Monte Carlo (MC) simulations were performed to assess potential directional dependence of nanoDot response using the EGSnrc MC code and the egs_chamber user code. To model the dosimeters, the C++ geometrical package available with the EGSnrc and information found in the literature and vendor manuals were used. For the measurement setup, RW3 (PTW, Freiburg, Germany) slabs of 13cm total thickness were used. The central slab was appropriately machined to hold the OSLDs in axial and coronal orientations. The phantom was CT scanned and two HT treatment plans were developed using the dynamic jaw delivery option of the HT systems. The first plan involved the irradiation of 5 targets situated along the central craniocaudal axis of the slab-phantom delivering 2Gy. In the second plan, 3 targets centered on the phantom craniocaudal axis lying across the Left-Right direction were irradiated delivering 2Gy on the side targets while a boost dose of 2.5Gy was planned for a sub-volume of the central target mimicking the delivery of simultaneously integrated boost (SIB). NanoDot measurements were performed at dose-plateau areas (plans 1&2), as well as regions of dynamic jaw movement (plan 1). Phantom alignment at irradiation position was performed using the image guidance capabilities of the HT system. Results MC simulations revealed mirror directional dependence of nanodot response, being up to 5% when irradiated from an angle of 90 o . Experimental dosimetry results are presented in figure 1 along with corresponding profile data exported from the PrecisionTM treatment planning system (TPS). As seen, nanoDot measurements agree with TPS predictions within experimental uncertainties of 5%.