ESTRO 2021 Abstract Book

S1322

ESTRO 2021

J. Smeulders 1 , T. Gevaert 1 , T. Everaert 1 , A. Gutierrez 1 , C. Ferro Teixeira 1 , A. Bom 1 , M. Boussaer 1 , B. Engels 1 , M. De Ridder 1 1 UZ Brussel, Radiotherapy, Brussels, Belgium Purpose or Objective To assess impact of two dose calculation algorithms, and minor differences in beam modelling, on single isocenter multiple brain met (MBM) stereotactic (SRS) patient-specific QA (PSQA). Secondly, assessing the feasibility of incorporating portal dosimetry into the SRS PSQA program. Materials and Methods Verification plans were calculated for SRS MapCHECK for two MBM SRS plans, created with Brainlab ® Elements v2.0 and v3.0. Each verification plan was calculated using different calculation methods: pencil beam (PB2.0), pencil beam with fine-tuned beam model (PB3.0) and Monte Carlo (MC). Eclipse’s Portal Dosimetry workspace was used to calculate portal image predictions. For both modalities, gamma evaluation with 10% threshold and 3%/1 mm passing criteria was performed field-by-field as well as on the composite measurement. Results Average gamma passing rates (GPR) for field-by-field measurements with SRS MapCHECK were 82.20% ± 24.55%, 84.11% ± 23.28% and 94.82% ± 4.48% for doses calculated with PB2.0, PB3.0 and MC respectively. Field-by-field EPID measurements yielded average GPR of 99.03 ± 1.38%, significantly higher compared to SRS MapCHECK (MC3.0) suggesting high ratios of false positives. Field-by-field GPR was dependent on the maximum delivered dose per arc for both PB algorithms. Composite measurements yielded similar results, however portal dosimetry GPR were not significantly different from SRS MapCHECK. In general, PB algorithms yielded significantly higher GPR compared to MC and fine-tuning of radiological leaf shift turned out to be significant in terms of GPR. Portal dosimetry measurements showed to be dependent on target size. Conclusion PB algorithms may not be sufficient to accurately predict dose distributions in single isocenter MBM SRS plans, especially in low dose regions. MC algorithms are recommended for plan verification. Minor fine-tuning of radiological leaf shifts in the beam model significantly improves delivery accuracy. Portal dosimetry is not suitable for field-by-field plan verification, however can be implemented with caution for composite plan comparisons. PO-1600 Quasi invariance of a point source image response in the conjugate view method with depth A. Montes Uruen 1 , A. López Romero 2 , C. Escalada Pastor 3 1 Hospital Universitario Puerta de Hierro, Medical Physics, Majadahonda, Spain; 2 Hospital Universitario Puerta de Hierro, Medical Physics , Majadahonda, Spain; 3 Hospital Universitario Puerta de Hierro,, Medical Physics , Majadahonda, Spain Purpose or Objective The conjugate view method uses the geometric mean of the counts recorded in the anterior and posterior views of the patient to estimate the accumulated activity in each region of interest. Due to its simplicity, it is still widely used in the dosimetry of many molecular radiotherapy treatments, such as CDT, hyperthyroidism, etc. The correspondence between the recorded counts distribution and the real distribution that produces it, depends fundamentally on the point spread function (PSF) of the gamma camera, which varies with the distance from the source to the detector and the depth within the scattering material, unknown parameter in this type of method. The objective of this work has been to find an alternative parameter PSF ', derived from the PSF but quasi- invariant with depth, which would allow determining the real planar distribution of the activity by deconvolution of the conjugate product. Materials and Methods All measurements were made with a General Electric Infinia Hawkeye II gamma camera. In order to obtain the PSF, acquisitions of a point source of 50 MBq of I-131 have been made, and the dispersing material was manufactured with solid water plates of RMI of different thicknesses. The DICOM images obtained have been analyzed by means of a script of own development in Python language. To obtain the PSF under different conditions, point source acquisitions were obtained by varying the source- detector distance and the thickness of the dispersing material. The acquisitions were made with the clinical parameters of energy and window 327-400 KeV and 200 s of time. The images were exported in DICOM format and entered in the script, which adjusted each distribution of counts into a two-dimensional bi-Gaussian function with parameters A0, s0, A1, and S1. From these parameters, an empirical function of the PSF was modeled for any acquisition condition within the determined range. Fig. 1. With the calculated PSFs, the anterior and posterior distributions of accounts produced by a point source in different distance and depth conditions were simulated, and the equivalent PSF was obtained for each conjugate product.