ESTRO 2024 - Abstract Book

S3426

Physics - Dose calculation algorithms

ESTRO 2024

Purpose/Objective:

Transmission through the round leaf ends in the Multi-Leaf Collimator (MLC) is a problematic issue to take into account when commissioning algorithms in a Treatment Planning System. Varian works with a parameter called Dosimetric Leaf Gap (DLG) and propose its own method of measuring it, but the actual value should be the one that makes the dose calculations as accurate as possible. The objective of this study is to evaluate how accurate the Varian-proposed method of measuring the DLG is, as well as comparing the value of DLG obtained to the one reached through four other ways. It is also discussed the variation of such parameter with MLC modulation and the planning technique (IMRT/VMAT).

Material/Methods:

Five different methods of measuring the DLG for Acuros (AXB) were used. 1) Varian-proposed method, which consists in several Sweeping-Gap plans (with 2 to 20 mm gaps) measured with an ionization chamber (PTW-Semiflex-3D). 2) Creation of 20 plans (10 IMRT and 10 VMAT) with different field size, pathologies, dose per fraction and MLC modulation. Calculate each of those plans with ten different DLGs (from 0 to 2 mm). Measuring those plans with portal dosimetry (PD) and evaluating a 2mm–2 % (local) gamma passing rate (GPR). Find the value for which the GPR maximizes. 3) Measurement of those same 20 plans with Octavius 4D (srs1600) and also evaluating a 2mm–2 % (local) GPR. 4) Measurement of a dose point of those same 20 plans with an ionization chamber (PTW-PinPoint-3D). 5) Measuring a Picket Fence plan with Octavius 4D (srs1600) The obtained DLGs for our treatment energies are: For 6X we obtained 0,84 mm (Varian method); 1,15 mm (PD); 1,35 mm (Octavius1600); 0,95 mm (ionization chamber) and 1,35 mm (PicketFence) For 6FFF we obtained 0,74 mm (Varian method); 0,05 mm (PD); 1,50 mm (Octavius1600); 0,98 mm (ionization chamber) and 1,30 mm (PicketFence). For 10X we obtained 0,95 mm (Varian method); 1,60 mm (PD); 1,20 mm (Octavius1600); 1,45 mm (ionization chamber) and 1,25 mm (PicketFence). Results: