ESTRO 2024 - Abstract Book

S4246

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

1257

Digital Poster

Assessment of the delivered dose discrepancies due to the use of gating systems in Versa HD linacs.

Artur Sampayo-Muñoz, Daniel Díaz-Martín, Abel Niebla-Piñero, Breixo Carmelo Portas-Ferradás

Hospital Universitario Nuestra Señora de Candelaria, Servicio de Radiofísica y Protección Radiológica, Santa Cruz de Tenerife, Spain

Purpose/Objective:

The aim of this study is to investigate whether the use of gating in Elekta Inc.'s Versa HD accelerators has effects on the delivered dose distribution in hepatic and pulmonary SBRT treatment plans and to evaluate whether the observed differences in the plans are clinically significant.

Material/Methods:

For this purpose, we have delivered 20 treatment plans -18 DCA and 2 VMAT- for pulmonary and hepatic SBRT without gating, and then with gating using Elekta's Response system, employing software called Response Service Tool from Elekta, which allows the establishment of irradiation and non-irradiation periods simulating radiation interruptions of a gating system. We set an irradiation period of 1005 milliseconds and a non-radiation pause of 2000 milliseconds, simulating a short breathing cycle to push the effect to the limit. Measurements were conducted using the PTW Freiburg's Octavius 4D phantom and the liquid-state ionization chamber array 1000 SRS. As this phantom rotates according to the position of a Bluetooth inclinometer anchored to the gantry, we understood it was appropriate to measure the plans by collapsing the treatment plan to the 0-degree gantry position to avoid possible measurement differences due to discrepancies between the gantry angle and the phantom angle due to the frequent pauses in the delivery. In this way, we account for variations due to potential fluctuations in dose rate introduced by frequent radiation interruptions, as well as those that might arise due to the position of the MLC leaves, but not those due to the possible discrepancy between gantry angle and phantom angle. To assess discrepancies in dose distributions between the two measurements of each plan, we employed PTW's Verisoft software using a gamma index, with a deviation criterion of 1 mm in position and a 2% deviation in local dose, as recommended [1] for the evaluation of clinically significant discrepancies in SBRT treatments.

Results:

Our findings show that, while it is possible to observe differences in dosimetric distributions of plans delivered with and without that simulated gating (which can be seen in the average dose variation point by point or the degree of compliance with stricter gamma criteria such as 1mm/1% or 0.5mm/0.5%, as shown in the figure), these differences do not seem to be clinically significant as compliance with the 1mm/2% gamma criterion is 100% for all analyzed treatment plans, regardless of plan modulation or whether it is DCA or VMAT.