Session Item

Over the past three decades, electronic imaging portal devices became a very important tool used for pretreatment patient verification and one of the most useful devices in the patient specific QA in radiotherapy. Potentially, using kilovoltage imaging offers better contrast of soft tissues, but the quality of the megavoltage imaging has been constantly improved. In the newest version of Varian TrueBeam medical accelerators, the energy of MV imaging beam has been reduced from 6MV to 2.5MV. New beam generated by 2.5MV nominal accelerating potential reduces the dose of radiation received by the patient during the imaging process by 50% and provides significantly better image quality. During the study we performed characterization of the 2.5MV beam by measuring percent depth-dose, beam profiles and outputs. To control the quality of the imaging we used TOR 18FG phantom for low‐contrast detectability and spatial resolution.

In order to measure PDD, Roos (type 34001) ionization chamber was used. To perform measurement of beam profiles, we used Semiflex 3D (type 31021) and to calculate outputs we used Farmer chamber (type 30013). Second method of dose verification was accomplished with EBT3 radiochromic films. We performed Winston-Lutz Test to check accordance of 2.5MV and 6MV isocentres. To evaluate image quality, TOR 18FG phantom was used. To simulate scatter effect in the patient body, slabs (5cm and 10 cm) of solid water were positioned on the beam path.

We characterized the 2.5MV beam by evaluating D_max (6.01mm), D_100mm (51.52%), D_200mm (21.83%), dose on surface (95.89%), and quality index (0.4770). In regard to control beam profiles for 40x40cm² field, we checked profiles intensity for distances ± 60mm and ± 180mm from the central beam axis. Obtained values were 96.2% and 73.3% respectively.

Fig.1 - Comparison between percent depth-dose curves of 2.5MV (blue curve) and 6MV (red curve).

We observed good agreement between radiation isocenter for 2.5MV and 6MV. In regard to imaging quality, 2.5MV showed much better image quality compared to the traditional MV beam. With 2.5MV beam it was possible to detect all low-contrast objects without any solid water layer and detect 13 more objects than 6MV beam for 10cm solid water layer. Megavoltage beam with reduced energy also demonstrated better spatial resolution.

Fig.2 - Comparison between image quality of 2.5MV and 6MV for TOR 18FG phantom, without any layer of solid water on the phantom.

This work characterized the 2.5MV beam used for patient imaging in the TrueBeam medical linear accelerators. The 2.5MV beam showed a much better imaging quality compared to the conventional 6MV energy. A significant advantage of the 2.5MV beam is the reduction of the imaging dose received by the patient during a radiotherapy session. Evaluated parameters describing the quality of the beam were used to develop quality control protocols for clinical practice.