Purpose: The aim of this work is to study the dosimetric parameters of newly introduced 2.5 MV imaging x-ray
beam used as inline imaging to do setup verification of the patient undergoing radiation therapy. As this x-ray beam
is in megavoltage range but comprises of a lower energy spectrum. It is essential to study the pros and cons of 2.5
MV imaging x-ray beam for clinical use.Methods: The mean energy was calculated using the NIST XCOM table
through MAC. Profile analysis was done using RFA to understand the percentage depth dose, degree of unflatteness,
symmetry, penumbra and out of field dose. Dose to skin for the 2.5 MV x-ray beam was analysed for field sizes 10x10
cm2, 20x20 cm2, 30x30 cm2. Leakage measurements for treatment head and at the patient plane were done using IEC
819/98 protocol. Finally, the spatial resolution and contrast were analyzed with and without patient scatter medium.
Results: The MAC at 15 cm off-axis was found to be lower than that at the CAX. Similarly, there was a decrease in
mean energy from 0.47 MV to 0.37 MV at 15 cm off-axis. The reduction of mean energy towards off-axis is lower than
the other high energy MV x-ray beams. The tuned absolute dose of 1 cGy/MU is consistent and within < ±1 %. The
relative output factors were found to be in correlation with Co-60. The beam quality of 2.5 MV x-ray beam was found
to be 0.4771. The profile parameters like the degree of unflatness of the 2.5 x-ray beam were studied at 85 %, 90 %,
95 % lateral distances, and the penumbra at different depth and field sizes are higher than the 6 MV treatment beam.
In addition, out of field dose also drastically increases to a maximum of up to 30 % laterally at 5cm at deeper depths.
The skin dose increases from 48.51 % to 88.15 % from 6 MV to 2.5 MV x-ray beam for the field size 10x10 cm2. Also,
the skin dose increases from 88.15 % to 91.78 % from the field size 10x10 cm2 to 30x30 cm2. Although the measured
leakage radiation for 2.5 MV x-ray beam at the patient plane and other than patient planes are with the tolerance limit,
an increase in exposure towards gantry side compared to other areas around treatment head and the patient plane may
lead to more skin dose to head and chest while imaging pelvis region. The MLC transmission of 2.5 MV x-ray beam
such as inter, intra and edge effect are 0.40 %, 0.37 % and 11% respectively. The spatial resolution of 2.0, 1.25 and
0.9 LP/mm was observed for KV, 2.5MV, and 6 MV x-ray beams. The spatial resolution and contrast of 2.5 MV x-ray
beam are superior to 6 MV x-ray beam and inferior to KV x-rays. Conclusions: The 2.5 MV x-ray imaging beam is
analysed in view of beam characteristics and radiation safety to understand the above-studied concepts while using
this imaging beam in a clinical situation. In future, if 2.5MV x-ray beam is used for treatment purpose with increased
dose rate, the above-studied notions can be incorporated prior to implementation.