SU-F-J-179: Commissioning Dosimetric Data of a New 2.5 Megavoltage Imaging Beam from a TrueBeam Linear.

PURPOSE: Recently a new 2.5 megavoltage imaging beam has become available in a TrueBeam linear accelerator for image guidance. There is limited information available related to the beam characteristics. Commissioning dosimetric data of the new imaging is necessary for configuration of the beam in a treatment planning system in order to calculate imaging doses to patients resulting from this new imaging beam. The purpose of this study is to provide measured commissioning data recommended for a beam configuration in a treatment planning system.

METHODS: A recently installed TrueBeam linear accelerator is equipped with a new low energy photon beam with a nominal energy of 2.5 MV which provides better image quality in addition to other therapeutic megavoltage beams. Dosimetric characteristics of the 2.5 MV are measured for commissioning. An ionization chamber was used to measure dosimetric data including depth-dose curves and dose profiles at different depths for field sizes ranging from 5×5 cm(2) to 40×40 cm(2) .

RESULTS: Although the new 2.5 MV beam is a flattening-filter-free (FFF) beam, its dose profiles are much flatter compared to a 6 MV FFF beam. The dose decrease at 20 cm away from the central axis is less than 30% for a 40×40 cm(2) field. This moderately lower dose at off-axis distances benefits the imaging quality. The values of percentage depth-dose (PDD) curves are 53% and 63% for 10×10 cm(2) and 40×40 cm(2) fields respectively. The measured beam output is 0.85 cGy/MU for a reference field size at depth 5 cm obtained according to the AAPM TG-51 protocol.

CONCLUSION: This systematically measured commissioning data is useful for configuring the new imaging beam in a treatment planning system for patient imaging dose calculations resulting from the application of this 2.5 MV beam which is commonly set as a default in imaging procedures.