Image quality and dose distributions of three linac-based imaging modalities.

Abstract

Linac-based patient imaging is possible with a variety of techniques using different photon energies. The purpose of this work is to compare three imaging systems operating at 6MV, flattening free filter (FFF) 1MV, and 121kV. The dose distributions of all pretreatment set-up images (over 1,000) were retrospectively calculated on the planning computed tomography (CT) images for all patients with prostate and head-and-neck cancer treated at our institution in 2013. We analyzed the dose distribution and the dose to organs at risk. For head-and-neck cancer patients, the imaging dose from 6-MV cone beam CT (CBCT) reached maximum values at around 8cGy. The 1-MV CBCT dose was about 63-79 % of the 6-MV CBCT dose for all organs at risk. Planar imaging reduced the imaging dose from CBCT to 30-40 % for both megavoltage modalities. The dose from the kilovoltage CBCT was 4-10 % of the 6-MV CBCT dose. For prostate cancer patients, the maximum dose from 6-MV CBCT reached 13-15cGy, and was reduced to 66-73 % for 1MV. Planar imaging reduces the MVCBCT dose to 10-20 %. The kVCBCT dose is 15-20 % of the 6-MV CBCT dose, slightly higher than the dose from MVaxes. The dose distributions differ markedly in response to the different beam profiles and dose-depth characteristics.