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Erschienen in:
01.04.2015 | Original Article
Image quality and dose distributions of three linac-based imaging modalities
Erschienen in: Strahlentherapie und Onkologie | Ausgabe 4/2015
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Background and purpose
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 6 MV, flattening free filter (FFF) 1 MV, and 121 kV.
Patients and methods
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.
Results
For head-and-neck cancer patients, the imaging dose from 6-MV cone beam CT (CBCT) reached maximum values at around 8 cGy. 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 13–15 cGy, and was reduced to 66–73 % for 1 MV. Planar imaging reduces the MV CBCT dose to 10–20 %. The kV CBCT dose is 15–20 % of the 6-MV CBCT dose, slightly higher than the dose from MV axes. The dose distributions differ markedly in response to the different beam profiles and dose–depth characteristics.