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Objective: In IMRT techniques, a Cone Beam CT acquisition preceding
the radiation therapy is essential for reliable patient positioning. Since it
presents a further dose to the patient and requires additional time, we
examined if it is possible to reduce the number of acquisitions from a
daily to weekly cycle in patients with small displacements in the first
therapy week defined as displacement of ≤ 4mm and a st.dev. of ≤ 2mm
in x-,y-, and z-axis.
Materials and Methods: For 34 consecutive head and neck patients,
fixed with a thermoplastic mask pretreatment Cone Beam CT have been
acquired at a Elekta Synergy accelerator. The patient displacements have
been recorded for at minimum five weeks in
x-, y- and z-axis; The mean deviation in the first week after therapy start
were compared to later deviations.
Results: The mean displacements in x-, y- and z-axis were 1.9 ± 0.9, 1.8
± 0.7 and 2.1 ± 1.1mm during the 1st week. 26 patients fullfilled our
criterions of small displacements. In 18 out of 34 Patients the mean
deviations in the first treatment week gave no reliable information about
displacements in the following weeks, so a patient specific positioning
reproducibility was not given.
Conclusion: From these data we decided to continue performing Cone
Beam CTs on a daily basis for the entire course of treatment in our head
and neck patients. Furthermore the decision was made to change the
fixation device in favour of a more stabile and accurate mask system.