Repeat CT for evaluation of inter- and intrafraction changes during curative thoracic radiotherapy; An exploratory pilot study.

Radiotherapy (combined with chemotherapy) is increasingly applied in the
curative treatment of tumours located in the thoracic region, including
esophageal cancer, lung cancer, breast cancer, and (non) Hodgkin lymphoma.
Accurate treatment planning and dose delivery is essential for radiotherapy to
be effective. However, accuracy is challenged by tumour and organ motion both
within the fraction (intra-fraction movements) and from fraction to fraction
(inter-fraction movements or changes). At present, radiotherapy treatment
planning is typically performed on one (3D or 4D) planning-CT scan before start
of the treatment. However, inter-fraction set up variations and organ motions
(including baseline drift) can lead to differences between the calculated dose
distribution on the planning-CT and the radiation dose actually received by the
tumour and normal organs (actual given dose) [1]. In photon radiotherapy,
assessment of these effects is essential to determine margins from clinical
target volume (CTV) to planning target volume (PTV) in order to irradiate the
tumour adequately while minimizing the dose to the organs at risk (OARs).
For proton beam therapy (PBT), knowledge of tumour and organ motion will be
even more important [2,3]. The major potential advantages of PBT for tumours in
the thoracic region in terms of prevention of radiation-induced side effects
are challenged by the respiratory motion of the tumour, breast, esophagus,
diaphragm, heart, stomach, and lungs. Setup errors and inter- and
intra-fractional organ motion cause geometric displacement of the tumours and
normal tissues, which blurs the dose gradients from target volume to normal
tissue. Furthermore, it can result in changes in tissue densities in the beam
path, which can alter the position of the Bragg peaks, in turn leading to
distorted dose distributions. If pencil beam scanning techniques are used to
treat moving tumours, there is interplay between the dynamic pencil beam
delivery and target motion. This phenomenon can cause additional deterioration
of the delivered dose distribution, usually manifesting as significant local
under and/or over dosage [4]. It is therefore essential to incorporate
motion-related uncertainties during treatment planning [5].
In this study, we want to evaluate the impact of inter-fraction tumour and
organ motion on photon and proton radiotherapy treatment planning - while
taking into account intra fraction movements - in order to create robust
intensity modulated photon- and/or proton treatment plans (IMRT, IMPT).

To evaluate the impact of inter-fraction tumour and organ geometrical
dislocation for moving tumours on photon and proton radiotherapy treatment
plans in order to create robust intensity modulated photon- and/or proton
treatment plans.

Pilot-study

During the radiotherapy treatment course, patients will undergo weekly repeat
planning CT scans in treatment position without contrast agents in order to
evaluate the impact of inter-fraction tumour and organ motion.
The additional radiation dose of these 3-6 extra 4D-CT*s and a maximum of 10
cone-beam CTs (7mSv) is extremely low (3-6 x 25-30mSv) compared to the
therapeutic radiation dose (30-60 Gy). The risks are therefore negligible and
the burden is low.