Imaging for radiotherapy essay (1500 words)
The MR-linac is a new, promising imaging modality that has been developed for adaptive
radiotherapy, allowing for extremely precise treatments with minimal side effects. It consists
of a linear accelerator (linac) combined with a magnetic resonance imaging (MRI) unit.
Through the fusing of these two aspects, the aim is to visualise the superior soft tissue
contrast provided by MRI during the course of treatment, enabling for the visualisation of
anatomical changes that may occur during the treatment process. Through utilising MRI on a
daily basis, position verification of the patient based on soft tissue contrast can be done,
alongside replanning if necessary. On the other hand, through keeping the beam on, MRI can
be used to analyse time resolved dose accumulation and finally, the functional aspect of MRI
allows it to be used to assess the response to treatment. The MR-linac is therefore a technique
that can aid in increasing the efficiency of radiotherapy treatment. By being able to see
anatomical changes in real time and subsequently adapting the treatment plan, the plan can be
optimised to provide a better conformity to the tumour, whilst sparing normal tissues.
Overall, MRI guided radiotherapy with the MR-linac is a new imaging technique that can
drastically advance the quality of treatments.
As with normal radiotherapy treatments, the initial treatment planning process is done the
same, by using CT scans with multiple imaging modalities to define and delineate the
volumes of the tumour and organs at risk. In sites where motion is prevalent, pre-beam
imaging involving techniques such as deep inspiration breath hold are employed to assess
inter-fractional motion changes. Although it is not available yet on clinical systems, 4D-MRI
has shown interest, due to its ability to reduce delineation uncertainties. Then, with the use of
the MR linac, the treatment plan is created – the plan can be adapted to shape or to position.
When adapting to position, usually the CT image that is taken before treatment is matched to
a MR image through a process known as rigid registration. Once the two images are
registered, the position of the iso-centre can be updated, allowing for the recalculation of the
plan. Adapting to shape involves adapting the plan based on changes in patient anatomy;
once again, the pre-treatment plan CT image is registered with the MRI, and if necessary, the
contours of volumes are edited. Finally, the treatment plan is reoptimized based on these
adjusted contours. This methodology is illustrated in Figure 1, in a study conducted by
Winkel et al. (2019), whereby they compared the adapt to position and adapt to shape
workflows on the Elekta Unity MR linac.
Figure 1: Comparison and overview of the “adapt to position” and “adapt to shape” workflows, which can be done with the
use of a MR-linac (Source: Winkel et al, 2019)
The MR-linac is a new, promising imaging modality that has been developed for adaptive
radiotherapy, allowing for extremely precise treatments with minimal side effects. It consists
of a linear accelerator (linac) combined with a magnetic resonance imaging (MRI) unit.
Through the fusing of these two aspects, the aim is to visualise the superior soft tissue
contrast provided by MRI during the course of treatment, enabling for the visualisation of
anatomical changes that may occur during the treatment process. Through utilising MRI on a
daily basis, position verification of the patient based on soft tissue contrast can be done,
alongside replanning if necessary. On the other hand, through keeping the beam on, MRI can
be used to analyse time resolved dose accumulation and finally, the functional aspect of MRI
allows it to be used to assess the response to treatment. The MR-linac is therefore a technique
that can aid in increasing the efficiency of radiotherapy treatment. By being able to see
anatomical changes in real time and subsequently adapting the treatment plan, the plan can be
optimised to provide a better conformity to the tumour, whilst sparing normal tissues.
Overall, MRI guided radiotherapy with the MR-linac is a new imaging technique that can
drastically advance the quality of treatments.
As with normal radiotherapy treatments, the initial treatment planning process is done the
same, by using CT scans with multiple imaging modalities to define and delineate the
volumes of the tumour and organs at risk. In sites where motion is prevalent, pre-beam
imaging involving techniques such as deep inspiration breath hold are employed to assess
inter-fractional motion changes. Although it is not available yet on clinical systems, 4D-MRI
has shown interest, due to its ability to reduce delineation uncertainties. Then, with the use of
the MR linac, the treatment plan is created – the plan can be adapted to shape or to position.
When adapting to position, usually the CT image that is taken before treatment is matched to
a MR image through a process known as rigid registration. Once the two images are
registered, the position of the iso-centre can be updated, allowing for the recalculation of the
plan. Adapting to shape involves adapting the plan based on changes in patient anatomy;
once again, the pre-treatment plan CT image is registered with the MRI, and if necessary, the
contours of volumes are edited. Finally, the treatment plan is reoptimized based on these
adjusted contours. This methodology is illustrated in Figure 1, in a study conducted by
Winkel et al. (2019), whereby they compared the adapt to position and adapt to shape
workflows on the Elekta Unity MR linac.
Figure 1: Comparison and overview of the “adapt to position” and “adapt to shape” workflows, which can be done with the
use of a MR-linac (Source: Winkel et al, 2019)
