Repeated CT study in patients with cervical cancer to assess organ motion for adaptive radiotherapy and proton therapy planning

At this moment, photon irradiation is the standard of radiotherapy treatment in
the Netherlands. Photon irradiation in patients with cervical cancer is
accompanied by substantial acute and late side effects. A new approach to
pelvic radiotherapy is proton irradiation. Due to the physical properties of
protons, it can be expected that proton therapy will offer improved sparing of
organs at risk compared with the state-of-the-art photon therapy. Hereby,
toxicity during and after radiotherapy can be reduced.
In previous studies it has been shown that the pelvic target volume in cervical
cancer has complex shapes and can be subject to large interfraction
deformations due to changes in bladder and rectum filling and changes in tumor
volume.
Since dose distributions in proton therapy are much more sensitive to
anatomical variations than in photon therapy, it is necessary to find a
strategy to account for these uncertainties. A promising approach is the use of
plan libraries. In this method a pretreatment generated plan library based on a
full and empty bladder CT-scan is used. Before each proton therapy treatment
fraction a CT-scan will be acquired, in which the target and organs at risk
will be segmented automatically. Based on the contour sets obtained the best
fitting treatment plan will be selected out of the plan library. Next the
selected treatment plan will be adapted to the observed variations in density
along the proton beam paths.
In this study we want to develop this abovementioned strategy.

To develop a strategy for online-adaptive radiotherapy with protons in patients
with cervical cancer.

Patients will undergo two CT-planning scans, one with full and one with empty
bladder as used in standard practice, which will be used for regular treatment
planning. These scans will also be used to generate a plan library for this
study. Four additional repeated CT-scans will be made for the study; these
additional scans will be made about once a week during the treatment period and
will be used to document the interfraction deformation and position of the
target volume during treatment due to changes in filling of bladder and rectum,
and changes in tumor volume due to tumor regression. Furthermore, these scans
will be used to test the accuracy of the pretreatment generated plan library
and to determine the amount of robustness that is still needed in treatment
planning to compensate for residual uncertainties.

Participation in this study will cost patients 15 minutes per extra CT-scan
made, so 60 minutes for the whole study. The additional radiation exposure due
to the repeated CT-scans will be negligible compared to the radiation dose
delivered by the radiotherapy. The estimated absorbed dose of a pelvic CT-scan
at our CT-scanner is about 10 mGy. Therefore the additional radiation dose for
this study will be about 4x10 mGy=40 mGy, which is about 0.09% of the total
dose received.
Since patients will be treated with the standard photon therapy, there are no
benefits to participating in this study, other than contributing to the
improvement of the treatment for patients in the future.