Feasibility study of intratumoral gold marker placement and cone beam CT for
image- guided radiotherapy in rectal cancer

Organ motion and patient set-up is an important source of error in
radiotherapy. A method to correct for patient set-up is Electronic Portal
Imaging (EPI). EPI is based on x-ray imaging and aligning the patient to the
beam by matching on bone structures. However, soft tissue targets, such as
rectum, esophagus, prostate, cervix/ uterus, bladder, liver and breast, are not
visible on the x-ray images. Matching on bone structures is not sufficient in
these indications because organ motion will not be detected. Although new
technologies have implemented kV cone-beam CT into daily image guidance for
set-up and organ motion correction, tumors of the large bowel, like rectal
cancers are often not easily seen on kV cone-beam CTs due to additional non-
tumoral rectal wall thickening. Fiducial markers placed into the upper and
lower border of the rectal cancer could therefore substantially simplify the
identification of the tumor location and thereby better guide the radiation
treatment.

The aim of this study is to establish the use of VisicoilTM fiducial markers
and kV cone beam imaging for image guided radiotherapy. Our goal is to develop
an accurate technique to correct for inter-fraction organ motion based on
fiducial marker based kV cone beam image information of the tumor itself and to
theoretically align the radiation boost beam to the target. The final goal is
to implement safe tumor dose escalations in the future, thereby improving tumor
responses

This is an interventional, registration and a planning study. The first step in
this study is to determine the accuracy and the reproducibility of intratumoral
fiducial marker placement for CBCT image guidance. The placement of 3 fiducial
markers in the upper and lower border of the rectal tumor will be determined
for its accuracy in target definition in CBCT. The inter- and intra- observer
variation will be investigated by randomly showing the daily CBCT scans of 3
patients to different observers.
Consecutively, the daily organ and tumor displacement will be analyzed and
recorded in the registration study. Finally, the planning study will evaluate
the feasibility of dose escalation to the tumor target in a theoretical
simultaneous boost treatment.
Finally, the standard TME surgical procedure of the rectal cancer will remove
the markers from the body to avoid any long-term side effects of the markers.

The additional CBCT scans do not require extra appointments for the patient.
One kV CBCT of the pelvis results in a weighted dose of about 22 mSv [18].
Taking into account 2 extra CBCTs three times a week this results roughly in an
extra dose of 17 x 2 x 22*10-3 = 0.75 Gy. The reference CT scan is not made for
diagnostic purposes, and will in principle not be judged by an experienced
radiologist. Furthermore CBCTs will cost extra time for the patient. The
acquisition of 1 CBCT costs aproximately 1*30*. This results in an extra time
of 3* per fraction.
The main risks of marker placement with endoscopy are bleeding and perforation.
These risks are low; the incidence reported in a large screening program using
sigmoidoscopy was 0.002% for perforation and 0.03% for major bleeding[19].
No direct benefits are expected for participants in this study.