Prone versus supine positioning for rectal cancer irradiation using volumetric modulated arc therapy

Neoadjuvant radiotherapy has an important role in the treatment of rectal
cancer patients, leading to a significant decrease in local recurrence rate.
Conventionally patients are treated with a 3-beam 3D conformal radiotherapy
(3D-CRT) technique, combining two lateral opposed fields with a posterior
field. In the past decade advanced irradiation techniques such as volumetric
modulated arc therapy (VMAT) have been developed. This is a new
intensity-modulated radiation therapy treatment technique employing single or
multiple radiation beams that rotate around the patient, resulting into highly
conformal dose distributions in the target volumes, better sparing of organs at
risk (OARs) and faster treatments.

The most important OARs in rectal cancer irradiation are the small bowel and
colon (i.e. bowel bag), in which acute radiation enteritis, chronic diarrhea,
and less frequent bowel stricture, perforation and hemorrhage can be caused.
Multiple studies have shown a relationship between the dose to the bowel bag
and the incidence of intestinal toxicity. VMAT substantially reduces high-grade
acute and late toxicity compared to 3D-CRT.

Since January 2018 VMAT is available for rectal cancer patients at the
Department of Radiotherapy in the Radboud University Medical Centre, with
approximately 70 patients treated per year. With the use of 3D-CRT patients
were irradiated in prone position, using a belly board and full bladder
protocol, in order to reduce the amount of small bowel in the high dose region
by pushing the bowel bag away from the target volume. However, in comparison
with supine positioning, the combination of prone positioning and a belly board
is known for less setup reproducibility and possible patient discomfort. Using
highly conformal VMAT, prone positioning with the use of a belly board may no
longer be superior to supine positioning when it comes to bowel bag dose. Only
few studies have described the role of patient positioning in VMAT for rectal
cancer, though including a relatively small number of patients, with
conflicting results

Concluding from the above mentioned items, we assume that:
* Rectal cancer irradiation with VMAT results into highly conformal dose
distributions and reduced intestinal toxicity, compared to conventional 3D-CRT.
* In terms of setup reproducibility and patient comfort, supine positioning is
superior to prone positioning using a belly board.

We hypothesize that:
1. Using VMAT, the combination of prone positioning and a belly board may no
longer be superior compared to supine positioning in terms of OARs sparing.
2. Optimal patient positioning may be different for male and female patients
due to variations in pelvic anatomy.

The primary aim of the study is to investigate the effect of patient
positioning on OARs sparing in rectal cancer patients treated with VMAT
(hypothesis 1).

The secondary aim of the study is to investigate the possible differences in
optimal patient positioning for male and female patients (hypothesis 2).

* All patients will receive 1 extra planning-CT scan in supine position,
alongside the standard planning-CT scan performed in prone position using a
belly board. No intravenous, oral or rectal contrast will be used
* Delineation of target volumes (CTV, PTV) and organs at risk (bowel bag,
bladder, femoral heads) on both CT scans
* For each CT scan a VMAT-treatment plan will be created
* For each patient treatment plans will be dosimetrically compared
* All patients will be irradiated in prone position using a belly board,
according to current treatment protocol.

In conclusion, all patients will be asked to undergo 1 extra planning-CT scan
compared to standard protocol. As a result of this scan, they will receive an
additional mean effective dose of 10 mSv. In theory this extra CT scan may
contribute to radiation-induced carcinogenesis, although the increased risk is
estimated to be negligible compared to radiotherapy dose. By way of comparison,
the average background radiation in the Netherlands is 2,5 mSv per year. No
extra hospital visits or invasive treatments will be necessary. All planning
scans will be performed in one session.

All patients will be irradiated in prone position using a belly board. In this
way, treatment is similar to those of non-participating patients.