Radiotherapy is used in cancer treatment to eradicate microscopic cancer cells to lower the risk of recurrence. The radiotherapy plan must account for organ movement inside the body to ascertain adequate dose is delivered to the target. Knowledge of the magnitude is crucial to radiotherapy treatment planning. This study aims to quantify the movement of the sigmoid colon between different fractions of radiotherapy treatment (interfraction motion) and within the same fraction (intrafraction motion). This knowledge will help us determine the optimal margin to use in radiotherapy treatment planning.

Day-to-day positioning variations contribute to uncertainty in radiotherapy. The International Commission on Radiation Units and Measurements (ICRU) report 62 recommended a margin added to the clinical target volume (CTV) to produce the planning target volume (PTV). In ICRU report 83, this concept is further extended and refined to yield the internal target volume (ITV), which is defined as CTV plus a margin to account for uncertainty within the patient. Image-guided radiotherapy (IGRT) has been developed as a method to lower the margin required in PTV expansion. In current practice, IGRT methods mostly rely on bony anatomy alignment. Due to its nature, internal organ motion cannot be compensated by IGRT. Therefore, an ITV expansion is still required. The optimal ITV expansion margin depends on the magnitude of internal organ motion; the smallest margin that provides adequate coverage is preferred.

In this study, we aim to quantify organ motion of the sigmoid colon anastomosis site during radiotherapy treatment, including movement between fractions (interfraction) and movement within the same fraction (intrafraction).

Inclusion Criteria:

• Patients with pathologically confirmed sigmoid colon cancer, who have received surgical excision (AR or LAR).
• Anastomosis of the colon must be performed with a surgical stapler, with the staple visible on CT imaging.

Exclusion Criteria:

• Gross recurrent disease (defined as visible mass on CT imaging) in the pelvis.
• Other malignancy within the pelvic cavity.
• Previous surgery to the pelvic cavity other than AR/LAR.
• Pregnant or lactating females.
• Subjects who, in the opinion of the Investigator, may be non-compliant with study schedules or procedures.