PurposeDuring external beam radiation therapy (EBRT), rectum position and shape undergoes daily change due to patient bladder and rectal filling. Accommodating these changes is a contributing factor for the planning margins applied for rectal cancer EBRT treatment. We aim to find the magnitude of these changes and design treatment strategies to accommodate these changes.Methods and MaterialsNine patients who underwent treatment with IMRT were selected for this retrospective study. KV CBCT was acquired on every fraction of the treatment. The rectum, bladder, and anus were contoured on one fraction of each week of treatment. The CBCTs were rigidly registered through bony registration and the contours were transferred to the planning CT. The rectum contours on the planning CT were expanded uniformly and the expansions needed to cover all the transferred rectum contours were obtained.ResultsForty-five CBCTs were used for this study. For 5 patients, an expansion of 1 cm of the simulation day rectum completely covers the treatment day rectum for all 5 fractions. The maximum expansion needed for all the patients is 3.25 cm. An expansion of 1 cm, 1.25 cm and 1.5 cm covers 71% (32 of 45), 89% (40 of 45) and 91% (41 of 45) of the treatment fractions respectively.ConclusionFor majority of fractions, an expansion of 1.25 cm of planning day rectum is sufficient to accommodate the interfractional rectal motion and deformation. However, large interfractional rectal movement exists and may need separate strategy, such as adaptive therapy, to accommodate. PurposeDuring external beam radiation therapy (EBRT), rectum position and shape undergoes daily change due to patient bladder and rectal filling. Accommodating these changes is a contributing factor for the planning margins applied for rectal cancer EBRT treatment. We aim to find the magnitude of these changes and design treatment strategies to accommodate these changes. During external beam radiation therapy (EBRT), rectum position and shape undergoes daily change due to patient bladder and rectal filling. Accommodating these changes is a contributing factor for the planning margins applied for rectal cancer EBRT treatment. We aim to find the magnitude of these changes and design treatment strategies to accommodate these changes. Methods and MaterialsNine patients who underwent treatment with IMRT were selected for this retrospective study. KV CBCT was acquired on every fraction of the treatment. The rectum, bladder, and anus were contoured on one fraction of each week of treatment. The CBCTs were rigidly registered through bony registration and the contours were transferred to the planning CT. The rectum contours on the planning CT were expanded uniformly and the expansions needed to cover all the transferred rectum contours were obtained. Nine patients who underwent treatment with IMRT were selected for this retrospective study. KV CBCT was acquired on every fraction of the treatment. The rectum, bladder, and anus were contoured on one fraction of each week of treatment. The CBCTs were rigidly registered through bony registration and the contours were transferred to the planning CT. The rectum contours on the planning CT were expanded uniformly and the expansions needed to cover all the transferred rectum contours were obtained. ResultsForty-five CBCTs were used for this study. For 5 patients, an expansion of 1 cm of the simulation day rectum completely covers the treatment day rectum for all 5 fractions. The maximum expansion needed for all the patients is 3.25 cm. An expansion of 1 cm, 1.25 cm and 1.5 cm covers 71% (32 of 45), 89% (40 of 45) and 91% (41 of 45) of the treatment fractions respectively. Forty-five CBCTs were used for this study. For 5 patients, an expansion of 1 cm of the simulation day rectum completely covers the treatment day rectum for all 5 fractions. The maximum expansion needed for all the patients is 3.25 cm. An expansion of 1 cm, 1.25 cm and 1.5 cm covers 71% (32 of 45), 89% (40 of 45) and 91% (41 of 45) of the treatment fractions respectively. ConclusionFor majority of fractions, an expansion of 1.25 cm of planning day rectum is sufficient to accommodate the interfractional rectal motion and deformation. However, large interfractional rectal movement exists and may need separate strategy, such as adaptive therapy, to accommodate. For majority of fractions, an expansion of 1.25 cm of planning day rectum is sufficient to accommodate the interfractional rectal motion and deformation. However, large interfractional rectal movement exists and may need separate strategy, such as adaptive therapy, to accommodate.