Setup Error Differences and Margin Expansion Among Rectal Cancer Segments: Consistency Evaluation Between EPID and...

To analyze the differences in setup errors among patients with rectal cancer in the upper, middle and lower segments and evaluate the consistency between electronic portal imaging device (EPID) and iSCOUT image-guided techniques.

We retrospectively included 277 patients with rectal cancer treated with radiotherapy at our center between January 2020 and June 2025 and divided them into upper (n = 23), middle (n = 115) and lower (n = 139) rectal cancer groups based on their pathological results. Setup errors and corresponding planning target volume (PTV) margins were calculated and compared across groups and between EPID and iSCOUT, with correlation and agreement between the two image-guidance methods further evaluated.

The required external margins (cm) in the left-right (LR, X), superior-inferior (SI, Y) and anterior-posterior (AP, Z) directions for patients with upper, middle and lower rectal cancers were (0.44, 0.83, 0.57), (0.50, 0.69, 0.59) and (0.43, 0.68, 0.53), respectively. The setup errors between the upper and lower rectal cancers in the Z direction (p = 0.013) were significantly different. The X and Y directions between the EPID and iSCOUT groups in the different segments of rectal cancer were significantly different. The registration results of the EPID in the X, Y and Z directions significantly correlated with the corresponding iSCOUT error data (p < 0.001). The 95% consistency limits of EPID and iSCOUT measurement results in the X, Y and Z were -3.65 to 4.38, -3.69 to 4.21 and -3.66 to 3.57 mm, respectively.

In the Y direction, different margin expansions should be adopted based on the different rectal cancer treatment segments. The iSCOUT guidance technology can replace the EPID when necessary.