Quantifying Interfractional Motion in Patients With Anorectal Cancer During Radiotherapy and Recommendations of Optimal Planning Target Volume Margins

Introduction

This study aimed to assess pelvic organ motion and volume changes during long-course radiation therapy (LCRT) for anorectal cancer and propose an optimal planning target volume (PTV) margin.

Method

This retrospective study analysed 526 daily cone-beam computed tomography (CBCT) from 22 patients with anorectal cancer undergoing LCRT. Patients followed a bladder-filling protocol by drinking water before treatment. The bladder, rectum and mesorectum were delineated on planning CT and CBCTs. PTV margins were calculated using the van Herk formula, and margins to cover 95% interfractional motion were also evaluated.

Results

Bladder volume exhibited the higher variability (mean ± standard deviations, 271.3 ± 130.5 mL), compared to the mesorectum (111.3 ± 26.2 mL) and rectum (50.6 ± 15.6 mL). Mesorectum motion showed no significant correlation with bladder volume changes but was significantly associated with rectal volume (r = 0.566; p < 0.001 in the superior mesorectum). Anterior mesorectum motion was larger compared to other directions, particularly in the superior region, with systematic and random variations of 4.4 and 3.5 mm. Using the van Herk formula, PTV margins for the anterior mesorectum were 5.8, 4.9, and 3.3 mm for the superior, middle and inferior regions, respectively. Similarly, margins to cover 95% interfractional movement extended up to 6.7 mm in the superior–anterior region.

Conclusion

Bladder volume varied significantly during LCRT, while mesorectum motion was influenced by rectal volume changes. A 5-mm PTV margin was sufficient for the mid to lower mesorectum, with a larger 7-mm margin needed for the superior–anterior mesorectum. Incorporating 95% interfractional motion coverage further refined these margin recommendations.