Inter-Fractional Spatial and Dosimetric Variations of Target Volumes and Oars In Rectal Cancer Patients Undergoing Radiotherapy on the CT-Linac
Abstract
Purpose
Inter-fraction variations in bowel and bladder filling during rectal cancer radiotherapy can cause positional and dosimetric deviations, potentially compromising treatment accuracy. This study aimed to evaluate the morphological consistency and dosimetric variations of target volumes and organs at risk (OARs) using an integrated CT-Linac system, providing data to optimize the timing for adaptive radiotherapy.
Methods
A retrospective analysis was conducted on 20 rectal cancer patients receiving 50 Gy intensity-modulated radiotherapy in 25 fractions. Fan Beam CT (FBCT) images were acquired after the 1st, 5th, 10th, 15th, 20th, and 25th fractions. All images were non-rigidly registered to the planning CT. Target volumes (CTV, PTV) and OARs (bladder, small intestine, femoral heads) were contoured. Morphological consistency was assessed using the Dice Similarity Coefficient (DSC) and Hausdorff Distance (HD), alongside volume and dosimetric parameter analysis. Paired comparisons were made across multiple time points.
Results
Target volumes (CTV, PTV) showed high spatial stability during the initial 1-10 fractions (DSC >0.89, HD ~8-9 mm), but stability significantly declined after 15 fractions (DSC 13 mm by the 20th fraction). Although target volumes progressively decreased, the D95 dose remained stable. Substantial variability was observed for OARs. The bladder exhibited significant morphological inconsistency (DSC: 0.52±0.06, HD: 35.19±1.98 mm) and a negative dose-volume correlation; a smaller volume at the 10th fraction coincided with a higher mean dose. The small intestine showed even lower consistency (DSC: 0.43±0.02, HD: 48.96±3.58 mm) and a significant negative dose-DSC correlation. In contrast, the femoral heads remained stable throughout treatment (DSC: 0.81±0.03, HD: 10.72±1.05 mm), with consistent dosimetry.
Conclusion
Significant inter-fraction variations were observed for target volumes and OARs (bladder, small intestine) in rectal cancer radiotherapy, with target stability notably declining after 15 fractions. FBCT-guided adaptive radiotherapy is suggested as a strategy to mitigate these variations and improve treatment accuracy.