ESTRO 2025 - Abstract Book

S4107

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

organ motion or anatomical changes. Daily cone-beam CT (CBCT) in Image-Guided Radiotherapy (IGRT) enables precise monitoring and adaptation, enhancing treatment accuracy throughout the course of radiotherapy.

Material/Methods: Twenty-five patients with locally advanced cervical cancer were treated with 45 Gy in 25 fractions using the target volumes and dosimetric constraints of EMBRACE-II protocol. MV-CBCT scans were performed daily on Halcyon Linac, and data from Days 1, 2, 3, 6, 11, 16, 21, and 25 were analysed. Changes in volume and centroid position were assessed, including shifts in the centroid of mass (COM) over the course of treatment. The Dice similarity coefficient and conformity index (CI), were calculated from Eclipse TPS v16.1 to assess CTV consistency. Dosimetric variations in parameters such as D2cc (Bladder and Rectum) were recorded throughout the course of radiotherapy.

Results:

The CTV-LR (Clinical Target Volume Low-Risk) was fully encompassed within the ITV in 90% of CBCT scans, in 19 patients throughout the course of radiotherapy. There was significant correlation between variation of Bladder volume and D2cc with the volume of CTV-LR out of ITV, on linear regression analysis. (p= 0.01 & 0.027). A trend of shift in CTV centroid position, particularly along the superior-inferior (y) and anterior-posterior (z) axes, were observed to be correlating to the variation in bladder and rectal volumes. (p=0.26 & p=0.61 respectively). The Dice similarity coefficient for CTV-LR decreased to 0.79 by Day 25 from the treatment planning scan, reflecting inter fractional variations in coverage. Similarly, the conformity index for CTV-LR showed a slight variation, decreasing to 0.88 by Day 25, indicating a consistent but gradual reduction in treatment conformity. Conclusion: Daily IGRT in cervical cancer radiotherapy validates the adequacy of ITV margins and underscores the impact of organ motion on target coverage and dosimetry. Bladder and rectal volume variations significantly influence CTV-LR consistency, as seen in centroid shifts and reductions in Dice similarity and conformity indices by Day 25. While ITV margins ensure coverage in most cases, these findings highlight the importance of integrating IGRT with adaptive strategies to address inter-fractional changes, ensuring precise and effective treatment delivery.

Keywords: CBCT, IGRT, Cervical cancer