Feasibility and impact of image-guided radiotherapy during chemoradiotherapy for anal squamous cell carcinoma

Objective Data on image-guided radiotherapy (IGRT) and interfractional anatomical changes during definitive chemoradiotherapy (CRT) for anal squamous cell carcinoma (ASCC) remain limited. This retrospective study aimed to quantify interfractional clinical target volume (CTV) changes and their geometric relationship to the planning target volume (PTV). Methods CBCT scans were retrospectively analyzed after alignment to bony anatomy for 34 patients undergoing definitive CRT for ASCC. Clinical target volumes (CTVs) were delineated on each CBCT and compared with the corresponding CTV and planning target volume (PTV) from the planning CT. The frequency, magnitude, and spatial distribution of CBCT-derived CTV extensions exceeding 10 mm beyond the planning PTV were quantified. Results A total of 212 CBCTs from 34 patients were analyzed (median 6 per patient). The mean CTV volume decreased by 6.9% compared with the planning CT, corresponding to a mean absolute reduction of 71.3 cm 3 (95% CI: 46.9–95.7; p < 0.001). Progressive reduction was observed over time, with the median deviation increasing from −1.5% at fraction 1 to −12.7% at fraction 30 ( p < 0.001). Overall, 36.8% of the analyzed CBCT scans demonstrated at least one CTV extension exceeding 10 mm beyond the planning PTV. Of these deviations, 66.3% involved the anal canal and 33.7% the inguinal region. Conclusion The observed geometric variability demonstrates interfractional anatomical variation rather than direct clinical or dosimetric consequences, suggesting that routine PTV margin reduction should be approached cautiously and requires dosimetric validation. Daily image guidance may facilitate the detection and correction of interfractional anatomical changes, and adaptive radiotherapy strategies may further improve target coverage while enabling individualized margin concepts.