Clinical application of cone beam CT in intensity‐modulated radiotherapy for the moderate to severe thyroid‐associated ophthalmopathy

Background Cone beam CT (CBCT) has been reported for intensity‐modulated radiation therapy (IMRT) of various cancers due to its real‐time imaging capabilities, but studies on its application in orbital diseases, particularly moderate to severe thyroid‐associated ophthalmopathy (TAO), remain limited. Purpose To investigate the clinical applications of CBCT in IMRT for TAO, with a focus on setup accuracy, dosimetric impact, and short‐term changes in eye prominence. Methods A total of 115 CBCT scans from 17 TAO patients were analyzed. Pretreatment CBCT scans were registered to the planning CT using bony anatomy. The resulting setup errors were quantified in three translational directions. Residual shifts after online correction were tracked to assess interfractional variation and trends throughout the treatment course. Dose‐volume histogram from plans recalculated with simulated setup errors (mean and maximum values) were compared to the original plan. Eye prominence was measured on serial CBCT scans, and its correlation with cumulative dose was analyzed. Results Mean setup errors were (–0.16 ± 1.09) mm, (–0.19 ± 1.18) mm, and (–0.55 ± 1.05) mm in X, Y, and Z directions. Errors significantly decreased planning target volume (PTV) conformity ( p < 0.05) and increased D mean , D max , D 2%, and D 50% ( p < 0.05). For organs at risk (OARs), maximum and mean doses increased, with left lens D max increasing by 21.3%, 32.7%, 33.8%, and 13.2% for mean setup errors and X, Y, and Z maximum errors. Right lens showed similar increases. A correlation between CBCT‐measured eye prominence and total radiotherapy dose was observed, with the binomial fit: y = 22.43 + 0.12x − (9.94 × 10 −3 ) x 2 ( R 2 = 0.96). Mean eye prominence decreased significantly from (21.74 ± 1.74) mm pre‐treatment to (20.94 ± 1.73) mm after 18 Gy of IMRT ( p < 0.05). Conclusion CBCT image guidance for IMRT in moderate to severe TAO improves target conformity and coverage while reducing OAR doses, particularly for lenses. CBCT also shows promise for short‐term efficacy assessment of eye prominence in radiotherapy patients which may alleviate patient anxiety with preliminary observation thereby improving patient compliance with radiotherapy.