Dosimetric comparison of lateral opposing fields in moderate-to-severe active TAO retro-orbital irradiation

Background: To evaluate and compare the dosimetric advantages of lateral opposing fields (LOF) and intensity-modulated radiation therapy (IMRT) in moderate-to-severe active thyroid-associated ophthalmopathy (TAO) retroorbital irradiation. Materials and Methods: Fifty-eight TAO patients who underwent radiotherapy from 2012 to 2018 were chosen. LOF and IMRT plans were separately developed for each patient. The independent samples t-test was used to compare the differences among conformity index (CI), homogeneity index (HI), dose received by the maximal 95% of the target (D95), and organs-at-risk (OAR) doses between the two groups. Pearson’s correlation analysis was used to analyze the relationship between exophthalmos and OAR dose. Results: CI and HI in the IMRT group were better than those in the LOF group, but crystal, eyeball, and optic nerve dose were higher than those in the LOF group (P<0.05). There was no significant difference in D95 (P=0.261). In the LOF group, exophthalmos was negatively correlated with crystal and eyeball dose (P= 0.000). However，there was no correlation between these indicators in the IMRT group (P>0.05). In the LOF group, the median observation time was 26 months; the CT value of post-ball adipose tissue, exophthalmos, and clinical activity score was lower than those before treatment (P=0.000), and eyesight was better than that before treatment (P=0.000); 10% (2/20) had dry eyes and 5% (1/20) had decreased vision. Conclusions: LOF is safer than IMRT and can make full use of the high exophthalmos of TAO patients to reduce OAR dose.