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

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) retro-orbital irradiation.

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.

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.

LOF is safer than IMRT and can make full use of the high exophthalmos of TAO patients to reduce OAR dose.